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Futaba Gold FP-T7FG/K FrSky DHT Conversion/Trim Issue

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Futaba Gold FP-T7FG/K FrSky DHT Conversion/Trim Issue

Old 01-12-2015, 07:45 PM
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ath5774
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I have recently completed the conversion to FrSky DHT 2.4gHz module on an older Futaba Gold Series FP-T7FG/K transmitter, S/N 11009852. This transmitter was originally 72.320 AM and DOES NOT have a removable module on the back cover.

The conversion was straight forward as per instructions found on the internet covering conversion of similar transmitters. The main deviation from online instructions was that I removed ALL electrical componants from the inside AM RF module board except the white multipin connector. I solder the three DHT wires to the respective traces on this circuit board then attached the DHT module to the board with velcro and snapped the board back into the black board holder. This method does not involved removing pins off the main circuit board. Anyone wishing to do the same, I highly recommend testing circuit traces to insure a solder bridge has not occurred on the needed traces during the componant removal phase.

I also installed the FrSky FLD-02 display which was the perfect size to attach to the lower left corner of transmitter front. I found a small rubber grommet used for larger Futaba servos that was the exact size to pass the FLD-02 cable after removing the pins from the black connector. I then drilled a small hole in the angled bottom front case as well a one in battery case large enough to pass and thread the FLD-02 cable through and up to the DHT module. I replaced the pins back into the black cable connector, then connected to the DHT module.

Now for the issue encountered. The bind process worked as advertised. I'm using the FrSky D8R-II Plus receiver. I have five operational channels on the receiver and servos respond as expected. HOWEVER... none of the trim tabs are responding to incrimental changes. Not even a tiny bit. This transmitter has two trim tabs associated with Ch6 and Ch7 which do not respond either. I was under the impression that the trim tabs were a "mechanical" connection to the gimbles, but this apparently is not the case.

I have spent a great deal of time researching the internet and have not found anything even remotely closely related. This makes me believe I have disrupted something inside the radio during the bowel removal that has broke or become disconnected that powers these trim controls. If no one else has had this issue and their trim tabs function normally after this conversion, will tell me that I need to start doing a trace to see what is common with the trim circuits. However, before tearing back into this radio, I thought I would inquire here just incase there is something else that needs to be done with the FrSky gear.

I have two other Gold Series T7FG/K AM radios I will convert, one has never been powered up. These two are newer with the removable RF modules in the back cover. I wanted to experiment on the older radio first. I took photos throughout this first conversion, so if others are interested, I'd be happy to share them. However, there are many photos already available and I don't know how mine would be of any meaninful difference.

What a way to get reacquainted with R/C after 20 years away... digging into the bowels of a 20 year old transmitter! ( smile )

Any shared info towards the resolution of this "gimble trim" issue will be highly appreciated. Hopefully, I'm not posting a duplicate query about a similar issue posted elsewhere here that I've missed.

Mike

Last edited by ath5774; 01-17-2015 at 11:24 PM. Reason: Re-posting original text
Old 01-17-2015, 06:26 PM
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As of today, 65 peoples viewed your post and it might take more than one day to get feedback! by deleting it you make it impossible for anybody to answer
Old 01-17-2015, 11:29 PM
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Thanks for replycoriolan... I've reposted original text.
Old 01-20-2015, 03:55 AM
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Mike, can you post some pictures of the insides of the transmitter so we can see the wiring connections to the trim levers. I'm running a FrSky telemetry module in my Futaba 9C and a FrSky D8R-II Plus receiver and works great. Of course that is a plug and play installation and much different than what you are doing.

One of the very old Kraft radios I have does have a mechanical connection to the control stick pots for the trim levers, but all my newer radios like the 9C the trim is more like a spring loaded switch that advances the trim function when it is held in ether trim direction. If your radio is using an electrical trim, I think the connections would be to the transmitters encoder section.

There is a Futaba FG Gold Series thread on that other RC form site (I don't believe RCU will allow me to post a link) where you also posted this information. There is a FP-T7 in the thread that is working. You might want to take a look at that thread.

