Rusty MC250 Project

RedRabbit

New Member
Hi Folks,
Long time lurker here, first time poster...
Picked up a very rusty MC250 today. Got it for a very reasonable price given the condition and thought it would be an interesting project knowing there would be gremlins. I need a winter project. First time working on a Mac power amp, but I have recapped my C26 before with no problems. Any pointers to a newbie working on these power amps would be much appreciated. I have much experience working on tube guitar amps but this is new ground for me. Gentleman I purchased it from acquired it from his job cleaning houses, he said he tried to get it to work but didn't get any sound out of it. I haven't tried to power it up yet.

This thing is very very rusty, but seems to mostly be surface corrosion. Is there a preferred method of taking the rust off? I keep seeing that Wenol or Barkeeper's Friend is the best move? I think a good majority of the screenprinting can be saved.

The internals all look very good, no scorches or burns, however one of the output power boards is cracked. This would be my guess why this thing was left to rust. There are 3 traces that are broken. Is there a better way of approaching this repair than just epoxying the board, stripping back the trace overlay, and flowing solder? I've used this approach a few times with guitar amps.The number on the board is 128-062. Is it crazy to maybe think someone may have a replacement board?

Can someone also please enlighten me on the circuit breaker? Should this read closed with no power?

Will start recapping in the interim...however, where would the experts here start? Or a better question, is this a project worth undertaking?

Forgive my ignorance on this subject and hope to get this 250 up and running again!

Many thanks!
 

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[QUOTE="RedRabbit, post: 12295162, member: 79
This thing is very very rusty, but seems to mostly be surface corrosion. Is there a preferred method of taking the rust off? I keep seeing that Wenol or Barkeeper's Friend is the best move? I think a good majority of the screenprinting can be saved.
Many thanks![/QUOTE]

I would use Naval Jelly (Phosphoric Acid) (Use gloves) and a Q-tip to remove the corrosion. Then you can give it the Wenol treatment.
 
Absolutely worth restoring. Go to audio classics website and search through their parts catalog. You may get lucky and find a parts board to replace the cracked one. If you can't, then epoxy and reconnecting the traces should work. These are easy amps to work on, and as long as you get it working nicely and you can polish up the logo side at least, you can bury it on a shelf and enjoy it.
 
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Cool project, keep us posted as you work on it.
That rust is very abrasive and will scratch whats left of the plating if you try and rub it off dry.
If you can't find a replacement board I would epoxy it back together with some kind of long channels running along the edges. Fix broken traces with bare copper wire soldered to the old traces after removing that green coating.
Welcome to the Mac world, that's a nice entry piece. You won't be disappointed by the sound of it either. Those early SS amps were made to sound like there tube counterpart from what I have heard. My MC40 monoblocks were rather rough when I got them to.

BillWojo
 
Give McIntosh a call....they might have that board available.

If the chassis still looks bad after a good deal of cleaning, you could have it powder coated. I have seen this done and it actually looked really good.
 
Appreciate the replies folks! Looking forward to getting this up and running with my C26.
I've put an e-mail into Audio Classics inquiring about a replacement output board. They didn't have the part listed on their online catalog. If they can't help I will inquire with McIntosh. In the mean time I've stripped back the broken trace paths to copper and will epoxy later today.

I swung by the local big box store and got a bottle of Naval Jelly. Will approach the rust cleaning outside when there's a warm day in the near future. If that doesn't work, powder coating does sound like a reasonable option.

The serial for this 250 is 36L17. I'm using the 250 service manual for "Starting with Serial No. 23L48." Is this the correct manual? I'm assuming this is later produced unit since the power section is on turret board that's not referenced in the manual.

I think that the circuit breaker has failed on this unit. Removed it from the circuit and there's no continuity after pressing the button many times.Would replacing it with a fuse holder be a better move or should I find a replacement circuit breaker?

Now a stupid question, is removing the cage as easy as just removing the 2 nuts on the far ends?
 
Navel jelly is what you want to use. It works well. However, avoid applying it to any lettering you want to keep.
 
Successfully epoxied the output board and fixed the broken traces with copper wire and solder. All of the broken paths have continuity now so looking good there!
I put in a fuse holder in the interim in place of the circuit breaker...which is confirmed as bad. Tried both fast acting and slo blo 2.5A fuses and both blew immediately. Where would people look first? Nothing looks burnt up. My guess is that the capacitors all need to be replaced, but would bad power transistors or a bad transformer manifest itself like this?
 
Successfully epoxied the output board and fixed the broken traces with copper wire and solder. All of the broken paths have continuity now so looking good there!
I put in a fuse holder in the interim in place of the circuit breaker...which is confirmed as bad. Tried both fast acting and slo blo 2.5A fuses and both blew immediately. Where would people look first? Nothing looks burnt up. My guess is that the capacitors all need to be replaced, but would bad power transistors or a bad transformer manifest itself like this?
Bad power transistors, yes.
 
Recap the entire amp while you're in it. I would also replace the main filter caps as well. The amp is at least 40 years old and due for it.
 
Recap the entire amp while you're in it. I would also replace the main filter caps as well. The amp is at least 40 years old and due for it.

I had thought this as well on my MC250, which was built the last week of December 1967, and ordered replacements from Mouser that fit and were recommended by different techs. I did part of the work to mine then took it to Little Rock to Durhams Electronics who has a long time Mac tech that works occasionally on the older equipment. He thoroughly tested the original caps and saw no reason to put the new ones in and ran the amp for a week with constant testing. I did, however, replace both of the smaller multi-caps with replacements from Hayseed which fit and look great. Both of those were shot. Also replaced all of the capacitors and quite a few of the resistors and Chris also changed a couple of things that brought it more "up to date" (although I can't recall what those were). Both of the gain potentiometers were way out of whack and were replaced with new pots.
 
power transformer ohms.jpg
I disconnected the power transformer as Mackie suggested and checked for continuity with an ohmmeter. Please see the attached photo.
Things look consistent with research into these and 2505s I've looked up for one exception.
On the GRN-GRN secondary, the center tap is roughly giving me the same resistance reading as both ends. Both sides from the center tap read 0.4 ohms, while the ends read 0.5 ohms. This doesn't seem right to me...

Thanks!
 
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As an update, found 2 broken diodes in the rectifier and a few resistors that are way out of tolerance. Just made a mouser order to replace all of the silicon in the power supply along with the smaller electrolytic caps. Will try to get correct voltages out of the power supply and see where we stand after that...

Wondering if the rectifier failure is indicative of a bad output transistor...we will see!
 
You will likely find blown output transistors and open circuit emitter resistors.
 
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