McIntosh C33 Problems

[B Laskowski]

I purchased a C33 pre-amp some time ago to mate with my MC2255 power amp. After getting the pre-amp I took it up to Mark Wilson at Absolute Sounds Labs in Minneapolis to have it restored/re-capped. I talked to Mark last week on the units progress and he is telling me that the board that supports the internal monitor amp and headphone circuit has a problem. It appears that something is opening up and causing intermittent operation. He has spent some time on it and cant track down the problem. I'm wondering if anyone knows where I might be able to get another board that could be swapped to replace the problematic one. Thanks in advance for your comments.
Bill

 

[c_dk]

The chances of finding a parts unit would seem quite slim, unit is too valuable with to few known problems to part out in the vast majority of instances, IMO.

The preamp can function without the internal amp, so opening up and causing a failure is to vague of a explanation.

What is the actual symptom or problem, you were experiencing, in your own words, to have him recap it.

If the issue is only with the headphone/ internal amp, and all the other outputs work, he will need to bypass the headphone amps muting circuit to test.

 

[jlovda]

I had an intermittent problem with the monitor board of my C34V (almost identical). There are three white connectors that bring all the wiring into the board. Each wire is pushed down into a slot with a little knife blade sticking up. This pierces the plastic cover of the wire so it makes contact with the copper. Two metal tabs are then wrapped over the wire to hold it down. Take a small flat bladed screwdriver and apply some pressure on the wire in each slot. I had one wire in which the blade did not go straight up into the wire. If you try to remove the connector from the board by pulling on the wires you can screw it up. The center connector provides power, ground and (I think) the input signals. The two outside connectors are the outputs. In my case the main input ground wire in the center connector was intermittent. I removed the wire, straightened the knife blade with an X-acto blade, pushed the wire back in with the tiny screwdriver, pushed the tabs back over the wire and then applied a tiny amount of epoxy in the channel. The problem has not returned.

The monitor boards on the C33 and C34V are functionally identical and can be directly replaced without any changes. The only difference is that the C33 uses TO-66 output transistors while the C34V uses TO-220 plastic tab transistors. TO-66 cases are almost impossible to find now so if you had to replace one, you would have to kluge the mounting.

John

 

[clinic-audio]

c_dk is right ! your C33 can be used without this board

 

[Classic_CJ]

The advice above is good, and I'd like to add that if necessary, you can shotgun the board if the intermittent simply can't be pinned down-- it's unlikely the resistors will go bad, so replace any electrolytics and all of the semis, and you'll be good to go. Not as hard to do as it sounds, the circuit isn't that terribly complicated. Last resort, yes, but-- I'd fix it if I could. That monitor amp is surprisingly chunky, you can drive a pair of normal efficiency speakers to a respectable volume with it, it's not just a headphone amp. You could run a second room off of it easily.

 

[c_dk]

CJ, good to see you back. It has been a while!

It could easily be a LDR in the muting circuit either in the pre section itself or the amp.

It sound like his local tech though is throwing in the towel.....

 

[jlovda]

The tech did not say if the intermittent was in both channels or one. The only part that is common to both is a "turn-on relay transistor." If only one channel is bad you only need to try changing components on that one channel. I still say look at the input connectors first. How often does the board go "dead?" Is it randomly dead when you turn it on or does it drop out after playing for awhile?

 

[B Laskowski]

Thank you all for your comments. I will gather a bit more information from Mark and share the notes here with him as well. He is a good tech and has done good restoration work for me on quite a few units. I believe his comments were given under advice to me as to keep a lid on costs. I plan to use this unit at my condo in Chicago and will rely on the headphone option for late night listening. I picked this up at an estate sale last summer and took it to him to check out and have the caps changed. I have experienced the improvement in sound on other units and have read comments in various forums that this particular unit does benefit from the refresh. Again, I appreciate everyone's help.