C28 lack of stereo separation

[CDinMB]

I bought a C28 for a good price, as-is condition knowing it would provide some opportunities to learn how it worked. Surprisingly all things I researched and expected to find are fine. The unit is quiet, no pops or hum, the volume is balanced left to right and I can calibrate the bass boost. It pairs well with the MC30s and LaScalas. I really like it. Ultimately the problem is it seems too balanced left to right, as in they seem to be exactly the same signal almost monoral. Streaming audio has a very collapsed soundstage and I don’t hear any imaging. Everything is in the middle. Any idea what would cause the unit to work fine in all other respects and fall so flat in stereo separation?

This didn’t show up in any of my research, so scratching my head. I am in this to fix it and keep the unit. Looking for some hints or ideas of where to start looking first. I downloaded and printed out the schematic and will start studying. Suggestions welcome.

 

[62caddy]

Try disconnecting the left channel from a source component, then play the source with the balance turned fully right. (Stereo mode selected)

Separation is excellent if there is barely an audible signal on the right channel.

 

[CDinMB]

Try disconnecting the left channel from a source component, then play the source with the balance turned fully right. (Stereo mode selected)

Separation is excellent if there is barely an audible signal on the right channel.

Three conditions tested (stereo mode)
1) Disconnect left channel input
- balance to left - no sound
- balance to right - equal sound both speakers
2) Disconnect right channel input
- balance to right - no sound
- balance to left - equal sound both speakers
3) Balance centered one input disconnected
- both speakers have equal sound

Both inputs connected and sound level increases incrementally.
Balance turned to either side only reduces volume, no shift of sound side to side

Hmm, balance pot bad?

 

[TSmith8605]

Check you connections on back before blaming the preamp. E.g., this preamp has 2 "center" main outputs; you may have the power amp connected to those instead a L and R output set. Also, is anything plugged into the tape outputs or Line Output?

 

[62caddy]

Correction to earlier post. Should have been:

Try disconnecting the left channel from a source component, then play the source with the balance turned fully LEFT. (Stereo mode selected)

Three conditions tested (stereo mode)
1) Disconnect left channel input
- balance to left - no sound
- balance to right - equal sound both speakers
2) Disconnect right channel input
- balance to right - no sound
- balance to left - equal sound both speakers
3) Balance centered one input disconnected
- both speakers have equal sound

Both inputs connected and sound level increases incrementally.
Balance turned to either side only reduces volume, no shift of sound side to side

Hmm, balance pot bad?

Sounds like you've taken all the correct diagnostic steps. The unit is definitely blending the channels. I suspect faulty repair work.

 

[CDinMB]

Check you connections on back before blaming the preamp. E.g., this preamp has 2 "center" main outputs; you may have the power amp connected to those instead a L and R output set. Also, is anything plugged into the tape outputs or Line Output?

Rear panel connections are correct. Only connections are phono 1 in, aux in (iPhone), and amp 1 out to the MC30s. Also, no amp, etc. power connections running through the preamp. Using a power box for all on/off power switching.

 

[for_p1]

Correction to earlier post. Should have been:





Sounds like you've taken all the correct diagnostic steps. The unit is definitely blending the channels. I suspect faulty repair work.

Seems to be stuck in MONO mode somehow.

 

[chef free]

Even if it was stuck in mono the balance knob should work. What happens when you switch the "mode" selector? I'd open it up and look for any evidence of 62Caddy's suggestion of "faulty repair work".

 

[c_dk]

To measure separation you need to inject a signal into a channel and measure how much bleeds over to the other channel. R then measure L. Then switch the input, L for R and do it again. Typically this is done with calibrated tones via a signal generator and monitored with a oscilloscope and a precision audio grade volt meter.

Once these baseline measurement is made via different inputs and multiple frequencies correcting a flaw is then possible by standard step by step diagnosis.

Your description seems to indicate a bad ground......only by centering the balance control a ground path is made. The ground paths for the C28 is convoluted and have been the subject to a number of posts.

 

[CDinMB]

To measure separation you need to inject a signal into a channel and measure how much bleeds over to the other channel. R then measure L. Then switch the input, L for R and do it again. Typically this is done with calibrated tones via a signal generator and monitored with a oscilloscope and a precision audio grade volt meter.

