MAC1900 with Left Channel Problem

[hirscwi]

Greetings, I'm having problems with my MAC1900. The left channel is weaker than the right channel - that is, it sounds a bit lower in volume. The left channel also sounds a bit muted or lacking in presence as compared to the right channel, but perhaps this is a result of the lower volume. I swapped the preamp boards, and the problem did not move (i.e. stayed on the left channel). I put the preamp boards back to their original position and swapped the amp boards. The problem stayed the same (i.e. stayed on the left side). So to my way of thinking, the problem is not in any of the PCBs. It could be in the volume or balance controls or in the output transistors. I'd appreciate any suggestions before I dive into it further.
Thanks.

 

[c_dk]

What is a "weak" channel? Too much bass in one channel compared to the other? Not enough mid range?

You have to do a spectral analysis to get beyond a wild ass guess to mere conjecture.

The tone circuits are years beyond needing a recap, and of coarse any contentious tech 10 years ago would have done a tone circuit realignment as a matter of SOP.

Did you swap the jumpers with a patch cord to feed LH preamp to RH amp?

 

[hirscwi]

By "weak" I mean it has less volume. And the mid range appears to be less, but that may just be the effect of lower volume.

I don't have the equipment or knowledge to perform a spectral analysis.

The entire unit has been recapped within the past month. But I don't know how to do a tone circuit realignment - or even exactly what that term means.

Yes, I did swap the jumpers and fed the left preamp to the right amp and vice-versa. Under that circumstance, the problem moved to the right channel. Given that swapping the preamp boards didn't cause the problem to change channels, it must be between the preamp board and the "Pre-out" RCA. That leaves the tone control PCB and the volume and balance pots, it I read the schematic correctly. I don't believe there are any electrolytic capacitors on the tone PCB so it was not recapped.

So your suggestion about where to look next would be appreciated. I would also like to learn more about a tone circuit realignment and spectral analysis if you can point me to some reference it would be appreciated. I'm retired and do this as a hobby so I enjoy learning as much as I can.

thanks for your assistance.

 

[hirscwi]

Any suggestions? I'm at a loss.

 

[hirscwi]

Well I may have solved the problem. The balance potentiometer appears to be defective. In the center position, the right channel shows about 8 ohms and the left channel is about 600 ohms. All the way to the right reads about 2 ohm but all the way to the left is 290 ohms. I hope the part is still in stock with McIntosh.

It seems that I've read that the volume and balance pots on this receiver tend to have a high failure rate.

 

[CohibaJoe]

Well I may have solved the problem. The balance potentiometer appears to be defective. In the center position, the right channel shows about 8 ohms and the left channel is about 600 ohms. All the way to the right reads about 2 ohm but all the way to the left is 290 ohms. I hope the part is still in stock with McIntosh.

It seems that I've read that the volume and balance pots on this receiver tend to have a high failure rate.

Yes...Volume control is the issue. Also the Push Switches are not very good. I have "2" 1900...Let me check the Parts box. I had a 1900 Parts unit.

 

[hirscwi]

Yes...Volume control is the issue. Also the Push Switches are not very good. I have "2" 1900...Let me check the Parts box. I had a 1900 Parts unit.

Thanks for offering to check your parts.

McIntosh still has the balance pot so I ordered one and I hope it's the solution - should know in about a week.

 

[dmalcom]

Hi did that fix your problem because mine is doing that. Mine worked fine until I cleaned it with deoxit now it doesn’t which makes no sense.

 

[dmalcom]

I have a Mac 1900 that worked fine except for some phono feedback on phone 1 . So I thought I clean all the switches and controls and now my balance control doesn’t operate the same my left channel is not working. Anyone have any thoughts.

 

[Bob the Tech]

I'm thinking you may have broken the connection at the balance pot for the L channel, but some questions:

Doesn't work for any input, or just phono?

Do you get both L and R outputs at the tape outputs?

If you switch the mode selector to mono or L+R to L, L+R to R do you get both channels?

Can you check the rear panel preamp outputs? Getting both channels?

 

[kwayde]

I have a Mac 1900 that worked fine except for some phono feedback on phone 1 . So I thought I clean all the switches and controls and now my balance control doesn’t operate the same my left channel is not working. Anyone have any thoughts.

Those balance pots are known to fail. Not the best quality.

A friend had one that was working somewhat, then after he cleaned it, it was garbage. I checked resistance with a meter and it was sky-high on one channel. Started out ok then just went very high like the resistive pad was gone.

It was bypassed so he could use it without the balance pot.

 

[dmalcom]

I'm thinking you may have broken the connection at the balance pot for the L channel, but some questions:

Doesn't work for any input, or just phono?

