Is this thump normal?

[Baron030]

Wow, I am having second thoughts. And I hate to have to say this. But, I really don’t think you have enough knowledge and experience working with electronics to safely test and possibly service your amplifier. So please don’t take this as personal offense. I just don’t think you realize the dangers and the risks you would be taking in attempting to service your amplifier. Adjusting the zero balance requires the amplifier to be running, which means there is 110 volts AC going into the power supply. And since this is a power amplifier, there would also be + and – 46.8 Volts DC on the power rails. And while 46.8 volts does not sound all that dangerous, actually it is dangerous, because across the two power rails the voltages add up to 93.6 volts DC. So, there is a real danger of electrocution. And even when an amplifier is turned off and un-plugged, there is still an enormous amount of power stored up in the capacitors. I have seen screw drivers get arc welded to a chassis when a power cap accidently gets discharged. And dead shorts are really hard on capacitors as it tends to lead to catastrophic failures.

And there is one other risk with adjusting R9. R9 is a wire wound trim potentiometer and its wiper has not moved since the amplifier has left the factory. Moving the shaft even slightly could cause the wiper to move from an un-oxidized winding to an oxidized winding. I think we have all experienced changing an L-Pad setting and hearing a tweeter cut in and out as the wiper is going across good and oxidized windings. Well, R9 is not all that different from an L-Pad. It is still prone to oxidation. But, in this case, it is carefully balancing currents in the differential input stage of the amplifier. A wiper cut out of the R9 trim pot could lead to a gross imbalance in the differential input stage and this gross imbalance would then be amplified by the “T-Circuit”. The result could be an excessive amount of current flowing through one of the output transistors. Unfortunately, this could lead to a very loud bang and a puff of smoke and one very dead amplifier. And I suspect it is going to be very hard to find exact replace transistors for an amplifier this old. So, do yourself a favor and have a professional look at your amp.

Baron030

[Wagner]

Wow, I am having second thoughts. And I hate to have to say this. But, I really don’t think you have enough knowledge and experience working with electronics to safely test and possibly service your amplifier.
Baron030

Do not know why you would "hate" to say it? It is honest, helpful and thoughtful advice (not to mention the correct advice) You and others (not attacking your post specifically, just the now all too common phenomena) make it sound as if it were some sort of personal attack or insult to make an honest (and responsible) statement or assessment based on the facts at hand

It is also good advice for someone you do not know personally, as many individuals just do not know when to STOP; that their knowledge of a specific field or trade is lacking (and that said same lacking could pose a potential hazard)

Unless of course the OP is interested in doing some self teaching from on-line resources, the library or textbooks; that possibility was mentioned (and plausible) except from a time perspective if he were hot for a quick resolution

The OP came to that realization and expressed it in post# 23

It is not as if you, or anyone, were insulting the guy

We all know what we know and do what we do

It is nothing to apologize for

What has this World come to, in which one has to quantify and qualify every single objective (and honest) remark made for fear of "hurting" someone's "feelings"? I am happy to report that I do not feel this is the case with this OP

The internet's vast resources of information does not qualify us all in all trades and professions although it falsely makes many think that it does (auto repair forums are some of the most entertaining)

I, for example, do not perform my own eye exams, teeth cleanings, minor surgeries or FM alignments

"Political correctness" and "sensitivity" is going to be the death of us all