Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
Anyone happen to know of a designed-in function that would switch out 1
amplifier of the 2?
Analogue signal of right levels is getting to the inputs of both amps,
which
both agree quiescent DC fashion but some control line is erroneous, I'm
assuming.
Anyone happen to know what switches the A/AB class function , via the 2x
DG211 analogue switches , I assume the user select, amp type, 16 way
encoder
switch, but which for A and which for AB ? but either way seems to
control
both amps simultaneously, not isolating one amp.
I thought it would be the 4 way power switch on the rear but that is
switched attenuators only to both amps, no control function. The phones
change over function via the MAX333 an. sw. should divert both amps by
the
single ground switch in the phones socket .




Looking at my database, one of these had a mainboard failure (i.e. digital
stuff), uneconomic to repair, One I changed the MAX333, a zener and
standard
diode, another was dirty switching contacts (power selector, headphones,
speaker sockets).

I would change the MAX if only to eliminate it. Could be an achilles
heel.
I remember years ago I usd to regularly find blown analogue switches - a
bit
like the old RAM chips in that they were obviously more prone to failure
than most other chips for some reason.




Gareth.



Looks related to An Sw failures, FET failures
No signal on the grids of V1 but signal on Con2 with only a couple of
long-tail FETs in between.
Like a stereo amp comparing channels, signals on V2 grids and con1. DC
around the 2 An Sw are the same, but 0,0.4,220V dc sort of values on the bad
FETs and about 60,60,140V on the good. How long do you scratch your head
before you realise that the schematic is wrong. How could there be 0 V on
one end of a trace at an an sw and 60V on the other at a FET gate of the
working amp?
Supplies to these FET are from the valve anode rail
2002 version
http://www.valvetronix.net/docs/AD12...l_Complete.pdf
about 1/3 down the pdf, left side
Now , with hindsight , it is bleeding obvious , a shorting connection
directly across both C51 and C48 is shown in the diagram but of course in
reality no such short.
Now to find some 240V,200mA rated FETs , dosn't seem right 240 V on a TO92
package