Marantz 250M help

[goldswimmerb]

Hey guys, I recently acquired a working Marantz 250M, it appears to ll be original, I would like to upgrade it as per the service bulletins. What is recommended to make this amplifier more stable, as I have heard of the 250s being "conditionally stable".

This unit appears to be a later model as well, one of the drivers looks like a sj2585, and the other one is a 2SA40445.
Any advice is appreciated. I just know that this model is usually more than a standard recap.

 

[airtime]

The 250M is truly one of the all time classics in audio. I have yet to hear even a modern amp sound better. Actually I'm a die hard tube guy and I STILL prefer this amp!!!!! Have it professionally redone and enjoy it forever. I posted a link to how mine was restored. What a difference!!!!!

The 250 is not an amp I would learn on. It's VERY stable once rebuilt. But if you're not familiar with rebuilding THIS amp then you'll learn why they refer to it as unstable. Contact Bob!!!

https://audiokarma.org/forums/index...m-rebuild-by-bob-speece.763026/#post-11885329

 

[ConradH]

Certainly wise to go to somebody that's been there before. Read the various posts over in http://www.DIYaudio.com

 

[goldswimmerb]

Certainly wise to go to somebody that's been there before. Read the various posts over in http://www.DIYaudio.com

Thanks, I'm fairly familiar with other marantz units, I've done a 2252, 2225, 2270, a model 26 and a model 140 so far, just wanted to see if anyone had advice here before I dove in. I want to service it myself as I have one that's been service in my main setup.

 

[goldswimmerb]

The 250M is truly one of the all time classics in audio. I have yet to hear even a modern amp sound better. Actually I'm a die hard tube guy and I STILL prefer this amp!!!!! Have it professionally redone and enjoy it forever. I posted a link to how mine was restored. What a difference!!!!!

The 250 is not an amp I would learn on. It's VERY stable once rebuilt. But if you're not familiar with rebuilding THIS amp then you'll learn why they refer to it as unstable. Contact Bob!!!

https://audiokarma.org/forums/index...m-rebuild-by-bob-speece.763026/#post-11885329

I saw your fantastic thread and was going to use that as a baseline for whatever I do to my spare 250M. Any reason why Bob left the original 10UF caps in there? Service manual states that they're lytics which typically means they should go, but knowing Bob he'd never overlook something like that. Compared to the 2325 and 2270 this seems like a fairly easy amplifer to disassemble to work on, but the circuit itself is very complicated. I was probably going to replace the differential input pair since the thermal bond is long past it's life expectancy, but I don't see much reason to swap the SJ2585 and SA40445 drivers if they're perfectly fine, and it seems the pre drivers are near impossible to find, so unless someone can explain why I need to swap them I'll probably keep them. This unit works great I just don't trust old electrolytics and thermal compound.

 

[airtime]

I think the reasons predrivers are changed are for lower noise. I'm clearly not a tech but it could have to do with matching the newer driver transistors. If not properly matched (not pairs but system wide) hFE/IT it could introduce oscillations - and boom. Do ask others that are more familiar with the 250 first.

The receivers had a different amp design so don't use that as a guide on the 250.

 

[rickl]

I was probably going to replace the differential input pair since the thermal bond is long past it's life expectancy, but I don't see much reason to swap the SJ2585 and SA40445 drivers if they're perfectly fine, and it seems the pre drivers are near impossible to find, so unless someone can explain why I need to swap them I'll probably keep them. This unit works great I just don't trust old electrolytics and thermal compound.

When I got my 250 running, most of the transistors were blown (including outputs). Something failed and it took out anything near. I replaced all of them. I have a thread there but it turned into more of me posting my build.

Since your amp works, replacing the old caps and thermal compound is a good call. I would also look at the protection board. I had to replace the main filter caps to remove PS hum. Getting at the cap clamp screws isn't easy with all the amp wires packed in.

Please use a DBT to limit current.