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Kraft '74 Single Stick Radio - Rebuild Thread

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Kraft '74 Single Stick Radio - Rebuild Thread

Old 03-01-2011, 08:24 AM
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doxilia
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Default Kraft '74 Single Stick Radio - Rebuild Thread

I recently decided to embark on the idea of trying out a single stick radio after several years of using the standard two stick transmitters common today. Since single stick radios are not currently sold commercially (that may change in the near future), I decided to purchase a vintage Kraft single stick '74 series transmitter and re-build it using modern circuitry and new components. One of the catalyzers in deciding to undertake this project has been the availability of a fantastic affordable computerized radio encoder made available to us by Gordon Anderson. The encoder is called the MicroStar 2000 (MS2K for short) and is a project that was started in 2000 using the design and programming of the Ace MicroPro 8000 (MP8K) as a basis. For those not familiar with the Ace MP8K, it is an 8 channel radio that was conceived, designed, built and sold in North America and offered (and still does) features that were considerably more advanced than many of the more standard commercial offerings even today. The MS2K has upped the ante with even more flexibility and features and is an all around great foundation for a custom self-built 8 channel radio control system. Gordon's wesbite is located here:

http://mstar2k.com/

In concert with the availability of the MS2K encoder, there has recently been considerable activity in the design and production of high quality three axis (aileron, elevator, rudder) single stick gimbals - notably by Andy Horka. I'm happy to say, a fellow Canadian. Andy's work was recently published in the Canadian Section of the March 2011 issue of MAN. I am not certain whether this section might not be included in American issues of MAN. If you do not receive MAN or your issue doesn't have Andy's article in it, you can follow his blog at the following website:

http://andysrecroomrc.blogspot.com/

This thread will be a basic documenting of the rebuild of the Kraft series '74 single stick transmitter - a KPT-5CS. The two main components that will be re-used are the transmitter case and the original Heathkit/Kraft three axis gimbal. All other component switches and knobs will be new and will be laid out in the case to suit my concept of a single stick (SS) transmitter. Whether the layout proves to be effective is somewhat experimental since I have never used a SS transmitter for actual flying. The layout is also somewhat dictated by the size of the case and the need to fit certain switches and components in specific locations in the case.

The encoder is available in both 10 bit (1024) and 12 bit (4096) resolutions. Kees Talen has been the key person to assemble and offer kits of Gordon's various radio related designs (beside the encoder, he has also designed a synthesized FM RF deck as well as an accompanying receiver) and makes the process of obtaining all the assorted parts for the encoder a very simple one. Kees website is located here:

http://www.qsl.net/k5bcq/MS2K/MS2K.html

It should be mentioned that Kees assembles these kits and offers them to us at no profit. His interest is in getting more modelers involved in building and flying their own self-built radios - a noble cause I might add.

While I wait for the encoder components to arrive, I have begun the planning and layout of the various switches and parts in the radio. I will also be re-finishing the exterior of the transmitter case which will do away with the vintage and classic "golden" Kraft look of yesterday. While I would have liked to preserve the original appearance, there are a number of good reasons for wanting to refinish the case including the need to "close up" some of the existing openings which will no longer be used. The existing finish is also not in optimal state. Below are a couple of drawings of my tentative transmitter layout and finished look.

As a final comment, while I'm aware that this is a classic pattern forum, I've decided to post the build here since I like you bunch and it seems that there might be some interest in other classic pattern buffs rebuilding their vintage radios. Mark (hook57), for one, might be posting alongside with me as he also has an interest in rebuilding a Kraft or Proline transmitter and is a single stick transmitter user.

'nough said for one post!

David.
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Old 03-01-2011, 08:47 AM
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doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

An overall picture of the original Kraft '74 radio is shown below. The main encoder circuitry of the rebuilt radio will go along with one of the modern 2.4 GHz RF decks for signal broadcast which allow for user interference free concurrent radio use. My tentative idea is to use a Spektrum 2.4 GHz RF deck but case size, RF module dimensions and installation options might require this particular radio to use another brand of RF deck. Posted below are also three battery options for powering the transmitter including:
[ul][*] 8s AA NiMH 1650 mAh pack[*] 3s LiFe 1500 mAh pack[*] 3s LiPo 1500 mAh pack
[/ul]
David.
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Old 03-01-2011, 09:10 AM
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doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

For those who enjoy looking at things like schematic diagrams of electronic circuits, here is a breakdown of the design of the MS2K encoder. The first pic shows the encoder PCB load map. The actual PCB is quite small 3.8" x 1.9" in size. The second pic shows the encoder block diagram illustrating how and where the various switches, RF deck and so forth interface to the encoder. The last pic shows the actual encoder schematic. The various IC2 blocks represent the single main programmable integrated circuit (PIC). It is separated into various blocks for simplicity in the drafting and reading of the schematic.

