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Old 23rd August 2013, 10:56 AM   #1
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Default McIntosh mystery resistor

I just purchased a '66-'72 McIntosh MA5100 integrated amp.

There is a variable resistor on each power output board. It is R254/R254 on the circuit board but the service manual doesn't list it as variable. What is the purpose of these variable resistors?

Here is the service manual:
Berner's McIntosh MA5100

Read note #2 and #12 on page 10 of the manual about the 'variable' resistors in question, though the schematic and manual doesn't indicate they are variable.
Resistor R254 & R255 are variable resistors on my L&R output boards. I thought they would adjust DC bias, but oddly there is nothing in the manual about it.
The manual mentions this resistor is either 120 ohms or 18 ohms depending on the serial number. My SN is #24H98 and don't know where it falls in their specs, but I suspect its an early production run.
The R254/255 variable resistors in my will only dial down to 248 ohms on the low end, to >1.2 K ohms on the high end.

Now to the problem with the amplifier.

The left channel has 6mv of DC at the speaker output and the right channel is fine near 0mv. I forgot to check it before I hooked up speakers and the amp fried the voice coil of my JBLs woofer on the left channel. What will cause this problem and how can I fix it?
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Old 23rd August 2013, 11:03 AM   #2
Mooly is offline Mooly  United Kingdom
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Which of those links are we supposed to look at

Some say server access password required, and one is a pdf marked 5100 but is of a valve amp.
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Old 23rd August 2013, 11:08 AM   #3
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6mV DC won't fry your speaker and sounds quite normal to me. I would suggest there is something else wrong.
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Old 23rd August 2013, 11:28 AM   #4
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Quote:
Originally Posted by KatieandDad View Post
6mV DC won't fry your speaker and sounds quite normal to me. I would suggest there is something else wrong.
I've heard anything over .5mv dc is dangerous.
Here is what happened:
CL purchase, but very bad news.
It had been playing all day at low volumes and turned it off for dinner. I turned it on and the left channel had hum only, but the right channel was fine. Then all of a sudden there was a high pitch squeal. My wife came running upstairs saying she never heard anything like that. It took a while before it registered with me before it turned it off. It fried one of my JBLs. There is a horrible smell of burnt electronics in the air, and if you put your nose near the woofer of the JBL. The voice coil is fried. Bummer.

Quote:
Originally Posted by Mooly View Post
Which of those links are we supposed to look at
Some say server access password required, and one is a pdf marked 5100 but is of a valve amp.
5th one down, 'MA5100 ser b.zip' worked for me, then open the pdf.
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Old 23rd August 2013, 11:32 AM   #5
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0.5mV, who told you that. 10-30mV is usually acceptable.
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Old 23rd August 2013, 02:40 PM   #6
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Sounds to me like you had an output transistor fail by shorting, blowing the loudspeaker and the other rail's output transistor. So the 6 mV is not a useful measurement.

As to the .5 mV maximum offset, you should be certain where you got this advice and actively ignore it and any other from the same source!
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Old 23rd August 2013, 09:22 PM   #7
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Originally Posted by simon7000 View Post
Sounds to me like you had an output transistor fail by shorting, blowing the loudspeaker and the other rail's output transistor. So the 6 mV is not a useful measurement.

As to the .5 mV maximum offset, you should be certain where you got this advice and actively ignore it and any other from the same source!
So should I go ahead and replace both output transistors on the left channel, or replace all 4?

Funny, I heard the .5mv from a Mac tech that did the free Mac clinics in the '70s. He said many said higher was fine, but he didn't subscribe to that himself. All my amps are below the .5mv range measured at speaker outputs.
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Old 23rd August 2013, 09:26 PM   #8
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Anyways, back to the variable resistors R254/R255. I suppose I should ask Mac what the value should be set at, because I see nothing in the service manual about setting dc
offset.
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Old 23rd August 2013, 09:37 PM   #9
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Pull the outputs, test them for shorts or opens. If any are bad test the drivers, if any are bad test the input transistors. Power it up through a 60W light bulb in series with the ac line.
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Old 23rd August 2013, 09:54 PM   #10
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BTW, I talked with my Mac friend and I misunderstood him. He said the 1/3 volt is fine even up to 1/2 volt max. I thought he said millivolts.

He said Mac had a lot of problems with this amp. When they worked they are great, but they had a lot of issues w/ their first generation ss integrated amps. And told me that if the output transistor are bad, it is almost certain the driver transistors need replacing to. So I'm going to check voltages this weekend.

I think you are saying to pull the input transistors and test w/ 60 watt bulb to the amp while in place?
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