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Kraft KPT-5c frequency problems

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Old 07-23-2003, 06:57 PM
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JAkeJeckel
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Default Kraft KPT-5c frequency problems

Hey guys, first post, so be nice to me!

I've recently aquired an old Kraft KPT-5c radio set from my father who was cleaning out his old r/c gear. It was manufactured in 1975. He never even took it out of the packaging, so despite the age, it's brand new.

The first thing I did was inspect the batteries for the transmitter and receiver, and they had not leked, so I charged the batteries overnight.

When I turned everything on, it seemed to come on ok, the rf meter on the transmitter shot right over to the '1' on the meter, which is correct according to the manual. I had attached 4 servos to the receiver, and when I powered it on, the servos did a little jerk, so it came on ok as well.

The problem is, when I move the sticks, I get no motion from the servos.

The radio and receiver are set for 72.240 MHz. I grabbed my frequency counter and found that the transmitter is transmitting at around 60MHz, a little higher or lower based on battery voltage, but never +/- 1MHz.

The transmitter has 8 cells in it, so at 1.2v/cell that should be 9.6v. With the charger attached, the batteries show around 10.5 volts, proper for charging, and around 9.5v immediately after removing the charger. I left the transmitter on for over an hour, and the voltage had not dropped below 9.3v, so I assume that the batteries are still in good enough shape to be used.

I opened up the 'transmitter pack', and there are no electrolytic capacitors to go bad.

My question is, is there anyting else that I should be looking at to find out my problem?

I know that this transmitter is no longer FCC approved for the frequency that it is working on, but I am not using the setup for planes. Rather, KAP, or Kite Aerial Photography, and I'm not using the gear anywhere near there is any flying going on.

So, that's my novel... Does anyone have any ideas on how I can get this gear fixed? My dad gave me the radio, four servos two 1100MaH receiver batteries, and a handful of other misc. gear that I can use, and I'd really like to get this working, rather than have to go out and buy a whole new setup.

Thanks for any help!!
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Old 07-23-2003, 11:17 PM
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strato911
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Default Re: Kraft KPT-5c frequency problems

Welcome to RCU! :thumbup: :sunsmiley
Originally posted by JAkeJeckel
The transmitter has 8 cells in it, so at 1.2v/cell that should be 9.6v. With the charger attached, the batteries show around 10.5 volts, proper for charging, and around 9.5v immediately after removing the charger.
If the pack only reads 9.5 immediately after removing it from the charger, then one or more cells have probably gone bad. An 8 cell NiCd pack fresh from the charger will be over 10V (sometimes even over 11V) for a while, then slowly drop to 9.6V and stay there for a long time before dropping sharply at the end of it's capacity. You can verify if it's the batteries or not by temporarily connecting 7 alcaline batteries (it can handle 10.5V no problem). Since you have already opened your battery pack, you can check the individual cell voltages, and you'll probably find one or more bad ones. Don't forget to measure while under load (Tx turned on).

Check out the Batteries & Chargers forum for more info on your batteries, but I don't think that is the cause of it not working. It sounds to me like something in the RF circuit has gone dramaticaly off-specification, resulting in it being off it's designed frequency.

Also, you didn't post the voltage of the receiver pack...

I won't even mention the legalities of using your radio equipment (a kite is still a form of aircraft), since you seem to already know this is illegal.
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Old 07-23-2003, 11:32 PM
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Default Kraft radio

You could always send it to Pete Waters who could probaly get it working and make it legal.
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Old 07-25-2003, 02:40 AM
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Default Kraft KPT-5c frequency problems

Quick update :

I had a chance today to do so more investigating on my radio.

BTW, thanks for the welcome Strato. I appreciate the help, and the lack of flaming. More than anything, I want to get this set working, build a prototype of a KAP rig, and if I like what I end up with, I'm going to upgrade to an FM rig anyhow. I run nitro cars, and my switch to FM from AM radios was a great one!

One to the dilemma...

On Strato's suggestion, I substitued 7 AA Alkalines in place of the NiCD's. The frequency is up to ~62MHz. Obviously not high enough.

Also, I checked each of the battery voltages after two days off the charger and much playing with the radio, and each cell was showing ~1.2 volts except for one cell which had reversed polarity, and was showing ~-.2 volts with the meter attached in the normal fashion. So, I guess there was the voltage problem with the pack.

So, since it seems it's not a voltage issue, I'm forced to look at the 'transmitter pack' as the culprit, as the battery wires go directly in to that. I'm wondering, can a crystal go bad after non-use, or go bad with time? I've been around radios in general for quite a while, and I've never seen it happen, but could be I suppose.