If you are planning to re-build a radio with the MS2K encoder, becoming familiar with these three pictures is highly recommended. An encoder can be built and made operational without understanding the schematic, but I would say that understanding the load map and block diagram is a must. I should mention that these drawings refer to the latest design revision of the encoder - version 5. This last revision is very similar to the previous v4 except that USB interfacing with a computer has been added directly on the encoder board. This is a very useful feature as many computers today no longer have the older RS-232 9 pin serial port which was used in previous versions of the encoder for programming and firmware update installation. In encoder v5, interfacing with a laptop can be done with a simple USB cable.

David.
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Old 03-01-2011, 09:50 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

David, this is cool. I recently obtained a Kraft single stick, thought I would send it someday to Radio South for updating but might not now. I wonder if I can accomplish what you are doing by following this thread.

Thanks a lot for the detailed posts.
Old 03-01-2011, 10:15 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

David,

if you can solder, you can do it!

Or at least I think so... first, I'll have to see if I'm able to do it before making bold statements! [:-] However, since I'm sure you are a proficient welder, you may well find this a walk in the park. Me, I'll have to spruce up some skills! No blow torches though in this project I've been told

Thanks for your enthusiasm regarding the project.

David.
Old 03-01-2011, 04:42 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

ORIGINAL: dhal22

David, this is cool. I recently obtained a Kraft single stick, thought I would send it someday to Radio South for updating but might not now. I wonder if I can accomplish what you are doing by following this thread.

Thanks a lot for the detailed posts.
You guys gotta quit buying those single sticks, I can't keep up with the price spikes!

If you want to make use of it now David, depending on the year (Gold Medal or Series type) an XPS universal deck is an easy hack into the encoder output; or you could mount the module inside with a cable to the antenna.

Mark

ps: I'll post the upgrades I plan to do as soon as I can; keep it coming David (the other David!).
Old 03-01-2011, 07:57 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Keep it coming and keep it technical. I was going to do this with a 7Z but sold it. Still would like to.
Old 03-02-2011, 03:19 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Thx Hook, now you all have me thinking.
Old 03-02-2011, 03:32 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

I have a half a dozen old Heath Kit single sticks in storage back in the USA, maybe I could do the same with one of them. I had considered converting them to narrow band FM, but never quite got around to it. With retirement only a year away, I may try the 2.4 conversion with the new encoder.

John
Old 03-04-2011, 08:15 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

John,

not sure if your radios are Heathkit or just your gimbals. In any case, I recalled seeing a Heathkit conversion by Danny Miller so here it is for fun.

These older gimbals strike me as being more robust and better designed than the newer Kraft sticks although the later ones included the trim in the knob. If you opt for digital trims (nicer IMO) though, then the analog trims will not be needed. Heck, with both the autotrim and preset buttons on the MS2K, you can do away with trims all together - something to think about for a scratch build using a case of ones own choice.

David.
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Old 03-04-2011, 08:56 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

David-

WAY COOL!!! I don't understand a word of what is written, but I love it.

I so wanted a single stick transmitter. Joe Bridi flew single stick!

Brian
Old 03-04-2011, 09:22 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

While continuing to wait for the encoder to arrive, I have been playing around with some other vintage Tx's to get a feel for things.

I received a Kraft KPT-7C yesterday with corresponding plug-in AM frequency module KPR-7C receiver. There were no batteries in the Tx so I tested things out with a spare set of Tx/Rx batteries to check the functionality of the electronics. The Tx seemed to be in order powering up with the RF meter also showing power output. The receiver however didn't seem to budge at all when powered up with a Kraft switch and servo which I know work. Either the Rx is shot or the module is. In either case, without being narrow banded, I think these AM RF decks are unfortunately of little use to us.

