Marantz 250M Help!!!!!

Love Vintage

Active Member
I have rebuilt 3 of these and am having trouble with the 4th, I am using the same substitutes as with the others but this one has stumped me. It is the AW115-1071 board power amp board, I am getting over -57 volts on the terminal 19 output which is turning off the protection circuit. I have checked all of the components and verified the pin outs of the replacement transistors. I'm not sure where to go from here. Any help would be greatly appreciated.
Terry
 
Terry, did it work before you rebuilt it? And what components have you replaced?

And Welcome to Audiokarma!

No it did not work before I started, I bought it from someone who had been working on it for several years and couldn't repair it. I started with the power supply/protection board, everything there was replaced except for the filter caps they measured in spec with esr of .00 & .01. I am getting +57.25 & -57.25 at 120 volt input.

The amp board has had all transistors replaced with appropriate subs, and hfe matched pairs where required. These are the newer boards with the darlington's. Also all out of spec resistors and capacitors were replaced.

I may have made some stupid mistake somewhere, but can't find it I don't know how to trouble shoot the circuit to locate the problem. So this is a learning experience for me.
Thanks
Terry
 
I'm not familiar with these. Based on your description though I have a couple of thoughts .
Mismarked or error resistor value
Possible transistor orientation issue sub pin out different than original
Solder bridge.
I know it's a pain In the ass to go piece by piece but it sounds like that's what's necessary

You said you've done 4
Are any of the other 3 available for comparison?
 
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This is why I advise fixing before recapping/rebuilding. Not that this helps you at this point!

You say you're getting -57 volts on the output of the AW115-1071 board, which is the output. Are you getting the same voltage on both channels, or just one?

And yes, this will be a great learning experience indeed. I recommend printing out a copy of the schematic, then taking voltage readings on the transistors on the affected channel(s). Fortunately the schematic for this unit shows what the normal voltages should be. Unfortunately I can't make some of them out on the schematic I'm looking at.

Make sure you're getting your +/- 58,5 volts at the output board(s), not just out of the power supply.

Oh, and whenever someone else has been into something before you there's no telling what you might find!
 
This is why I advise fixing before recapping/rebuilding. Not that this helps you at this point!

You say you're getting -57 volts on the output of the AW115-1071 board, which is the output. Are you getting the same voltage on both channels, or just one?

And yes, this will be a great learning experience indeed. I recommend printing out a copy of the schematic, then taking voltage readings on the transistors on the affected channel(s). Fortunately the schematic for this unit shows what the normal voltages should be. Unfortunately I can't make some of them out on the schematic I'm looking at.

Make sure you're getting your +/- 58,5 volts at the output board(s), not just out of the power supply.

Oh, and whenever someone else has been into something before you there's no telling what you might find!

Yes I get it, I don't have any of the others available, just pictures of the boards. I have checked the board for bridges and breaks also reflowed all parts after finding a cold joint with loose part.
I have only done one board, and yes I am getting +\- 57.25 volts at the board. I will check for proper subs in proper locations along with correct pin outs. Is there a recommendation for starting the voltage check in any kind of order?
Thanks All
 
Great thanks for that, I will go back over everything and start checking voltages, then report back. Thanks for all the welcomes and suggestions.
I checked and tested all the transistors, also for Pin out and correct substitutions, all were fine and I started taking voltage measurements and I am getting that appropriate +\- 57 volts all over the place. I will finish taking the voltages tonight and post.
Terry
 
I try not to troubleshoot under power, but sometimes you have to. Power off first- are resistance readings across all resistors of each channel the same? Resistances from major circuit nodes to ground the same? Everything visually the same? Check color codes carefully- it's easy to miss a factor of ten error. Power on. Look for about 0.7 VDC between the base and emitter of every transistor, save for protection circuits. Any reading common to both channels suggests power supply or common circuitry stuff. Any reading different says something on one channel is a problem.
 
Do
I try not to troubleshoot under power, but sometimes you have to. Power off first- are resistance readings across all resistors of each channel the same? Resistances from major circuit nodes to ground the same? Everything visually the same? Check color codes carefully- it's easy to miss a factor of ten error. Power on. Look for about 0.7 VDC between the base and emitter of every transistor, save for protection circuits. Any reading common to both channels suggests power supply or common circuitry stuff. Any reading different says something on one channel is a problem.
Do you mean .7 volts difference, like 57 volts / 57.7 volts or literally .7 volts?
Terry
 
transistors when powered up should read about 600 700mv between base and emitter . in some circuits this way of testing wont work . base emitter might be at same voltage according to circuit .
 
transistors when powered up should read about 600 700mv between base and emitter . in some circuits this way of testing wont work . base emitter might be at same voltage according to circuit .
My transistor voltages are all around -/+ 57 volts on most of all 3 legs of each transistor. I will spend some time this weekend verifying all the resistors and capacitors. Any off hand ideas why I would be getting similar voltages on all the transistors?
 
I try not to troubleshoot under power, but sometimes you have to. Power off first- are resistance readings across all resistors of each channel the same? Resistances from major circuit nodes to ground the same? Everything visually the same? Check color codes carefully- it's easy to miss a factor of ten error. Power on. Look for about 0.7 VDC between the base and emitter of every transistor, save for protection circuits. Any reading common to both channels suggests power supply or common circuitry stuff. Any reading different says something on one channel is a problem.
Ok I just got the 2nd channel running, I will go over the other and compare power off first. I probably will have questions as I go, thanks all.
Terry
 
Ok I just got the 2nd channel running, I will go over the other and compare power off first. I probably will have questions as I go, thanks all.
Terry
YAHOO I got both channels running and they sound great, but...., I can't get the bias adjustment low enough on one channe 47mv is the lowest I can get it, need to be around 15-20mv is my understanding. I Need help troubleshooting. I have checked R556, R558, R559, R526 trimmer pot, and C513 and they are all in spec. Any ideas would be greatly appreciated.
Terry
 
check emitter resistors . if going high you will see a false reading ..this is presuming you measured voltage across them .
do the outputs get hotter that side ?
 
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