McIntosh MC 2105 Isues

[wanders]

I could use some help thinking through an issue. A couple of years ago I recapped this unit and it was operating to spec. After a while the right channel meter stopped working and later the right channel started losing output power. At the time I was busy and didn't really need the amp, so I put it on the shelf until now.

Here's the current situation: At the head phone jack, with no load, there is 29 vac rms on both channels. H'ever at the speaker outputs, with 8ohms dummy load,, left channel is OK, but right channel is distorted and won't create power. From the Power Output Section schematic, it seems that there is likely something wrong with/at the right channel OPT.

Does thing make sense? And, if so, ideas on how to proceed would be sincerely appreciated.

 

[llwhtt]

Usually when the headphones seem to work but speakers don't there's a problem in the output stage. Headphones don't draw much current and the drivers are able to provide the necessary current. Check your output transistors and emitter resistors to start. Check the voltages in the output stage also.

Craig

 

[llwhtt]

The speaker switch is in the signal path also, try exercising the switch.

 

[wanders]

I had not thought about that. I'd been suspicious of the speaker switch, but, until your 1st post, I assumed the headphone results eliminated it. Turning it on and off sometimes seemed to affect ac output in both channels, but it did not ever produce full power to the right channel. I could remove the switch for testing, but it is a bit close quarters. I wonder if there is a way to test it in-circuit. As a practical matter, the easiest thing would be just to replace it and eliminate the variable - assuming replacements were available. I'll check around before moving on.

Thanks for the help.
Warren

 

[llwhtt]

IIRC that particular switch is no longer available. The power switch and speaker switch are the same I think. If a new power switch is needed McIntosh will tell you to put the speaker switch in the power switch location and they will sell you a new but different speaker switch. I think that's what I did a few years ago, things might have changed since then.

Craig

 

[wanders]

Craig,

Here's a link to a substitute for the power/speaker switch. They have them a DigiKey. I'll order one and install it before I tackle the power transistors. I'll pos results.

https://audiokarma.org/forums/index...witch-replacement-found.971611/#post-15056875

Thanks,
Warren

 

[llwhtt]

That's the one! If you have a scope try looking at the signal going in and coming out of the switch.

 

[wanders]

As noted, I have a clean sine wave for both channels at the headphone jack. Operating the switch during play can cause significant distortion, in both channels, at both outlets. Logic indicates that the switch is a likely culprit. (But logic is not infallible,) I'd prefer to postpone pulling the output boards until I've replaced the switch.

 

[llwhtt]

Clean in and dirty out would indicate the switch as the culprit.

 

[wanders]

Couldn't get a clear answer testing the switch so I replaced it. The output seems more stable now - as working the switch has no ill effects; but, alas, it did not fix the right channel power issue. Possibly the switch had some problem that made diagnosis tricky. I'll have to pull the power board and check the components. I did that a couple of years ago and found a bad resistor. However, I don't know any way to put ac, or dc, into the board when it is unplugged from the chassis. I suspect that the factory has a powered gig that would spot the problem quickly.

 

[rayma]

I don't know any way to put ac, or dc, into the board when it is unplugged from the chassis. I suspect that the factory has a powered gig that would spot the problem quickly.

The factory may just replace the entire board, so ask if they or a local dealer will sell you a new one.
Or just test all the parts on your board, there are not that many.

 

[llwhtt]

The factory probably had a jig at one time but now it's 50 years in the future so I doubt they still use it or have it. There's not much on those boards so should be easy to troubleshoot.

Craig

 

[mterbekke]

Check the service manual, if you don't have one (mind the different versions):
https://www.hifiengine.com/manual_library/mcintosh/mc2105.shtml

3 options:
1 faulty amp works at high impedance load
2 transformer defect
3 bad switch

Since you replaced the switch, only 2 remain.
Does the headphone work when you load the amp with a low impedance resistor (e.g. 4 ohm on 8 ohm output, turn it up a bit!)?

 

[wanders]

Made a quick trip through both boards: only found one bad component (resistor) on the left (good) board. Decided to swap the boards, and the previously bad, right board, worked properly in the left socket. And the previously good board ran bad in the right socket. Duh, it ain’t a bad board; something’s wrong in the right channel.

As posted, both channels work properly at the headphone with no load. Tomorrow, I can put the headphone output through a 4 and 8 ohm loads and post results.

Using the latest manual available at HiFi Engine.

 

[rayma]

Good work, that saves you a lot of time.
If those are plug in style boards, clean the fingers and sockets on both sides.

The headphones connect through a 220R resistor, so loading it probably won't prove anything.
You have tested all the low value resistors in the output stage?

 

[wanders]

Tested all resistors on both output boards. Found only R135 at 10M ohms in and out of circuit. Just replaced with new 1/4 w 100R.

 

[rayma]

How about the resistors that are not on the board, just connected to the output transistor emitters?
There are seven of them. Those can often fail with age.

 

[llwhtt]

Definitely something on the heatsink module.

 

[wanders]

Yeah, I can see seven on the heat sinks for each channel. All are 5W cement, all seem to be marked .33 ohm. I'll try to get a probe on them.

 

[rayma]

For each channel, two are 0.56R and five are 0.33R.