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Tram D201A Very low receive after using a pre-amp

Lkaskel

Lkaskel

Well-Known Member
Aug 4, 2017
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Hi Everyone,
A local on air guy brought his D201A by for me to check out. This is the 1st tube radio on the bench.... Mikes Radio did a restoration on it 3 years ago.

The owner said that he hooked up an amp to the radio and at one point turned on the pre-amp. When he did, the receive dropped and did not come back. I hooked the sig gen up to it and pumped in -73db (S9) into it on AM and it registered an S2 on the Tram meter. If I crank the sig gen up the signal does increase in the radio. SSB receive is almost non-existent. Any thoughts?
 

Damaged RF Gain control, maybe?

Quickest test is to turn it full clockwise with the signal gen cranked high enough to hear. Then bridge across the wiper lug and clockwise lug. If the control has failed, the signal level will jump up.

And a pair of 470k half-Watt resistors feed pin 2 of V300. If that pin reads much lower than the 127 Volts DC shown on the schematic, both R302 and R303 get changed.

I doubt his preamp had much to do with the loss of receiver sensitivity. Spontaneous point failures are to be expected in a 45 year-old tube type radio. Those are the two most-popular of them that I see over and over.

Any chance the guy tweaked it off the air? Simply turning random adjustments while the mike is keyed can cripple the receiver.

73
 
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nomadradio said:
Damaged RF Gain control, maybe?

Quickest test is to turn it full clockwise with the signal gen cranked high enough to hear. Then bridge across the wiper lug and clockwise lug. If the control has failed, the signal level will jump up.

And a pair of 470k half-Watt resistors feed pin 2 of V300. If that pin reads much lower than the 127 Volts DC shown on the schematic, both R302 and R303 get changed.

I doubt his preamp had much to do with the loss of receiver sensitivity. Spontaneous point failures are to be expected in a 45 year-old tube type radio. Those are the two most-popular of them that I see over and over.

Any chance the guy tweaked it off the air? Simply turning random adjustments while the mike is keyed can cripple the receiver.

73
Click to expand...
Hi Nomad,
So, I tested the voltage at pin 2 and it read 130vdc. Here is where it got interesting, I shorted out the the 2 terminals on the RF Gain pot. It instantly came up to S9. I had the RF Gain at 100% and the sig gen at -73db (S9). The meter was reading about S2-S3 then S9 on the short. I thought....YAY!!!! After I removed the short it remained S9 for about 10 seconds then drifted back to S3. Ok, maybe not YAY yet. I was curious what that was about. I could at times tap on the bottom of board around V300 and the radio would jump back to S9 for a second or two. I reseated the tubes but it did not seem to matter. I plan on re-flowing that part of the board tomorrow but I was curious on why it would jump to S9 and stay there for a few seconds on the RF Gain short. Any thoughts here?

Thanks as always!!!!
 
I really think your RF Gain control is bad. Don't know what would make it hesitate falling back when you remove the short. Then again, we have seen a bad control "patched" by soldering the wiper lug to the clockwise lug. The control will still turn down. For a while.

73
 
nomadradio said:
I really think your RF Gain control is bad. Don't know what would make it hesitate falling back when you remove the short. Then again, we have seen a bad control "patched" by soldering the wiper lug to the clockwise lug. The control will still turn down. For a while.

73
Click to expand...
Roger that. Any idea where those controls are available? I will remove it tonight to see what the part number is.

Thx Nomad!!
 
Ok, well I learned some more today. I decided to take the potentiometer apart to see if it is fixable. This is what I found....

Carbon explaination.jpgMissing carbon area.jpg
 
If memory serves, the value was 2.5K ohms. Kind of an odd value but maybe a 2K or 3K could substitute??

73
David
 
Dmans said:
If memory serves, the value was 2.5K ohms. Kind of an odd value but maybe a 2K or 3K could substitute??

73
David
Click to expand...
I think the bigger challenge is finding one that "fits" the radio correctly. The shaft size and length would need to be the same so it des not look incorrect. The owner wants it right if possible. I've researched the heck out of this and there are no solutions I could find other than a used one which is great if they work. Maybe the RF Gain control does not go bad very often? I wish there was a way to repair the carbon on the original but I don't think that possible. I wonder if maybe there was a lot of current going through this and it just burned it off.

Thx David!!!
 
I've seen the trick of paralleling a resistor as well. I was able to secure a replacement and it is in the mail. I'll update everyone once it arrives and it gets installed.

This hobby has become an interesting process. I originally intended to set up a bench and restore stereo's and 2-way radio's. Something to keep my head engaged outside of daily life. A few years ago I aligned a local operators Cobra 139 as a favor to him (and others on air) and the amount of people wanting repairs blew up. Last year I had maybe 5+ radios here all the time from locals wanting everything from major repairs to an occasional recap and restore. I am now only taking one or 2 max at a time and there is a line of folks waiting. I've had to create a schedule and place folks in a que. I do enjoy it though. I would really be in trouble if it were not for all of you pouring your experience into me!!! I may need a hobby to get away from my hobby. Anyone know anything about fishing or golf :censored:......

Thanks everyone!!!!
 
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The resistance value of that control is not at all critical. We have used 1k, and 2k controls. Can't remember the last time I found a 2.5k pot listed for sale.

And a 5k control with a 4.7k across the wiper and counterclockwise lugs has worked in a pinch. Seems to work more smoothly than placing the fixed resistor across CW and CCW.

73
 
Well, its all fixed and back to the owner. The issue was the bad RF Gain control (pot). Thanks to everyone for your help with this one!!!! The owner was so thrilled he wants me to work on his Mark 4. Oh My.......
 
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