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Marantz 250 Left Channel VU Meter Problem, Could Use Some Expert Advice

Davidmac5

Davidmac5

AK Subscriber
Subscriber
Hi all,

I have a 250 that I completely restored recently thanks to the help from several AK members who guided me through it. The amp works flawlessly except for one issue that it has had ever since I got it.

The needle movement on the left channel VU meter is less than half of the right channel VU meter needle movement. I did the meter calibration as per the service manual and the right channel calibrated just fine but the left channel won't calibrate, it will only adjust to about half way to 0db. When playing music if the right channel is at -2db the left channel is at -12db that's how much of a spread there is between the two needle movements.

Both channels are putting out equal volume so its not the left channel. I rebuilt everything in this amp except the meter board. I also noticed if I turn the left meter off, it doesn't turn off, it drops a little but still has movement when music is playing, but if I turn off the right VU meter then they both turn off.

The left meter is not sluggish or sticking and I can peg it if I increase the output of the 1KHz sinewave, so I don't believe the meter is defective.

Is it time to rebuild meter board or could it be something else causing the left channel VU meter to not be performing equally to the right channel? Any input would be greatly appreciated.
 
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Could you swap the meters over between channels and see if the problem remains on the same side or shifts? This will help narrow your search potentially having said that the meter board is pretty straight forward to rebuild so you could do it anyway for peace of mind note the larger resistors do get warm on them and often show out of spec here’s a photo of my board as a reference
 

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The behavior of the meter vs the switch is odd, makes me think there is a problem on the meter board or with the wiring.
 
I guess I should just go ahead and rebuild the meter board, I'm sure the electrolytic's have probably lost their value over the years and since I have already rebuilt everything else in this amp, might as well finish it up. This 250 is a later model S/N 9497 and the four transistors that on the meter board now are two 34-1008 and a two MPSA20, in the 2nd addendum for S/N 6549 and above the schematic calls for two MPSA20 and a two 2N4250, should I go ahead and replace both 34-1008 with the 2N4250 that the schematic calls for?

Also the there are four 50uF 16V caps on this board, the schematic calls for 47uf 10V, I have plenty of 47uF 25V caps but no 50uF and they are fairly expensive, is it OK to use the 47uF? and lastly there are two 2.2uF 40V caps on the board which I do not have but I have plenty of 3.3uF 50V caps is it OK to use them instead of the 2.2uF?
 
Davidmac5 said:
I guess I should just go ahead and rebuild the meter board, I'm sure the electrolytic's have probably lost their value over the years and since I have already rebuilt everything else in this amp, might as well finish it up. This 250 is a later model S/N 9497 and the four transistors that on the meter board now are two 34-1008 and a two MPSA20, in the 2nd addendum for S/N 6549 and above the schematic calls for two MPSA20 and a two 2N4250, should I go ahead and replace both 34-1008 with the 2N4250 that the schematic calls for?

Also the there are four 50uF 16V caps on this board, the schematic calls for 47uf 10V, I have plenty of 47uF 25V caps but no 50uF and they are fairly expensive, is it OK to use the 47uF? and lastly there are two 2.2uF 40V caps on the board which I do not have but I have plenty of 3.3uF 50V caps is it OK to use them instead of the 2.2uF?
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I would upgrade the transistors. You can use 47uF for 50uF, it's very close. Not looking at the schematic but they probably control the ballistics. Won't have any effect you can see. The 2.2uF are probably coupling caps. You can probably use 3.3 but it may make the meters react more to turn on and turn off transients. I would order 2.2uF. You should have them in stock anyway, it's a common value. Get 50V or 63V caps.
 
The MPSA20 transistors are easy to find and Mouser has them, but the 34-1008's are not and neither are the 2N4250's. What is a good replacement for the 34-1008 transistor?
 
Good news, the left channel meter function is sorted out and working perfect. The problem was MPSA20 and 34-1008 transistors on the left channel side of the meter board, both tested bad. I replaced all the transistors for both channels with MPSA20 and 2N3906 from Mouser, I also replaced all the capacitors with same value and higher voltage Vishay axial caps also from mouser, now both meters are working as they should.

Thanks to everyone for your valued help and suggestions. This 250 is now totally restored with all the factory updates done and all new components through out. She sounds awesome and should last for another 50 years.
 
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