How do you bypass balance pot in a c-28 preamp

Good morning all,
I am starting a new thread on this question hoping to get more specific info, hopefully from some of the McIntosh tech experts out there.

3/4 of the carbon trace has peeled up on the right channel of my balance pot. The part is not available from Mc parts or any of the other common sources. My only option is to bypass until I can find a donor replacement. I don't completely understand this pot's configuration or why it is wired the way they wired it. I would like to know if anyone has done this or knows how to do this and may be able to provide some detailed instructions on how I can do this. for now I have Jumpered the bad side with some resistors to get the values close to the original side and it kind of works but not properly. I would just like to completely de-solder the pot on both sides and sub in proper resistors so both channels are balanced.

Thanks in advance for any help that can be provided.

Please.......Anyone?

Just took a look at the C28 schematic. The Balance pot is a 500K, dual channel pot with the 2 channels wired backwards so that you can have one channel reduced to minimum value while the other channel is at max volume.

So, you should be able to bypass the old pot with 2 250K resistors. At the junction of the 2 resistors, you should be able to connect the wiper contact, which would put both of the pots (L and R channels) in the middle position.

This way, you can then have both, effectively, taken out of the circuit for all intents while maintaining the proper loading of both the driving and receiving circuits.

So much for the theory. The biggest implementation problem is getting all this to work in tight quarters of the balance control itself to implement such a scheme.

Cheers,

David

dshoaf said:

Just took a look at the C28 schematic. The Balance pot is a 500K, dual channel pot with the 2 channels wired backwards so that you can have one channel reduced to minimum value while the other channel is at max volume.

So, you should be able to bypass the old pot with 2 250K resistors. At the junction of the 2 resistors, you should be able to connect the wiper contact, which would put both of the pots (L and R channels) in the middle position.

This way, you can then have both, effectively, taken out of the circuit for all intents while maintaining the proper loading of both the driving and receiving circuits.

So much for the theory. The biggest implementation problem is getting all this to work in tight quarters of the balance control itself to implement such a scheme.

Cheers,

David

Click to expand...

Thank you for taking the time to respond David. That is what I initially thought, however 2 odd things I noticed.

1. The way these are wired, yes they are wired opposite of each other so I understand that part. But the usual way I have seen pots like this wired is signal comes in to one side of the carbon trace, we will say pin 1 - then the wiper, we will say pin 2 is the signal grounds - then the other side of the trace, we will say pin 3 is signal out.

This one is wired signal - pin 1, signal - wiper (pin 2) - signal - pin 3. Can you help me wrap my head around that?

2. I took resistance measurements from the good channel and it shows the pot as being only ~400k (one end of trace to the other) so a bit out of spec, but then when it is centered I only get ~30k between the two signal leads pin 1 and wiper (pin 2).

So I don't know if this is a custom pot and not fully linear or if I just don't understand what is going on here. My bet is I just don't understand.

This is very frustrating to me as I just spent a lot of hours and a good chunk of change replacing the volume pot and doing a full recap, plus fixing some of the ground issues. All I want to do is listen to it and enjoy it now. I am really starting to wonder why the balance pot which is rarely used suffered this same fate as my volume pot. I suspected strong cleaner, hopefully its not my cleaner. I have been using the rat shack version of de-oxit.

Anyway sorry for then rant at the end there, just want to get this thing going and keep it that way.

Any further info you can provide would be very very appreciated.

Thanks,
Chris

Look at the schematic and find where the Balance pot is wired in. As I think about my prior answer, first look at the block diagram.

See how the Balance control is wire just after the Mode switch? This means they are dependent upon each other. We need to be sure that the Mode switch isn't affecting the symptoms.

Next, as you look at the schematic for the Balance control, you see the that the pots are not grounded as we normally expect them to be. The Balance control is wired into the Volume control's common point.

Can I assume you changed C39 and C40? Leakage on these caps would cause an imbalance but would in no way cause the carbon to peel up as you have found.

If you are getting 30K out of 500K on the good channel, this may be an audio taper pot. Does it track exponentially across the arc? If so, then it is an audio taper.

Based on the findings so far, if you have a good pot on one channel, you'll need to approximate the bad channel by building a resistor divider network to emulate a fixed center position and then leave it alone.

I point out that the stages are interacting and to eliminate potential problems upstream that may also be affecting the symptoms.

Lastly, given the C28s I've worked on, also suspect those pesky little ground wires that the _really_ experienced guys talk about. All the moving of wires can certainly break them inside their sleeve. Indeed, both C28s I've restored had cases where I did this while moving bundles of wires around during the recap.

Cheers,

David

dshoaf said:

Look at the schematic and find where the Balance pot is wired in. As I think about my prior answer, first look at the block diagram.

See how the Balance control is wire just after the Mode switch? This means they are dependent upon each other. We need to be sure that the Mode switch isn't affecting the symptoms.

Next, as you look at the schematic for the Balance control, you see the that the pots are not grounded as we normally expect them to be. The Balance control is wired into the Volume control's common point.

Can I assume you changed C39 and C40? Leakage on these caps would cause an imbalance but would in no way cause the carbon to peel up as you have found.

If you are getting 30K out of 500K on the good channel, this may be an audio taper pot. Does it track exponentially across the arc? If so, then it is an audio taper.

Based on the findings so far, if you have a good pot on one channel, you'll need to approximate the bad channel by building a resistor divider network to emulate a fixed center position and then leave it alone.

I point out that the stages are interacting and to eliminate potential problems upstream that may also be affecting the symptoms.

Lastly, given the C28s I've worked on, also suspect those pesky little ground wires that the _really_ experienced guys talk about. All the moving of wires can certainly break them inside their sleeve. Indeed, both C28s I've restored had cases where I did this while moving bundles of wires around during the recap.

Cheers,

David

Click to expand...

Ok yes these must be audio taper pots. Testing the resistance on the good channel values across pin 1 and wiper (pin2) are ~25ohms to ~400k ohms and centered at ~30k so this is not linear at all.

Mode switch worked fine prior to this with perfect channel balance. part of the trace was sticking out of the side of the pot so there is no question that is what I am dealing with.

All measurements were taken with mode switch in stereo.

I don't have the schematic in front of me at the moment, so I am not sure which caps C39, C40are but if they are electrolytics they have been replaced.

Here is what I did that kind of works (See very crude drawing:sigh.

Does this look right to you from a mechanical/wiring perspective?

Maybe I need to play with resistor values because after doing this both channels are back, somewhat unbalanced though and that is what I am trying to resolve.

Since there was a clear open between wiper and trace in the centered position I soldered these in place with the wires still connected to the pot. There was not enough room to do otherwise. If you think I am on the right track with the wiring then I will go ahead and sub in some different resistors to try and balance the channel.

Thank you so much David.

David, hopefully you saw my last post. Your thoughts?

David?

Check out Rod Elliot's site Soundwest for an example of how to build a balance control with linear taper wafers.