Marantz PM-66SE Burns Transistors

[hyva_ana]

Hello,

While ago I replaced those big filter caps to that amplifier. After that started to happen. I connected the amplifier normally to the mains and almost immediately several transistors burned out. I really didn't do anything else (at least on purpose) than replaced those big caps C801 and C802.

After that I replaced those burned transistors (I can't remember exactly what they were) and measured all the others, and now everything should be ok. But, when I power up that amplifier part with two bench power supplies connected in series there is obviously still something wrong. The voltages fluctuates up and down and (NPN's) Q761, especially Q762 gets very hot - of course depending on the input current. I just can't figure this out. I have measured practically everything on that board but I just don't see, what's wrong with it.

When I remove (PNP's) Q763 and Q764 from board, the voltages is no longer fluctuating, but when I connect either of those, then the fluctuation and heating starts again.

I'm not good at explain this situation, but I hope that you get something out of this, and I'm more than glad to give you more information. If needed I can try to remember out which transistors burned out in first place.

Schematics:
https://share.icloud.com/photos/086M...9Uj8AAYIxiI5QA
https://share.icloud.com/photos/034p..._Muf5RWPQc-ukg

Board:
https://share.icloud.com/photos/0c6u...yW6oiKThMKGtRQ

[eccerr0r]

Re: Marantz PM-66SE Burns Transistors

Again as a broken record, those main filter caps rarely fail on audio equipment... Sometimes they make them with a fake bulge.

So Q763/Q764 are still good/not shorted?

You should check the transistors upstream in replacement cases like this, especially Q757-Q760) to make sure they're likewise good and not shorted.

You did check all the voltages printed on the schematic (with Q763/Q764 removed so you don't fry any more stuff)? Any hot resistors?

[petehall347]

Re: Marantz PM-66SE Burns Transistors

i take it you had the transistors screwed down to the heat sinks and insulated ?
did you put the caps in the right way round ?

[eccerr0r]

Re: Marantz PM-66SE Burns Transistors

geesh, i don't know why i give people benefit of doubt when reassembling power transistors... you're right, have to assume they don't, and this indeed does make more sense why it would all fail at the same time...

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

i take it you had the transistors screwed down to the heat sinks and insulated ?
did you put the caps in the right way round ?

Yes. screwed down to the heatsinks and properly insulated. Caps inserted as they should be.

I have not found any shorted transistors and all transistors I have changed are likewise.

About voltages, they seem to be all over the place:
B; C; E
Q802 -18.5 V ; -43.4 V ; -18.0 V
Q801 25.1 V ; 44.3 V ; 24.6 V

Diodes:
Cathode; Anode
D805 0 V ; -18.7 V
D804 25.2 V ; 0 V
D803 -18.8 V ; -18.5 V
D806 ~32.9 V ; ~8 V
D802 25.0 V ; 25.4 V

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

R806 and D804 gets hot. From transformer comes 32.8 VAC. After bridge rectifier comes + / - 44.8 VDC. All of the voltages are higher than printed on the schematic.

Voltages from QN02:
B C E
-22.1V ; 44.7V ; -22.1V, so far away from what those should be.

[petehall347]

Re: Marantz PM-66SE Burns Transistors

QN02: appears shorted base to emitter as no volt drop .

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

QN02: appears shorted base to emitter as no volt drop .

No it's not. Just removed it from the board and measured.

[petehall347]

Re: Marantz PM-66SE Burns Transistors

is it the KI or AE model ?

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

I'm not sure what you mean, but in front panel is a plate which says "KI Signature", so maybe it's KI model?

[petehall347]

Re: Marantz PM-66SE Burns Transistors

ok i see now at least one channel has railed ..
see what voltage at QN01 emitter or L751 .

[petehall347]

Re: Marantz PM-66SE Burns Transistors

what i would do at this point is remove all output transistors for safe keeping .then add resistors from where the bases and emitters connect to keep the feedback path intact . for those 1k usually works for me .. its important to not have any loads connected to speaker outputs for now until its going to work again with the outputs back in .

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

ok i see now at least one channel has railed ..
see what voltage at QN01 emitter or L751 .

Voltage is -0.9V

I'm pretty new to audio amplifiers, so what do you exactly mean by "add resistors from where the bases and emitters connect"?

And, I have not connected any other thing than transformer leads in to it.

[petehall347]

Re: Marantz PM-66SE Burns Transistors

first remove the output transistors then connect 1 resistor in place of each output transistors .. ignore the collector connecting point and solder resistor where base and emitter legs came from .

[eccerr0r]

Re: Marantz PM-66SE Burns Transistors

ignore xNxx components for now, apparently they failed to do their job or at least they rather protect the speaker only, ignoring the driver transistors. Also their readings don't mean much if you have Q761 or Q763 out.

What are all the voltages at the bases of all the transistors (Q753 Q755 Q757 Q759 Q761 Q763) and the voltage across capacitor C751?

OK I Hate icloud, uploading here:

[petehall347]

Re: Marantz PM-66SE Burns Transistors

ignore xNxx components for now, apparently they failed to do their job or at least they rather protect the speaker only, ignoring the driver transistors. Also their readings don't mean much if you have Q761 or Q763 out.

What are all the voltages at the bases of all the transistors (Q753 Q755 Q757 Q759 Q761 Q763) and the voltage across capacitor C751?

i think this channel is ok and its the other one thats gone to negative rail . yes it does have -900mv showing at the output but i think thats coming back from the protect circuit .

[eccerr0r]

Re: Marantz PM-66SE Burns Transistors

also some oops mistakes that you should check for is swapping npn and pnp...

I guess the other channel is worse off but neither channel should get hot when quiescent.

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

first remove the output transistors then connect 1 resistor in place of each output transistors .. ignore the collector connecting point and solder resistor where base and emitter legs came from .

And these are the output transistors?

[petehall347]

Re: Marantz PM-66SE Burns Transistors

outputs are the 4 big ones on the heat sinks

[hyva_ana]

Re: Marantz PM-66SE Burns Transistors

I removed those output transistors and soldered 1kohm resistors between base and emitter places. Now resistors "in" Q762 and Q764 gets burning hot after couple of seconds when I turn the power on, and same goes to Q759 but it warms slightly slower - anyway, temperature in it rises so much, that I must shut power off after a few seconds.