Last edited by 8178; 01-20-2015 at 04:03 AM.
Old 01-20-2015, 09:31 AM
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8178
RCU doesn't practice censorship This is the thread in question:
http://www.rcgroups.com/forums/showt...FG+Gold+Series
Old 01-20-2015, 01:55 PM
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Hello, 8178

I will post three photos I submitted to RCG this morning. My Futaba FP-T7FG/K radio is an older version which did not have the removable module like the newer versions did. However, it still has an RF board inside which is removable. I took this RF board and removed all the electronic componants except the white five pin connector. I have many more photos I took during this conversion, so I may have an image that shows more relevance to what you are searching for.

The photos showing the trim pots was taken after conversion, but nothing was removed or changed on the main circuit board. Some suggest removing the five pin sticking up from the main circuit board where the RF board connects, but that did not set well with me so I opted to remove the componants and re-use the RF circuit board as a mount for the FrSky DHT module.

Mike

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Old 01-20-2015, 07:23 PM
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Mike,

After looking at your pictures, I agree that your RF section changes should not have any impact on the trim pot functions. From the pictures the trim pots look like they are all connected to the main board.

It might be worthwhile just as a quick trouble shooting test to remove the old RF board and connect the FrSky unit directly to the main board with some individual slide on connectors. I can't believe that is the problem but it would be good to eliminate it.

The only other thing that I can offer without having a schematic for the radio is that maybe in handling the main board a wire related to the trim pot power or ground has become disconnected. How one wire could impact all the trims is hard understand.
Old 01-20-2015, 10:52 PM
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As you suggested 8178, I removed the wiring from the old RF circuit board and connected them with wire nuts using a power supply hard drive power connector which fit the pins exactly. Unfortunately, there was no change.

While I was into the system again, I decided to disconnect the FLD-02 cable and test again. Again, no change. I have the usual five oprational channels, but no trim function or Ch6 or Ch7. No harm in testing in the effort of eliminating potential problems areas.

I have gone all the wiring in this radio two or three times, originally thinking that a wire somewhere got disconnected or broke during the conversion. So far, I've found nothing suggesting a wiring issue.

I have a half notion to order a duplicate FrSky system for testing this setup while preparing to convert a second FP-T7FG/K radio I have. It's not a total impossibility that the unit I currently have may be defective in some way. However, I believe there is still more research to be had concerning this issue. I still have to read more about this system with respect to upgrading possibilities that may cure this issue.

Thanks for the suggestions 8178,

Mike

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Old 01-20-2015, 11:14 PM
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Originally Posted by 8178
There is a Futaba FG Gold Series thread on that other RC form site (I don't believe RCU will allow me to post a link) where you also posted this information. There is a FP-T7 in the thread that is working. You might want to take a look at that thread.
I forgot to comment on the above, 8178. I've scoured most all those forum threads on RCGroups, here and elsewhere before committing to this conversion. They provided me the necessary information and inspiration to push me off center to "go for it". ( smile ) During my research on this converison, I found suprisingly little to suggest anyone was having any major issues converting these Futaba Gold Series radios.

Needless to say, after reading nothing but success postings, I felt I had committed a grave error on my system. I searched and searched to no avail to resolve my situation, thus my posting here and on RCGroups for some guidance or help when frustration began to mount.

There is an answer out there somewhere... just gonna take time to research and see what falls out of the network tree. ( smile )

Mike
Old 01-21-2015, 01:24 PM
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Mike,

I do not believe there is anything related to the FrSky module that could cause the trim problem. It just modulates the pulses coming from the main board. The trim function pots give input into the main board so the encoder on the board will change the neutral position of the pulse output to the FrSky module.


You might try working with just one of the trim pots with the radio on and using a volt meter see if you can detect voltage changes at the pot terminals when you move the trim. To start I think I would start with one lead on ground and then use the other lead on the pot terminals and see if there is any voltage on them. If you do not find any voltage at all then you would need to trace the pot wires to the board and check voltages there. Then it gets tricky because without a schematic you would need to follow the traces on the board until you find the power loss point.
Old 01-22-2015, 12:03 AM
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8178, I used a VOM to check voltages on the trim pots this evening. I check at the trim pots as well as at the circuit board where the number coorespond to the channel numbers (see photo). I found between 10 to 12 volts at these locations, so there is no loss of voltage at the trim pots or the circuit board. However, these voltages DID NOT change with movement of the trim tabs.


