"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:-(



--
Richard Knoppow
Los Angeles
WB6KBL