Old amp crackling noise problem when turned on

[radix]

I have a McIntosh MC2205. It started making a crackling noise in the R channel on power on that lasts for about 20-30 seconds, then is gone. It is most prone to do this when cold. Sometimes it barley (if at all) does it.

Is this likely a capacitor problem, or the relays? Once it warms up, the sound is OK to my ears, but I'll measure it when I get it to the bench. I just want to be prepared for what the problem might be, as it's a long haul to the workshop with a heavy amp.

Thanks

 

[Don Hills]

Before turning it on, try rotating the gain controls back and forth a few times. (Or while it's crackling.) The resistive tracks get noisy with age, and generate noise when DC current passes through them. The current is caused by the coupling capacitors charging up, and ceases once they are charged.

 

[DonR]

Could also be a dry solder joint which clears up once it heats up. @restorer-john might be able to shed more light.

 

[Doodski]

I've seen the fault caused by bad transistors, bad caps, bad solder, bad switches and controls. It seems your McIntosh requires some TLC and a tune-up.

 

[restorer-john]

Crackling like static that disappears is likely to be a noisy junction transistor. Could possibly be an electrolytic someplace with a degraded interior. Unlikely to be a dry joint as 30 seconds isn't really enough time to heat anything much enough to expand, but it is possible I guess. Could even be an intermittent trimpot someplace. Check all your input pots, switching and any filters/outputs- cycle any and all physical switches and pots to rule them out.

Swap input channels R-L, to ensure the source isn't the problem.

Like @Doodski said, she needs a bit of TLC.

 

[DonR]

... and the light has been shed...

 

[Doodski]

wowowow! This baby is heavy. 44kg is about 97 pounds when packed in the box.

Weight

38.6kg 44.0 kg (when packed)

McIntosh MC2205 Specifications Macintosh

 

[radix]

wowowow! This baby is heavy. 44kg is about 97 pounds when packed in the box.

Weight

38.6kg 44.0 kg (when packed)

McIntosh MC2205 Specifications Macintosh

@Doodski yes, that's why I only want to make one trip out back to the shop with it. I might wrap it up in a moving blanket and secure it to a hand dolly.

@Don Hills I will try fiddling with the gain control tomorrow morning when I turn it on again.

@restorer-john I've tried it with the preamp volume turned to 0 and it still makes the noise, but I don't think I tried it disconnected. If that's not it, I'll need to isolate where in the signal path the crackling starts to see if it's the current gain transistors or voltage gain transistors or something else. In any case, I'll likely replace signal path electrolytic and the big power supply guys while I'm in there and clean the pots and switches. I think there's just two coupling caps per channel, unless I missed one on the schematic.

Hopefully it's just the transient protection relay or gain pots.

Thank you

 

[restorer-john]

If that's not it, I'll need to isolate where in the signal path the crackling starts to see if it's the current gain transistors or voltage gain transistors or something else.

I would be looking at whether the issue is up front, before the differential input stage.

Have a look at the output of IC 201 at turn on and see if it glitches with the noise. It drives the attentuator (optocoupled LDR).

 

[radix]

I would be looking at whether the issue is up front, before the differential input stage.

View attachment 309801

View attachment 309802


Have a look at the output of IC 201 at turn on and see if it glitches with the noise. It drives the attentuator (optocoupled LDR).

Ok. And thanks for pointing out the LDR, I missed that. I'll probably just need to binary search to see where the crackle comes in. Unfortunately, it does not do it every time, especially when it's warmed up, so this might take a while.

 

[radix]

The winner is... @Don Hills for it being the pots. I wiggled them around before powering on and no crackle. After it was on, there was some crackle adjusting them. I'll try this for a few more days, repeating the process to make sure it was not just coincidence.

I might be able to open up the amp for a cleaning without hauling it out to the shop in the back, though I guess I should inspect the caps.

Thanks everyone.

 

[radix]

Well, not positive its the pots. The crackling volume tracks with the amp gain control, so it must be there or before. I will disconnect the preamp next time and see if that's it -- maybe it is the preamp after all, and not the amp.