Early Marantz 250 problem.

[elgrillo]

Hello guys, I have a Marantz 250 with serial number #2109. I have a few dilemmas.

I need to replace transistors q507 and q508, I already have 2n5415 (q507) and 2n3440 (q508) . But this are replacements from the bulletin M-250-3 where they specify "for serial numbers ABOVE 6549.

Can I still use them or this is not recommended and I need to find 2n5680 and 2n5682 ?

My second dilema , In the schematics of the original 250 (first schematics) there is no diode under Q510, and in the addendum 1 there is a diode under Q510. Should I need to "upgrade it' and put a diode. This is a germanium diode as far as I know.

Just for more info the amp at this moment has this:

Q510 and Q511 have Toshiba's 2sc783 and 2sa483

As output transistors there are MJ21193G & MJ21194G.

Thanks in advanced.

Gonzalo.

 

[catrafter]

There were several variations of the 250 amp.
I have even seen 250's where one amp module is different than the other, maybe one was replaced at some point, who knows.
I identify what variation of amp module I am working on by comparing it to the service data photos.
Some 250 amp modules used a 3 terminal epoxy 'block' containing a diode and transistor of bias sensing, others an MJE180(?) transistor.

I would be particularly concerned with SB-250- (#2?) about Hfe matching (colored dots).
I don't think 2SC783/2SA483 are the original drivers, they may not be matched.
I have never had a problem using those ON-Semi outputs.

Good Luck! These can be difficult, but sound wonderful and are pretty tough once you get them sorted out.
Tom

 

[elgrillo]

There were several variations of the 250 amp.
I have even seen 250's where one amp module is different than the other, maybe one was replaced at some point, who knows.
I identify what variation of amp module I am working on by comparing it to the service data photos.
Some 250 amp modules used a 3 terminal epoxy 'block' containing a diode and transistor of bias sensing, others an MJE180(?) transistor.

I would be particularly concerned with SB-250- (#2?) about Hfe matching (colored dots).
I don't think 2SC783/2SA483 are the original drivers, they may not be matched.
I have never had a problem using those ON-Semi outputs.

Good Luck! These can be difficult, but sound wonderful and are pretty tough once you get them sorted out.
Tom

Thanks for the response Tom, mine its the first model or mk1 y think. It is exact as the original 250 service manual. The 2sc783/2sa483 are not originals but are good replacements for the predrivers. Do you know if its ok to put on this original version this transistors? 2n5415 (q507) and 2n3440 (q508).

Thanks in advanced.

 

[rickl]

I have a 250 restore thread with a lower serial number. You can find it here. I listed transistors subs.

 

[elgrillo]

I have a 250 restore thread with a lower serial number. You can find it here. I listed transistors subs.

Thanks Riki , that's super cool!

By the way how is the 250 behaving?

Gonzalo!

 

[elgrillo]

Thanks Riki , that's super cool!

By the way how is the 250 behaving?

Gonzalo!

I looked to your restoration , and see you didn't have problems changing Q507 & Q508 with 2n5415 & 2n3440 so I will give them a try , I hope this will make my 250 stable when biasing. Thanks man I will report tomorrow if I have the time to swap the transistors!

Gonzalo.

 

[elgrillo]

Well so I changed Q507 & Q508 with 2n5415 & 2n3440, and it looks it helped but still I think its not completely stable , after more than 30 min bias drift around 4 mini volts on each Chanel . So between 11mv and 15mv. there is a behavior that when one Chanels goes up in Mv the other one goes down in Mv. Its that ok?

Im biasing the amp without preamp connected and without dummy load. I don't know if its completely necessary to use a dummy.

Any help?

Thanks in advanced

Gonzalo.

 

[elgrillo]

Next step will be to check all carbon resistors, long journey but im all the way in with this amp and I really want to make it stable.

It already have new caps in all board minus the big filter caps because they are too expensive. maybe I will change them when I get the amp runining stable.

Any help will be appreciated .

Thanks,

Gonzalo.

 

[catrafter]

Do you have a heat gun (not a paint stripper) or a hair drier? You may have some overly heat sensitive components although it is odd for it to be in both channels. I would also look for oscillation with no signal input.

