Marantz 250M

[quadmatrix82]

Man, you are really working me.... So, I'm doing the yellow caps... So, what about the mica caps? do you change all of those out also?

I'm trying to put my last Mouser order together for this project... I know, wishful thinking...

I am really indebted for the help you've give me...

Mica caps should be fine but if you have any that have burn marks might be worth changing

 

[GRonald]

Man, you are really working me.... So, I'm doing the yellow caps... So, what about the mica caps? do you change all of those out also?

I'm trying to put my last Mouser order together for this project... I know, wishful thinking...

I am really indebted for the help you've give me...

Yes Mate. I run my units hard and all day. I do a full rebuild on these ones. Not a single one has failed since I started doing a full rebuild. Do you need a mouser part list?
Having said the above, you don't need to replace those Mica caps, they are reliable and, in most case, spot on in values.

 

[hedroom]

Yes Mate. I run my units hard and all day. I do a full rebuild on these ones. Not a single one has failed since I started doing a full rebuild. Do you need a mouser part list?
Having said the above, you don't need to replace those Mica caps, they are reliable and, in most case, spot on in values.

Yes, I would love a parts list... Does it include transistors...?

Ok, So I replaced all of the yellow Philips caps and everything was working great and then I decided to replace transistors and I think I've made things worse...

Here's where I am: I replaced all of the transistors (the pre-driver, drivers and outputs are already replaced. Once i hooked it back up, the bias voltage continues to climb up to about 20mV with the bias pot turned all of the way down. Tried putting back the ones the were way different hFE but still unstable. Wound up putting all of the original transistors back and now it's working.

So, I'm running with the original transistors until I have a problem... I did leave in the new differential pair because I didn't feel like reworking them...

The list below are replacements that I made... Could not get the bias voltage to stabilize with the new parts. always climbing to the 30+ mV range for some reason.
Can anyone explain why these didn't work... The darlington (Q503) has the largest gain difference. When I put the original back, no difference in stability. The rest were pretty close in hFE.

Ref	Type	Original Part	Replacement Part	Original hFE	Replacement hFE	Datasheet hFE
Q501	NPN	2N5961	2N5961	263	300	150-1000
Q502	NPN	2N5961	2N5961	309	300	150-1000
Q503	PNP Darlington	MPSA65	MPSA65	20200	76200	20K - 50K
Q504	NPN	SPS1951	BC547B	232	277	110-800
Q505	PNP	2N4125	2N3906	124	173	100-300
Q506	NPN	2N4123	2N3904	94.7	225	100-300
Q516	NPN	MPSA43	MPSA43	83.6	54.1	50-200
Q517	PNP	MPSA93	MPSA93	60	105	40
Q518	PNP	MPS6519	MPS6519	252	391	150-500
Q519	NPN	SPS1928	BC550C	595	603	100-800
Q520	NPN	2N4123	2N3904	85.8	223	100-300
Q521	PNP	MPS6519	MPS6519	133	389	150-500

Cheers and Thanks,
Wade

 

[GRonald]

Yes, I would love a parts list... Does it include transistors...?

Ok, So I replaced all of the yellow Philips caps and everything was working great and then I decided to replace transistors and I think I've made things worse...

Here's where I am: I replaced all of the transistors (the pre-driver, drivers and outputs are already replaced. Once i hooked it back up, the bias voltage continues to climb up to about 20mV with the bias pot turned all of the way down. Tried putting back the ones the were way different hFE but still unstable. Wound up putting all of the original transistors back and now it's working.

So, I'm running with the original transistors until I have a problem... I did leave in the new differential pair because I didn't feel like reworking them...

The list below are replacements that I made... Could not get the bias voltage to stabilize with the new parts. always climbing to the 30+ mV range for some reason.
Can anyone explain why these didn't work... The darlington (Q503) has the largest gain difference. When I put the original back, no difference in stability. The rest were pretty close in hFE.

Ref	Type	Original Part	Replacement Part	Original hFE	Replacement hFE	Datasheet hFE
Q501	NPN	2N5961	2N5961	263	300	150-1000
Q502	NPN	2N5961	2N5961	309	300	150-1000
Q503	PNP Darlington	MPSA65	MPSA65	20200	76200	20K - 50K
Q504	NPN	SPS1951	BC547B	232	277	110-800
Q505	PNP	2N4125	2N3906	124	173	100-300
Q506	NPN	2N4123	2N3904	94.7	225	100-300
Q516	NPN	MPSA43	MPSA43	83.6	54.1	50-200
Q517	PNP	MPSA93	MPSA93	60	105	40
Q518	PNP	MPS6519	MPS6519	252	391	150-500
Q519	NPN	SPS1928	BC550C	595	603	100-800
Q520	NPN	2N4123	2N3904	85.8	223	100-300
Q521	PNP	MPS6519	MPS6519	133	389	150-500

Cheers and Thanks,
Wade

This is the Transistor part list which is the most stable, I have had no issues when using these replacements. What replacements are you using for the pre-drivers and drivers?
The issue you are mentioning is typical of the bias transistors.

 

[hedroom]

So, good news! My 250M is back together and working properly. Once I re-installed the original transistors, everything got way better. So, I did not change any of the TO-92 transistors with the exception of the differential pair on the left channel.

Thank you AK for the help (especially GRonald)! This stuff, meaning electronics, is not my thing but the AudioKarma community really helped me figure it out. I bought this system in Oct. 2023 and spent the first month just figuring out if a DYI repair was even feasible. Needless to say, AK gave me the knowledge to proceed.

I haven't tallied up the bill yet but off the top of my head, I think I have about $600 (USD) in the amp for parts and incidentals. I did not use everything that I purchased. So knowing what I know now, I think the bill might have been quite a bit lower..

I still need to clean up the face plate etc. and put the rest of the screws back in it but I'm so happy with the results.

Now it's onto the 3200 Console/PreAmp. It has one blown channel and I'm just getting started with diagnosing the problem(s). I'll start a new thread once I get started.

If anyone has any experience with the 3200 Console, I would appreciate input...

Again, thanks, AK and Cheers

Wade

 

[hedroom]

Here are a couple of shots of the unit all buttoned up... Even the VU meters work!

W

 

[hedroom]

So, after I got done "high-fiving" myself and thanking everyone yesterday, I fired up the 250M this morning and it would not come out of protection mode... So, after taking the thing apart again, it was the left channel that was not letting the power supply come out of protection. Funny thing was that it would work for a little while and then boom, the relay would drop out. Checked the diff pair, Q501 & Q502, they were fine... Checked the voltage on the zener diode CR501 and it was ~19.5 volts. Changed both CR501 & CR502 (the only things I didn't replace). Looks like I'm back in business.

I'll keep the "high-fiving" on the down low for now...

W