Pioneer A-80 clipping troubleshooting

[MBuras]

Been working on a basket case Pioneer A-80 for a while. One amp board and outputs were completely trashed. Replaced the obvious and then tracked down a few more severely out of tolerance resistors. That channel is working just fine and begins clipping at 223wpc using direct mode, 1000hz signal and 8-ohm loads. On the other channel I replaced the exact same components to keep everything matched. Testing that channel it started clipping at around 30wpc. All amp board components and outputs check out ok.

Digging in with voltage measurements I found pin 16 & 17 feeding the output assembly are running over voltage (-79.5vdc when they should be -58vdc like the good channel). Those pins feed from a fast recovery common cathode dual diode D10 (CTU-21S). Both anode sides of that pin measure -79.5vdc. The common cathode side measures the appropriate -58vdc as does the filter capacitor further back in the circuit.

From these measurements I'm thinking D10 is bad.

Question #1 - Does anyone agree with this assessment?

Question #2 - Any suggestions for a replacement on the CTU-21S common cathode fast recover diode? I have a data sheet but not sure what the most important specs are. Also was hoping I might use 2 available UF4005 or similar diodes to fashion something instead of ordering one part.

I've attached the board map and circuit diagram for the power supply in case they're needed.

Thanks for the help!
Mike

 

[MBuras]

Doing a bit of my own research. To my eye the Sanken FMU-22S (they don't make a 21S) seems pretty close to fitting the bill. Only things I can see that don't match are the VF max on the old part was 2.0 and the new one is 1.5, and the forward voltage is a little higher. Critical specs or close enough?

http://www.digikey.com/product-detail/en/sanken/FMU-22S/FMU-22S-ND/4289271

 

[llwhtt]

Since this amp has a tiered power supply, Class G or H, those dual diodes are to keep the high voltage tier from feeding back into the low voltage tier. The only time you should have high voltage on those diodes is when the signal thru the amp calls for it. It looks as though your negative high voltage is turned when it shouldn't be. I would go back and check the switching circuit for a shorted "pass?" transistor, Q5, 6, 7, or 8 whichever one is for the negative rail on the bad channel and check it's surrounding circuitry also. I think the dual diodes are doing what they are supposed to do, protecting the low voltage power supply.

Craig

 

[zebulon1]

As long as the amp is behaving. Swap the suspect components. I do this to speed the parts order. I can't stand waiting for a part only to find its one I didn't order. Verify.

 

[MBuras]

those dual diodes are to keep the high voltage tier from feeding back into the low voltage tier

That makes sense. Still building my troubleshooting skills, especially on complicated stuff like this. Thanks for the info!

 

[MBuras]

check it's surrounding circuitry also

Surrounding circuitry for the win. R202 on the right side output assembly was bad. Looked fine but measured in the millions. Popped in a new 10-ohm resistor and everything is well. Right side now measured over 200wpc with 8-ohm load which matches the left. And no strange clipping on the scope. Can't wait to get it completely put back together and hooked up to my main listening system.

Thanks!!!