I worked on the Tx with the idea of keeping the original encoder and simply installing a low cost 2.4 GHz deck. The batteries in these radios were of the 2/3 sub-C NiCd type and took up a fair deal of space in the transmitters. I removed the plastic battery case which provided a base for mounting the encoder board (these are much smaller than the early 70's boards since they make use of 4 IC's) and soft mounted the encoder back against the front of the case. This will provide plenty of space to mount a flat LiFe or 8s AA pack against the back of the case with velcro. I also went ahead and removed the functioning AM RF deck module apparatus since it will no longer be used. I was hoping to see if there was a way to use the plastic housing for the module to accommodate the 2.4 module but I'm not sure this will work. Removing the module housing leaves the top of the Tx with a large rectangular opening in front of the holes made for the antenna. With the help of a little aluminum plate, this area could be useful to produce an interface area for the 2.4 deck bind and status module. While there is some concern (or is it disdain) for products made in Asia for a number of reasons, I have decided to give a 2.4 unit a try on this Tx since it is a bit of an experimental/learning exercise. Since one is modding this oneself, it is easy to build in an option to install another better known and trusted RF deck if the Tx is put into serious use.

I have located the battery power leads, the corresponding charge leads (these will display voltage when the Tx is off for charging and will have no power when on - reason for which DPDT power switches are used on Tx's) and have made sense of the 6 leads going to the AM RF plug-in deck. The positive (red) and negative (black) power leads for the deck are routed via the two outer pins. One lead is for the antenna and one for the RF deck power meter which left two leads for the PPM signal. There are two leads, one yellow and one orange/black and I haven't figured out the purpose for the orange/black leads since it exhibits no voltage signal when powered up. I'll have to dig a little deeper. In any case, the yellow pin 4 lead is the PPM signal as confirmed by the signal variation as the switches and sticks are moved. Total potential variation for each channel seemed to be 0.06V with 0.03V change in voltage when deflecting a stick from neutral to full travel. Although I didn't check, trims are probably on the order of 0.01V or less (~25 to 30%).

Armed with this knowledge, if one takes the yellow, red and black leads and feeds them to the 2.4 deck, one should be golden in the conversion. There is presumably also a PPM I/O signal at the trainer/charge/Rx voltage multi-pin jack in order to receive singals from a buddy box and pass them on to the RF deck if the Tx is being used in master mode. Come to think of it, this is what the orange/black lead may be for. When the trainer switch is pulled, the RF deck no longer is fed from the resident encoder via the yellow lead and instead it receives the PPM signal from the slave (student) Tx over the multi-pin jack and sends it to the RF deck over the orange/black lead. I should have taken pictures of the rear of the encoder prior to affixing it against the front. It's just double sided tape so it can be done if needed.

The trainer jack consists (should be past tense) of 6 leads too. Two of them as mentioned are for battery charging (blue+ & purple-) and I imagine also to send current to the slave transmitter encoder and four other leads are, and I'm kinda guessing a bit here, for encoder send (yellow/bk), encoder receive (orange/bk), and, encoder enable - trainer switch powers it up (gray/wht). The fourth lead is black and is ground.

There is also a 4 pin jack in the bottom left of the Tx and judging by the colour of the leads, this not only can be used as battery charge jack as it is used in the sport series Tx's but it also might serve to pass Rx battery voltage to the Tx for display on the meter. In fact, it may also act as a direct servo controller (DSC). If it is, I'm just not sure how the signal is passed on to the Rx since the switch interface which presumably does the PPM I/O to the Rx would need to be at least a three pin connector. I received no switch with the Tx/Rx and the switch that comes with sport series radios is a two pin (+/-) Rx battery charge connector.

I'll have to do a little more voltmeter prodding to see what's what. In any case, the long discourse above regarding PPM outputs is also for the purpose of installing a simple 1/8" mini-jack for PPM output to a simulator. JR uses a very simple PPM out DSC/trainer interface with a 1/8" mono jack tip to pass PPM and somehow it also manages to engage the encoder without having to turn either the master Tx on (DSC function) or the slave Tx on (trainer function). I'm not sure how that is done but perhaps someone like Andy could illuminate here so we may implement the same simple means using a 1/8" jack. I have a particular dislike for those multi-pin cables. I don't know why.

