Marantz 250 DC Balance question

zebulon1

Working on my own stuff. Finally
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I got the amps operational after replacing a few resistors and some transistors. I wish I could be more specific but it's been a year since I worked it.
Fired it up on the DBT and I can't get the right amp to balance. It was reading -.120v's with the trimmer full CW's. I replaced the diff pair with the 2N5961's as recommend from this forum and it got worse to -.310v's.
I checked all the resistors for value and the solder connections are good.
Any experience on this set with this issue?
I didn't gain match the pair thinking that the trimmer would have enough give to compensate.
I was thinking of swapping the pair wondering if it would go the other way? (To the plus side).
The protection relay is working.
Also how do you set the idles? Reading the manual for the 240 makes it look like a chore (I don't have the full SM for the 250). Is there a different procedure that one can use?

A image of the turd:

DSC04394.JPG
 
After reading the Bias procedure again, It doesn't look so bad.
I got to get past the balance first.
 
No first hand knowledge but was wondering if you still use the DBT to power up, there will be a volt drop across the DBT so the amp does not see full voltage, maybe
try without it. Also has cap C507? 220uf/6.3V been replaced, may be some leakage? Same on both channels???
 
I'll check the caps in the morning.
The left amp is perfect. Zero and stable
 
I replaced both C503 and C507, replaced the diff pair with a matched set of 2N5961's.
No help.
-.500v's DC Balance with the trimmer full CW's.
I measured the voltages at the base of the drivers and there is the difference I was looking for. +0.200v at the NPN Q510 and 1.060v at the PNP Q508. They both should be 0.6(+-) at the base.
So, I think it's in the pre-stage somewhere.
I'm going to start pealing this amp apart piece by piece.
 
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Nothing wrong with this board except the diff pair.
Putting the original pair in makes the offset closer to zero (-0.150v's)
I need a curve tracer.

What about this?
eBay listing
 
I got it finally.
Reinstalled the original diff pair, only I put them in opposite of the original configuration. Replaced the other two e-caps 10uf/25v, C502 and C504 and it zeroed with the trimmer centered.
Now other issue: Now an then it will jump to .053 and fluctuate +- but 99.5% of the time it's stable. Trimmer? I worked the trimmer but tomorrow I'll remove it and give it good Deoxit treatment.
I'm sure reflowing all the connections probably helped as well.

A newer image of the turd dialed in:

DSC04396.JPG
 
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I just noticed it's wired for the 200v AC line voltage.
After looking at the Google images, most are wired for the 200 volt.
Not sure what to do, might just leave it :dunno:

DSC04397.JPG
 
Looking at the SM(fig 10), wiring looks OK for 100Vac. If it was 220Vac wired primary you would only get half the secondary voltage on rails etc...
 
Edit:
See post #14 for the correct primary wiring.

I wired it for the 100 volt but ended up with +-70 volts at the filters. So I'm going to put it back. When it was wired as seen in the image above, the unregulated at the filters was +-58.
It's not per the SM but I see what your ("mbz") getting at.

110v conversion marantz 250.PNG 220 volt conversion marantz 250.PNG

The 100 volt conversion image:

DSC04398.JPG
 
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I'm gonna say that diagram is wrong, it doesn't match yours or mine. it's shows 2 & 3 jumpered and ours have 3 & 4 jumpered for 220.

Mine is to dusty to follow transformer wire colors.
Untitled-2 copy.jpg

I have another one, but haven't opened it up to look. Mine are just 250's, not 250M
 
I think I figured it out:
The 100v is two halves (in parallel), of the two primary's. And the 220v, is a winding and a half of both primary's (in series). According to the diagrams.
My original winding is a 220v conversion, only it was using a different set of winding's.
The 100 volt conversion is too much for the line power as it runs the unregulated/rails to +-70 volts.
I'm going to rewire it to the 220v conversion.

The image above looks like its wired to use only one of the two primary's full winding's (It is wired for 100v's(?)). Maybe it works better on 120v lines(?). You would have to check the rail voltage.
 
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After some experimentation I got this line voltage wiring figured out for the Marantz 250.
With 115-120v AC Line voltage:
Wired for the 200 volt conversion the rail voltage is -+31v's.
Wired for the 100 volt conversion the rail voltage is -+70v's
Wired like this image below, it runs a perfect +-58.0v's:

DSC04397.JPG
 
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Good deal.
And it looks like the one I took a pic of is wired just like you say it should be. (except my hot and common are swapped - doesn't matter in this amp).
Looks like we just need to leave the transformer taps alone.
 
Copy that.
Now We have a better understanding of the conversion settings.
The DC balance on both channels are adjustable and stable with the trimmers near center.
But:
I'm still having trouble getting the amps to adjust for idle.
Both are acting the same, where I have no idle adjustment and their not drawing any current. I can see good base voltage adjustment at the drivers Q510 and Q511 but no activity at the outputs?
Took a break from it for the holidays.
Started back at it this morning.
 
Good luck with that.
Neither of my two are in use - one has been updated with the service bulletins (I think 3 of them?) by a "professional tech" at an audio shop here in Des Moines. It should still be in good working order.
My other isn't even really tested - picked it up kinda cheap and it's just sitting - for what, I'm not sure. Maybe someday I'll go thru it myself.
 
I'm telling you this amp is a little turd. I spent the whole evening yesterday reading past AK posts, the ones concerning the repairs, Most of which were never finished. The thread would just abruptly stop.
The notes I did take indicated a detailed approach to the repairs. Matching the transistors and adhering to strict tolerances concerning the resistors. I see all this as I've been trudging through this amp.
I ordered up some replacement drivers and will sub in some On Semi outputs later. I had to walkway for a day.:whip:

Here, I have two of the RCA outputs installed with the collectors jumpered to the sockets. This was one tip I found when scouring the past posts. It has helped considerably when probing for voltages.

DSC04400.JPG
 
Ya, I noticed that as well - maybe posts on this appear that the owner gave up or something.

The one I had repaired and updated - I actually paid a pro to do it.
In fact, I tried to pay one guy who said (and he does have a repair shop, but not dedicated to audio) he was an former Marantz repair guy, whatever that means. He totally failed and wouldn't give my amp back for about 5 months. Finally got it back.
Took it to a dedicated audio shop that I'm familiar with and the repair guy has been repairing high-end audio for 30 years. He said he had it repaired, but it only lasted for about a day before blowing - something. Finally, I took it to a third guy with a great reputation with the local bands and he did successfully repair and update per SB. Whew.
So ya, this amp is a turd.
Good luck, I understand having to walk away, I've done that too.
 
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