MC 2255 Issues Thread

[Gregory]

1/3 of the way 'up scale' is far beyond any zeroing screw adjustment. Even if you can't find the right meter, I'm thinking it's possible to swap meter movements, but that may be just wishful thinking. The meters are simply 1 Volt DC scale, aren't they? If you have a power supply, you can drive the meter disconnected from amp... but with <1 volt.


-Greg

 

[captouch]

1/3 of the way 'up scale' is far beyond any zeroing screw adjustment. Even if you can't find the right meter, I'm thinking it's possible to swap meter movements, but that may be just wishful thinking. The meters are simply 1 Volt DC scale, aren't they? If you have a power supply, you can drive the meter disconnected from amp... but with <1 volt.


-Greg

Well, I confirmed that the driving circuit is working fine by using the bad meter plug to drive the good meter.

So something is wrong with the meter itself. Hoping that meter experts can fix whatever is wrong, whether by swapping meter movements or somehow fixing whatever is wrong.

 

[clinic-audio]

Hello captouch

here is a picture of the MC2255 meter front view . The zero adjustment is in my red circle :

 

[TSmith8605]

. . .

5) Yes, I'm looking into possible repair centers. Only one Mc authorized repair center in the area (about 30mi away) - they charge $175/hr labor, so am checking into other reliable (maybe not factory authorized, but experience with Macs and good overall reputation).

I assume you mean R&B in Redwood City - listed as a service center on the McIntosh website. I, and I suspect other locals, will be curious to see how they do. The Bay Area lost a great Mc Tech when L&M Electronics in Daly City closed (retired) a couple years ago. A fundamental problem is that unless you make $250K / year (or already own a house), you cannot afford to live here - so hard for techs to exist, I guess.

The McIntosh website listed 2 authorized service centers in Sacramento - which might not be bad if you're up in the north bay or north east bay somewhere.

 

[captouch]

I assume you mean R&B in Redwood City - listed as a service center on the McIntosh website. I, and I suspect other locals, will be curious to see how they do. The Bay Area lost a great Mc Tech when L&M Electronics in Daly City closed (retired) a couple years ago. A fundamental problem is that unless you make $250K / year (or already own a house), you cannot afford to live here - so hard for techs to exist, I guess.

The McIntosh website listed 2 authorized service centers in Sacramento - which might not be bad if you're up in the north bay or north east bay somewhere.

Yup, R&B. They're expensive as you would expect. I'm not sure who I'm going to take it to. The 2120 I have now (working fine) has an L&M sticker on it, so they worked on that one at some point in its life. But indeed, too bad as more choices provides a little more competition and perhaps competitive prices.

From your sig, it looks like you've opted to send everything to Terry to get worked on.

 

[TSmith8605]

. . .

From your sig, it looks like you've opted to send everything to Terry to get worked on.

About 12 years ago, I started "collecting" older McIntosh stuff for fun. I wanted that stuff restored / recapp'd / upgraded - made reliable with preventative component replacements. Terry offered those services and came highly recommended. L&M would only repair to spec.

My first purchase for the "collection" was an MR71 from the auction site. It turned out to be a mess with pitting on the chrome, the wrong, non-panloc cabinet and was not working correctly. I decided that would not do for my "collection" so I purchased a second, nearly perfect MR71 from Tom Manley.

In the mean time, I took the funky MR71 to L&M who repaired and aligned it to spec. I put it in a proper panloc cabinet hiding the cosmetic issues and proceeded to use it and the "good" MR71 for the last 10 years. Turns out the wrong, non-panloc cabinet that came with the MR71 is the correct, slant-legged cabinet for either my MAC1500 or 1900. Lemonade from a lemon.

Anyway, the point in there somewhere is that services Terry provides are different and more comprehensive than a simple repair from a local tech.

 

[Gregory]

Here's a power supply set at 0.1 Volt DC connected to the LEFT 2205 meter wires to see if it would cut out over several hours.



Which demonstrates the meters are basically 1-volt DC full scale.


If you remove BOTH nuts that fasten the wires onto the studs, the movement will easily come out. That is, it will fall out if both nuts are removed carelessly. IOW, when servicing or replacing the terminals, only remove ONE SIDE at a time!




This is similar to what you described, swapping the RED wires to the meters.



The LEFT meter cut out, so it was the meter and not the circuit.


-Greg

 

[captouch]

Here's a power supply set at 0.1 Volt DC connected to the LEFT 2205 meter wires to see if it would cut out over several hours.

View attachment 1317093

Which demonstrates the meters are basically 1-volt DC full scale.


If you remove BOTH nuts that fasten the wires onto the studs, the movement will easily come out. That is, it will fall out if both nuts are removed carelessly. IOW, when servicing or replacing the terminals, only remove ONE SIDE at a time!

View attachment 1317098


This is similar to what you described, swapping the RED wires to the meters.

View attachment 1317102

The LEFT meter cut out, so it was the meter and not the circuit.


-Greg

Were you able to fix the meter?

 

[Gregory]

Yes, by resurfacing the brass terminals with a flat jeweler's file on both sides.


-Greg

 

[captouch]

Wanted to update on progress on the amp in the original post:

I recapped all the boards (I was going to do this anyway even if original problems didn't exist) and replaced both darlingtons that fit into the heat sinks. The overly-hot heatsink issue has been resolved and both heatsinks get mildly warm when the amp is on for awhile at moderate volume. No doubt the bias isn't optimized and I'll seek help on that since I don't have big enough dummy load bigs, but at least the amp appears to be out of danger.

Thanks to @c_dk and @clinic-audio for pointing me toward the darlington. Thanks also to @jeffs79 who answered some questions I had via PM on some details.

The bad meter still isn't working, but I can see signs that it sometimes tries to move. It twitches slightly at times in time with the music and sometimes when I move the meter knob, it'll glitch a bit. I'll send it somewhere to be looked at. With any luck, maybe it's just dirty and needs to be cleaned versus rebuilt.

The glass replacement will come last after everything else is done.

 

[Corktowner]

Resurrecting this thread to add another potential issue - high heat levels from the +/- 15V regulators. When working on the 2255 I bought with the broken glass, I saw evidence of heat damage around Q703. The heat sink, surrounding circuit board and capacitor C705 were all discolored, as shown in the photo. Fortunately the transistor pair seem common, so I ordered new replacements from Mouser when getting the capacitors. I then cleaned everything and added a generous amount of thermal paste. Everything seems to be working well after reassembly.