Marantz PM350 repair

[Lucamonzaz]

Hello everyone, this is my first post here although I've been lurking for a while and have gathered a lot of great information.
I'm pretty much a newbie regarding repairs, though I can solder and have done some small simple works (guitar electronics upgrades, speaker crossovers, built a couple guitar pedals kits, cables, easy turntable repairs etc). I definitely want to understand more about amplifiers and audio circuits and I'm trying to learn as much as I can as a hobbyist.

So a friend asked me to restore this old Marantz PM350 which belongs to his father (as a gift for his retirement) and I felt pretty confident, but man I was wrong lol.

At first it didn't power on, I checked the switch, transformer, rectifier and fuses and they were all good so I swapped the two filter capacitors with Panasonic's same value slightly higher voltage. Now it turns on but within 30 seconds the heatsink gets very hot so there's obviously too much current I guess.
I've been carefully taking some voltage readings powering on for a few seconds and they're all over the place.
I've checked most components for shorts but I haven't found any. The output transistors are original and I think they test good (I'd like to ask some clarifications on how to test them though).

Luckily here's the service manual https://www.google.com/url?sa=t&sou...MQFnoECAgQAQ&usg=AOvVaw2lu5fJ7vvQC5qbi6HuZxHo
and I only have a digital multimeter for troubleshooting. Hope someone can help me

Many thanks in advance.
Luca

 

[Lavane]

The output transistors probably should be tested at the very leased. You can buy a cheap Chinese made tester that can test transistors, diodes, capacitors etc.
They aren't very accurate, but at leased will tell you it the parts are shorted or open. I've used one to test a couple transistors and the bad ones tested like as a diode. Partsexpress sells a cheap go, no go transistor/diode tester also.
Does the PM350 have a protection relay? It will not click on if one or more outoutsbare fried.

 

[Lucamonzaz]

The output transistors probably should be tested at the very leased. You can buy a cheap Chinese made tester that can test transistors, diodes, capacitors etc.
They aren't very accurate, but at leased will tell you it the parts are shorted or open. I've used one to test a couple transistors and the bad ones tested like as a diode. Partsexpress sells a cheap go, no go transistor/diode tester also.
Does the PM350 have a protection relay? It will not click on if one or more outoutsbare fried.

Thanks for the suggestion, I'll definitely buy one of those LCR testers soon.
For now I can tell the transistors aren't shorted and they measure about the same voltage drop in diode mode so I assume the problem is elsewhere. Do you think desoldering the leads from the PCB and testing with a multimeter would be useful? I'm quite sure this amp has no relays in it.

 

[Lavane]

You can desolder one end of diodes, caps and resistors and measure.
How do the emitter resistors on the outputs look? Maybe pull and test any transistors and regulators in the power supply. I've run across bad regulators due to them running at their upper limit with no heat sink attached.
Check for any discolored, leaking or bulging caps in the power supply also.
Im betting its something in the amplifier section though.

 

[Lucamonzaz]

You can desolder one end of diodes, caps and resistors and measure.
How do the emitter resistors on the outputs look? Maybe pull and test any transistors and regulators in the power supply. I've run across bad regulators due to them running at their upper limit with no heat sink attached.
Check for any discolored, leaking or bulging caps in the power supply also.
Im betting its something in the amplifier section though.

The emitter resistors look and test ok.
Some electrolytics tested questionable so I decided to start recapping the board and here's something I found (feel stupid for not seeing it before). Two resistors are pretty burned up, they are R809 and R810 on the schematic. They still test close (37.9 and 80) but they're obviously getting too hot. Needless to say I'm not powering this up anymore until I get something fixed.

 

[Lavane]

Something up the chain is causingw them to get that hot. I always look at and test the power supply first.

 

[Lucamonzaz]

Something up the chain is causingw them to get that hot. I always look at and test the power supply first.

Thanks for your suggestions, I'm following the schematic but still haven't found anything. I'm kinda giving up on this one until I have more knowledge but your help is much appreciated!

 

[ecluser]

These resistors are in series with the Zener diode Q812. It is normal that they get very hot. Do you have 2.5V at the Zener? Sometimes, Zener diodes get shorted when they fail. This 2.5V regulator is just for the power meters. Malfunction in this part of the circuit will not generate too much heat on the power transistor's heatsink. If the heatsink gets very hot it is probably because there is too much bias (idling) current in the power transistors (there should be 10mV between the emitters of the power transistors), or there is parasitic oscillation at high frequency (you need an oscilloscope to confirm this).

 

[Lucamonzaz]

Q812 tests good, I read 2.7V on one side and 0 on the other. Those resistors look charred though, should I replace them?
Between the emitters of the power transistors I have 500mV and rising and 11V measuring base to chassis ground, which is obviously too much, if that's useful information.

 

[ecluser]

There is no need to replace R809 and R810 for the moment. Concentrate your action on what is defective. Reduce the bias as much as you can. To do this, turn the bias trim pots R725 and R726 in such a way that their resistance is at minimum. Take care because some times in amplifiers we must turn one trim pot clockwise and the other trim pot counter-clockwise to reduce the bias. Measure again the voltage between the emitters of the power transistors, for both channels.

 

[Ray Gianelli]

There is no need to replace R809 and R810 for the moment. Concentrate your action on what is defective. Reduce the bias as much as you can. To do this, turn the bias trim pots R725 and R726 in such a way that their resistance is at minimum. Take care because some times in amplifiers we must turn one trim pot clockwise and the other trim pot counter-clockwise to reduce the bias. Measure again the voltage between the emitters of the power transistors, for both channels.

I don't know why, but the OP failed to mention that the bias is way too high, although he didn't mention which channel he measured. He asked me on my YouTube channel for assistance and I recommended he post in the Marantz forum on AK. The request he wrote was this:

First of all many thanks for sharing your knowledge. I'm a beginner and I'm trying to fix a marantz pm350, I powered on for a few seconds to check the bias and on one channel it went instantly over 500mV, immediately turned it off. I'm looking for obvious shorts and blown components with a dmm, the outputs aren't shorted and resistors match on both channels, do you have any other suggestions for possible causes?

 

[Lavane]

The Marantz forum may help. However, you don't see a lot concerning the 80's stuff.