With that said, I should be able to enable this Tx in legacy hybrid mode upon obtaining a fresh battery for it and a 2.4 deck for broadcast. It would be nice to add servo reversing as well as dual rates which is something which can be done on this encoder. This might require a little further inquiry with those who have narrow banded servo reversing D/R versions of the KPT-7C.

Here are a few pics I took while sorting things out.

David.
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Old 03-04-2011, 09:26 PM
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jlingrel
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

That is the same basic transmitter I have, but I have to wait until I get back to the USA to get them out of storage. I had planned to restore them but found that having them narrow banded was almost out of the question. I thought I could have justed added a Hitec or Futaba RF module. I was told that there would be a "type acceptance" issue if I did it and the cost of getting it approved by the FCC would be cost prohibitive.

Anyway, I am going to start aquiring the parts that I think I will need. I am going back for Chrismas and will drag one back to Abu Dhabi with me. Should be a fun project. When I get around to it, I will post here in this thread how it goes.

John
Old 03-04-2011, 09:47 PM
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doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Sounds good John. Single stick conversions are quite unique since they are few and far between. The MS2K is also a gold nugget of RC tech - especially when one considers that it was started over 10 years ago!

I should have added in the post above that the 7C and Phil Kraft Signature Series Tx's made use of excellent Spectrol pots on the three main axes. These can be seen in the photos above. They are of the 5K ohm 1/4" shaft type. The throttle stick uses a lower quality inexpensive pot of the same type used on the Aux channels 6 & 7. Lower quality smaller pots are also used on the vertical axis trim controls (throttle/elevator) while the horizontal axes of the gimbals have the trims engaging the master Spectrol pot itself - a nicer more precise arrangement I suspect. I'm not sure actually why Kraft didn't just use 4 good Spectrol pots for stick and trim and dispense with the vertical axis trim pots. It would have required a little stick re-design but I believe that this is just what Proline did. I might be wrong and they also used 3 pots per 2-axis stick assembly.

In any case, this 30 year old Tx has very smooth feeling sticks so I'm hopeful that the results when converted will have been worth while. This is all a very educational experience for me since I haven't played with the innards of radios for over 30 years!

David.
Old 03-05-2011, 04:59 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Doxilia,

Kraft designed the buddy box function to have both master and slave transmitters powered up when the cable is connected. When the buddy box switch is enabled, signal inputs from the slave transmitter is passed through to the master. Take a look around our favorite auction site for a copy of the Series 76 through 79 service manual which has this information. When I can get to a scanner, I can post some switch wiring information for you.

Batteries America sells a Sanyo 2/3 sub C and a generic cell that will fit in the Kraft battery box.
Old 03-05-2011, 07:55 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

David,

The 7C has a cable that plugs into the slotted connecter on the bottom of the TX for checking the receiver batteries. Don’t know about the direct servo control. I think there was a separate box for that.
Old 03-05-2011, 09:46 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

ORIGINAL: Michaelj2k

Doxilia,

Kraft designed the buddy box function to have both master and slave transmitters powered up when the cable is connected. When the buddy box switch is enabled, signal inputs from the slave transmitter is passed through to the master. Take a look around our favorite auction site for a copy of the Series 76 through 79 service manual which has this information. When I can get to a scanner, I can post some switch wiring information for you.

Batteries America sells a Sanyo 2/3 sub C and a generic cell that will fit in the Kraft battery box.
Thanks for the info.

I went ahead and measured a few more leads to check for voltages. I made a sketch of the lead arrangement in the multi-pin plug. The purple and blue leads go directly to the switch and are powered up when the Tx is off - these are the battery charge leads with the blue being Vcc+ and purple being ground. The Grey/white and black leads go to the encoder and are powered up when the Tx is on - these also have Vcc+ (grey/white) and ground (black) and are presumably to power up the slave Tx when the cable is plugged between two Kraft Tx's.

If the Kraft system works like all other trainer systems I'm familiar with, the master Tx is turned out in order to provide voltage to the RF deck and the slave Tx receives power from the master via the trainer cable. Since the grey/white lead has battery voltage when the Tx is powered up and goes to the encoder, it makes sense for it to provide power to the slave encoder.

