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Marantz 240 - DC Offset unstable after recap

ocdecio

ocdecio

The proof of the restoring is in the listening.
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Please take a look at this video:

https://drive.google.com/file/d/141BK1ALmQPpahIRRAuvCNeKXTbZ8kvbB/view?usp=sharing

This is the reading off the speaker terminals on left channel, after a full recap (no silicon was replaced though). I also added the flyback diodes as per service bulletin M-250-3. The service manual says defective Q501, Q502 or Zener CR501/CR502, leaky cap C502, open resistors R501, R502 or R503.

From your experience, what would you think is most likely? Push comes to shove I can replace everything but if it could be something simpler that would be easier. Thanks!
 
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Based on my minimal experience I say check the resistors first. Lift a leg and test them.
Same with the diodes if your Multi-meter can do a basic diode test? Or do you have a component tester?
 
@Magnus Back I do have a diode tester and I will check them out. Funny thing is that it gets worse after it warms up (and the 240 seems to be notorious for running hot even though the bias is spot on).

Let me disassemble it and test. Thank you!
 
It's not comparable I think since Marantz and Sansui build differently but I just pulled out 14 resistors from a Sansui.
I had four 150Ohm resistors coming in at between 2kOhm and 17kOhm ...
Every single resistor was off by at least 2X.
 
@ivandezande good point, I did that for my 1120 and didn't have any issues afterwards. Turns out that I believe they are the same and I have a batch of them to match hFE. BC550C if memory serves me - originally SPS1928
 
ivandezande said:
Every restore/recap I do, I replace the input differential transistors. Doesn't matter if they're fine or not, they're going. I suggest you do the same here, and clean the hell out of the offset adjustment control.
Click to expand...

Oh yeah, deoxit on the pots. They're probably oxidized outside of the spot they were set to.
 
@Magnus Back that's a possibility for sure, I hope it isn't the Zeners because it may be hard to find subs for them.
 
I'll try to deoxit them first without removing from the board to see if it gets better. That's an easier thing to start with.
 
Magnus Back said:
It's not comparable I think since Marantz and Sansui build differently but I just pulled out 14 resistors from a Sansui.
I had four 150Ohm resistors coming in at between 2kOhm and 17kOhm ...
Every single resistor was off by at least 2X.
Click to expand...

I believe you're talking about the fusible resistors, which is a must replace on the Sansui.
 
@petehall347 good idea, I have a pair. I just checked and the other channel is stable which may point towards the differentials
 
Yeah I don't think it will hurt to replace the diffs. I I should have done that, they are the same as the 1120 which I replaced with BC550C and it worked well - unless someone has a better suggestion.
 
So guess what I found as the diff pair... 2N5961's and one of them had an hFE of 283 and the other 486. Regardless if they are the problem or not they have to go. BC550C has inverted pinnage so I will go with 2N5210 - these are used as the diff pair in the 1120 power section, hope they will work as a replacement for the SPS1928.
 
As per the schematic it is around 20V so maybe I could use the 2N5210 's ? I have plenty of these matching at around 500 hFE. Thanks!

upload_2021-1-23_9-26-4.png
 
Swapped them this morning, DC is now holding steady and no static after listening the whole day!

Thank you all for your help and guidance!
 
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