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In Reply to: RE: voodoo posted by Lew on December 16, 2011 at 07:18:49
With power OFF, take your ohmmeter and see whether there is any resistance between the two phases at the anodes (or plates) of the phono input stage. (If you can identify the plate resistors for the dual-differential cascode, place the probes where those resistors go to the anodes of each half of the upper tube in the cascode.) R should equal roughly 2X the value of each plate resistor, because the meter will see them in series since they both connect to B+ (or maybe the reading will be dominated by some resistive components in the RIAA). If R = 0, there is continuity between the two phases which would cancel the output signal entirely.
Follow Ups:
This suggestion forced me to take the everything apart so that I could remove the circuit board. Found the problem in a bad solder joint that became apparent once I had the underside of the board in a good light.Fixed and all is working well. Now, just have to get all the lead dress and tie wraps back to factory spec (in case I ever send it back to Ralph for upgrades - don't want to be embarrassed).
Thanks again for you help and suggestions.
Inquiring minds want to know where the bad solder joint was, but I am going to guess that there was loss of contact between the cathode resistor and the cathode(s) of the phono input stage or between the negative supply rail and that resistor (or could be a CCS instead of just a resistor between B- and cathode of the dual-differential circuit; I don't know what's inside an MP3 these days).
I have had stuff like that happen where a "perfectly good" solder joint goes sour.
Embarrassed to say I don't know, other than to say it was associated with the second pair of tubes in the phono channel. Because I was looking at the bottom side of the board I am not sure what was on the other side. But the solder joint had opened up completely. Maybe from pressure inserting and removing the tube from the socket. Just had to re-melt the solder (nice and shiny now) and all was well.
What has made me feel really stupid is having removed the board part way to inspect and then getting Ralph's message above that I could have removed the bottom plate instead. Anyway was able to get everything re-assembled so it looks factory new. Also went through and cleaned everything. Sounding sweet with a little Coleman Hawkins.
Again thanks for the help and have a great holiday season
Thanks Lew will try this.
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