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Marantz 250M Rebuild Project- Advice & Suggestions Welcome!

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domokelly

New Member
Hi everyone,


I’ve just taken delivery of a Marantz 250M I ordered online from Japan- SN 30308. I’ve seen a number of posts on this site about 250M repairs so I thought I’d run you through what I’m doing in the hope of getting your tell your advice and suggestions. So here goes!

The unit appears in decent shape- with the major exception that someone cut two big holes on the back to fit a pair of XLR sockets for the speaker connections. ☹

I’m not to impressed by that… so my plan is to fix a backing plate on the inside then I’ll fill the holes with body filler, then paint over the top. It won’t be perfect, but it’ll be better than the way it looks currently! I’ve managed to remove the silver pen marks with plastic polish.

As for the internals- they actually seem fairly decent to me. I tested the big caps C1 and C2 and they only had a leakage or 1.3mA at their working voltage of 63V- which seems quite ok, but I think I’ll replace them with Chemicon 200V / 22000uF caps.

As per usual I’ll replace all the electrolytics- and I’ll go about testing all the foil capacitors and resistors to ensure they’re in tolerance.

Otherwise it looks like its been set up for 100V use, for some reason the fuse holder has been bypassed (not ideal) but it does have a fuse hard wired to the transformer. Either way I’ll replace the fuse holder to bring it back to spec and i'll rewire it to 240v to suit my part of the world.

It also looks like the flyback diode mod has been done which is good news- and perhaps some other mods too.


IMG_7434.JPEG IMG_7435.JPEG IMG_7436.JPEG IMG_7440.JPEG IMG_7442.JPEG IMG_7445.JPEG
 
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Replace those big caps no matter what, I've had one be a dead short of the exact same brand and era. I replace those rectifier diodes too sometimes, some UF5405's would be good.
Replace all the ceramic wire-wound resistors, even on the protection board. I also replace the carbon compositions.
Replace the input pair transistors at minimum.
You can fit a MY2-DC24 relay for the huge one, you just have to jumper in two wires.
DeOxit the bias and offset trimmers.
I'd get a black power cord on it, unless that's original but I don't think so.

It's got the diodes on the outputs already, that's good.
 
domokelly said:
Hi everyone,


I’ve just taken delivery of a Marantz 250M I ordered online from Japan- SN 30308. I’ve seen a number of posts on this site about 250M repairs so I thought I’d run you through what I’m doing in the hope of getting your tell your advice and suggestions. So here goes!

The unit appears in decent shape- with the major exception that someone cut two big holes on the back to fit a pair of XLR sockets for the speaker connections. ☹

I’m not to impressed by that… so my plan is to fix a backing plate on the inside then I’ll fill the holes with body filler, then paint over the top. It won’t be perfect, but it’ll be better than the way it looks currently! I’ve managed to remove the silver pen marks with plastic polish.

As for the internals- they actually seem fairly decent to me. I tested the big caps C1 and C2 and they only had a leakage or 1.3mA at their working voltage of 63V- which seems quite ok, but I think I’ll replace them with Chemicon 200V / 22000uF caps.

As per usual I’ll replace all the electrolytics- and I’ll go about testing all the foil capacitors and resistors to ensure they’re in tolerance.

Otherwise it looks like its been set up for 100V use, for some reason the fuse holder has been bypassed (not ideal) but it does have a fuse hard wired to the transformer. Either way I’ll replace the fuse holder to bring it back to spec and i'll rewire it to 240v to suit my part of the world.

It also looks like the flyback diode mod has been done which is good news- and perhaps some other mods too.


View attachment 2635757 View attachment 2635758 View attachment 2635759 View attachment 2635760 View attachment 2635761 View attachment 2635762
Click to expand...

The T066s that thing uses are rare. I rebuilt 2 of them a few years back. Fun amps.
 
ivandezande said:
Replace those big caps no matter what, I've had one be a dead short of the exact same brand and era. I replace those rectifier diodes too sometimes, some UF5405's would be good.
Replace all the ceramic wire-wound resistors, even on the protection board. I also replace the carbon compositions.
Replace the input pair transistors at minimum.
You can fit a MY2-DC24 relay for the huge one, you just have to jumper in two wires.
DeOxit the bias and offset trimmers.
I'd get a black power cord on it, unless that's original but I don't think so.

It's got the diodes on the outputs already, that's good.
Click to expand...

Thanks for all your tips- that's great info.
I had planned on replacing the rectifier diodes too- I had planned on Vishay 1N4003E's but I'll take a look at the UF5405's.
Yes I've just pulled out a few resistors and have found a few that are way beyond tolerance- so I might even just replace them all to be done with it!
 
In fact on the subject of the transistors- I pulled off the left channel to test them and here’s what I came up with:

Q501 / Q502:

Schematic:
Q501/502: 2N5961 NPN

Installed:
Q502: F462 7066 603 NPN
B=285 Uf=694mV
Q501: F462 7056 601 NPN
B=429 Uf=684mV

The B is way off… So I’ll buy a bag of 2N5961s to replace them- and I’ll aim for a matched pair with a B/HFE of >500.



