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McIntosh - Hints and Kinks

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Terry,

I replaced the resistors, but the channel is still running hot. Replaced R I5 with a Ratshack 22 ohm 5% carbon. Couldn't find a carbon 18 ohm for R-17, so used a 2% metal foil 18 ohm Phillips ECG. Used a 2% metal foil NTE flameproof for the 22 ohm R-39. Checked the three new resistors on the DMM before soldering them in--they gave readings within .3 ohms of spec--probably within margin of error on my meter. I swapped the two power output boards again and the heating moved to the rear sink. The transistor on the bottom of the sink was hot to touch after about 3-5 minutes; the one on top was still cool. The amp sounds fine; before I switched the boards around, the L channel was still emitting the slight DC voltage as before. (Brian Scott at Mc Parts Dept states all three resistors have been replaced with the same part #, but they don't have any in stock.)
 
D103 bias diode, possibly bad, is equivalent to 2 diodes in series,, have substituted 2 1N914 or 1N4148 diodes in series as a trial instead of D103, this will not be super stable but is good enough to tell if the original is bad.
 
Terry,

Am feeling really dense here, but I don't see a D103 anywhere in the amp schematic. Don't see any circuitry with 100-series numbers. Diodes on this board are D1, D5 and D7. D3 is not present. D1 connected between R15, R13, and Q5. D5 between Q7 and Q11. D7 between R17 and C7. Corresponding diodes on the right channel board are even numbered single digit. Power supply diodes are D20X-series. No other diodes shown.
 
Am wondering if at some point McIntosh renumbered MC 2100 components on the board. By driver board I assume we're talking about L and R channel power output boards. Mine are both part number #043-899. On my schematic at the location you're talking about--top left corner of the board (furthest opposite from the 15 pin connector), the transistor is Q11 with D3 sandwiched into a plastic tube underneath and between B, C, and E leads for Q11. Directly above Q11 are C9--ceramic and R15, 22 ohm. There is no diode in the corner. No components numbered in the 100-series.
 
Other info: Serial number ink-stamped on back panel is no longer legible. Someone must have used a household cleaner on it far in the past--what is legible-- A(?) V 23... doesn't seem to track with the 10W serial number pattern. There is a track of light pitting over the area which looks like the moisture trail left by a sponge. There is a number metal stamped into the bottom flange of the rear panel--11632.
 
Nortonrider said:
Am wondering if at some point McIntosh renumbered MC 2100 components on the board. By driver board I assume we're talking about L and R channel power output boards. Mine are both part number #043-899. On my schematic at the location you're talking about--top left corner of the board (furthest opposite from the 15 pin connector), the transistor is Q11 with D3 sandwiched into a plastic tube underneath and between B, C, and E leads for Q11. Directly above Q11 are C9--ceramic and R15, 22 ohm. There is no diode in the corner. No components numbered in the 100-series.
Click to expand...
Hmmm. Actually, I'm looking at a schematic for a 2105 which I guessed would be largely the same as the 2100. Anyway D103/4 should be under the heatsink at the corner of the board or perhaps on the opposite side. One end of the diode is connected to one of the 22 ohm resistors, R119/120 on my schematic.

Murray

Edit: Ok, I've got the 2100 schematic now. Diode is D3/4 in the same place, right next to Q7/8 on the schematic. (Corresponding to D103/104 and Q107/108)
 
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Thanks Murray, that kind of what I thought. That one is going to be murder to get to; think you have to remove the transistor to get to it. Why on earth they put it underneath the transistor I don't understand.
 
I'm using the 2105 late production manual, the bias diode is the one under the driver, using a Simpson 260 a normal diode will have a forward R of 10 to 20 ohms, the bias diode will have 60 to 100 ohms for a forward R.
Worked on a MC2105 that had one channel running hot, found the diode was broken. It is mounted this way to act as a heat sensor that as the driver gets hot it drops the bias current to cool things back down.
 
Terry,

Replaced diode D-3 with two Radioshack 1N914-iN4148 diodes with positive end toward the 18 ohm R-17. The channel governed by this board is still running hot--heating quicker than with the Motorola 070-46 diode. I did not, however, mount the two directly under the Q11 heatsink but routed them next to the Q11 leads because of the extra length. Mounted with the positive end toward R17. Swapped the board from L to R channel, and the heating moved with the swap. Heating is starting with the transistor at the bottom of the heatsink, either 2d or 4th, depending on which channel the faulty board is running. Amp still sounds fine. Dave
 
Terry,

I rerouted the diode so it is under the Q11 heatsink; however, no change in the amp's performance--it still runs L channel hot, with the lowest tr5ansistor on the second heatsink heating first. Would it be possible to send you the board only?
 
No problem, may have to wait till I get an amp in to test the board, I get them in often for updates so it should not be a problem.
 
Subject: Panel lamp for C28

My C28 suddenly lost one of the cute little Red dot bulbs along with a couple of the panel lamps. In a call to the McIntosh Parts Dept., I learned that these little red 'wheat' lamps are considered No Longer Available. The panel lamps, however, are still available as part # 0580-1400.

A bit of digging turned up the fact that the 'wheat' lamps are 6 volt, ~25ma bulbs and that Radio Shack maintains a similar lamp as part #272-1140. They should install just fine in the C28 _but_ the leads are not long enough on the replacement bulbs. You will need to splice in the new bulb and use some heat shrink tubing to connect them into the exisiting wiring on the front panel.

At least we can get replacements for a reasonable cost.

Hope that will help someone else in the future.

Cheers,

David
 
Question for the McIntosh Guru:

I just acquired an MA 6100 and it doesn't have the jumpers to connect the pre-amp out to the power amp in. I tried regular audio cables but it makes an awful crackling noise at a relatively low volume. The power amp works fine on it's own. I have seen pics of jumpers and they appear to only jump the inside section of the jack as opposed to an audio cable jumping both sides.

Am I correct in that assessment and where can I get the proper jumpers?

Thanks.
 
Sounds like you've got a problem with the preamp section of your 6100. Can you plug the preamp outputs into another amp and report results?

Since this is a diagnosis, I'd suggest you post this as a regular thread rather than in Hints and Kinks. You'll likely get more responses.

Cheers,

David
 
Jumpers have been covered several times before, #8 ground wire from Home Depot, tough to bend and cut but as good as the originals.
 
my eq on my c33 isn't working

I'm a newbee, this is my first post. my eq on my c33 isn't working. I've tried all modes. it's like flat all the time no matter how much i turn them. any suggestions? Thanks.
 
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