Mike

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Old 01-22-2015, 06:10 AM
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Mike,

Again not having a schematic makes this a big guessing game. I would say having voltage at the pots is a good thing, but it not changing when you move the trim lever is not good. The pots on the trim levers are basically variable resistors. If the voltage is the same across a resister it means that it is not dropping any voltage because it is not connected to a load. In your case the load should be the logic on the main board that is inputting the trim position into the encoder logic.

We know the encoder is working because the servos move when you move the transmitter sticks. Somehow something has disconnected the trim post voltage input from the encoder logic. It would be hard but you might try using a magnifying glass and see if you can follow the traces on the main board and figure out where the trim pot connections go on the main board. I would work with just one pot first. After you get one done then you could map out the others and see what is common between them .

It is also possible that the voltage you see is because somehow all the trim pots have lost their ground source and the voltage you see is coming back from the logic on the main board that they should be controlling. As any rate see what you can track down on the main board.
Old 01-22-2015, 10:10 AM
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8178,

I performed a trim pot voltage and continuity test on this FP-TFG/K transmitter this morning. This time my ground lead was connected to the pots directly and not connected to main chassis ground as I did in the first test.

I have the same 10-12 volts available at each pot. However, the voltage DOES NOT vary when operating the pots. Removing power, I tested for continuity at each pot, touching directly on the pots. I have continuity at the pot leads, but the continuity DOES NOT vary when operating the pots.

Next, I tested voltage/continuity at the channel pads on the bottom circuit board that was shown in the previous photo. I have the same voltage and continuity on these solder pads as I do directly on the pot, but still... no variation when operating the trim tabs.

I'm fairly confident that I will NOT see a variation while operating the trim tabs due to the fact that these pots are obviously connected together in some way, thus preventing them from being monitored functioning as they should if being OUT of the circuit.

I am extremely reluctant to disconnect any of the pots for testing purposes due to the fact that ALL pots are testing the same and probably the best reason is that when I stopped using this radio, it was NOT exhibiting any type of malfunction.

This morning I've sent an email to futaba support requesting schematic and/or service manuals. Yes, I too can hear the laughter when the email is received and read. ( smiling )


I appreciate your continued interest, 8178

Mike

Last edited by ath5774; 01-22-2015 at 10:35 AM. Reason: spelling correction
Old 01-22-2015, 01:19 PM
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Mike

I agree that disconnecting a pot would not help for testing. It sounds like pots do not have a ground and you are seeing voltage feed back from whatever they control or they have ground and whatever they control is a open circuit.

Hope you can find a solution.
Old 01-24-2015, 08:14 PM
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Hi 8178

I will not leave you out of this saga. I've finally been in communications with another R/Cer who has performed this conversion on his gear. Sharing notes, I believe a definitive answer to this "no trim" issue of mine is within a week or so away. I will report back after I've had a chance to test and make sure the "fix" in fact actually eliminates my "no trim" issue. I'm as anxious as you are. ( smile )

Mike
Old 01-24-2015, 09:01 PM
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I'll be watching for your reply as well..........I have a Futaba Gold 7AM narrow band. It would be nice to press it back into service for certain aircraft.....always like the feel of that radio.
Old 01-29-2015, 08:57 AM
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Hi 8178,

I received my D8R-XP receiver and upgrade cables yesterday. Unfortunately, I must report that the new and different receiver DID NOT change anything. The trims still do not function. At this point, I do not see any immediate reason to update the D8R-II Plus receiver to a D8R-XP using the upgrade cables. This is really frusterating. However, I'm still tickled to have what I have (the five usable channels) because I can trim at the control surfaces. It would still be nice to have Ch6 and Ch7 available for say flap controls or some such, but I can live without them.

Now my thoughts are towards AM and FM encoders differences must be at play here. However, Futaba is so closed lipped about their radio schematics and I do not have the expertise nor tools to troubleshoot the encoder differences. I understand the ardrino fellas have been doing some experminations with DIY encoders, but thats getting into heavy electronics and programming skills. Me, I want to build airplanes and fly! My ability to dive in and decipher minute technological details is slowly slipping away, I would like to enjoy the building and flying aspect of what's left my brain. ( smiling )

So, I'm closing out trying to resolve this FrSky/Futaba AM issue and go FLYING!! ( smile )

Maybe someone else will pickup and continue the search for a solution.

