Eden WT800C Power Amp module failure

[nevetslab]

When is the last time you've encountered a simple SS power amp module fail, taking out EVERY transistor in the circuit? Besides that a handful of resistors, both zener's, leaving behind three 1N4148 Signal Diodes that survived. Eden WT 800, one of the later generation models having the Power Amp PCB's that use three pairs of NPN/PNP's....though the last time this had come in, again with further foil / solder pad damage to the output stage, I had to restore it with 2 pairs of MJL21193/MJL21194's in place of the 3 pair of 2SA1492/2SC3856 power xstrs. Both modules had been done the same.

In this instance, the amp appears to have been running in Bi-Amp mode. I can't recall a time when EVERY xstr had failed. The other module.....I found a single 1N4148 diode open (D4), for some reason. I did end up with some scorched fiberglass, so after removing all that carbon, it's now curing with a coating of Corona Dope, so I'll have to wait until tomorrow before populating the board.

The power amp schematic on this one is the PM500, last page in the file.

WT800C schematic.pdf

[J M Fahey]

When is the last time you've encountered a simple SS power amp module fail, taking out EVERY transistor in the circuit? Besides that a handful of resistors, both zener's, leaving behind three 1N4148 Signal Diodes that survived.
I can't recall a time when EVERY xstr had failed. The other module.....I found a single 1N4148 diode open (D4), for some reason. I did end up with some scorched fiberglass, so after removing all that carbon, it's now curing with a coating of Corona Dope, so I'll have to wait until tomorrow before populating the board.

Easy, just replace incumbent fuses with >20A ones taken from nearest car or motorcycle or wrap burnt ones with aluminum foil, then turn on 20 times and watch parts smolder while scratching your head but NOT turning it off.

Plan B: test diode and transistor continuity with a 120V 60W lamp connected between Mains and a couple test probes or crocodile clips.

Not a joke: I saw a 40īs magazine article suggesting that "Short/Continuity Tester" for such useful purposes as repairing washing machine motors, switches, wiring, relay contacts and even AM radio tuning caps, where it not only detected shorted plates but also burnt debris which made tunning scratchy.

[Leo_Gnardo]

Easy, just replace incumbent fuses with >20A ones taken from nearest car or motorcycle or wrap burnt ones with aluminum foil, then turn on 20 times and watch parts smolder while scratching your head but NOT turning it off.

Plan B: test diode and transistor continuity with a 120V 60W lamp connected between Mains and a couple test probes or crocodile clips.

Not a joke: I saw a 40īs magazine article suggesting that "Short/Continuity Tester" for such useful purposes as repairing washing machine motors, switches, wiring, relay contacts and even AM radio tuning caps, where it not only detected shorted plates but also burnt debris which made tunning scratchy.

At my techy college, the humor mag gave electrical advice. Definition of a transistor: a circuit component that when applied properly, prevents fuses from blowing.

Also had a fuse tester pictured: a line cord with Edison plug at one end and a pair of alligator clips at the other. Clip onto fuse than plug in. If fuse goes up in a flash, then it was good. Emphasis on the "was."

[J M Fahey]

A friendīs brother who received Electronics lessons from him and resented it, designed an "OFF Switch" .... a N.O spst switch wired across Mains: you flip the toggle and turn everything OFF .

[Leo_Gnardo]

A friendīs brother who received Electronics lessons from him and resented it, designed an "OFF Switch" .... a N.O spst switch wired across Mains: you flip the toggle and turn everything OFF .

Similar idea was used by legendary Tele slinger Danny Gatton: a foot button switch plugged into the AC line. If a gig was going sour, or if he was just getting bored, hit the button - stage blacks out - shouts (without microphone) "That's all folks!" - and heads for the dressing room.

[J M Fahey]