C28 lack of stereo separation
- Thread starter CDinMB
- Start date
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- c28 stereo imaging
The pre-amp is behaving just as if you had the amps connected to the central channel outputs, but you don't.
So just a long shot but maybe somebody rewired the two top main outputs to the center channel internally. Maybe someone wanted to run several mono amps for sound in several different rooms for background whole-house audio where stereo was not needed. I noticed some labels stuck on the back next to the top main outputs; what are those for?
So just a long shot but maybe somebody rewired the two top main outputs to the center channel internally. Maybe someone wanted to run several mono amps for sound in several different rooms for background whole-house audio where stereo was not needed. I noticed some labels stuck on the back next to the top main outputs; what are those for?
nj pheonix
Lunatic Member
I would remove the L+R output circuit, disconnect the resistors, to remove the last common connection between the two channels as a trial.
Then confirm the grounds to chassis from the main out RCAs.
You will need to systematically work your way through the unit to confirm which section is shorting the two channel together, or causing the section to seek a ground reference via the other channels signal carrier.
Repairing the ground drains between circuit boards is just SOP with this vintage of Mac preamp, it must be done so it should be confirmed before any purchase.
Then confirm the grounds to chassis from the main out RCAs.
You will need to systematically work your way through the unit to confirm which section is shorting the two channel together, or causing the section to seek a ground reference via the other channels signal carrier.
Repairing the ground drains between circuit boards is just SOP with this vintage of Mac preamp, it must be done so it should be confirmed before any purchase.
CDinMB
Active Member
Following the schematic I see a lead connecting the
I am only using the Aux for testing at the moment so I replaced the input jacks and traced the wiring through the selector switch. I replaced the resisters between the L and R output connectors to the center channel and measured then resoldered the grounds at the output jacks. One side ground was at 30ohms. I removed the center fade connector wire and tested. All no change. Continuing to solder ground wires. Haven’t found any that look corroded externally.
I am only using the Aux for testing at the moment so I replaced the input jacks and traced the wiring through the selector switch. I replaced the resisters between the L and R output connectors to the center channel and measured then resoldered the grounds at the output jacks. One side ground was at 30ohms. I removed the center fade connector wire and tested. All no change. Continuing to solder ground wires. Haven’t found any that look corroded externally.
CDinMB
Active Member
I don’t know what those labels are referencing. I traced the input and output sections and there was no miss wiring and everything looked original. There is some rewiring internally. I see some wire wrap labels where some wires were replaced or reattached. They are in the switched power section not the signal. Still looking for that obvious miss soldered wire and tracing the schematic.
Whether you work systematically from the input to the output or from the output to input, pick one, you will have to start lifting input connections to the circuit boards to isolate where the channels are cross connected. You can not skip around. Following the schematic you need to work one connection at a time. Tedious yes, expensive time wise, yes but sometimes that is what tech work is all about.
CDinMB
Active Member
Found it!!!! Moving around the unit, fixing the grounds, I took out the volume control and found this glob of solder across the two input connections. Desoldered it and Wow this unit sounds great! Separation is what I expected and the all functions including balance work correctly. Love it!
Someone must have wanted to bridge left and right and not use the built in center channel, because I cannot imagine soldering this bad by accident!
Thanks for all the help, suggestions and support, guys!
Someone must have wanted to bridge left and right and not use the built in center channel, because I cannot imagine soldering this bad by accident!
Thanks for all the help, suggestions and support, guys!
damacman
Resident Psycho
Doh! Some people have no business wielding a soldering iron ...