Q505 / Q506:

Q505:

Schematic: 2N4125 PNP
Installed: 2N 4125R PNP
B=71 Uf=689mV

Q506:

Schematic: 2N4123 NPN
Installed: 2N 4123 NPN
B=60 Uf=709mV

The HFE difference of 11 seems excessive- but I couldn’t find any guidance for what the HFE needs to be… does anyone have any hints? My plan is to replace them with 2N3904 / 2N3906 and I’ll aim for a 1-2% match.



Q507 / Q508: (Pre drivers)

Q507:

Schematic: S39613 NPN
Installed: Motorola 462 2016 624 NPN
B=47 Uf=616mV

Q508:

Schematic: SJ2582 PNP
Installed: Unknown PNP
B=52 Uf=641mV


The schematic shows that their HFE needs to be 90-200 (for blue) or 200-300 (for grey). So these seem to be way too low! But what is the colour code anyway? I assume that just how they grouped the HFE range right? The best suggestion I've seen seems to be to replace them with 2N3440 and 2N5415.


Q510 / Q511:

Q510

Schematic: SJ2586 PNP
Installed: Motorola 461-2011 7639 (TO-66 package) PNP
B=75 Uf = 604mV

Q511

Schematic: SJ2585 NPN
Installed: Motorola 462-2022 7639 (TO-66 package) NPN
B=38 Uf=591mV

These are also way off so I’ll have to replace those too. From what I can see I won’t be able to find TO-66 substitutes, but I can get TO-220 substitutes in 2SA1011 and 2SC2344 so they might have to do.



Q802 / Q804 / Q803 / Q805 (Drivers):

Q802 / Q804:

Schematic: SJ2520 PNP
Installed: F 461-2010 631 PNP
B=88 Uf=545mV
B=90 Uf=549mV

Q803 / Q805:

Schematic: SJ2519 NPN
Installed: F 462-2021 631 NPN
B=22 Uf=499mV
B=19 Uf=498mV

This one seems odd- because the NPNs are matched- and the PNPs are matched- but they’re way off each other! The schematic shows the HFE ranges as being 25-50 (for the orange code) and 50-100 (for the yellow code). I’m still not sure how the colour codes work- but I’m assuming the HFE for the drivers needs to be closer than this right? From what I've found so far it looks like the best candidates are going to be the MJE15030 and MJE15031.
 
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domokelly said:
Yeah you're not wrong... They're HFEs are way off each other so it looks like they'll have to be replaced. So far I can only find TO-220 options. Do you have any suggestions on what to use?
Click to expand...

A fellow AKer gave me a set for the ones I was working on but that was back in 2014 so it’s been a while. I don’t have a source for them now.
 
domokelly said:
Thanks for all your tips- that's great info.
I had planned on replacing the rectifier diodes too- I had planned on Vishay 1N4003E's but I'll take a look at the UF5405's.
Yes I've just pulled out a few resistors and have found a few that are way beyond tolerance- so I might even just replace them all to be done with it!
Click to expand...
That's only a 1A diode....not a good choice.
For the TO-66's use MJE15032/3G
 
When I've rebuilt my 250m I've used the complimentary pair MJE15032G/33G. You essentially cutoff the center pin which is the collector and you can bend the 2 outer pins to fit in a T0-66. Should be a drop in no modification necessary. When you purchase the new subs the PNP will be a bit higher than the NPN which is normal. The hfe will more than likely be higher than the original spec, that is ok, higher hfe is better as engineers design circuits for the lowest possible Hfe for a particular model as manufacturing process varies from lot to lot. Have fun, it's a great amp!

Ben
 
Ben LaVigna said:
When I've rebuilt my 250m I've used the complimentary pair MJE15032G/33G. You essentially cutoff the center pin which is the collector and you can bend the 2 outer pins to fit in a T0-66. Should be a drop in no modification necessary. When you purchase the new subs the PNP will be a bit higher than the NPN which is normal. The hfe will more than likely be higher than the original spec, that is ok, higher hfe is better as engineers design circuits for the lowest possible Hfe for a particular model as manufacturing process varies from lot to lot. Have fun, it's a great amp!

Ben
Click to expand...

That's awesome- thanks Ben! I'll look into that.
 
So a quick update- I've started work on the right channel. Most of the wire wound resistors have drifted a long way out of tolerances- especially the 5 watt types. But I've found suitable replacements from Mouser and they're all installed- I've also replaced a few 1/4 watt ones.

View media item 28738
 
So now I'm on to the transistors- and first off is the replacement for Q510 / 511.

I followed some previous recommendations to use MJE 15032 / MJE 15033 as replacements (in a TO-220 package) but the gain for the two types are significantly out. I bought five of each and here's what I have:

MJE15022 (NPN): HFE = 88 for all

MJE15033 (PNP): HFE = 199, 219, 199, 208.

So unfortunately I don't think they're going to be useable. I couldn't fine the 'G' versions- is the gain for those significantly closer?
 
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