MC2200/MC2205 Power Guard/Input Limiting on 1 Channel

[timkritz]

Hey guys! Having a few issues and was hoping for some advice from the experts. I've been a lurker for around 5 years and this forum has helped me on many projects, but I finally encountered an issue I wasn't able to find a thread on.

I recently recapped and replaced all the output transistors/resistors on a MC2200 due to a few problems. When I got the amp it had one channel quieter than the other, excessive voltage on the current sense of the quiet channel (~165mV), shorted caps/bad diodes for the limit lamps on the power board, high power draw at idle (~0.65A 120V with bias pots completely CCW). The repair fixed the current sense voltage, but not the high idle power draw or quiet channel.

After playing around with the scope and function generator, I was able to determine that the power guard on one amp board is misbehaving. When I disable power guard via switch, the input level is not attenuated. However, when enabled, no matter what the input level is, the optocoupler/LDR appears to be engaging and reducing the input level. The limit lamp is NOT activated (bulb is new and flashes on cold power up) so this leads me to believe the comparator is working properly. Does anyone have any advice on what to look at? Could it be as simple as a bad optocoupler/LDR? I did try and measure resistance of both working and non-working channels in-circuit (power off) with my fluke and it couldn't get a reading on either.

In addition to the power guard issue, anyone have any advice on the high idle power? I honestly have no idea where to even begin

-------Tim


PS: I did clean the power guard switch and confirmed the issue by swapping the boards. Problem followed with the board.

 

[c_dk]

While the comparator circuit or the LDR could be malfunctioning it first must be determined if the amp is distorting in excess of the threshold distortion level of 1% or in there might be a possible partial short triggering the sentry monitor circuit that would lead to excessive distortion.

The Clarix branded LDRs are known to fail in a high resistance mode not a short mode therefore it would not be shunting input voltage to ground in its most common failure mode.

 

[timkritz]

While the comparator circuit or the LDR could be malfunctioning it first must be determined if the amp is distorting in excess of the threshold distortion level of 1% or in there might be a possible partial short triggering the sentry monitor circuit that would lead to excessive distortion.

The Clarix branded LDRs are known to fail in a high resistance mode not a short mode therefore it would not be shunting input voltage to ground in its most common failure mode.

What's your recommended way to check without a distortion meter? I have a nice tek digital scope and agilent function generator, but haven't picked up a hp 8903 yet.