Marantz PM34 amplifier repair advise

[Nevillet]

Hi All,

I have a Denon PM34 HiFi amplifier that presented with no output on the right channel.

On examination the main drive Darlington transistor pair was faulty together with a smaller NPN driving them.

I've obtained the matching transistors but have not soldered them in place and would like some advise before I do:

The faulty componants are:
Q762 2SD1894 Darlington NPN
Q764 2SB1254 Darlington PNP
Q752 2SC3419 NPN

My question is why did they blow in the first place and are there any componants I should check that may have led to their demise?

What I don't want to do is power up the amp with the replacement parts in place and instantly kill them.

Any thoughts would be most welcome.

I've attached the service manual and a picture of the amplifier board.

[petehall347]

Re: Marantz PM34 amplifier repair advise

shorted speaker or wires can do that .. failed bias control circuit can also do same . turn the bias trimmer down before power up and set if all ok . limit the current somehow when powering up first time .

[redwire]

Re: Marantz PM34 amplifier repair advise

It does have output short-circuit and SOA protection, and the output transistors are rated 140V. Hard to tell why. Open Q752 would do it and many other reasons.

Check C804 47uF 10V, C752 100uF 16V, C716 22uF 50V, C706 did not get damaged which can happen when O/P transistors fail and the output pegs with HV DC.

[Nevillet]

Re: Marantz PM34 amplifier repair advise

I've checked the caps mentioned and they all measure fine (using a Peak ESR70)

Powering the amp via a variac and monitoring the current leaving the transformer and supplying the PCB the current is about 400mA on both side when the variac o/p is about 25V.

If I remove the 3 transitors and repeat the above test is only draws 20mA at 25V.

Any ideas?

[petehall347]

Re: Marantz PM34 amplifier repair advise

try turning the bias pots down , but there is likely a shorted transistor .

[Nevillet]

Re: Marantz PM34 amplifier repair advise

would turning them down be anticlockwise?

[petehall347]

Re: Marantz PM34 amplifier repair advise

not sure on that one ,is there a service manual available ? failing that leave the outputs removed and power up and measure voltages where the bases go in the pcb reference ground and set this as low as it goes .. with it being darlingtons expect -+ 1.2 v at each outputs base .
you will need the little transistor going back in as its for bias control

[Nevillet]

Re: Marantz PM34 amplifier repair advise

Many thanks Pete

In my original question I attached the SM.

[petehall347]

Re: Marantz PM34 amplifier repair advise

manual says to set to central positions

[redwire]

Re: Marantz PM34 amplifier repair advise

Bias trimpot R756 at max ohms should give lowest bias.
You can measure the bias voltage (not current) across the VBE multiplier Q752 C-E it must be around 2.4V anything higher even 2.6V I think is trouble and stop.

I'll set for minimum bias, even completely off to Class B when doing checkout.

If Q752 has previously died, I would check Q704 and the nearby resistors haven't fused open R722 or the 100R base resistors "Q760, Q758".

[Nevillet]

Re: Marantz PM34 amplifier repair advise

I'm sorry I haven't been back due to pressure of work.

I will though...