I had assumed that I would measure PPM voltages at the yellow/red lead to send the signal to the master Tx when in slave mode and would "see" PPM voltages on the orange/black lead to receive PPM signals when in master mode. Until two Tx's are connected together, it will be difficult to assess whether the orange/black lead is for this purpose. I didn't however measure PPM at the yellow/red lead whether the trainer switch was enabled or not. One way or another though, these leads need to pass PPM in some configuration as they are the only two leads that can send encoder signals when in slave trainer mode. I suspect that the cable needs to be connected in order for the respective encoders to output or receive PPM signals.

A scan of the service manual of this particular area would help to sort this out. Pages concerning the wiring for the switch reversing, dual rate and snap/roll buttons would also help.

David.

P.S. I'LL HAVE TO REVISE THIS POST SINCE IT IS NOW CLEARER THAT IT CONTAINS WRONG "GUESSES" !!
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Old 03-05-2011, 09:55 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread


ORIGINAL: 8178

David,

The 7C has a cable that plugs into the slotted connecter on the bottom of the TX for checking the receiver batteries. Don’t know about the direct servo control. I think there was a separate box for that.
Mike,

that makes sense. I read that a cable existed for this and the appropriate voltages are indeed shown on the ESV for the Rx battery voltage. What I don't understand is why there are 4 leads at that jack in the Tx. Perhaps they are needed as the signal needs to pass via the encoder in order to provide adequate load on the Rx battery for proper voltage measurement. However, the plug at the other end on the model's switch presumably still only has two leads - red & black - so I'm not clear why 4 leads are needed. If you have such a cable it would be helpful to see how it is wired if it is not shielded by an outer sleeve. It would also be helpful to see the wiring of the KP-7C Rx switch if it has any more than the two standard power leads.

David.
Old 03-05-2011, 11:49 AM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

There is a separate Kraft contol box for directly controling the servos and I have one. It is like a servo tester in its operation and needs to be plugged directly into the servo. I have a Kraft Series 73 7C single stick transmitter which no longer works. And I have a Kraft Bicentennial Series '76 single stick system which is gold stickered and runs great. I have both AM and FM rf decks for it. I also had it updated in the '80s with servo reversing and ATV. The Ace MP8000 single stick tranmitter I still use is great and it currently has switch selectable AM and FM modules. I have been thinking of updateing that to Futaba Fasst via Radio South since I currently have Futaba 7C and 8FG Fasst systems. I have also been thinking of converting it to the MS2000 encoder board. I use to do all of my own radio work back in the '70s and '80s but no longer have the steady hand necessary for soldering. Therefore I would need someone to do the updating for me.[&o]

Bruce

Old 03-05-2011, 01:31 PM
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Bruce,
I just got a Series 73 (does that mean it was built in 1973?) and have questions.
What is the pushbutton next to the throttle used for? (Picture enclosed)
Why is the Tx battery center tapped?
The DIN plug for charging has 6 wires to it. What are they for?

I assume the charging voltage is 12v or less rather than line voltage with an onboard diode like in my Heathkit.

I was surprised about this radio. I thought I'd just use the case & gimbals for a MS2K upgrade, but this thing has been gold sticker'd. I could using it as is on channel 40.
The reciever has 6 plug-ins and a separate cable (4 wire) coming out of the case. Which is the power plug for the Rx?

If I go 2.4 conversion I'll have to scrap the Kraft encoder board because it would block the module.

Do you know of anyone who has a service or owners manual?

I'm not trying to hijach this thread, just need some answers. Anybody out htere help?

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Old 03-05-2011, 01:37 PM
  #21  
doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

ORIGINAL: landeck
There is a separate Kraft contol box for directly controling the servos and I have one. It is like a servo tester in its operation and needs to be plugged directly into the servo...
Bruce,

good to know about the DSC unit. It sounds like it is a slightly more primitive version of Ace's system. With 2.4 nowadays we don't really need DSC anymore but the method used by the MP8K and MS2K is nice since it allows you to setup servos without a receiver or servo power - it is all driven by the Tx.

