Tram D201 AM RX problem

[9fb177]

I've made good progress with my Tram D201 project.    I've done a complete recap.  Replaced and raised power resistors, solved a number of faults, and removed unwanted mods along the way.   My radio is the 23 channel printed circuit board version.

I'm down to what I believe is my last problem... and I took a bit of a break from the project in favor of summer activities.

Anyway, working on the last issue.     Everything working well except AM RX.   Using a signal generator and scope I've traced the failure down to the 2nd stage of AM IF, immediately before the AM detector.   Checking voltages around the 6GH8A V401A I find that the screen voltage is very close to the plate voltage instead of the 95v according to the schematic.  

There are two circuit board trace cuts that I previously wrote off as possible factory edits, because the traces were cut so surgically.   But I now see that one of these cuts leads from the V401A screen to to A4 connection of the PCB "AA" auxiliary board.   This would effectively break the screen voltage divider, which would lead to the screen voltage just below that of the plate.   I can see how this might be the failure point.

Are there any legitimate factory edits to the boards?    I see there is another cut trace that I'd better explore.

**EDIT**   I see that my PCB "AA" is blank where R407 should be... and that R406 is 82k ohms.   So the circuit trace cut would appear to be factory.  I see that this part of the circuit more closely resembles the D201A schematic.   I still don't understand why my screen voltage is so high...

Thank you for any assistance!
73,
Kevin   

[9fb177]

Ok... kind of embarrassed to have bothered you guys with this post, but I've found the solution...

It had nothing to do with the screen supply, and yes there are factory edits to my board.  The screen supply is laid out the same as on the D201A, and my radio is the D201.

Anyway, I discovered that connection A1 at the Receive auxiliary board was poor and was isolating R405 from the mode switch, allowing the cathode of V401A to float free, effectively turning off the stage.

I'm embarrassed, cuz I knew to reflow all of those contacts, and in fact that was one of the first things I did, following the advice of others..  There were a number of broken connections... but even though I'd reflowed them all, one connection was still bad and somehow it got past my ohmmeter tests the first time.     Found it with the ohmmeter the second time around.  Removed the old solder, and remade the connection.   

All good now!
73,
Kevin 

[2600]

Rock on!


Congratulations for sticking with it. Far from being a waste of our time, this is valuable info to anyone that hasn't had to trace through this section of that radio.

More than once I have tracked down a fault with a 'scope, stage by stage until it leads me to a bad solder joint, or a failed socket connection.

The plug-in boards on that radio have a chronic headache with the thick round pins where they solder to the large audio or receiver board. I blame it on the temperature coefficient of expansion. The metal inside the pins expands and contracts at a different rate than the solder. After enough hot/cold cycles this disrupts the solder bond and causes the fractured "ring" in the solder around the pin.

Good work!

73

[9fb177]

Nomad, 
I very much appreciate the help and encouragement!

Your theory about the expansion and contraction of the pins sounds right on.    I have now resoldered all of those pins on the expansion cards.

One more question...  I've realigned the radio carefully, and so far, the receiver is always hotter down on the original 23 channels, and starts to fade as I move up to .385.   Its still pretty good, but I would have hoped to be able to tune the RX to favor frequencies up around channel 38.   No dice.     It's only the stages prior to the first mixer that could affect that right?  Any thoughts?

Thanks again!
73,
Kevin