DC offsrt on a preamp ranges from approx 10mV to 350mV, climbs and drops in a cycle that takes a minute or two to complete. (SM says it should be between 0 and 200mV.)
How should I go about finding the cause of this?
Freeze spray?
Trimpots have just been replaced by Bourns, so it's not them.
Power source voltage is rock steady at 30.2V
And which section of the amp does it likely stem from? Power supply?
Solder joins have been checked and reflowed but caps are still the originals, although the few that have been checked were well within spec.
Any suggestions?
Amp is a Yamaha C-4 preamp.
How should I go about finding the cause of this?
Freeze spray?
Trimpots have just been replaced by Bourns, so it's not them.
Power source voltage is rock steady at 30.2V
And which section of the amp does it likely stem from? Power supply?
Solder joins have been checked and reflowed but caps are still the originals, although the few that have been checked were well within spec.
Any suggestions?
Amp is a Yamaha C-4 preamp.
Do both channels exactly DC track each other, or do they behave differently but both still drift?
I thought this was DC coupled, but turns out that there are output coupling capacitors, so that won't work.
It may be within spec, which is +/-200mV, and you get 0-350mV, so good range, but an offset.
Try tweaking the appropriate pots (VR123, VR124) slightly, so each channel goes +/- about equally.
It may be within spec, which is +/-200mV, and you get 0-350mV, so good range, but an offset.
Try tweaking the appropriate pots (VR123, VR124) slightly, so each channel goes +/- about equally.
Last edited:
Ok, thanks anyway for commencing fault finding.
If it is reasonably synchronised.. and this most likely means its PSU related.. a more thorough testing of the PSU Caps (or wholesale replacement) seems in order?
My next question, for now, with the level maxing out at 350mV is that ok/bearable within a system?
The C-4 connects to a Yamaha P2200 power amp, speakers vintage Celestion Ditton 66's.
Naturally, I'll be wanting to rsolve this DC fluctuation very soon though.
If it is reasonably synchronised.. and this most likely means its PSU related.. a more thorough testing of the PSU Caps (or wholesale replacement) seems in order?
My next question, for now, with the level maxing out at 350mV is that ok/bearable within a system?
The C-4 connects to a Yamaha P2200 power amp, speakers vintage Celestion Ditton 66's.
Naturally, I'll be wanting to rsolve this DC fluctuation very soon though.
Again, it may actually be in spec with just a slight offset, so try the DC adj pots.
The RCA outputs all have coupling capacitors, so no DC can reach the power amp.
The RCA outputs all have coupling capacitors, so no DC can reach the power amp.
Thanks Rayma, so the C-4's 'pre out' output can't have any DC on it?
Also, the adj pots can make the DC rise or fall, but this just shifts this 340mV range higher or lower..
Also, the adj pots can make the DC rise or fall, but this just shifts this 340mV range higher or lower..
Right, exactly 0VDC will be at all of the RCA outputs.
Yes, the pot will shift the range of DC drift up and down wrt ground, that's what you would expect.
You can tweak each phono stage pot so the drift up and down wrt ground is about equal.
The drift does not seem to be power supply related.
Yes, the pot will shift the range of DC drift up and down wrt ground, that's what you would expect.
You can tweak each phono stage pot so the drift up and down wrt ground is about equal.
The drift does not seem to be power supply related.
Last edited:
Thanks Mike, fair point.
I'll double check on the snych precision level..
If it's not precisely synched, where best to start looking?
Would freeze spray be good in this situation?
Never used it before, can one literally spray it onto the board with the power on with no risk of damage / danger?
I'll double check on the snych precision level..
If it's not precisely synched, where best to start looking?
Would freeze spray be good in this situation?
Never used it before, can one literally spray it onto the board with the power on with no risk of damage / danger?
Rayma said:
These two pots only relate to MC/MM input pre-amp, so the question is this offset on all inputs ?