Macrom M1A-4150 hissing on all channels

[Nappe]

Hi,

I have tried to find the problem with my Macrom M1A-4150 amp that I recently bought new in box, it's naturally an old amp, it has never been powered before as far as I understand and it came with all the plastics and tapes intact.

The problem is that it has a terrible noise floor on all channels however the noise on channel 3 is clearly lower than on the other channels. The noise is high enough to be heard over the music at medium listening volumes.
The noise floor does not change with any of the adjustments on the amp like gain or filters etc. and plugging in/unplugging the inputs does not affect the noise, running on car battery (car not running) or power supply on the desk, the noise is the same.

After some probing on the board I found that if I probe the line from opto U2 to resistor R28 using an oscilloscope probe, the noise is gone.

Is this a dried out capacitor problem or where do I start with this?

Thanks!
- Nappe

Pictures attached:
1. Speaker output with no input signal
2. Amplifier board
3. Oscilloscope probe placement
4. Closeup of track from R28 to opto U2

 

[Perry Babin]

That appears to be part of the muting circuit. In many amps like this, the opto-coupler drives positive voltage to the muting transistors during the mute delay. When the mute is defeated, the 1M resistor pulls the gates of the muting transistors towards -15v.

 

[Nappe]

Ok, thanks.

My search for the noise source continues.

 

[Perry Babin]

It could be that the muting transistors are simply not being turned completely off by the 1M resistor.

 

[Nappe]

Finally got back to this and lifted one of the legs on the 1M resistor, the noise is gone, or at least drops to a hard to hear and quite acceptable level.

With the 1M resistor back in place, measuring at the 1M resistor on the Opto-side (using volt meter, black to GND), the voltage at the 1M resistor with no remote connected is +4,36, with remote voltage applied the voltage at the 1M resistor rises to 7,55V to drop to +7,33V before going down to -13,45V to un-mute.

When the amplifier un-mutes or powers down the tweeters make a "squealing" noise, when powering up the "squealing" have a rising frequency and when powering down the "squealing" have a decreasing frequency.

Any suggestion on what might be wrong?
Thanks!

 

[Perry Babin]

What happens if you install a 100k in place of the 1M resistor?

 

[Nappe]

With a 100k resistor in place of the 1M res. the voltage on the opto-side of the resistor goes to -14.15V and the hissing noise increases in volume quite significantly, clearly visible as noise on a test tone.

With the resistor lifted, the voltage on the opto-side of the resistor shows a fluctuating voltage around -1.5V ... -2.0V, the amp plays a nice clean test tone.