Restoring a small transmitter

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Bow
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Restoring a small transmitter

Post by Bow » Wed Aug 25, 2010 6:00 am

Hello all,

I'm restoring a small transmitter that was removed from Back-up/night time/stuffed in a back corner service a few years ago. It is an interesting critter, a Cunningham CM-3050. I believe it was made by a retired broadcast Engineer, Mr James R Cunningham, in Stonewall, OK. From what I understand, after retirement, Mr. Cunningham built these for missionary use in foreign countries..

It is a 3 tube (6DQ5, 6V6, 12BY7) small transmitter rated for 2-50 watts output and Cathode modulated.

http://riley-music.com/BowsStuff/CM3050/01Front.JPG
http://riley-music.com/BowsStuff/CM3050/04Rear.JPG (those filter caps are not original)
http://riley-music.com/BowsStuff/CM3050/10IDTag.JPG

Through the internet, I located a fellow HAM operator that had one with the schematic, so that helped.
Schematic: http://riley-music.com/BowsStuff/CM3050 ... ematic.jpg

The previous owner had attempted to retro-fit a replacement power transformer, which put about 750V B+ on the 6DQ5 plate instead of the circuit recommended 450V.. I got that figured out and was able to get it down closer to the 450V

The next challenge is the Modulation circuit. Using the internal modulation (Tip 100 or IRF822) I was only able to get about 60% modulation before the signal starts to flat-top. I'm looking to make sure the Transistor is biased correctly... Then I'll go from there.

I guess my question is...

Have any of you gents ever seen one of these things before?

Thanks
Bow

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Deep Thought
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Re: Restoring a small transmitter

Post by Deep Thought » Wed Aug 25, 2010 10:06 am

I've heard of them but never seen one. Interesting...

I looked up the FCC ID and found this:
4c74edd4.jpg
I wouldn't expect much more than 20-30 watts out of this transmitter, and since it is cathode modulated you won't be getting much more than 50%-70% modulation.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Bow
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Re: Restoring a small transmitter

Post by Bow » Wed Aug 25, 2010 10:29 am

Thanks Deep Thought.

Yes, they seem to be quite rare, and I understand the Builder, Mr. James R. Cunningham, passed away a number of years ago. I looked up the FCC ID number, but I found no information... I must not have looked in the right place? :oops:

I have a copy of the manual, and it says it is capable of 125% Positive Peak.. Maybe in theory? It also does state that at 50 Watts output, it will not achieve 100% Modulation, but around 30 watts and below, it will.

Here is the Manual I have, if you are interested:
http://riley-music.com/BowsStuff/CM3050 ... CM3050.pdf

Thanks for the input.
Bow

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Deep Thought
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Re: Restoring a small transmitter

Post by Deep Thought » Wed Aug 25, 2010 12:10 pm

Interesting...I read the whole manual and it is a neat little transmitter. I still doubt the claim of 100% modulation using the cathode of the 6DQ5 at anything above 20 or so watts though. Can you get it to do better than 60% with the power set at say, 10 watts? The manual is pretty specific about getting the grid drive set right on the final in order to optimize the modulation capability. I assume you went through that exercise already.

How does it do through the modulation transformer?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Bow
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Re: Restoring a small transmitter

Post by Bow » Wed Aug 25, 2010 12:19 pm

I actually just got the carrier back under control yesterday, so now I need to got through all the procedures to set it up, and then recheck the modulation.

I was modulating it yesterday through the "Internal Modulation" with a signal generator and a 1kHz tone. If I get a chance today, I'll try the Modulation transformer. I think we have a small audio amplifier around here somewhere...

Have you ever seen this type of modulation circuit before?

Thanks for taking a look.
Bow

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Re: Restoring a small transmitter

Post by Deep Thought » Wed Aug 25, 2010 2:16 pm

I haven't seen that specific configuration but cathode modulation is pretty common in low power tube transmitters. The PA efficiency will be lower than with plate modulation but usually that's not an issue, and it is hard to get to 100% mod with decent distortion performance. Using a transistor as a modulator probably helps this design a lot since you need a low impedance in the cathode.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Bow
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Re: Restoring a small transmitter

Post by Bow » Wed Aug 25, 2010 6:41 pm

DT,

Thanks for the assistance and knowledge.

