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MAC4100 channel imbalance and clipping light on meter

A

Artopsy

Active Member
Here is the deal.
this Mac 4100 works great as a preamp but when used as an amp...
one channel is weak. originally the left channel. Audio and Meters, weak meter also has red clipping light on. When I swap the amp boards the weak channel changes along with the meter reactions. So I assume it has to be on one of the amp boards. I replaced caps and two transistors on the amp boards (I did not see any of these reading really bad so that was a waste of time). I don't think it is any better. There is definitely no difference with the meters.
I think it has to be in the one amp board. The 132188 and 132189 seem most likely, since I can make it go from left to right by moving the amp boards left to right.

I will probably swap the output transistors from left heat sink to right heat sink to verify it is the 132188 or 132189

Question one, what is the modern equivalent for RCA 132188 some threads suggest certain MJ####/# but other people say these are not correct.
Question two, what is the modern equivalent for RCA 132189 some threads suggest certain MJ####/# but other people say these are not correct.
Question three, am I on the right track? Is there elsewhere to look?
Question four, (stupid but necessary one) when looking at the amp from the front, is the right amp board on the right and the left amp board on the left? I ask because from the front when looking at the speaker terminals the left speakers are on the right and the right speaker terminals are on the left.
 
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SaturationPt said:
Have you traced the signal through with an oscilloscope and checked voltages on these boards?
Click to expand...
after i started this thread i put a scope to the speaker terminals. I now know when looking at the receiver from the front the left bank amp board is the right amp board. so that answers question four.
it also verified the right channel is weak. when warm top of the wave can be balanced to match the left channel but the bottom cannot get go down far enough to match the left. when cold I can almost balance them peak to valley. I also saw the waveform on the right is not as clean as the left ( a bit of a bump coming up off the low point in the wave). This is not on the left channel. this bump can be seen as on pins 3 & 4 and other pins off the board.
I did voltage checks at the pins into the amp board, and most are ok except pin 4 should read -18V it was reading -15.87V everything else was in range. When reading the 12 pins on both the amp boards most are almost exactly the same pin for pin. except 9 & 10 one bank on the left reads -8.1 mV the other (right bank as you look at the receiver from front) reads 1007mV.
I am now switching the amp boards and running the scope again, then probably switching the rca transistor around on the heat sinks. to verify or rule them out.

edit; swapped amp boards "red" is now on the left bank, basically the same readings... left bank and right bank pin readings didn't change.
and of course channel volume mismatch and peak indicator went from right to left
for future reference I am going to call the amp board originally in the right slot "red"
 
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i swapped the RCA 132188's transistors from left board heat sink to right board heat sink, along with the 132189's nothing changed... so those are ok.
 
I replaced Transistors Q821 to Q824. the clipping stopped... until I decided to tighten the mounts for the left amp board to the chassis.
I touched the Phillips head screw, there was a small spark and the clipping was back ????
Is this a clue someone can deduct a cause?
 
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