McIntosh 250 buzzing/oscillating

[goldswimmerb]

Hey guys, so I have an MC250 on my bench that emits a terrible buzzing sound out of one channel. It was brought to me as it allegedly had bad Capacitors, so I went through and Recapped it to no improvement. After the recap I noticed that once channel would occasionally oscillate and had what sounded like a leaky or noisy tranistor.

I went through and replaced the pair on the input board with matched KSA992s, and one KSC1845s.

I also replaced the differential pair in one amp board with matched KSA992s, which made no difference.

On the DBT the unit powers on, lights the DBT, the bulb dims a bit, then glows brightly when the left channel starts buzzing. I was wondering if anyone had experience with these units oscillating and any ideas where to check. I did see that the resistors are known to drift in these, could that be causing it? Or was my KSA substitution a bad idea?

Mine appears to be a less common revision as the power supply is not on a circuit board.

Inside pic:

 

[Pio1980]

Check integrity of chassis grounding in bad channel.
Just a hunch.

 

[goldswimmerb]

Check integrity of chassis grounding sin bad channel.
Just a hunch.

Chasis grounding seems fine, assuming that BLK and one of the mounting screws are my chasis grounding.

 

[Pio1980]

Go back, check again and verify with resistance measurements. If it still has the swedged-in RCA Jax, check them for security and resistance ground integrity. Any other riveted/swedged grounds should get attention.

 

[c_dk]

Have you checked the output transitors?

Has the bias resistor drifted?

Can you see it oscilate on your scope, visable notch distortion will point to a drifted bias resistor.

 

[goldswimmerb]

Go back, check again and verify with resistance measurements. If it still has the swedged-in RCA Jax, check them for security and resistance ground integrity. Any other riveted/swedged grounds should get attention.

So tightening the screws seems to have done away with the horrible buzzing, however the DBT is still bright (it's a 75w bulb) and I am able to measure 2v or so of DC on the output, which shouldn't be possible through the transformer, unless I am measuring something else.

 

[Pio1980]

See post re c_dk.

 

[goldswimmerb]

Have you checked the output transitors?

Has the bias resistor drifted?

Can you see it oscilate on your scope, visable notch distortion will point to a drifted bias resistor.

Not seeing any notch distortion, do I need to have the output loaded as well?

I am seeing the waveform shift up and down on the Y axis with every "pop"

 

[Pio1980]

How hot is the heatsink idling?

 

[goldswimmerb]

How hot is the heatsink idling?

Haven't left it running long enough to get warm, didn't want to explode anything.

 

[Pio1980]

Haven't left it running long enough to get warm, didn't want to explode anything.

The autoformers will usually crowbar the fuses if anything is seriously wrong in the driver/output section. Check to ensure the recommended value fuses are in place.

 

[c_dk]

I think he might have a output issue, I hope you did not repace the sentry monitor transistors with the Fairchild devices you posted.

There is only the main fuse no individual rail supplies like the units with a actual crowbar circuit.

Lastly a olding but a goodie, make sure someone in the past has not replaced a output strip screw with a too long screw that can short to the chassis.

 

[goldswimmerb]

I think he might have a output issue, I hope you did not repace the sentry monitor transistors with the Fairchild devices you posted.

There is only the main fuse no individual rail supplies like the units with a actual crowbar circuit.

Lastly a olding but a goodie, make sure someone in the past has not replaced a output strip screw with a too long screw that can short to the chassis.

Which transistors are the output sentry monitor tranistors? The only ones replaced in the amplifier board were the two that make up the Differential pair on the input of the amp.

 

[c_dk]

Too many assumptions and shotgunning, not enough basic repair technique.

With the amp under a load restricted by the DBT restricting the input voltage, you need to use your o'scope probe to observe each circuit boards input and output to find the offending section, then within each stage, and finally each device making up that stage so you then know what part is malfunctioning.

Us remotely guessing or relating past issues we have seen is not good repair discipline except for maybe the real goofy ones like someone subbing in a way too long screw because they lost the original 1/4 inch long one and the toolbox had a 3/4 inch long replacement "to get by". The band must go on you know.

 

[MACKIE1975]

Agree.
Issue likes this most likely will involve several components in a circuit.