HELP - McIntosh 250 Solid State Amplifier Problem

[m1k35r]

Hi all... I have a strange problem with a McIntosh 250 solid state amp. No sound on one channel, very low distorted sound on the other. If I remove the input to each autoformer, the sine wave at the output of the power transistors looks symmetrical and great. Hook up the transformer, and it stops working. Any idea what the measurement (ohms) should be from the input to the 4,8,16 output taps? They all measure very low, about 1.4 ohms each on both transformers. Also, with the output transformers hooded up, the sine wave at the driver transistor (TO-66) on the driver board shows the top half of the waveform to be good, the bottom half is almost a flattened triangle wave. Although I believe it to be rare, I think the autoformers are bad. Any ideas would be greatly appreciated.

 

[Pio1980]

Look at the DC offset with the autoformers disconnected.

 

[m1k35r]

Look at the DC offset with the autoformers disconnected.

At the output of the power transistors, one channel is 9.6mv and the other is 10.1mv. (No load, no signal)

 

[Pio1980]

Hmmmm----. That looks pretty good. Does offset shift with test signal input under load?
Try testing on a regular resistive loading sans autoformers.

 

[Pio1980]

The DC resistance of the autoformers is very low. If any appreciable offset appears at the output, the amp would likely go into protection and misbehave.
I'm definitely not an expert on these Mac amps, but the point is to give the experts enuff data to determine likely fault modes.

 

[m1k35r]

Hmmmm----. That looks pretty good. Does offset shift with test signal input under load?
Try testing on a regular resistive loading sans autoformers.

Loaded output transistors @ 4 ohms. Only as I vary the input signal (1khz) level does the offset change by a few mV, then stabilizes (both channels) to around 4.5-6mV.
Sine looks very good.

 

[Pio1980]

Try feeding a lowish 60 hZ voltage into each autoformer from a transformer and look at the inputs and outputs without/ with loading to parity check them.

 

[m1k35r]

Try feeding a lowish 60 hZ voltage into each autoformer from a transformer and look at the inputs and outputs without/ with loading to parity check them.

Will do that tomorrow morning and get back. Thanks for the advice!

 

[m1k35r]

Try feeding a lowish 60 hZ voltage into each autoformer from a transformer and look at the inputs and outputs without/ with loading to parity check them.

Using a low current 12V transformer, I injected into the primary of both autoformers. It kills the transformer output and no signal is seen on the output(s) of the autoformers. Looks like bad news.

 

[Pio1980]

Using a low current 12V transformer, I injected into the primary of both autoformers. It kills the transformer output and no signal is seen on the output(s) of the autoformers. Looks like bad news.

Yes, it does. Very strange, these things are supposed to be quite robust.
I'm out of ideas past this point if this is it. I wonder if they somehow had high current source DC put on their output terminals. Any sign of overheating?

 

[m1k35r]

Yes, it does. Very strange, these things are supposed to be quite robust.
I'm out of ideas past this point if this is it. I wonder if they somehow had high current source DC put on their output terminals. Any sign of overheating?

No sign of overheating or any smell near the autoformers either. I really can't imagine, especially with the sine being so pure at the output of the power transistors, what happened.
You would think the outputs would fail and kick the breaker before the autoformers would short out. All original parts too. Thanks for the advice.

 

[Pio1980]

No sign of overheating or any smell near the autoformers either. I really can't imagine, especially with the sine being so pure at the output of the power transistors, what happened.
You would think the outputs would fail and kick the breaker before the autoformers would short out. All original parts too. Thanks for the advice.

It's possible they got hit somehow with an external DC source that didn't get transformed to the output devices, a lightning strike to a PA speaker setup or somesuch.
These autoformers should be much more easily and economically rewound than the unity coupled output transformers. Check the usual reliable services for testing and prices.