Hi all

I've struck an infuriatingly obscure problem with my AR88 and the AVC line.
Are there any true experts here? If so, I'll post the details. Don't want
to bore you all otherwise!

"MoiInAust" wrote in message
...
Hi all

I've struck an infuriatingly obscure problem with my AR88
and the AVC line. Are there any true experts here? If so,
I'll post the details. Don't want to bore you all
otherwise!
I probably don't qualify as an expert but you have
roused my curiosity to the breaking point! Please post the
details. Those who are bored can just not look. (As in
_Saving Private Ryan_ "Dont look! Don't look!").



--
Richard Knoppow
Los Angeles
WB6KBL

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

"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

"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.

"MoiInAust" wrote in message
...

"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

MoiInAust ha scritto:
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


Hi,
probably a gassy 6H6 ?

Cheers,
P.

"Piero Soldi" wrote in message
news Hi,
probably a gassy 6H6 ?

Cheers,
P.

Thanks. Yes, I thought that possible but according to my AVO 160, no gas on
any valves! I'll try that again with some replacements...

"MoiInAust" wrote in message
...

"Piero Soldi" wrote in message
news Hi,
probably a gassy 6H6 ?

Cheers,
P.

Thanks. Yes, I thought that possible but according to my AVO 160, no gas
on any valves! I'll try that again with some replacements...
Further to that Peiro, there is no provision on either of my valve testers
to test diodes for gas! I always understood that was a problem if there was
grid current but of course in a 6H6 there are no grids!

Although I do not have a schematic of the AR-88, I just started reading the
thread, so I thought I would jump in more to kick you into changing your
detective methods than to hand you an answer (which I do not know)

If everything was ok before the surgery, and things are now bad, then
something happened during the surgery. That is probably painstakingly
obvious. But, it is important, because it probably rules out changes that
might suddenly have occurred outside of your surgery - such as bad tubes.

So, the question is what happened during the surgery that could cause the
problem. It could be a defective capacitor, or an electrolytic instlled
backwards (especially in negative line). Or, it could be a wire from one of
those components sticking through a terminal and shorting something. Or, it
could be that a resistor got moved and broke in two pieces, but is not
visible. It might be useful to do a resistance check if the manual has one,
or check individual components. Another possibility is, when soldering to a
pot, that the internal connection was broken by the movement of removing and
reconnecting the wires. Same thing on power resistors, especially
adjustable power resistors.

The AVC circuit is usually a long string that can be disconnected in
sections. It sounds like you have been doing that. I think one problem I
had like you are having was when a mica coupling cap in the if stage was
bringing in a small positive leakage into the avc circuit. I kept
addressing the avc circuit, but it was not there at all. Sort of a Trojan
Horse.

The good thing about this is that you will feel very good when you find the
problem. Enjoy the old receiver.

Good luck.

73, Colin K7FM