Tom

 

[elgrillo]

Do you have a heat gun (not a paint stripper) or a hair drier? You may have some overly heat sensitive components although it is odd for it to be in both channels. I would also look for oscillation with no signal input.

Tom

Hi Tom , thanks for answering, Is there somewhere to look hoe to test this amp for oscillation? My knowledge in electronics its ver very basic , but I could try.

Gonzalo.

 

[catrafter]

Hi Tom , thanks for answering, Is there somewhere to look hoe to test this amp for oscillation? My knowledge in electronics its ver very basic , but I could try.

Gonzalo.

To look for oscillations, connect an RCA jack with the shield connected to the center pin (shorted) to each input.
Use an o'scope and look for anything on the speaker terminals, there should be very little there.

Tom

 

[elgrillo]

To look for oscillations, connect an RCA jack with the shield connected to the center pin (shorted) to each input.
Use an o'scope and look for anything on the speaker terminals, there should be very little there.

Tom

Thanks Tom, but unfortunately I dont have an oscilloscope and also I don't know how to use it. either .Is there a way to check it with a multimeter?

 

[rickl]

Thanks Riki , that's super cool!
By the way how is the 250 behaving?

I reread my post and I missed a few things. Oh well.

After I got it working fine per the thread I had to add new PS caps to remove a hum. I'll try to update the thread with pics of the new caps and cap part numbers. But if I don't, just ask.

 

[elgrillo]

I reread my post and I missed a few things. Oh well.

After I got it working fine per the thread I had to add new PS caps to remove a hum. I'll try to update the thread with pics of the new caps and cap part numbers. But if I don't, just ask.

Thanks Rick , did you matched the trsnsisitors? and power transitors? im afraid that mine are to Far away on hfe and the reason of the instability .

 

[rickl]

Thanks Rick , did you matched the transistors? and power transistors? im afraid that mine are to Far away on hfe and the reason of the instability .

No. I just used what I got from Mouser. I really don't know what the problem you are having.

without a scope, it is always difficult to narrow down the problem.

 

[elgrillo]

No. I just used what I got from Mouser. I really don't know what the problem you are having.

without a scope, it is always difficult to narrow down the problem.

I am checking left side boards resistors , at the moment I found 1 out of spec (R539) it should be 27 ohms , and it’s reading 71ohms. I dont know if that could be one of the culprits.

Gonzalo.

 

[elgrillo]

So now I changed the bad resistors and decided to isolate one channel (left) to check it first. The bias is still not stable . I checked the power rails on the board (red cable and blue cable) both are fluctuating between 61V -63V . There is a Yellow cable that is coming to the amp board that its readings are fluctuating also around 7 - 12 Mv. I don't know if this is normal behavior or I have to check the relay board. Zener diodes are given 17.8v and 17.5v so I asume they are ok.

This is makeing me nuts...

Any suggestion?

Thanks in advanced.

Gonzalo.

 

[rickl]

not sure if it helps, but i also recapped the relay board and added new ($$) main PS caps.

 

[elgrillo]

There were several variations of the 250 amp.
I have even seen 250's where one amp module is different than the other, maybe one was replaced at some point, who knows.
I identify what variation of amp module I am working on by comparing it to the service data photos.
Some 250 amp modules used a 3 terminal epoxy 'block' containing a diode and transistor of bias sensing, others an MJE180(?) transistor.

I would be particularly concerned with SB-250- (#2?) about Hfe matching (colored dots).
I don't think 2SC783/2SA483 are the original drivers, they may not be matched.
I have never had a problem using those ON-Semi outputs.

Good Luck! These can be difficult, but sound wonderful and are pretty tough once you get them sorted out.
Tom

Hi Tom, as you said , 2SC783/2SA483 are not the original drivers, I bought them on internet a few years ago , and its correct that I don't know if they are matched or not. Saying that and after changing almost every component on the amp board, My next step will be to change them with MJE15030G & MJE15031G (on semi ). I don't have the right tools to check Hfe on to-220 type transistors. I assume that my cheap multimeter can't handle this bigger transistors with the Hfe

 

[elgrillo]

not sure if it helps, but i also recapped the relay board and added new ($$) main PS caps.

Mine its also recapped , I changed the diodes , resistors, and the relay also with the same mod as you. The only thing I think that I left on the relay board was the original transistors.