Pioneer A-80 over current protection

QSilver

Super Member
I got it this week and at first it worked well. DC offset is 4mV on one side and 14mV on the other. I checked and set the idle current to 22mV as per the manual. But Then it started going into protection.

I checked the idle current and dc offset and it was still ok, so I changed the capacitors over on the protection board. Still no luck. I then decided to measure the voltage on the over current protection pins (5 and 6 on the output assmebly) and here is the strange thing. When I measure it with either of my two DMMs, the amp immediately goes into protection and stays there until I remove the meter probe.

The voltage readings starts out at 6VDC then drops down to between 1.8 and 2.5.

I next lifted the legs of D3 and D4 on the output board and if I probe anything from these two diodes, either side of R23 or the collector of Q2, it instantly flicks into protection.

Can anyone explain whats going on? My meters are set up correctly... does it point to a faulty transistor?
 
Here's the protection circuit... if I probe anything connected to the base of Q4, it goes into protection. Even with D3 and D4 lifted. If I probe either D3 or D4 it also goes into protection. My scope shows a strange peaked waveform on the cathode of D11. Could a faulty zener cause this or is the protection circuit just so sensitive it picks up my probe?
 

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So after a lot of component replacing and tracing, I shelved it for a while. Just taken it off the shelf again and so far, all I can find is that the collector voltage on Q4 is 8V isntead of 10V.

I've got some weird waveforms on my scope when I probe Q4 Collector and if I then try to take a voltage measurement from the Base of Q4 I get a huge amount of noise on the scope and it goes into protection. I can't get a reading in volts because the moment I probe it the voltage disappears.

Q1 voltage check out according to the schematic.

D11 is low at around 8VDC and 2VDC as apposed to 9.5VDC and 3.5VDC

I've changed R6, Q2, Q3, Q4 and D11 but no change.

Any ideas? I'm really stumped... or stupid.
 

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Probes could be causing some oscillations, use a ferrite bead or choke in series with the probe tip. Even some cap at the probe tip 100n or so.
 
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