Once these baseline measurement is made via different inputs and multiple frequencies correcting a flaw is then possible by standard step by step diagnosis.

Your description seems to indicate a bad ground......only by centering the balance control a ground path is made. The ground paths for the C28 is convoluted and have been the subject to a number of posts.

Thanks! I have not taken voltage readings yet. Sound level readings show 100% bleed. Looks like I will be opening it up and tracing all the grounds, resoldering them and testing. I planned to replace all the input/output jacks anyway, so the soldering iron will be hot. I read all the posts and didn’t see this as a symptom, but I will naively say, I am glad it maybe “just” the grounds. For what I’ve read, I may be regretting those words, as I read the grounds are a lot of work, but replacing transformers, transistors, and volume controls, etc was my worst case scenario. I don’t have a scope anymore, so I may be needing some local tech help.

 

[CDinMB]

Even if it was stuck in mono the balance knob should work. What happens when you switch the "mode" selector? I'd open it up and look for any evidence of 62Caddy's suggestion of "faulty repair work".

Other mode settings and results:
L+R to L
- balance Center, equal sound both speakers;
- balance left, equal sound both speakers; no sound reduction
- balance right, silent
L+R to R
- balance Center, equal sound both speakers;
- balance right, equal sound both speakers; no sound reduction
- balance Left, silent
So looks like balance is working

ALL other settings act the same BUT different than listed above. Equal sound levels L and R, slightly reduced when balance knob is moved off center either direction.

So, c_dk may be right, flaky grounds.

 

[c_dk]

If you sit down with the schematic you can trace out the paths to ground via tha ground drains of the interconnecting cabling. They rout through the presence/ loudness circuit as well as the mode ans hi and low preamp boards.

SOP for all these vintage units is to slit the shrink wrap and flow solder into the ferrule that crimps the drain to lead to the circuit board. At amp clinics Dave O'Brien would then black tape them, I have had units from Landon that he just left open, I will reinstall new heat shrink. Budget 5 to 6 hours to get them all the first time you take them on. DOB could get all the most important ones done in 45 minutes but having done so many at clinics he could most likely do them in his sleep.

I had a MAC 1900 that showed similar symptoms 25-30 years ago but via the speaker outputs......drove me nuts. Turned out to be a bad ground rivet to the chassis from the speaker terminals, it was hidden under the power supply board. Thankfully, Mike, the then service manager for Mac gave me some guidance during our monthly chat about service issues and sailboats.

 

[CDinMB]

Gone through a few grounds and no improvement. I found another odd control reaction. Working with the trim pots on top of the unit. Turning the level down on the right output level trim reduces the left speaker and visa versa. Started looking for crossed wiring from a prior service connected incorrectly, swapping left to right. I traced the connections from input to the mode switch and the circuit boards and all agree with the schematic. So not a simple wiring issue. I connected headphones since they use the same line input but have their own amp and trim pots. Amazing they work correctly. Right trim affects the right speaker, etc.

Scratching head again! Will continue to work on grounds but considering sending it to an expert.

 

[62caddy]

Swapped action of trim pots suggest further evidence of tampering. Might be time to call in the Calvary on this one.

Being a recent purchase begs the question whether the seller pulled a quick one just to get it out the door....

 

[mrz80]

Might be time to call in the Calvary on this one.

What, lay hands on it and pray for it?

Calvary = the crucifixion
Cavalry = horse soldiers with test benches and schematics

Sorry, I couldn't resist.

 

[928fool]

 

[c_dk]

There have a few intermittant units over the years that I thought I would have to call in a priest to perform a exorcism.....

 

[CDinMB]

I am slowly attacking the grounds, saying a few rosaries, and testing. No improvement yet. I will try anything,

 

[c_dk]

So if I understand correctly,

If you have the balance control centered you have a mixed identical signal out of both the left and right main output, however if you rotate the balance off center both channels go up in level but still the same signal.

 

[CDinMB]

So if I understand correctly,

If you have the balance control centered you have a mixed identical signal out of both the left and right main output, however if you rotate the balance off center both channels go up in level but still the same signal.

Balance centered creates equal mono signal, Yes. But, When I rotate the balance, the volume decreases both channels by a small amount. Very small but noticeable. That is true for stereo mode.

I can cut off a channel by placing the mode switch in L+R to L or L+R to R and moving the balance left or right. I think that tells me the balance control is working.