Troubleshooting Muddy buzzing receive D&A Mavrick

[jtrouter]

Hello
I have been trying to figure out the problem with a D&A MDX200/Mavrick that has been
sitting in my fixit stack. When its pluged in and turned on it works fine for a while and then
after sometime when listening the audio from the radio starts to become muddy or buzzy?
Its not a 60cycle hum but when it starts it becomes hard to understand what people are saying.
If i power it off and turn it back on its clear for a short while and then it starts again.
Now i have ordered all of the electrolytic caps that are needed for replacment, But when i test them
they all do test ok. Is there anything else that could be causing this problem or am i on the correct path?
I look forward to your comments as always. Thank you.

 

[Dmans]

Dirty relay contacts.

73
David

 

[jtrouter]

Well i replaced all the caps and it did not fix the problem. I also cleaned both relays
with a dollar bill and Deoxit and the problem still is there. Anyone else have any ideas? Can a tube cause that kind of problem? Its driving me crazy trying to troubleshoot?

 

[brandon7861]

I'm not a tube guy, but those symptoms are similar to what I read about tube problems in guitar amps. I would be checking the tubes.

 

[jtrouter]

I'm not a tube guy, but those symptoms are similar to what I read about tube problems in guitar amps. I would be checking the tubes.

thank you, it does seem that that is the next area for me to try. I am planning on switching tubes as soon as i rebuild the filament power. I do have one spare set of old NOS TUBES in six volts, I will test those and then if that does fix it i will then do the upgrade to higher voltage tubes.

 

[DoctorAudio]

(After you discharge the high voltage) Check the relay that switches on the high voltage that feeds the plate chokes. If one of the contacts is welded closed, you can get "diode noise" from the tubes being energized while you are in receive mode. Diode noise usually sounds like a "bubbly" buzz/hiss that rides in there with the signals you are hearing from stations on the channel.
If the relay does have a stuck contact it can sometimes be popped apart but it usually happens from having well worn pitted/carbonized contacts. I'd recommend just replacing the relay when they get so worn they weld shut. It'll just happen again if you don't.

 

[Dmans]

The tubes have nothing to do with receive. (Unless you have a tube based RF sniffer circuit using a 6BQ5 or 6AQ5-usually in the very old Mavericks. If so you would have 9 total tubes above the chassis.)

Check the path from the antenna connector to the transmitter connector and you will find your problem.

73
David

 

[jtrouter]

The tubes have nothing to do with receive. (Unless you have a tube based RF sniffer circuit using a 6BQ5 or 6AQ5-usually in the very old Mavericks. If so you would have 9 total tubes above the chassis.)

Check the path from the antenna connector to the transmitter connector and you will find your problem.

73
David

 

[jtrouter]

Thank you for the response, What should i be looking for? Some kind of hint?
I do not see anything that looks burned or damaged. I can follow schematics and i can test parts but after that i am lost.

 

[nomadradio]

Hmm. Might start with narrowing down which specific amplifier this is. The MDX was an attempt to get the thing sold under the FCC radar by pretending it was a crystal-controlled ham CW-only transmitter. Not so different from what Texas Star does these days. Didn't work. D&A closed anyway.

The MDX had two 12-Volt relays and a transistor with two diodes to key them.

The Maverick would more likely have a skinny 6AQ5 tube to sense the radio's transmit RF, and close two relays with 110-Volt DC coils.

If it has the tube keying circuit, try pulling out that tube and see if it stops.

73