Marantz PM-64 repair

[dummy0]

I have an old Marantz PM-64 MKI (The older model). It' s been kinda broken for a while - Only one channel works, and, at first it has a giant DC offset (in volts). The DC offset gets gradually lower and the relay clicks after ~10 minutes.😕 In my understanding the relay clicks when the DC offset is low enough... Am I right? It can work in mono but it' s very annoying. It didn' t work at all at first, then i swapped a few small caps near the TA7317P. Now only one channel works, and i have no idea where to start diagnosing... Any hints would be appreciated🙂

 

[MikePP]

Dry joints most likely...

It didn' t work at all at first, then i swapped a few small caps near the TA7317P. Now only one channel works,

Confusing symptoms:


Was it only one channel faulting before you changed protection components ?


Anyway, first things first, check/confirm PSU voltages
Then check bias levels, also check base voltages on all the output devices at the same time.
Inspect the main board very carefully under a strong light with the aid of a loupe if possible.

 

[dummy0]

No, both channels were malfunctioning before i changed the PS caps... I'll try what you suggest. 🙂

 

[dummy0]

Also, where do I measure the bias levels, and what is the expected level? Sorry if that's a noob question

 

[MikePP]

Also, where do I measure the bias levels, and what is the expected level? Sorry if that's a noob question

This setup is confirmed to be in the service manual 😎

 

[dummy0]

Oh I literally found the instructions 10 mins before you replied😛

 

[dummy0]

Hello


I set up the idling/bias voltage to exactly 8.8mV. The problem is it Gets higher, real slow. I set it to 8.8 and after 5 minutes it gets to something like 10.


I managed to make it stay at 8.8 for a while, and then it just started randomly jumping arounf from 6 to 10 mV...


I suspect the trim-pot itself, or some sort of parallel component. What do i do?

 

[MikePP]

Hello


I set up the idling/bias voltage to exactly 8.8mV. The problem is it Gets higher, real slow. I set it to 8.8 and after 5 minutes it gets to something like 10.


I managed to make it stay at 8.8 for a while, and then it just started randomly jumping arounf from 6 to 10 mV...


I suspect the trim-pot itself, or some sort of parallel component. What do i do?

It will do, leave the amp on for 30 minutes , no audio.

Set both and then leave for 5-10 minutes , and recheck/readjust.. (and repeat until you can see it’s on a plateau that resembles the factory value)

Every coarse adjustment by you will settle immediately as you adjust it, and then drift and stabilise.

So make finer adjustments once you’re close to target and you’ll see the post adjustment drift is a small amount

It’s normal

If it’s jumping about (with no audio) then you’ve got a fault, or the pot is tired…good practice on an old amp to replace (try a Bourns 3386 family)

 

[dummy0]

Hello.


I managed to set up the bias.



Now I found out there is a giant DC offset at the output. I decided to measure the STK voltages first, and the PS voltage is around 60V (62V on Vcc+ and 60V on Vcc-) I found out that there is 3.5 VDC between the output pins on the STK on one channel, and 0V on the other. That leads me to the conclusion that the STK is bad. Also, there are some ceramic caps soldered to the pins of the stk on the bottom. I don' t think they should be there bc the soldering is low effort and messy. The STK was probably bad even before i got the amp and somebody tried to make a quick fix...

 

[MikePP]

The STK’s are well known to fail.

Lots of fakes out there, so buy the seller before buying the part….
Also if it’s clipped onto a sheet metal heat sink, make sure you clean and renew the thermal paste, the spring clip just moves to one side and comes off completely