McIntosh MA 6100 Problems

Hey guys,

It's been a while, only because I haven't had any problems to report with either of my amps until now. The Dynakit ST-70 has been going strong, but I've been using my MA 6100 more since it has an integrated volume control (and I don't have a preamp built to use with the ST-70 yet).

However, a few days ago, the McIntosh started making some scratching sounds on startup and shutdown. Also, the background hum has increased tremendously. After getting out the 'scope and seeing about .1V of ripple at 120Hz across the capacitors, I am guessing the two big 9300µF 50V capacitors are going. It looks like it will be $30-$40 to replace them, depending on quality and ratings.

Which of these two will be better?
Digi-Key - 565-3324-ND (Manufacturer - E36D500LPN273TC79M)
or...
Digi-Key - 565-3329-ND (Manufacturer - E36D630HPN223MC67M)

Thanks!

Kyle
 
Hi there.

If you said you are seeing 120hz oscillation on the output, this will probably be because the main PSU filter capacitors are failing. Their resistance is too high to allow ripple currents to short to ground so the ripple from the rectifier gets into the output stages and onto the speaker output. If the capacitors you are talking about are the power supply capacitors then you are probably best replacing those for a start.

Although the originals are 9,300uF 50V items, then you can replace them with 10,000uF 50V parts which will be much easier to find and cheaper thn the originals which are a rather strange value.
 
I don't want to pay upwards of $30 for Cornell-Dubilier parts, so will those two linked in my first post work? They are 22000uF and 27000uF, respectively. They cost about half as much as C-D. I know it might put more strain on the diodes and power transformer at startup, but I'm hoping that won't be too much of a problem.

Kyle
 
Hey guys,

I replaced the power supply caps, and the problem has gone away. However, I'm reading ±36V, instead of the ±42.5V. That seems like a substantial difference, especially since elsewhere I'm reading about 10% higher voltage. What do you think could be the problem, if there is one? It sounds pretty good as it is, but I seem to recall the voltage being more normal when I first replaced the transistors a few months ago.

What are the suitable replacement rectifiers? I think 1N4007 will work for the regulated 75V supply, but I'm not sure about the others...I would think since this is rated at 70 watts per channel, it would need something that could handle more than an amp. Thanks!

Kyle
 
Nope, I'm afraid it's not working up to par. The left channel got very hot, while the right remained room temperature. That would explain the voltage drop and the increased hum. I replaced the left channel's transistors with some spares, but the voltage is still only ±38V. They don't seem to be getting very hot just yet, but I've only had it on for 5 minutes or so. I'll report back when I have some more information.

Kyle

EDIT: Things haven't started heating up, but the transistors I took out tested just fine. Pretty strange, eh?
 
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If it is heating up and the rail voltage is sagging, it could be due to incorrectly biased output transistors. If they are pulling too much idling current, they will heat up even when not doing anything and will possibly (depending on the current rating of the transformer) pull down the rail voltage too.

When you swapped the transistors and said that they didn't heat up, it probably is due to the bias current. The gain of different transistors (even the same type) can vary so the bias setting that is correct for one pair might not be quite right for another.

If you have the service manual, it should say how and where to adjust the bias. It is normally done by measuring the voltage across the two emitter resistors (or both) which is proportional to the current being passed through the output devices.
 
I don't think there is a way to adjust the bias on this amp. I think it's self-biasing. There are no pots other than to control tone/volume.

Whatever the case, the rail voltage is staying around ±38V and the transistors are no longer getting hot. I also removed the fuses, two at a time per channel, to see if the rail voltage changed. It didn't. I presume everything is in working order yet again. It sounds good, so I suppose I'll leave it until something else goes bad.

Kyle