ORIGINAL: landeck
I have a Kraft Series 73 7C single stick transmitter which no longer works.
Perfect candidate for an MS2K and 2.4 upgrade. That project would be very similar to the '74 SS in this thread except that you would already have the trim sliders on the side for aux channels. I will be locating the aux channels there too but will use rotary knobs since mine was a 5C and the levers are missing.

ORIGINAL: landeck
The Ace MP8000 single stick transmitter I still use is great and it currently has switch selectable AM and FM modules. I have been thinking of updateing that to Futaba Fasst via Radio South since I currently have Futaba 7C and 8FG Fasst systems. I have also been thinking of converting it to the MS2000 encoder board. I use to do all of my own radio work back in the '70s and '80s but no longer have the steady hand necessary for soldering. Therefore I would need someone to do the updating for me.[&o]

Bruce
Great it is indeed. Ace also can provide a shift inverter or, with the latest eeprom, the shift is part of the system like with the MS2K. This allows you to operate it with just about any Rx out there whether it be Futaba, Hitec, Airtronics, JR or Spektrum. Adding 2.4 into the mix would be quite straight forward it would just require a three position selector switch (AM, FM, 2.4) instead of the 2 position which I imagine you have at the moment. For simplicity though, you could consider replacing the AM RF deck for the 2.4 deck and you might not need much in the way of soldering other than the three RF leads to the 2.4 deck. It is likely that the decks are wired with 3 pin deans plugs so if you added a 3 pin deans to the 2.4 module, that would be just about all that's needed. The 2.4 antenna would have to be installed though. The easiest option might be to use a friction fit 2.4 antenna from Horizon which would just go atop the FM antenna housing and would be retained with a wide rubber band over the inside housing. This would make it very quick and easy to replace 2.4 to FM antenna's if you needed - even in the field. Otherwise, the 2.4 antenna could be installed to the side of the FM antenna for permanent mounting.

Converting it to MS2K would be an additional nice notch above the MP8K encoder but I'd restore the Kraft first if I were you - the MP8K is a great system as it is.

David.
Old 03-05-2011, 01:45 PM
  #22  
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread


ORIGINAL: doxilia


ORIGINAL: 8178

David,

The 7C has a cable that plugs into the slotted connecter on the bottom of the TX for checking the receiver batteries. Don’t know about the direct servo control. I think there was a separate box for that.
Mike,

that makes sense. I read that a cable existed for this and the appropriate voltages are indeed shown on the ESV for the Rx battery voltage. What I don't understand is why there are 4 leads at that jack in the Tx. Perhaps they are needed as the signal needs to pass via the encoder in order to provide adequate load on the Rx battery for proper voltage measurement. However, the plug at the other end on the model's switch presumably still only has two leads - red & black - so I'm not clear why 4 leads are needed. If you have such a cable it would be helpful to see how it is wired if it is not shielded by an outer sleeve. It would also be helpful to see the wiring of the KP-7C Rx switch if it has any more than the two standard power leads.

David.
The cable I have for checking the RX batteries has a 4 pin connecter for the TX and two pins for the RX. The cable appears to have only two wires. They do not appear to be shielded and as I recall the cable came with my TX.

When I bought the 7C in 77 I did not buy the Kraft flight pack because I was using it with ACE kit receivers and kit servos that I built. I do know that the 77 RX power harness did have a wider charging plug for use with the direct controller use as describe in my manual.

The direct controller described in my manual plugged into wider RX plug and controlled the throttle or retract channel one at a time. The box also had a mode 1 and 2 switch. A lot of old timers back then left over from the reed days flying mode 1. Only few left now. The manual does not say anything about another cable that would connect from the TX to RX to direct control the servos. Seems like that would be complicated because the encoder would need to power up with the RF deck off. I cannot remember why the TX has extra wires on the slotted connecter. I guess you could trace the wires to determine what they could be used for.
Old 03-05-2011, 01:52 PM
  #23  
landeck
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

ORIGINAL: Prairie Mike

Bruce,
I just got a Series 73 (does that mean it was built in 1973?) and have questions.
What is the pushbutton next to the throttle used for? (Picture enclosed)
Why is the Tx battery center tapped?
The DIN plug for charging has 6 wires to it. What are they for?

I assume the charging voltage is 12v or less rather than line voltage with an onboard diode like in my Heathkit.

