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MA6100 problem.

pmsummer

pmsummer

simul justus et peccator
Doing a cleaning of my MA6100 (purchased new by me in 1973).

The unit worked fine, but with some hum that indicated the need to look inside after 36 years. That's when we discovered this.

Any ideas what's going on here? As I said, the amp has never missed a beat, and I don't recall a dramatic event (but I did have teenagers).

ma6100before002.jpg
 
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I swear that someone posted a pic just like that, sometime in the past. Definitely drawing too much juice through those 1.5k and 600 ohm resistors.

The 1.5k resistor is part of the 75 volt supply (regulator board) and the 600k resistor is part of the 110v supply and they go to different places.

The schematic I have is fuzzy and hard to read but you've got a (some) serious problems there.

Murray
 
On the low voltage side of the resistors (don't know which side that is on your pic) you should have 105v after the 1.5k resistor and 110v after the 600 resistor. S/B about 150v on the other side of each resistor.

Murray
 
Thanks, Murray. Like I said, the darn thing has shown NO problems in use, and the ONLY problem I ever had was in 1974 when my college room mate hooked up a Fender Bassman speaker box to one channel and blew the outputs on one side (fixed under warranty by High-Fidelity Inc in AUSTX amazingly). I doubt this is damage they didn't touch back then. Meter to it soon.
 
Also, check the current draw with amp at idle. S/B about 50 watts according to the schematic.

I guess it could have been that way for the last 35 years but it seems unlikely - it would have been obvious and it seems like the guys working on it would have covered up those naked wires.

Murray
 
Looks exactly like mine. One of the solder joints finally failed due to the heat resulting in a dead unit. I raised the resistors off the circuit board and put some space between them for better air circulation and haven't had any issues since although they do get HOT!

Since it seems like a common issue, I'd bet it's just undersized resistors that can't dissipate the heat.
 
It just so happens I have my MA6100 apart, giving it a clean-up, and I looked at mine. Now I have had mine since I bought it new, also. I played it for a while, and my resistors get a bit warm, and the wires going to them look a wee bit stressed, but there is still insulation on them. I checked my offset, and it is very good, and the output heat sinks get only slightly above room temp.
 
They have always gotten hot as they drop the voltage from 150 to 105 and 110 volts. Never have seen two get this bad. Sign of a failing c312 multicap? How much Ac on 110 and 105 supply?
 
There is a new post with a person who has a MA5100 and he is experiencing the same thing, and I think they are the same "equivalent" resistors, although the layout of the amp is slightly different.
 
Mac used this circuit on many of their classic amps. Have got a 1900 on the bench right now and they have been and get hot. Yours got real hot and I have to say I have not seen any that hot before. That is why I suspect the multicap which is common to both.
 
I think we have this one nailed; good call on C312, c_dk - it's toast. Might not be dead stock, but improvements are sometimes in order . . . more to follow, Film at Ten . . .
-pm's tech
 
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Sorry to thread ressurrect - but what was the outcome? I just popped the bottom on my MA6100 and have the identical issue - although the amp is working fine currently.

Was it the multicap?
 
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