I recently had to repair a Marshall Bass 60 speaker amplifier, which came to have the scratchy pots cleaned. Easy job, but there remained some problems in the sound. I made the usual checks and found a huge imbalance between the negative and positive voltage rails. +50 vs -20 Volts!!.
I had to scratch my head awhile but finally took the mains transformer out. The problem was clear: the amplifier ground lead was not soldered to the transformer center tap rather than to the adjacent, empty solder lug. That was clearly a manufacturing fault.
The owner had this amplifier for 35 years and never suspected any major problem. I believe there must have been asymmetric clipping when higher volumes were used. Another wonder was, that the primary smoothing cap was rated only to +35 V yet it was continuously seeing +50 volts.
Has anybody seen a similar case? How could this go undetected for so long??
Hi Harri,
I read your post, I have some curiosities. First, when you say the voltage on the rails were +50v and -20v, and the ground was not connected, what were those measured relative to? A floating connection to the transformer? An unconnected earth lead? I think part of the question I am asking comes down to an ambiguity in how you (and much of the electronics world) use the word ‘ground.’ I wish people would move away from using the term ‘ground’ to describe something that is simply a point in a circuit that voltage measurements are measured against in the way the circuit is described. It communicates an assumption that that point in the circuit somehow “should” be, in fact, referred to ground. It’s not obvious that that should always be the case.
So, practically, my curiosity is: When you use the term “amplifier ground,” do you mean the circuit’s zero-volt reference point, or a ground connected to earth, or both? Which was connected, and which was not? Was the center voltage connected to the body of the amplifier, but not the correct transformer tap? Assuming that it wasn’t connected to the center tap in the transformer, I’d guess that the reason for the filter cap having higher voltage than it was rated was simply because perhaps the amp was supposed to have 35v on each rail, referenced to the center tap? I am adding up the absolute value of the +50 and -20, and then dividing that by two to get that figure. It seems to me that a floating center point in the circuit would give the same rail-to-rail voltage as with the neutral point connected, but, the voltage between + and – rails and the neutral would be that divided by the ratio of the momentary impedances between the two rails and the intended neutral.
Anyway, just some food for thought.
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