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Marantz 250M Questions

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
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Hey guys, so I have a 250M on my hands that seems to be acting up, I actually had another local shop work on it while I was busy with other projects, however it's been in and out of their shop for 3 weeks now. It originally had a completely exploded channel. It is now at a usable state, though I cannot say I am satisfied with the work. I supplied output transistors and caps for the blow up channel, but it still seems to be problematic in some ways.

What I have noticed is:
With no input, on power on, the "repaired channel" has 500mv to 2v of DC offset, and it shows up on the meter.
When a source is plugged in the DC offset drops to normal range, the shop is telling me this is normal, which having had 2 other 250Ms I am almost certain this isn't the case. I don't think the unit is oscillating though as the bias is completely stable once it warms up.

When I have a source plugged in the DC offset drops to somewhere around 0mv but fluctuates wildly even after the bias has stabilized. Like varies from 0 to 30 rather quickly. See video below:

I noticed they definitely replaced a bunch of transistors, but it looks like they only replaced one driver, and one predriver, last I checked you had to replace them in pairs in this unit. Is that correct? Could the mismatch there be the issue?

I also gave them MJ21195/96s to use as output transistors since it was what I had at the time, assuming those are okay as well?
 
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The input and first stage of the 250 are both capacitor coupled, so you should not be able to effect the offset from the input jack. Are you sure you are not just getting some hum, and the DC meter is resolving it as an offset. Try measuring AC mV on the output, and see if there is just some signal there with no input plugged in.
 
Last edited:
AE Bob said:
The input and first stage of the 250 are both capacitor coupled, so you should not be able to effect the offset from the input jack. Are you sure you are not just getting some hum, and the DC meter is resolving it as an offset. Try measuring AC mV on the output, and see if there is just some signal there with no input plugged in.
Click to expand...

I would but the other channel just nuked itself on me right out of the shop.

What transistors need to be matched? I wonder if they're not testing the HFE if the output devices as marantz specified.
 
I would glean as much info as I could by looking at all the documents on the 250 series and their service bulletins. Some 1200 integrated amps used power amp similar to the ones in some 250s.

The SB's about matching I have are too large to attach. PM me an email addy that can take a big file.

Tom
 
The outputs MUST be matched, the drivers should be matched. Generally if a transistor is replaced (especially if the replacement is a substitute and not the exact transistor) the corresponding complimentary transistor should also be replaced.

The 240/250/250M are all pretty marginally stable, especially the early serial numbers. If anything is out of whack they will self-destruct...
 
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