Marantz PM64II repairing

[Aquarius72]

Hi,

I bought a faulty Marantz PM64II amplifier which had blown its power transistors (2SC3182/2SA1265). I replaced the power transistors with 2SD1047/2SB817. Are these ok for replacement?

The biasing transistors (2SC3964) were also bad and I couldn't find a replacement to them. So I replaced them with a 2SD882.

The amp works now but I have a problem setting the bias. Here is the procedure from service manual:

1. Short-circuit the two pins in the AVSS board (PU01) marked CLASS A and CLASS B: (there are actually pins marked CLASS AB, so I guess it means these)

2. "Adjust the main board (P701) semi-fixed resistors R719 and R720 so that the voltage between the ends of the emitter resistor is within 14 to 15mA"
(I guess it should read mV).

(The emitter resistors are "2-pack" resistors with 2x0.18ohm resistors connected to the emitters of push-pull (NPN and PNP) transistors and the test point are connected at the ends of resistor pack so the voltage is measured across 2x0.18=0.36ohm resistor and the measured voltage is actually the sum-"current" of the push-pull circuit, right?)

3. "Immediately remove the sort and adjust semi-fixed resistors RU35 and RU36 for a voltage of 100mV between the ends of the emitter resistor (Do this in the shortest time possible)."

4. "Short-circuit the two pins in the AVSS board (PU01) marked CLASS A-25W"

5. "Adjust semi-fixed resistors RU33 and RU34 for a voltage of 174mV between the ends of the emitter resistor"

6. "Finally, remove the short"

I set the bias according to these instructions but as the amplifier warms up the bias voltages drops about 50% from original setting. Is this normal or is it because the original bias transistor (2SC3964) is a temperature compensated transistor and the ones I used (2SD882) are not? I've always thought that the bias should actually increase as the temperature get higher?

How about the bias current with these new power transistors that I used (2SD1047/2SB817) since I couldn't find the original, should it be actually something different than with the original transistors?

Cheers!

-Sami

[bekim]

[QUOTE] 14 to 15mA"
(I guess it should read mV).

[QUOTE]


Hi,

it is mA, not mV. PM64II has different bias setting from PM54II, and you did it properly as is in the service manual.

regarding output transistors 2SC3182/2SA1265, to find replacement sometimes I use 2SD1047/2SB817 as you did, depends from amp. Because original Toshibas is impossible to find, but you can use 2SA1186/2SC2837 original Sanken or Mospec is good too.

I just cannot find your biasing transistor on sm 2SC3964.. which nr is?
If is Q701 & Q702 I've marked in sm as 2SC1815, logicaly might be 2SC3964... If so, I think direct replacement is 2SC3419 or 2SC3416.


Do you have any problem now with your amp after fixing...?


regards,

bekim

[Aquarius72]

Hi,

" PM64II has different bias setting from PM54II, and you did it properly as is in the service manual."


yes, I followed the PM64II bias setting procedure.

The Biasing transistors are Q701 & Q702 as you figured out. They are 2SC3964 in PM64II and 2SC1815 in PM54II.

When the amp is cold, bias voltage is about 120mV and it drops to about 90-95mV. I'll adjust it to 100mV as described in the sm.

The amp sounds fine and it runs hot just like my PM84II. The PM64II draws 96VA at idle and the PM84 draws 120VA. I haven't check the bias from PM84II but it will be done soon...

[bekim]

Very good.

Well, to check the bias is proper aligned in your amp and after all this procedure you did, if other channel is ok you measure it and do comparing, otherwise if both channels were dead and you fix it then is other story.

I've repair many marantz amps in past but, I forget "one procedure with these old models" how to make sure they are fine after repairing.

You can try with dummy load in output i.e. 8 ohms in full power and see how is behaving



regards'

bekim

[Aquarius72]

Originally posted by bekim

You can try with dummy load in output i.e. 8 ohms in full power and see how is behaving


regards'

bekim

Hmm, I might have some hi wattage resistors somewhere. If I find them, I'll give a nice little stress to my amp...

BTW, I measured the temperature of the heatsink. It shows 70C (155F) at idle

[reddish75]

i would suggest that the idle current is set too high, 100mv when the amp is cold seems an awful lot, try aiming for about 5mv, test the amp see how the waveform looks before hooking up to speakers, also monitor idle current and d.c offset under load

chris

[Aquarius72]

The 100mV is from service manual. This amp works in class A at low power rates. The 100mV is required for the bias to be high enough. At 5mV the bias is so low that the amp cannot work in class A.

[bekim]

That's true.

The only thing is, best to leave it on and playing with more power and see how is behaving..
Sometimes you cannot just determine faults even with sm.


bekim

[reddish75]

nearly all intergrated amps work in class A at low volumes, are you sure the manual says 100mv because in one of your previous posts you said it says 14-15ma, that certainly doesnt convert to 100mv!

[poynton]

Please post the manual (somewhere) so we can attampt to help you.

Andy