Marantz PM17 Repair

[solan]

Hello, my amplifier right channel stopped working. I decided to fix it myself, I don't know if this will be possible, but I want to try it. If someone is willing to help me I will be very grateful.
I got the service manual, and I checked the power amplifier components. Result: Two transistors shorted (Q715, 2SC3284 and Q716, 2SA1303), 4 bad resistors (R735, R737 R738, 68 Ohm, and R741, 0.22Ohm x 2, this adjust the bias).
I have the amplifier with the good channel mounted and I have measured the voltage at the inputs of the bad channel (I use a protection bulb(60W)) . And these were the results:

-J702( +43v, -43v, Out R)-----+36v, -36v
-J703( J703 (PROT, GND, +60v, -60v)_____+36v, -36v

Why does such low values?, according to the diagram should be: +43v, -43v, +60v, -60v. Before ordering components I would like to know if I should change more parts. I must check if there is a short somewhere else.

I'm doing things right?, what parts should I continue checking?. I have basic knowledge, but I want to try it. Very grateful, thank you.(sorry for my English)

 

[rayma]

I have measured the voltage at the inputs of the bad channel (I use a protection
bulb(60W)) .Why does such low values?

The bulb tester could be causing the low readings.

 

[solan]

So, do I do the tests now without the bulb?

 

[rayma]

So, do I do the tests now without the bulb?

There's always some risk in any repair, but if you have checked all the parts for defects,
that's probably the next thing, maybe using a fast blow, smaller line fuse than normal,
and a fused Variac if you have one. Do the capacitors all look ok?

 

[solan]

Yes, the capacitors look good. If the input voltage to the power stage measures well, can we think that the problem is in the power stage? What should I do? I light it without the bulb and I measure that voltage ?. Thank you.

 

[rayma]

If the input voltage to the power stage measures well, can we think that the problem is in the power stage?

Could you post the power supply schematic?

 

[solan]

I have removed the bulb, the readings are: + 45v, -45v. They are correct?.

 

[rayma]

I have removed the bulb, the readings are: + 45v, -45v. They are correct?.

That seems ok.

 

[solan]

Hello, I have already installed the components. I lit with the bulb, the bulb goes off and the relay jumps. But there is a problem, the bias is 0Mv. I upload the variable resistance and it does not move from 0Mv. What can happen?. Thank you.

 

[rayma]

I lit with the bulb, the bulb goes off and the relay jumps. But there is a problem,
the bias is 0Mv. I upload the variable resistance and it does not move from 0Mv.

The dual 0.1 ohm emitter resistor unit could be open circuit. See if there is continuity
in both halves. This is either part # R641 or R741.

 

[solan]

Checked, it's correct, there's continuity in R741. Those resistances were replaced.

 

[jaycee]

The driver transistors probably need to replaced too. Also, where did you get the replacement transistors from ? Sanken have discontinued them.. which makes it highly likely you will get fake parts

 

[rayma]

Checked, it's correct, there's continuity in R741.

Both halves measure 0.1 ohm? Also do you measure 0.2 ohm total between the two outside terminals?
If so, then there may be a problem in the bias or driver circuit. Did you test the other semiconductors?

 

[solan]

Q713, Q702, Q714 were replaced by Toshiba originals. Q715 and Q716 bought on Ebay, they assured me they were Sanken, now I have many doubts. Could that be the problem?

 

[solan]

Yes, , I have measured the two terminals

 

[rayma]

Yes, , I have measured the two terminals

Ok, then test and replace semiconductors where necessary, avoiding ebay.

 

[solan]

Which are equivalent with Q715-2sc3284 and Q716-2SA1837 ?. Thank you.

 

[jaycee]

I would use 2SC3519/2SA1386. Get them from an authorized seller such as Farnell, Digikey or Mouser

 

[solan]

Thank you very much. I'm going to order the transistors and later I'll continue counting.

 

[solan]

Hello, while the order of 2SC3519 / 2SA1386 arrives, I have installed the transistors of the good channel in the bad channel. The result is the same: 0mV of Bias. I'm frustrated, what can I do? Thank you.