Do you get both L and R outputs at the tape outputs?

If you switch the mode selector to mono or L+R to L, L+R to R do you get both channels?

Can you check the rear panel preamp outputs? Getting both channels?

Those balance pots are known to fail. Not the best quality.

A friend had one that was working somewhat, then after he cleaned it, it was garbage. I checked resistance with a meter and it was sky-high on one channel. Started out ok then just went very high like the resistive pad was gone.

It was bypassed so he could use it without the balance pot.

View attachment 3263669

Hi I now have both channels but not in the center if I turn to left at about 10 o’clock the left channel picks in and I have to turn mostly to right to get right channel. One thing is if I tap on the top of the receiver on the front cage it cuts in and out almost like a ground problem maybe . It weird work fine until I cleaned the balance and volume and switch’s which makes no sense .

 

[dmalcom]

Hi I now have both channels but not in the center if I turn to left at about 10 o’clock the left channel picks in and I have to turn mostly to right to get right channel. One thing is if I tap on the top of the receiver on the front cage it cuts in and out almost like a ground problem maybe . It weird work fine until I cleaned the balance and volume and switch’s which makes no sense .

Bob it works on all inputs it’s like it has dead spots on the balance dial . I did noticed on the shaft as it goes into the pot had green tarnish I sprayed depot into the shaft . I’m hoping I didn’t just shoot all that into the pot.

 

[dmalcom]

Bob it works on all inputs it’s like it has dead spots on the balance dial . I did noticed on the shaft as it goes into the pot had green tarnish I sprayed depot into the shaft . I’m hoping I didn’t just shoot all that into the pot.

I meant to type deoxit into the shaft.

 

[kwayde]

If you have a meter, check resistance on the balance pot, both channels. That will tell you if the pot isn't working right.

Somebody said that if the pot was already compromised, cleaning it may make it worse. This seems to be what happened to the one given to me to look at.

After I cleaned it again the pot got worse. I'm thinking the resistive pad was already bad and got much worse after cleaning again.

The center taps are the wipers which are connected to the caps.
Put one lead on the center tap and the other on either the green wires or black ground wires.
Slowly work the pot from one end to the other watching for resistance to move in a smooth gradual motion with no drop outs.

 

[Bob the Tech]

Given you're getting mixed results in the more you mess with that pot I'd a) check the connections and make sure nothing is frayed, weak, broken at the solder lugs and b) use faderlube and flush again. Maybe you did spray some junk down into it, dust, whatever, so give it another go. Then exercise it up and down. See if working it a bit after cleaning doesn't resolve the issue.

I'd work the other switches and controls in a similar way, cycle things a hundred times just to be sure everything is cleaned off good.

 

[dmalcom]

If you have a meter, check resistance on the balance pot, both channels. That will tell you if the pot isn't working right.

Somebody said that if the pot was already compromised, cleaning it may make it worse. This seems to be what happened to the one given to me to look at.

After I cleaned it again the pot got worse. I'm thinking the resistive pad was already bad and got much worse after cleaning again.

The center taps are the wipers which are connected to the caps.
Put one lead on the center tap and the other on either the green wires or black ground wires.
Slowly work the pot from one end to the other watching for resistance to move in a smooth gradual motion with no drop outs.

 

[dmalcom]

Thanks for your response, it does move slow on left channel till about 10 o’clock then goes dead a more I turn meter is jumping around . Could the cap be bad and what is the value of these caps . But it worked fine until I cleaned it .

 

[dmalcom]

Could it possibly be a ground because if I rap on top front cage the it cuts out and crackles if that’s a good choice of a word .

 

[kwayde]

Thanks for your response, it does move slow on left channel till about 10 o’clock then goes dead a more I turn meter is jumping around . Could the cap be bad and what is the value of these caps . But it worked fine until I cleaned it .

The caps have no affect on what you read when measuring across the balance pot tabs. When you tap on it the wiper may be connecting to part of the resistive pad that's either dirty or compromised.
If you attempt to clean it more I'm guessing more of the resistive pad will be removed and work less than it does now. But since it isn't working you may as well try it and see what happens.
Another thing you could do is pull the pot, take it apart, then look at the wiper and resistive pads to see what's wrong.
When an attempt to clean it makes it worse, it's more than likely ruined and needs to be replaced, but there aren't replacements currently available. That's why I bypassed the balance control for guy whose unit I worked on.
It works fine, you just have no balance control.

Found this thread:
https://audiokarma.org/forums/index.php?threads/mac1900-balance-control-information-needed.756296/

Those two caps are fragile so be careful when around them. The legs break off easily.