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I have a MP-3 which has recently lost all signal output through one channel for the Main (phono) input. Aux 1 & 2 work fine. On the Aux channels the sound is equal on both channels. On the Main one channel sounds fine and the other is silent. Would appreciate thoughts on the best troubleshooting approach i.e. what should I look at first. The tubes have less than 100 hours on them. Was all working great last week and the problem manifested itself when I turned the preamp on yesterday. I appreciate any help given.
Thanks
Follow Ups:
What is your level of DIY competence? The easiest thing to do is to remove the cover and just look to see whether there are any loose leads. If there are no obvious problems, then I would look upstream from the MP3 next. Balanced devices are relatively unlikely to go silent in one channel, because even if one half of the signal is lost due to a bad component, the other half will continue to deliver and you would hear something in the affected channel, even if not so good quality or as much gain.
Check tonearm wiring and cartridge, in other words.
Hi Lew,
Thanks for the response; built the M-60s I run the MP-3 with, so moderate DIY competency. Have checked and not found any loose connections or cold solder joints. Switched tubes from channel to channel. Tried my turntable (with balanced output) with another balanced system I have and all works fine. As do balanced inputs into Aux channels 1&2. Also unbalanced RCAs seem to work as well.
What is difficult for me to understand is that I was playing records through it a few days before and now there is absolutely no signal in one channel. Seems that if a capacitor or tube were to fail the change would not be that abrupt. So I am guessing it has to be something that would completely disrupt the signal in one channel without immediately being visually obvious.
Will get in there with a meter next, but suggestions of where to start would help. Even without knowing too much or having a schematic I am figuring that readings should be approximately the same in each channel, point to point, and that I should be able to at least track down where the readings differ.
Try the 12AT7 in the socket by itself in the rear. If there is a Chinese tube in that socket, it can cause one or both channels to quite suddenly go dead. You need an American or Euro tube in there so that won't happen.
You could also look at the remaining tubes and see if any of them have turned white inside- indicative of lost vacuum.
Thanks Ralph,
Using Mullard 6201 gold pins in all 7 phono positions. Swapped them out for a new set (for trouble-shooting purposes) and they all seemed to test well on my Weston tube checker.
Any suggestions before I try and remove the circuit board to check the underside? All looks clean and copacetic with all connections I can easily see. The fact that it happened so suddenly leads me to believe it must be something broken and not something failing.
And i swapped channels with the phono cable to make sure that was not the problem. Both worked on the left channel and neither worked on the right.
All caps look as new (no discoloration) and now all tubes have been swapped.
You don't remove the circuit board, FWIW- instead just remove the bottom panel.
The plate resistors for the first stage are located by the second 12AT7 in each channel, followed immediately by the EQ components. The plate voltage should be near 150 Volts there and also on the 12AT7 that is the output of the phono section. There are two plate voltage per gain stage as it is a differential amplifier in each case. They may not be all that close to each other; if they are within 20 volts its working. If the plate voltage is really high, at the B+ voltage, then the tube is not conducting.
In each channel you will find pads on the board marked 'tp K1' and 'tp K2'. These are test points for the cathode circuits of each gain stage (so tp K1 occurs on the 1st 12AT7 from the rear panel). The voltages should be ~+0.3 Volts on K1 and K2 for MkIIIs and earlier. Mk.3.1s will have about 1.5V on K1.
If the preamp has these voltages then you can assume the phono section is operational.
Thanks for the detailed help. Boy do I feel stupid. Caught this message a little late as I had already started to remove the board. Was able to find an open solder joint, fix it and re-assemble every thing back to the way it was; new tie wraps and all. Working great now. Left the new tubes in and all sounds wonderful.
Can't express how good it feels to know I can get such responsive and knowledgable help here on this board at the Asylum. You have become one of the few constants in this business; something that has become sorely lacking these days. My best to you in the holiday season.
You might now try swapping R to L leads at the cartridge pins. If there is an issue in the wiring anywhere between cartridge and phono input, obviously the problem will move to the opposite channel. (I am thinking tonearm wiring, since you did rule out IC issues.)
I may have mentioned earlier that I tried the turntable with another balanced system an it worked fine so I don't think it can be the tonearm wiring. But thanks for the suggestion. Still thinking about what it might be.
Sometimes the cause is right in front of one's nose.
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.
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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