Hello everyone,
I was given an old non-working Marantz PM250. Not a great amp, but it looks smart & would still be nice to get it working. The issue is no amplified sound, although there is a very quiet sound from both channels of the input music (tuner in my case) , even with the volume at zero, which is strange. Turning up the volume increases the volume very slightly but it is a tiny weak sound. The volume potentiometer obviously works, but is boosting virtually nothing
I made a few checks :
Can anyone advise what the next step should be ? Unfortunately I don't have a scope.
I can't figure out the reason for no gain. Either there is no signal to amplify , or the power amplifier isn't working, although I expect the power amp isn't the issue. Also how can sound be coming through with the volume knob at zero ?
I was given an old non-working Marantz PM250. Not a great amp, but it looks smart & would still be nice to get it working. The issue is no amplified sound, although there is a very quiet sound from both channels of the input music (tuner in my case) , even with the volume at zero, which is strange. Turning up the volume increases the volume very slightly but it is a tiny weak sound. The volume potentiometer obviously works, but is boosting virtually nothing
I made a few checks :
- The bulb on the DBT goes out almost immediately after switching on
- less than 2mV at both speaker terminals with a load connected
- easy to adjust the bias across the emitters of each pair of output transistors to 11mV as per the manual. stable bias
- all fuses good, no sign of any heat damage anywhere, in fact the inside looks like new
Can anyone advise what the next step should be ? Unfortunately I don't have a scope.
I can't figure out the reason for no gain. Either there is no signal to amplify , or the power amplifier isn't working, although I expect the power amp isn't the issue. Also how can sound be coming through with the volume knob at zero ?
Wrote too soon. It turned out to be a simple fix - a 220ohm fusible resistor was open in the power supply section.
Changed it out and the amp works perfectly.
Question : I only have 1/4W 220ohm fusible resistors in stock. I can't see the rating of the resistor in the parts list. I know for Sansui it's generally 1/4W if not stated, but for Marantz I am not sure.
Is it OK to leave the 1/4W in there or do I need to buy some 1/2W fusible resitors ?
Changed it out and the amp works perfectly.
Question : I only have 1/4W 220ohm fusible resistors in stock. I can't see the rating of the resistor in the parts list. I know for Sansui it's generally 1/4W if not stated, but for Marantz I am not sure.
Is it OK to leave the 1/4W in there or do I need to buy some 1/2W fusible resitors ?