TRAM D201 no modulation issue

[BBB]

My Tram 201D has lost modulation. This is a circuit board model that has good receive audio on AM or SSB and dead keys 4 watts right on frequency in either crystal or VFO modes. Again no AM modulation or SSB modulation.

Mic checks out fine, swapped all 4 tubes on the audio board and swapped the BA board. High voltage at the filter caps is good. Just wondering where to look next. Will study the schematic after I get one enlarged enough to see haha Maybe the service manual has a flow chart?

[Sandbagger]

Feb 5, 2014 16:55:28 GMT -5 BBB said:

My Tram 201D has lost modulation. This is a circuit board model that has good receive audio on AM or SSB and dead keys 4 watts right on frequency in either crystal or VFO modes. Again no AM modulation or SSB modulation.

Mic checks out fine, swapped all 4 tubes on the audio board and swapped the BA board. High voltage at the filter caps is good. Just wondering where to look next. Will study the schematic after I get one enlarged enough to see haha Maybe the service manual has a flow chart?

The fact that you have receive audio, and do not have either AM or SSB transmit audio, eliminated a ton of circuits. You are basically confined to the stuff around V601A and V600A. Check voltages to expose potential open resistors, and there may be an open electrolytic cap in the audio chain, or a shorted bypass cap. Look for a really hot resistor.

[BBB]

Thanks for the reply. I'll definitely dig in deeper since I have a working unit to help trouble shoot with.

Modulation issue aside, this radio still has the original yellow .22 uf 400 volt caps and I've heard they should be replaced. I'm assuming they're non-polorized film caps even thought they have a black stripe on the on end. I found some .22 uf 400 Volt MMDR CAPACITORS. I think MMDR stands for a metal film capacitor. Will that work? I didn't know there were so many different types of film capacitors and just want to make sure I get the right replacements.

Some different types of film caps; www.tscgroup.com/acrobat/cr02001b.pdf

I should have posted this in the Browning/ Tram section.

[Sandbagger]

Feb 6, 2014 12:54:03 GMT -5 BBB said:

Thanks for the reply. I'll definitely dig in deeper since I have a working unit to help trouble shoot with.

Modulation issue aside, this radio still has the original yellow .22 uf 400 volt caps and I've heard they should be replaced. I'm assuming they're non-polorized film caps even thought they have a black stripe on the on end. I found some .22 uf 400 Volt MMDR CAPACITORS. I think MMDR stands for a metal film capacitor. Will that work? I didn't know there were so many different types of film capacitors and just want to make sure I get the right replacements.

Some different types of film caps; www.tscgroup.com/acrobat/cr02001b.pdf

I should have posted this in the Browning/ Tram section.

The type of cap is less important than the value and voltage rating. Those .22 uF caps have a tendency to short out (I found 3 of them that way in my D201 (but they were metal electrolytic types). There are a bunch of smaller electrolytic types in the mic preamp stages, so it shouldn't be a big deal to track down the culprit.

[2600]

Since you have no transmit audio on AM or SSB, this suggests a fault upstream from where the audio splits off in two directions for each separate modulator.

First thing to check would be R620, a 470k 1/2-Watt resistor feeding juice to pin 3 of V601, the mike preamp stage. A voltmeter probe to pin 3 of V601 should show between 25 and 30 volts DC. Good chance you'll read about zero if R620 is bad. We use a 1-watt resistor to replace it, but that's probably overkill. Next up would be C610, a 4uf 150V cap that's connected from V600 pin 3 to ground. If it shorts, you'll have zero Volts on pin 3, and the same "no audio" symptom. We have put R620 onto our "100,000-mile tuneup" list of parts to change even when they check okay. C610 is on that list because it is an electrolytic capacitor. If your radio has ANY original electrolytics in it they are ALL on borrowed time, and the only mystery is why ANY of them still work at all.

Next-most likely culprit might be C634, a 10uf 450-Volt cap that is the next one 'upstream' from the C608 and R620. The 47k 1/2w R619 will almost certainly smoke any time C634 fails as a short circuit.

Naturally this assumes that V601, a 6GH8A is okay.

73