NEUMANN V475-2A problem- help needed

Hi all, I just built a DIY summing mixer with a NEUMANN V475-2A card. What I found during calibration and check, is that although the 1st channel works as expected. i.e I feed the input with a 1kHz sine at balanced line level -18DBFS through a pair of 5.1k resistors as Re, and I 've installed a stereo 20k linear for Rg. I get 0db unison when the pot reaches 6.2k and I can get headroom from -12db at 0Ω, all the way to +12db just over 15k. I understand that is correct and according to Neumann. On the 2nd channel though when Rg reaches +3db I am starting to get clipping, and just over +3db the output starts to fade down, like something overloads the amp and it shuts down. It seems to me, like a degraded or burnt component somewhere on this channel, or maybe something is wrong with the bias of the amp, but as I don't have any knowledge on electronics- I am a sound engineer myself- I wouldn't know where to look or what to measure with a multimeter or even replace. I thought of asking the community, in case someone happens to know the insides of these cards and can direct me in some way.
Regards,
Nikos Kefaloyannis
 
Nikos
Since you said you do not have any knowledge on electronics, you should first try switching the channels on the 475, just to rule out a possible construction error on your part. If problem goes to the other side, then you know it is the 475 for sure.
If the TDA1034 op-amp on the 475 board is socketed. you could try swapping with the other channel. Other than that, you will need a scope to troubleshoot. The good news is that you have a good working channel as a reference so finding the problem with a scope should be fairly easy.

Your 475 card must be at least 30 years old, could possibly have a failing capacitor on the power lines. You may want to change those out - and you may get lucky and that resolves your problem. Do both channels as one side with fresh new caps may sound different than the side with old drying out caps. Don't worry about changing the sound as it should only sound better with new power line caps. The desirable sound from the 475 cards is mainly from the transformers and secondly from the discreet output amplifier.
 
first of all, a big thank you to fellow member dvd7900 for his input to my problem. This is what I have done so far to no avail, unfortunately. I replaced the 2 10uf electrolytics on the power lines of the problematic channel, no change.I replaced the 3 roederstein electrolytics plus the Philips one but nothing new happened. Then I started swapping parts between the channels. I swapped the whole MO-3 power board, nothing. I swapped the MO-1 board, no change. I swapped the MO-2 board, nothing. I swapped the tantalum cap, no change. I swapped the 2 WIMA caps, no change. My PSU is an 150-0-150 to 2x12 toroidal to a regulator with LM317/LM337 outputting 24.0V measured. I swapped it with a laboratory PSU in case there was an issue with current but nothing changed. I suspect that the limited headroom in the 2nd channel dictates limited voltage or current somewhere in the circuit but at this point I am really running out of ideas. Any help will be much appreciated.
 

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