Help - F2B Ch. A No Output

[smuprof]

So, my vintage F2B has no output on Ch. A.  B is fine, both tubes are warm (so not an HV problem).  Swapped out the Ch. A tube with a new one - still no output.  No scorch marks, no trace damage, no broken leads.


Worked fine recently.  Any suggestions for signal path trouble shooting?  Really scratching my head on this one.  Handy with a meter, don't own a scope.  Appreciate any help!


John

[David Houck]

I imagine you've already troubleshot input and output cables, but mentioning it just in case.

[David Houck]

Rereading your post, I thought I would ask (not that I would know the answer), is the problem with no output from channel A occurring with both the channel A output and the mono output?

And then there's this.

[smuprof]

Hi Dave -


Thanks for the suggestions.  This is my second F2B, so I did try those things first.  It does seem like it should be something simple, but I haven't figured it out yet.  (FYI, B Ch works through B out and Mono out; A Ch works through neither.


I've wondered about an open cap in the signal path?

[xlrogue6]

The tube being warm means that the heater supply is working. The B+ supply is obviously OK since the other channel is OK. Check DC voltages on pins 1,2,3,6,7,8. I'd check the V1 plate load resistors first--one of them may be open.

[mica]

See if the tube pin voltages are similar on channel A and B, and report back. Let me know what the voltage is on pin 1 of channel A and pin 1 of channel B, etc. This will be the best clue and inform the next steps.

[smuprof]

Hi All -


Thanks for the help.  Mystery solved.  Apparently the A Ch signal lead going to the output (which feeds the mono output) broke right at the solder joint.  The jacket held it in place so it wasn't obvious until I had it on the bench poking around (actually, I dropped the multimeter lead on it and it popped off).  A quick strip and resolder and back in business!


Just curious:  The HV seems to be about 220v DC on both tubes at the tube socket (and on my other F2B) when the schematic calls for 300 or 320.  Am I measuring this wrong?  Seems right since they are all the same, but not quite sure I understand why.


Thanks again.  As always, Alembicians are THE BEST!


John 

[xlrogue6]

300V or so is the supply voltage--that's what you'll find on the other side of the plate load resistors (i.e., the side not connected to the socket pin). 200ish is about right for voltage at the socket with the tube installed.

[smuprof]

Thanks Kent!  Appreciate the guidance.


John