Mike
Old 01-30-2015, 10:59 PM
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Mike,

It is too bad that it did not work out. Sadly if a schematic was just available it would be pretty easy to find the problem.
Old 01-25-2024, 02:53 PM
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Post Just acquired Futaba Gold with same issue - Solved it - Mechanical

Hi all

I know this thread is ancient, however I just acquired an FP-T7FG/K from E Bay with the intention of converting it to 2.4GHz.

So not to waste anyone's time - this problem is a MECHANICAL issue not an electronic one. My unit was exhibiting the exact same issue but with one exception. I had one trim lever out of 6 working I also noted that the working lever was slightly easier to move that the other 5. The problem seems to be that the potentiometers get so sticky that when the levers are moved the pots stay still. Each trim lever has a friction slip arrangement, the actual pot shaft was not moving!

My fix: Unscrew the main board, un-solder the 72Mhz Antenna connection, then swung the main board down. Each of the 6 trim lever pots can be removed one a a time with 2 Philips screws. Spray contact cleaner into the opening of the pot and work the pot back and fourth about 50 times. NOTE: Work the pot using the slotted screw at the pivot point of the lever not the lever handle, once the pot is loose enough, moving the lever will also move the pot shaft (Slotted Screw). Once done re-test all functions. I used a scope to look at the PPM waveform to make sure each lever made a change in the pulse train. I expect that a final centering of each trim lever assembly with be required. Now I can install my DIY 2.4 GHz module.


That's it! I hope it helps someone.
Old 01-29-2024, 08:56 PM
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Hello remsat1
I specifically and possibly others, appreciate you sharing the mechanical solution for the trim pot issue on your FP-T7FG/K. Thinking back when I first posted about the mod on my Futaba radios, they had sat around on the shelf for 20 some years, quite possibly same as your recent EBay purchase. Every reason those pots eventually, over time, seized up. Awesome find! Thank you! *smile*

I put a lot of enjoyable flying hours on my FrSky DHT 2.4gHz module conversion on my Futaba Gold Series FP-T7FG/K AM transmitter. Never had any issues with the conversion other than the in-op trims in all that time, trimming was minor, after first flight, re-trim and go on about the joys of flying.

Dormant to RC after 20 years, I got a hankering to do some flying after running across FlightTest.com site and became intrigued with their electric foam aircraft. I had all my old RC gear but radios were no longer legal to use so researched upgrades for older Futaba radios and found zilch with one exception, FrSky DHT 2.4gHz module for older Futaba Gold Series FP-T7FG/K transmitters. I thought to myself back then, like Jimmy's World quotes today, "what could possibly go wrong". *smile*

I found some great directions online covering the FrSky DHT 2.4gHz module conversion into the Futaba radios. While modding the radio, I ordered from FlightTest.com plans for their FT Simple Soarer which seemed a perfect introduction into building electric foam aircraft. The FrSky/Futaba conversion turned out great.

My wife and I moved onto an 8 acre property which is located in the middle of North Eastern Oregon wheat fields on mostly flat ground. I mowed a 30'x 100" strip in the pasture where I flew my foam creations from for a few years without issue. A farmer down the flat owns a Super Cub and for the longest time, never flew directly over our property. Then seeing my RC activity, flew directly over the property while I was flying my foam creations. Given what the FAA has been shoving rules and regulations down the throats of RC'ers, I figured it was only a matter of time there would come an altercation of his flying and my flying. I ceased my RC activities on our property three years ago. Given the current political, lawyer infested legal climate, I simply would rather not risk an FAA legal hassle over flying rights. It's a shame, but that's the reality today.

I will however, check the trim pots on my Futaba radio. You have my curiosity up now, remsat1. *smile* I really do thank you for sharing your discovery.

Happy Flying!!
Laters...

P.S. A shout out to RCUniverse forum staff for emailing notification of remsat1's replay to my original postings. Thank you!
Old 02-11-2024, 10:08 PM
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Hello ath5774

I just noticed your response. Sorry to hear that you had to cease flying at your acreage! Bummer. I hear what you are saying about all the rules, it really sucks. I also have a small acreage in BC. Park flyers were one of the reasons we bought here. I also discovered the FlightTest foamies a few years back - built (and crashed) a couple of the "FT Versa" flying wings. Easy enough to fly, but then I wanted to try fancy tricks.

Thanks for the encouraging words and I hope you find a place to fly models again!

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