Fender deluxe 85 frying power transistors

[mccutter]

Hi there,
I have a difficulty with the repair of my old Deluxe 85.
Power transistors have fried , RIP ...
One of the 1N4448 diodes seems to be the cause. I changed all the diodes, zener, caps. Power voltage is ok.
As soon as i put new couple of transistors, they fry ... sometimes I hear a buzz before. Any specialist ?
Thank you !

[Tassieviking]

Schematic:

[g1]

These amps will burn their output transistors if the heatsink is not completely mounted to the chassis, including the bar.  And the removable 'bar' has to be oriented properly or it will not mate up right with the heatsink.

[mccutter]

Ok thank you, effectively  I didn't mount heatsink completly. Let's make a trial !

[g1]

You said you heard a buzz before when powering up, so I assume you had the speaker connected.
It is best to first power-up with no speaker or load connected.
Then check that transistors are not overheating and there is no DC voltage at the output.
If all that checks out, then connect speaker and test.

[mccutter]

Thank you for this wise advice ... I tought that the amplifier must be loaded by a speaker. Today, I  made different tests : it seems that everything is okay before the power amp section. Something seems wrong with one of the vishay diodes ... (BYV26D). I'm out of these components. Should I order BYV26E (1000v vs 800v) ?
I'll change every diodes, zener and transistors of the last stage, it's not too expensive ;-) and let you know the results !

[g1]

What is that BYV26D diode called in the circuit?  CR18 or 19?
Disconnect one end of it and recheck.

[mccutter]

It's called CR13 (one of these in the heat sink)
Seems to be good but the vf value is different from the others (616mv vs 840mv)

[phatt]

You might be wise to build a limiter before you power up next time as it will save you blowing transistors. All info needed can be found here;
https://www.ssguitar.com/index.php?topic=2093.0
Phil.

[g1]

It's called CR13 (one of these in the heat sink)
Seems to be good but the vf value is different from the others (616mv vs 840mv)

CR10 to 13 are critical to the bias circuit as they compensate for the heatsink temperature.  The Fender part # for them comes up as BYV26E.  You should replace all 4 just to be safe.

Also, what I said earlier about checking for DC with no load won't work on this amp, because of the IC driver.  You will need some resistor on the output instead of the speaker, 1K should be safe for testing.

[mccutter]

Thank you phatt, Only have to find a old bulb  and not led  !
And thank you  g1 , I let you know the progress !

[joecool85]

Thank you phatt, Only have to find a old bulb  and not led  !
And thank you  g1 , I let you know the progress !

If you are having trouble finding an incandescent bulb for this, try looking at heat lamps.  You'll pay more, but they are readily available.  If you don't see them in a hardware store, try a pet store and get a reptile heating lamp.  As long as it glows red, it's the right type (they have some ceramic ones that don't emit any light and wouldn't work the same).

Good luck!

[mccutter]

Hi guys !
It's been a long time but I was ill, so I couldn't do anything on my amp.
Now I'm recovered !
I changed all vishay diodes, and put a brand new pair of transistors. I do have change all diodes of the power amp stage.
Well, it works ... but !
But now I have a pretty loud buzz, ca 100hz. I can hear any input ...
I don't know if it's come from preamp or power amp ... Any idea ?

[g1]

Plug your guitar into the 'power amp in' jack, is the hum still there?

[mccutter]

Yes , it's coming from the power amp section ... I'm losing my mind : all caps are new and tested, all resistors values are ok, all diodes are new. voltage values are ok. It's not a lack of earth noise. I also tried different transistors ... The only way would be a breakpoint in the circuit i think.