I was surprised about this radio. I thought I'd just use the case & gimbals for a MS2K upgrade, but this thing has been gold sticker'd. I could using it as is on channel 40.
The reciever has 6 plug-ins and a separate cable (4 wire) coming out of the case. Which is the power plug for the Rx?

If I go 2.4 conversion I'll have to scrap the Kraft encoder board because it would block the module.

Do you know of anyone who has a service or owners manual?

I'm not trying to hijach this thread, just need some answers. Anybody out htere help?

The pushbutton next to the throttle is for using a buddy box and is used on the master system to give control to the slave transmitter.

I don't know why the Tx battery is center tapped other than the design may have required it. SR Batteries and Radio South can still provide batteries for the Kraft Txs and Radio still services them. While the Series '73 used the (at the time) new 3 wire servos, it still supported to older 4 wire servos.

The DIN plug is both for charging the battery and the buddy box cord.

My receiver has 7 plugins and the cable. The cable is for ailerons. I belive the battery and switch plug into the first port next to the cable. It only accepts a 3 prong plug.

I have the owners manual but it does not contain any wiring diagrams.

Hope this helps.

Bruce

Series '73 means it was released in 1973.
Old 03-05-2011, 02:08 PM
  #24  
doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

Mike,

that Tx is exactly like mine. I should have posted some close up pictures for reference. I'll answer some of your questions since I've had some time to figure things out.

ORIGINAL: Prairie Mike
Bruce,
I just got a Series 73 (does that mean it was built in 1973?) and have questions.
Yes, probably 1972 actually. I suspect they marketed radios a bit like cars. 1973 was sold in late '72.

ORIGINAL: Prairie Mike
What is the pushbutton next to the throttle used for? (Picture enclosed)
Trainer push button. When depressed it passed control over to the slave Tx (encoder) and broadcast those controls over its own RF deck. Standard trainer setup.

ORIGINAL: Prairie Mike
Why is the Tx battery center tapped?
Good question. Mine is a series '74 and I don't believe that it was center tapped. However, older Kraft systems worked with two voltage levels which is why they had 4 pin connectors: ground, center tap, full voltage and signal. The four pins from the bottom had 1) battery voltage (red), 2) ground (black), 3) center tap voltage (orange I believe), and 4) signal (yellow). Servo's needed these three levels but I'm not sure why Tx's did. A little digging would probably refresh my recollection on this.

Come to think of it, I believe the center tap voltage was used as a reference PPM (Tx end) or servo (Rx end) signal voltage. Higher (positive) voltage departures from this reference indicated rotation of the servo in one direction while lower (negative) departures indicated rotation of the servo in the other direction. Modern systems don't need this reference voltage as they create their own reference internally. For example, the '80 dual stick 7C system above outputs a PPM signal which hovers around 8V reference (so lower than battery voltage). Departures from this reference accomplish the same thing as with the "center tap" system.

ORIGINAL: Prairie Mike
The DIN plug for charging has 6 wires to it. What are they for?
This is discussed in detail in post #17 above. It has a diagram of the multi-pin 6 wire DIN plug. I am yet unclear how the yellow/red and orange/black leads are used but I'm pretty sure they are for trainer PPM signal I/O.

ORIGINAL: Prairie Mike
I assume the charging voltage is 12v or less rather than line voltage with an onboard diode like in my Heathkit.
Yes, I believe that all "Series" radios used 9.6V Tx voltage (8 cells in series). The battery boxes had two sets of 4 x 2/3 sub-C batteries. The chargers were just like modern chargers with two voltage levels for Tx and Rx. It could be that just around '73 to '74 Kraft dropped the "fast charge" option in their chargers. I have an "old" version charger with the fast-slow charge rates as well as the more recent black wall wart chargers with standard slow rate charging. Whatever the case, you are going to want to replace those cells!

ORIGINAL: Prairie Mike
I was surprised about this radio. I thought I'd just use the case & gimbals for a MS2K upgrade, but this thing has been gold sticker'd. I could using it as is on channel 40.
The reciever has 6 plug-ins and a separate cable (4 wire) coming out of the case. Which is the power plug for the Rx?
You could but the advantages of a $50 MS2K encoder cannot be compared to the functionality of a '73 encoder. I would also be a little concerned with the reliability of the components on the radio - mine had a number of capacitors which were fried. The quality and state of the components on the more recent '80 dual stick 7C Tx above are much better. 30 years (~1980) might be a reasonably cut off date for using a radio with its vintage components. Anything older unless cared for throughout the years, would be questionable in my mind.