Any thoughts?
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Old 07-25-2003, 10:41 PM
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Default Kraft KPT-5c frequency problems

Originally posted by JAkeJeckel
I'm wondering, can a crystal go bad after non-use, or go bad with time? I've been around radios in general for quite a while, and I've never seen it happen, but could be I suppose.
I've never seen a crystal go bad from sitting idle in old computers (they used to use crystals for the CPU clock), so I don't think that's the problem either. Is it possible your frequency counter is reading an intermediate frequency instead of the final output? What is the voltage of the receiver pack?

Although just as illegal a venture, why not use the old AM radios you removed from your cars (if you still have them), since this is just a prototype? Just make sure you are in a VERY isolated area when you test your setup this way. Having that old radio repaired may not be worth it if you don't intent to keep it.
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Old 07-26-2003, 02:40 AM
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Default Kraft KPT-5c frequency problems

Originally posted by strato911
I've never seen a crystal go bad from sitting idle in old computers (they used to use crystals for the CPU clock), so I don't think that's the problem either. Is it possible your frequency counter is reading an intermediate frequency instead of the final output? What is the voltage of the receiver pack?

Although just as illegal a venture, why not use the old AM radios you removed from your cars (if you still have them), since this is just a prototype? Just make sure you are in a VERY isolated area when you test your setup this way. Having that old radio repaired may not be worth it if you don't intent to keep it.
I'm at work right now, so I don't have the voltage handy for the receiver pack But, I'll put it back on the charger tonight to top it off, and let you know tomorrow.

As far as the car radios go, AM is still legal for use on some bands. I've got an AM radio that is less than two years old that came with an HPI NitroStar. I'd have used one of the car radios before now, but they are only two channel radios, and I need at least three channels: one for elevation, one for rotation, and one to fire the shutter release on the digital still camera.

It's possible that the freq counter is reading a harmonic. When I get the receiver packed charged up, I'll try to run the receiver again. Since I found that it waas not on frequency, I have not even used the receiver for testing at all, just my volt meter and freq counter. However, using your idea of alkalines, it would be good to give it another try.

Thanks for the continuing info!
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Old 07-26-2003, 02:51 AM
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Default Kraft KPT-5c frequency problems

Yes, I know AM is still legal. I was refering to use of 75Mhz on an airborne kite....

If you find the receiver pack voltage is wrong too, you can temporarily replace the defective cell(s) with alcalines like you did for the transmitter.
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Old 07-26-2003, 11:44 AM
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Default Kraft KPT-5c frequency problems

To get a correct reading with a frequency counter you have got to get inside the transmitter on the osc circuit or else turn off the modulation.The so called AM radios are really carrier on carrier off so the counter gets all mixed up unless you shut off the modulation
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Old 07-26-2003, 09:16 PM
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Default Kraft KPT-5c frequency problems

Originally posted by ALbert.S
To get a correct reading with a frequency counter you have got to get inside the transmitter on the osc circuit or else turn off the modulation.The so called AM radios are really carrier on carrier off so the counter gets all mixed up unless you shut off the modulation
Thanks for the tip Albert. I guess I should have known that, being a ham operator... Will I damage any part of the radio if I leave the rf circuit attached to power, and just remove the power for the sticks and control bits?

Strato, I didn't mean to step on you with that last comment, sorry if I came off wrong.
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Old 07-26-2003, 09:34 PM
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Default Kraft KPT-5c frequency problems

No offense taken. Have you checked your Rx voltage yet?
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Old 07-27-2003, 05:57 AM
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Default Kraft KPT-5c frequency problems

Yeah, I did check the voltage on one of the packs that I have. It is a 550 MAh pack, and it shows 5.3 volts after charging last night, and being off the charger all day today. I am reasonably certain that all the cells in it are good.
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Old 07-27-2003, 03:27 PM
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Default Kraft KPT-5c frequency problems

5.3V is good. :thumbup: Now that we have eliminated the power sources, I will bow out to those more knowledgable in RF circuits. Since you say you are a Ham opperator, you probably know more in that field than I.

Good luck. Hopefully you'll get it figured out soon.
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Old 07-28-2003, 05:18 AM
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Default Kraft KPT-5c frequency problems

I'll let this thread sit here for a few more days, but if I haven't accomplished anything, I think I'm going to just put the whole set up on EBay to sell to a collector. I know there are some guys out there who are interested in buying Kraft gear for the historical value of it.

I'd really like to get this set going, but more than anything, I want to get a KAP rig in the air for the International Kite Festival in Long Beach, WA this August.

Thanks for all the help Strato, and the tip Albert. I really appreciate it, and especially for not flaming me for it not being legal gear!
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