Hi, I’ve got a Marantz PM 35 amplifier which I’m trying to restore. It was in real bad condition when I got it full of dust, and there was no output. Cleaned the whole thing touched up the dry joints. It looks like some else has gone in before me as I can see lifted pcb traces which has been fixed and some parts are filled with dried up solder flux. I wasn’t able to get a service manual for it however I was able to find a detailed schematic which was very accurate except for some componet values. It is at
http://akdatabase.org/AKview/albums/userpics/10007/Marantz%20PM-35%20Schematic.pdfAnyway here is what I have done so far.
1. Touched up all the dry joints that I could find (feels like I have re soldered the whole board back again).
2. Changed the 24V regulator.
3. Changed some leaky electrolytic capacitors.
4. All the output transistors and everything seemed to be in working order meaning couldn’t find any shorts etc.
The problem
The audio output is distorted on the left and right channel. Same for the speaker and headphones.
Voltage measured across components/test points seems ok relative to the schematic nothing too out of the ordinary. But I am not able to get 35V at the main bridge rectifier output. It only went upto 25V! Normally it’s even below that around 20V. This is what I have found so far – It goes upto 25V when one side of the amplifier was not turning on (left output side). I measured 300mV at (Q711) which is half of what it should be. When it rises up to 600mV the output of the rectifier goes down to 20V. I touched up the solder joints of the amplifier IC (uPC1270H) and now I get steady voltage of 600mV at the base of Q711. But now I get 20V at the bridge rectifier.
Following this I get 18V at the input of the 24V regulator! And only have 16V at the output of the regulator :S. I didn’t measure 24V at the output of the regulator at any instance the highest it went was around 20V.
Another thing I remarked was R805 which is a safety resistor gets quite hot and it has got burn mark. It checks out ok I have checked after it and before it but couldn’t find any shorts. I measured 10V across it which seems reasonable, its well within the power rating of the resistor.
When I first powered it on the speaker protection relay didn’t engage, but now after all the work done the relay turns on but it takes quite a significant time to turn on.
I checked the output of the transformer - it measures at 19Vac in circuit and out of circuit too (measured across winding and the center tap)
I am using the series light bulb trick - the light bulb turns on briefly when the amp is turned on but remains off afterwards.
I would really like to get this to work please advice on what else to do and other test I could do.
Thanks so much for the help.