The Fiser Saga: Power Amplifier Autopsy

 Yeah, it's been a few weeks, and a busy time it was with the gorgeous weather we've had for the last week.

And then I got clobbered with the cold/flu/whatever. Anywhoo...feeling much better.

Back to the Fisher RS-2010 Stereo Receiver I've now had almost a year. Yes, I'm embarrassed, but it turned out to be a LOT more work than I estimated. I'm now down to searching for ONE transistor, with all the rest having been sourced, and on-hand. The issue is that it's one of what's called a Complementary Pair, and while a sub for one of the two is readily available, it's compliment is not. Soooo....I have to dive into either finding a vendor with some in stock (NO eBay or Amazon....too many fake parts!), -OR- start going through the "sub-for-the-sub" routine, and find a complementary pair that's available. 

Bags of new transistors awaiting their long overdue compatriot.

It gets a bit tedious....

 

So what killed several transistors, and let the smoke out of three resistors? Who did the dirty, dastardly, despicable deed?

'Twas a shorted Zener Diode, it was.... that's what who dunnit! 

The diode in question is labeled "DO6" in this clip:

 In this application it's being used to develop a voltage which is 12 Volts less than the supply voltage, rather than developing "just" 12 Volts. They do this because the entire amplifier is isolated from "DC Ground", and floats between the +60 Volt and -60 Volt power supplies. The only transistor in this circuit connected to the chassis, is Q06. When the diode failed, it put Q04 into saturation (FULLY turned on), meaning it had only .65V drop across it, which in turn took out everybody connected to it by putting them into an over-voltage condition. A classic example of a Cascading Failure.

So that's where I'm at right now, changing transistors, and getting ready for a brief test when they're all replaced. After I verify all the voltages are back to where they should be, I'll be ready to run it at 50% power for a few hours while I monitor the temperatures to make sure nothing goes bonkers, and then run it Pedal-to-the-Metal for a couple of hours to see how it well it performs.

 

Gettin'n close, finally, to a resolution on this one.

Groan.....Got Clobbered By Something

 Been flat on my back since Monday. Aches like I've never had (even my knees hurt),  coughing, sneezing, going between sweats and chills, and all sorts of other things.

Finally got out of bed today for more than 30 minutes, and while I'm still a bit wobbly, at least I'm feeling somewhat better.