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

 

[Dmans]

STEP 1-UNPLUG THE AMPLIFIER AND MAKE SURE ALL HIGH VOLTAGE HAS BLED DOWN!!

This amplifier (and any amplifier) should appear to your receiver as a solid piece of coax cable with no attenuation at all.
With this in mind, a quick and simple test would be to hook up your transmitter in reverse -output of tx to antenna connection of amplifier and transmitter connection of amplifier thru a trusted SWR meter terminated into a dummy load. If you are not seeing the same SWR reading as if only the dummy load is connected, you have an issue.

Based on your description of the problem, I suspect you will see the same reading until some sort of heat has built up. I would first manually actuate the relay(s) that the signal from your radio is going thru with this set up several times while looking for any bump in SWR. Relay contacts “connect” to each other with only a small amount of surface area. The relays may not return to their proper “at rest” connection points due to age, misuse, heat etc.

If this does not show your problem, I would then use a heat gun or heat source on the relay coil area and relay contact points to try and simulate a running condition. (RE-READ STEP 1 ABOVE!!) I’m not talking about a propane torch or trying to melt any material under the chassis just enough heat from the wife’s hair dryer. This would reveal a relay coil “short circuit” when current is applied or a loss of tension on the contact arms/leafs of the relay. Again manually actuating the relays while a moderate heat is applied re-checking SWR into your dummy load.

A simple continuity test between your 2 S0-239 connectors of the amplifier may be misleading as continuity does not guarantee a circuits ability to pass current.

As a simulation of the amplifiers current passing ability since it should be able to pass a very low voltage RF signal (.7 microvolts) you need to insure all connections from 1 coax connector to the other is totally unobstructed. I.e. mechanically and electrically perfect.

RE-READ STEP 1!!!
Using a 9 volt battery and a potentiometer set it up so that you have about a 20Ma current that you can connect to the antenna connector thru to the TX connector of the amplifier and measure the current at every connection point starting at the antenna connector under the chassis to the TX connector under the chassis. Anything that measures less than you initial current you set up from your 9 volt battery should be repaired. You should be able to check this current thru the relay(s) and know the condition of the contact points. Actuation of the relays again and measuring current should be done here as well.

If the relay coils are 12vdc actuated you could apply 12vdc to make sure they do actuate completely with no mechanical bind. Also check voltage drop across the relay coil and resistance of the relay coil. It shouldn’t be too difficult to find a data sheet on the coil part number to confirm your readings.

You can do the same with a 110/115 vac relay but RE-READ STEP 1 first. I would use an alternate power source (a variac) because of STEP 1.

You should also insure that the amplifier is supplying the correct voltage to the relays though typically the relay is “off” or at rest during receive and when energized your receive signal is “disconnected”.

If this amplifier has a “Pre-Amp”, your receive signal could be “getting lost”in this circuit. Might be wise to bypass this circuit all together. I can’t tell you how to do that without the amplifier or a schematic in front of me but careful tracing of the circuit from antenna coax connector to TX connector may reveal an easy to way do this. Better to leave it this way in my opinion but it is your amplifier.

RE-READ STEP 1!!!!
There is no need to plug this amplifier in to troubleshoot this problem. You can simulate many conditions without putting yourself at risk.

I suspect a bad solder joint or mis-aligned relay contacts or a relay coil that is “misbehaving” under power. At the age of this amplifier I would replace the relay(s) without question.

I am on vacation this week and the blue-green waters of the Gulf of Mexico are calling!!!

RE-READ STEP 1!!!!!

73
David

 

[jtrouter]

STEP 1-UNPLUG THE AMPLIFIER AND MAKE SURE ALL HIGH VOLTAGE HAS BLED DOWN!!

This amplifier (and any amplifier) should appear to your receiver as a solid piece of coax cable with no attenuation at all.
With this in mind, a quick and simple test would be to hook up your transmitter in reverse -output of tx to antenna connection of amplifier and transmitter connection of amplifier thru a trusted SWR meter terminated into a dummy load. If you are not seeing the same SWR reading as if only the dummy load is connected, you have an issue.

Based on your description of the problem, I suspect you will see the same reading until some sort of heat has built up. I would first manually actuate the relay(s) that the signal from your radio is going thru with this set up several times while looking for any bump in SWR. Relay contacts “connect” to each other with only a small amount of surface area. The relays may not return to their proper “at rest” connection points due to age, misuse, heat etc.

If this does not show your problem, I would then use a heat gun or heat source on the relay coil area and relay contact points to try and simulate a running condition. (RE-READ STEP 1 ABOVE!!) I’m not talking about a propane torch or trying to melt any material under the chassis just enough heat from the wife’s hair dryer. This would reveal a relay coil “short circuit” when current is applied or a loss of tension on the contact arms/leafs of the relay. Again manually actuating the relays while a moderate heat is applied re-checking SWR into your dummy load.

A simple continuity test between your 2 S0-239 connectors of the amplifier may be misleading as continuity does not guarantee a circuits ability to pass current.

As a simulation of the amplifiers current passing ability since it should be able to pass a very low voltage RF signal (.7 microvolts) you need to insure all connections from 1 coax connector to the other is totally unobstructed. I.e. mechanically and electrically perfect.

RE-READ STEP 1!!!
Using a 9 volt battery and a potentiometer set it up so that you have about a 20Ma current that you can connect to the antenna connector thru to the TX connector of the amplifier and measure the current at every connection point starting at the antenna connector under the chassis to the TX connector under the chassis. Anything that measures less than you initial current you set up from your 9 volt battery should be repaired. You should be able to check this current thru the relay(s) and know the condition of the contact points. Actuation of the relays again and measuring current should be done here as well.

If the relay coils are 12vdc actuated you could apply 12vdc to make sure they do actuate completely with no mechanical bind. Also check voltage drop across the relay coil and resistance of the relay coil. It shouldn’t be too difficult to find a data sheet on the coil part number to confirm your readings.

You can do the same with a 110/115 vac relay but RE-READ STEP 1 first. I would use an alternate power source (a variac) because of STEP 1.

You should also insure that the amplifier is supplying the correct voltage to the relays though typically the relay is “off” or at rest during receive and when energized your receive signal is “disconnected”.

If this amplifier has a “Pre-Amp”, your receive signal could be “getting lost”in this circuit. Might be wise to bypass this circuit all together. I can’t tell you how to do that without the amplifier or a schematic in front of me but careful tracing of the circuit from antenna coax connector to TX connector may reveal an easy to way do this. Better to leave it this way in my opinion but it is your amplifier.

RE-READ STEP 1!!!!
There is no need to plug this amplifier in to troubleshoot this problem. You can simulate many conditions without putting yourself at risk.

I suspect a bad solder joint or mis-aligned relay contacts or a relay coil that is “misbehaving” under power. At the age of this amplifier I would replace the relay(s) without question.

I am on vacation this week and the blue-green waters of the Gulf of Mexico are calling!!!

RE-READ STEP 1!!!!!

73
David

THANK YOU!!! You have given me a whole lot to do and try. I really appreciate the step by step guide. I will go though each step very carefully and will wait a week and post back and i really hope i will be thanking you even more. Off to my workbench. And It is a MDX200 it does not have a rec amp and does not have a keying tube, I just always call it a mdx/mavrick for those who may not know ita just a later version of the mavrick. And lastly the relays are still the original 115volt versions. Until next week.