I am happy to report:

I have Carrier:
Image

I have Modulation:
Image

This was done with an 8 watt mono amplifier feeding the External modulation connection (through the modulation transformer) with a 1kHz sine wave into a dummy load.

Now that I know it will modulated properly, I can work on the internal modulation circuit more.

As for the distortion in the audio, I am not sure how it sounds, I didn't have a receiver handy when I was testing... but it looks good on the Scope...
Bow

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Deep Thought
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Re: Restoring a small transmitter

Post by Deep Thought » Wed Aug 25, 2010 9:07 pm

Cool...according to the scope you've got about 114% positive modulation there ((144.4-67.37)/67.37) and the negative peaks are being rounded as expected with this kind of circuit. If that 67 volts P-P of carrier is across 50 ohms you're also getting better than 10 watts out of it too. What was the load?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Bow
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Re: Restoring a small transmitter

Post by Bow » Thu Aug 26, 2010 5:43 am

50 ohm dummy load

I'm curious to see what it sounds like... If I get a chance, I'll try it today.
Bow

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Patrick De Landsheer
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Re: Restoring a small transmitter

Post by Patrick De Landsheer » Thu Nov 27, 2014 3:44 pm

Regarding those small transmitters, 3 of them was brought to me and I'm planning to use them to rebroadcast on diffeent point of the territory. My first problem is to change the frequency they are set for 1610 and 1620 khz AM. My frequency is 990 khz I need to replace the crystals, I need to find a place in the USA to order them. I hope someone could help me locate a place where I can buy them.

Based on the infomation provided by Mr. James Cunningham that I had the chance to talk to over the phone a few years ago, he had sent to one of my partners the small 30/50 transmitter as an exciter and also a 2 kw linear power amplifier to boost the signal. I was able to put the 30/50 transmitters on the ai,r but never could drive the power amplifier the way it was suppose to be. I would like to know if any one could help me acheive that to drive the power amp. I have to say that I have previously used a di-pole antenna on 1610 or 1620 khz, now I have plan to use a vertical antenna. Thank you!

TPT
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Re: Restoring a small transmitter

Post by TPT » Sun Nov 30, 2014 7:45 am

International Crystal in Tulsa, OK (icmfg.com) can probably make you a crystal.

A picture of one of the crystals (to show size/ type of socket) and a copy of the oscillator schematic may be enough; they're pretty easy to work with.

COMMENG
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Re: Restoring a small transmitter

Post by COMMENG » Mon Dec 01, 2014 11:37 am

I have been able to find some crystals for legacy transmitters from the following supplier:

http://af4k.com/crystals.htm

COMMENG

Patrick De Landsheer
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Re: Restoring a small transmitter

Post by Patrick De Landsheer » Mon Dec 01, 2014 6:09 pm

Thank you very much, i'm going to contact them. Thanks again

COMMENG
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Re: Restoring a small transmitter

Post by COMMENG » Wed Dec 03, 2014 11:45 pm

Patrick De Landsheer wrote:Regarding those small transmitters, 3 of them was brought to me and I'm planning to use them to rebroadcast on diffeent point of the territory. My first problem is to change the frequency they are set for 1610 and 1620 khz AM. My frequency is 990 khz I need to replace the crystals, I need to find a place in the USA to order them. I hope someone could help me locate a place where I can buy them.

Based on the infomation provided by Mr. James Cunningham that I had the chance to talk to over the phone a few years ago, he had sent to one of my partners the small 30/50 transmitter as an exciter and also a 2 kw linear power amplifier to boost the signal. I was able to put the 30/50 transmitters on the ai,r but never could drive the power amplifier the way it was suppose to be. I would like to know if any one could help me acheive that to drive the power amp. I have to say that I have previously used a di-pole antenna on 1610 or 1620 khz, now I have plan to use a vertical antenna. Thank you!
The first thing I would check is the SWR between the 30/50 and the input to Linear Amplifier. If the Linear Amplifier has a tuned input, it may not be tuned to the frequency of interest.

What kind of circuitry does the Linear Amplifier have, tube or solid state? What is the brand?

COMMENG

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Dale H. Cook
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Re: Restoring a small transmitter

Post by Dale H. Cook » Thu Dec 04, 2014 7:35 am

Another thing to look at is the load impedance of the linear. Depending upon that impedance, the topology of the linear output network, and the power amp design, the linear may not be capable of developing full output.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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