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MC250 3v Voltage Spikes One Channel

C

CaseyWKC

Active Member
Hey all, I had an MC250 brought to me with transformer buzz and severe hum in both channels.

I found the 500/200uf multi section filter leaking and after replacing that both buzz and hum are gone.

However; I am still getting a nasty voltage spike of 3v dc in the left channel on power on and off.

All caps have now been replaced in the unit (on both amp boards, both main filter caps, and both multi section filter caps).

All bridge rectifier diodes as well along with new Wiim films.

Other than this pop the unit now sounds great, bias dialed in and got offset to under 10mv in both channels.

Any advice or best bet on where to look would be appreciated, I have not worked on too many McIntosh amps!

Thanks everyone!
 
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I suggest you recap the whole thing. The capacitor going bad is indicative there are others that are borderline and going bad as we speak.
The spike is the transition, in early DC amps was a common benign phenomenon but then they found some ways to eliminate it, starting from delay-relays connecting the speaker and ending by some RC in the right places.
I would start with full recap.

Also, check your output autotransformer. Maybe even swap them temporarily, see if the problem migrates.
 
Morzh said:
I suggest you recap the whole thing. The capacitor going bad is indicative there are others that are borderline and going bad as we speak.
The spike is the transition, in early DC amps was a common benign phenomenon but then they found some ways to eliminate it, starting from delay-relays connecting the speaker and ending by some RC in the right places.
I would start with full recap.

Also, check your output autotransformer. Maybe even swap them temporarily, see if the problem migrates.
Click to expand...
As stated in the original post, all capacitors in the unit have been replaced, including the main filters and both multi section filters.
 
Missed that.

The pop occurs with or without the MC250 connected to the pre-amp?
The differential input amp is worth looking at.
Using a scope seeing where else you could see the transient in the same channel.
 
Morzh said:
Missed that.

The pop occurs with or without the MC250 connected to the pre-amp?
The differential input amp is worth looking at.
Using a scope seeing where else you could see the transient in the same channel.
Click to expand...
will take a look at that. and I will take my scope to these areas as well.

I did find a broken lead on the rainbow fish cap on the left amp board. My tester reads it at 50nf.

I'm not sure what the original capacitance was. Looking at the schematic I am guessing it was .047uf therefor a 47nf cap.

need to confirm this before I proceed.
 
It is in both channels I was incorrect, I am only hearing a slight pop in the left channel but I see 3.-3.9v on each channel during power off and around 800mv in each channel on power on.
 
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CaseyWKC said:
It is in both channels I was incorrect, I am only hearing a slight pop in the left channel but I see 3.-3.9v on each channel during power off and around 800mv in each channel on power on.
Click to expand...
What are you using to turn the amplifier on and off with?
 
Those tropical fish caps are Mylar film caps. A lot of people replace them with polypropylene film. I replace with mylar if they are in the signal path. I feel that the slightly higher distortion characteristics of the Mylar are possibly part of the sound signature that we enjoy in these amps.
 
CaseyWKC said:
Power strip

I guess I should also mention if I power it off while it’s playing music it sizzles out without a violent pop.
Click to expand...
Ok. You said "all capacitors in the unit have been replaced, including the main filters and both multi section filters" however you also said "did find a broken lead on the rainbow fish cap on the left amp board". Have the capacitors, regardless of their value and religious denomination, been replaced on all the boards?
 
Why I doubt this is due to the TF caps, do check them for being on spec.

As for you pop: since you said first it was one channel, that threw me off.
Now we know it is in both channels (you might wanna figure out why, despite looking electrically the same, you hear it more on one channel than on the other), here's the thing: without specialized ways to stop it, a power-up click is normal for solid state amps. The only reliable way to stop it was using a timed relay that switched the speakers on after 2-3 seconds.
So, if it plays well, and you have done the full recap, I would leave it be, should it be someone elses set, and would introduce 3-sec timed NO relay to the speaker, were it mine.
 
ramjet said:
Ok. You said "all capacitors in the unit have been replaced, including the main filters and both multi section filters" however you also said "did find a broken lead on the rainbow fish cap on the left amp board". Have the capacitors, regardless of their value and religious denomination, been replaced on all the boards?
Click to expand...
You're right, good for calling that out. All caps have been replaced except for the small silver foil looking caps on each amp board, and the rainbow fish mullard caps, whatever they are referred to as.

Edit : Oh and the ceramic disc caps. - so ... I guess it would have been more accurate to just say "most important caps have been replaced" I suppose.

Morzh said:
Why I doubt this is due to the TF caps, do check them for being on spec.

As for you pop: since you said first it was one channel, that threw me off.
Now we know it is in both channels (you might wanna figure out why, despite looking electrically the same, you hear it more on one channel than on the other), here's the thing: without specialized ways to stop it, a power-up click is normal for solid state amps. The only reliable way to stop it was using a timed relay that switched the speakers on after 2-3 seconds.
So, if it plays well, and you have done the full recap, I would leave it be, should it be someone elses set, and would introduce 3-sec timed NO relay to the speaker, were it mine.
Click to expand...
TF caps? Can you please clarify , transformer caps?

The power up click I do not hear, it's the 3v on power off in each speakers that looks concerning.

It does play well, but I don't think leaving it be is necessarily right.
 
Yeah, what I thoght.
This all said, it is usually not these caps that would influence the transient anyway.

Transient is inherent to SS amps. Either bi-polar (DC) or unipolar (speaker connected through a large cap).
The higher end amps had those relays.


Q; I do not think you answered it: does the pop still occur when the amp is by itself and is disconnected from the preamp?
 
Morzh said:
Yeah, what I thoght.
This all said, it is usually not these caps that would influence the transient anyway.

Transient is inherent to SS amps. Either bi-polar (DC) or unipolar (speaker connected through a large cap).
The higher end amps had those relays.


Q; I do not think you answered it: does the pop still occur when the amp is by itself and is disconnected from the preamp?
Click to expand...
Correct, it happens regardless of input or source. Isolated to power on and off of the MC250.
 
If it is someone else's unit and was brought to you for repair, or just given to you but you are still in touch with the previous owner, ask about the behaviour before these malfunctions started happening. It is quite possible that that pop was the case before (if the owner remembers).
 
Morzh said:
If it is someone else's unit and was brought to you for repair, or just given to you but you are still in touch with the previous owner, ask about the behaviour before these malfunctions started happening. It is quite possible that that pop was the case before (if the owner remembers).
Click to expand...
I am in contact with the owner of the unit, but regardless of whether or not he remembers being a pre-existing issue I would still like to get rid of it if possible.
 
I doubt you should be looking to mod the sch in order to do so.
I suggest you make a time-relay, using a DPST one, with the appropriate current rating (your speakers), I guess 4A/contact and an RC with the relay's winding or, better, use a MOSFET with an RC in its Gate (make sure you do not exceed the Vgs max, either using divider or a Zener) and the relay as its load, and also make sure a diode is in parallel to that relay and is in reverse with the polartity of the power applied to it. It is literally 5-6 parts circuit . Make the RC in the gate about 3s or so (say 10uF / 300K or 100uF 30K). ALso, if your Vgs has to be divided (say it is +/-6V so you have to divide it by 7), remember that the R in the RC calculation is the PARALLEL of the two divider's parts, not serial.

And the contacts go in series with the speaker.

This is the way hi-end amps did it.
 
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