"MoiInAust" wrote in message
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"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.
The RCA schematics are sort of combination schematic
and wiring diagram with all the pins shown in actual order.
A lot of schematics for home radios were drawn like this.
Its confusing because there is no natural flow. My other
favorite is Western Electric transmitter diagrams with all
the filaments and ground wires shown so they look like a map
of a large freight yard.

BTW, I downloaded a bunch of stuff from the VMARS site,
lots of R-390 and other books there and a military handbook
on the older Super-Pro models.

--
Richard Knoppow
Los Angeles
WB6KBL