[Scott Dorsey]

jim wrote:

I have bought a photocopied Service Manual for the Amp. Made in 1972,
a good year, my year of birth. It does not seem to mention anywhere
about setting the bias or how to test the flow to resistors and
transistors.

No, all these things are on the schematic. Somewhere on the schematic
there is a voltage value across the emitter resistors. You adjust the
trimpots on the base of the output stage transistors to get the right
voltage value across the resistor.

I can see where the power transistors are on the diagrams, there are
4. I could just replace these 4 if it would do it any good.

No, invariably that will make any problems worse. If the output stage
isn't biased right, changing transistors will just change the bias point
it's set to, but probably not improve it.

2 are described as: Driver transistors which operate as emitter-
followers to provide the current swings demanded at the output stages
and the necessary phase inversion. Phase inversion is performed by
using PNP and NPN type transistor.
The other two are the Power transistors connected directly to the
power supply of about +-23V. One supplies power to the load during the
positive half cycle and the other the negative. All stages are
directly coupled and are designed to obtain zero potential at the
output terminal. The large coupling capacitor at the output is
eliminated.

Please rephrase to me if you can.

This is the standard output stage everybody has used for years and years
now. It is called a "push-pull amplifier" and it is discussed in a good
introduction to electronics (like Horowitz and Hill, or the ARRL Radio
Amateur's Handbook).

However, your problem isn't the output stage. If it were, you'd have
the output stage fuses blowing. The output stage _should_ be rebiased
as part of the usual service, but that's low on the priority list.

I have noticed on the top of one cap there is a marker pen mark,
could this be the offending cap which has been replaced before now?
Doing this is maybe a way of showing to other sparkies that this has
been fettled with?

Who knows? Is it the same brand cap? Invariably all the electrolytics
are going to be bad todey.

There is a list of components defining which are electrolytic caps
which will help.

That is important. And, of course, be sure to put them in the right
way.

I seem to remember having an earth strap from the unit to the bench
whilst working with electrics, is this going to be necessary, i
presume some of the caps will maintain a whack of voltage still,
before i go replacing and soldering anything what precautions need to
be made. Obviously i will not have it plugged in to the power supply!!

Not really. None of this stuff is as static sensitive as modern MOS
stuff. You should still touch something grounded before you sit down
and work, though.

There is also a list of technical specifications, Dynamic Power,
RMS Power, Rated Power, etc.

All these numbers come out of someone's ass in the marketing department
and can be freely ignored.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."