The Rx will have 7 jacks plus the separate cable coming out of the case. This last is the aileron channel if I'm not mistaken and is sort of a built-in extension. The other 7 jacks are for the remaining 6 channels and the one closest to the center of the Rx has male pins and is for the battery switch.

ORIGINAL: Prairie Mike
If I go 2.4 conversion I'll have to scrap the Kraft encoder board because it would block the module.
This is part of the problem with using the existing PCB of these older Tx's. The RF deck is built on the same board as the encoder. You would have to disable the AM RF deck. It is fairly clear what parts form the RF deck on the PCB but you would have to close a few circuits for the encoder portion and locate the PPM stream to feed it to the 2.4 deck. All together a little too much hassle. The 2.4 module doesn't have to "live" inside it's case. It can be internal to the Tx. More on that in the next post but my conversion drawing in the first post shows the layout of the various parts for a MS2K/2.4 part layout. The 2.4 deck is not within it's "module case" - it has been taken out so the radio case has no exposed "warts" of sorts. The 2.4 antenna is installed where the older AM antenna was and the 2.4 deck is located wherever convenient inside the case. A plausible spot is shown in my drawing. If one doesn't use digital trims, one gains a lot of space inside the case allowing one to locate things with more ease.

ORIGINAL: Prairie Mike
Do you know of anyone who has a service or owners manual?
Apparently the auction site. Other older Kraft owners may also have such documentation.

ORIGINAL: Prairie Mike
I'm not trying to hijack this thread, just need some answers. Anybody out there help?
Not a problem - that's what the thread is for. You might want to re-read my lengthy posts though. Some of your questions were answered in there in more detail.

David.
Old 03-05-2011, 02:31 PM
  #25  
doxilia
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Default RE: Kraft '74 Single Stick Radio - Rebuild Thread

It seems that this thread is starting to stir up some interest in vintage Kraft radios and how they worked. I have a functional 4 channel system which will allow me to take some pictures of the Rx plug-in setup. It isn't much different than how things work today except that nowadays we can power up Rx's by plugging power into any channel jack. In the '70's the Rx's jacks were specific so the battery had its own jack and the other channels had different male/female connectors to insure one didn't plug in the battery to a channel servo jack and vice-versa.

It seems that it might also be a good time to shed some light on 2.4 decks. Below is a 2.4 deck which is referred to as a "hack" module which basically means it is designed to go into "any" Tx which has a PPM output. There is no module case per se which are typically designed for Futaba/Hitec or JR/Spektrum systems. 2.4 systems work with three leads to the RF deck - 1) Ground (black), 2) Battery voltage (red) and 3) PPM signal (yellow or white or some such). The PPM signal is the voltage modulated signal and is altered from a reference voltage to instruct the RF deck which channel to modulate (send signal to) over the air. The Rx detects this deviation from the reference voltage and knows which channel (servo) it needs to send a corresponding signal to.

2.4 systems need to have the RF deck and Rx lock on to each other. This is what the binding (or marrying) routine is for. The deck below has a separate PCB with the binding button and informational LED which mounts to the Tx case for binding the Tx to a specific model Rx. Other than that, the module just needs an antenna signal lead to go to a ~2 dBi (5 dBi or stronger is sometimes used on telemetric system or for military purposes but 2 dBi provides us with an ample one cubic mile of range or so) 2.4 GHz antenna.

From what I have read and research done by others (including RF engineers), the FrSky (pronounced Freesky) 2.4 modules are among the best of the lower cost 2.4 RF alternatives. Of course they work in concert with Rx's of the same brand as each brand uses its own design variation of mapping operating frequencies to the 2.4 GHz spectrum (e.g., DMS2, FASSST, ACCST, etc.). Shown are two Rx's that work with this module - a 7 channel and an 8 channel. A 4 channel is also available. More info on the ACCST 2.4 system and FrSky can be found on their web site:

http://www.frsky-rc.com

David
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