Marantz PM-57 repair for a newbie

[jifop]

Hey all, I was wondering if someone could help me diagnose a broken PM-57 amplifier.

the power light comes on but its stopped working, no relay click, no nothing, I opened it up and using a thermal camera I noticed that the 2SB1353 transistor was very hot and then that resistors 3302(1k) and 3304(330R) had burned out, I also noticed that the two caps were bulging so i replaced these components (except the transistor) and powered it back up to see smoke coming off the board.

can anyone recomend the best procedure for fault finding this amp without damaging it further, I have a scope, frequency generator and multimeters but haven't had much experience with amplifier repairs.

Thanks in advance,

Tim

 

[tikiroo]

I can't help with troubleshooting, but schematic is attached that includes expected voltages. The components you mention are associated with the output driver transistors. I have one of these that it still working well after more than 20 years.

 

[nigelwright7557]

You can do a lot of testing without powering it on and causing more damage.
Go around transistors and test for diode function.
Test all resistors for marked value.
Check for shorted caps.
Then once everything checks out ok disconnect amp from power supply.
Turn it on and check voltages out of power supply.
Might be a diode gone or a shorted cap.
Then reconnect power supply.
Put a mains 40w light bulb in series with mains to the transformer.
The lamp should flash briefly then go out.
If not there are more problems.
If it does then try some low audio into the amp but check for DC on output first before connecting a speaker !
DC should be < 100mV + or -.

 

[jaycee]

Most likely 7268 is also dead. I'd replace 7260 as a caution as well

 

[Oksana]

nigelwright, do I assume correctly these are generic techniques that can be used to troubleshoot most solid state amplifiers? I have seven Marantz MA500U monoblocks and three of them have the same problem. I've never attempted an amp repair.