"Richard Knoppow" wrote in message
m...

"MoiInAust" wrote in message
...

I have an early AR88 that developed a very elusive fault while I was
coincidentally changing some capacitors and resistors. Of course I
suspected I may have miswired something but I have checked and rechecked
and it doesn't seem so. May be just a coincidence.

This fault has defied many weeks of painstaking work. I wonder if members
of the list have any ideas

On the excellent test table provided in EMER 773 (on the VMARS website),
under the specified test conditions (including AVC off) there should be
voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate)
and cathode of V8b, taken to the slider of RV3 (RF gain) when at max.
This is of course the AVC line, and for the voltage as shown to the
slider of RV3 there must be 1.2v across resistor R42 (390K). Well those
readings used to be obtained, but after my work they are very slightly
positive! (I emphasise that the readings are not to chassis but to the
slider of RV3 as instructed. looking at the circuit diagram, to get that
voltage between those points, there must be a very small current flowing
through R42. It would have to be as a result of voltage from the bias
dropper chain of R45, R44, and R43. These have all been checked OK and
there is approx -30 volts to chassis at the end of the chain (total HT/B
current for the set 110mA). With v8b shorted (per EMER 773) the other end
of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have
been checked OK but no volts across R42!

I have tried disconnecting the AVC main feed to V1 etc from SW22, but no
difference so the problem is not in any of the circuitry before v8b.

Next problem (connected?) There is mild AVC action *whether the AVC
switch is on or off*!

Any fresh ideas?

BTW, to get -1.2 volts across R42 (390 K) would require a very small
current of .003 mA. How is that obtained if there's a chain through R42,
R47, RV1, R49?

Wouldn't strain your mind with this, but you did ask! Cheers

I'll follow along the schematic and see if I have any ideas. RCA's
schematics can be confusing:-(

Well I agree. Being a Pom, I used the Anlglicized version of the circuit
which is on EMER772. I find the convention of having high voltage lines
higher than low ones (and the negative AVC line below the chassis 0) easier
to understand. The US diagram in the original handbook has the valves upside
down to my mind (!) Having said that, I consider the AR88 (US design that
it is) to have been decades ahead of any Pom effort (like the infamous 38
set!). 1940 or even earlier and it's still a beaut performer.