MoiInAust ha scritto:
"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!



Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.

"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"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!

Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.



Well that's something new I learned! I've got a few and will try different
ones! Thanks heaps!

"MoiInAust" wrote in message
...

"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"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!

Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.



Well that's something new I learned! I've got a few and will try
different ones! Thanks heaps!
Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same
results. So, still puzzled...

MoiInAust ha scritto:
"MoiInAust" wrote in message
...
"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"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!
Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.



Well that's something new I learned! I've got a few and will try
different ones! Thanks heaps!
Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same
results. So, still puzzled...



OK, nothing useful.

When you have restored the capacitors and resistors, substituted any
valve ? If so, check for a *slightly* gassy one !
( RF amplifiers, mixer, MF amplifiers )

Cleaned carefully where soldered new components ?

Capacitors are of the *best* type ? ( low loss types ).

Impedances are very high, a current of 3 *microAmp* is nothing with a
grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable.

Wires are well insulated ? No cracks or degrading rubber ?

Cheers,
Piero.

Hi Piero

Many thnaks for your ongoing hep re this. I have interleaved comments in
your posting ***

"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"MoiInAust" wrote in message
...
"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"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!
Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.



Well that's something new I learned! I've got a few and will try
different ones! Thanks heaps!
Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same
results. So, still puzzled...

OK, nothing useful.

When you have restored the capacitors and resistors, substituted any
valve ? If so, check for a *slightly* gassy one !
( RF amplifiers, mixer, MF amplifiers )

*** Many new valves BUT I have disconnected the AVC line from V1,2,5,6 and
there is absolutely NO current flow across that gap or chnage in the wrond
voltage at V8, so I think we can rule out gassy valves?

Cleaned carefully where soldered new components ?

Capacitors are of the *best* type ? ( low loss types ).

*** They are a darned sight better than the leaky (in both ways!) tubs that
I removed.

Impedances are very high, a current of 3 *microAmp* is nothing with a
grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable.

Wires are well insulated ? No cracks or degrading rubber ?

*** not to allow foreign contact

Cheers,
Piero.

*** still scratching skull (hair out!)

MoiInAust ha scritto:
Hi Piero

Many thnaks for your ongoing hep re this. I have interleaved comments in
your posting .

When you have restored the capacitors and resistors, substituted any
valve ? If so, check for a *slightly* gassy one !
( RF amplifiers, mixer, MF amplifiers )

*** Many new valves BUT I have disconnected the AVC line from V1,2,5,6 and
there is absolutely NO current flow across that gap or chnage in the wrond
voltage at V8, so I think we can rule out gassy valves?
Cleaned carefully where soldered new components ?

Capacitors are of the *best* type ? ( low loss types ).

*** They are a darned sight better than the leaky (in both ways!) tubs that
I removed.

Impedances are very high, a current of 3 *microAmp* is nothing with a
grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable.

Wires are well insulated ? No cracks or degrading rubber ?

*** not to allow foreign contact
Cheers,
Piero.

*** still scratching skull (hair out!)



OK, make some measures.

With a VTVM or other *high* impedance voltmeter, make a reading at
juncton between R43 and C97 ( 4uF ) and chassis ground.

Must be approximately *minus* 3-4 volt.

Again, make a measure at junction between pot R46 pin 3 with R55 and
chassis.

Must be approximately *minus* 0,3-0,4 volt.

After done, post readings.

HTH,
Piero.

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

"MoiInAust" wrote in message
...

"Richard Knoppow" wrote
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.

Good site, isn't it! Now Richard, stop salivating over
the R-390 stuff and solve my problem with the AR88!
I redrew the circuit. The AVC circuit is fairly
conventional. The AVC voltage comes from the detector and
the second half of the 6H6 is used to generate a "delay"
voltage to prevent the AVC from acting until the signal
strength reaches a certain minimum level. This is done to
maintain the RF and IF stages at full gain. There is a
negative bias supply obtained from a series of resistors in
the center-tap return of the rectifier. The RF gain voltage
comes from the RF gain control which is in this string.
About the only way I can see for a positive voltage to
appear on the AVC-RF gain bus is if this whole string is
being pulled positive. If that's so it should also show up
on the audio tubes which are biased from this source. Unless
you made a gross error in wiring I would be suspicious of
C-96 and C97, the first two filter caps since they are
returned to the top of the negative supply rather than to
chassis ground. C-98, the last cap, goes to chassis ground
so can't cause trouble. If one or both of the the filter
caps is leaky it could be pulling the negative supply to a
positive value.
I wouldn't worry about gassy tubes. Positive voltage
can come from one of the controlled tubes becoming gassy but
they are pretty well isolated in this circuit. I can't
account for the slight AVC action with the AVC switch in
manual but if the negative supply has been pulled positive
it might affect this.
The AVC switch essentially shorts out the AVC and puts
the controlled stages directly on the negative supply.
However, the whole AVC bus floats on the negative supply so
that the RF gain control works in both AVC and Manual.

I certainly agree with those who suggest going over
your work carefully. If you know another knowlegible person
have them look also since its possible for someone to
overlook the same thing over and over where someone else
will spot it right away. But, check the filter caps first.



--
Richard Knoppow
Los Angeles
WB6KBL

Thanks again Piero.

Interleaved as before

"Piero Soldi" wrote in message
...
OK, make some measures.

With a VTVM or other *high* impedance voltmeter, make a reading at
juncton between R43 and C97 ( 4uF ) and chassis ground.

Must be approximately *minus* 3-4 volt.

That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I
get 27-30, which would be expected with a total current of 110 mA and a
total AVC chain of 275 Ohms.

Again, make a measure at junction between pot R46 pin 3 with R55 and
chassis.

Must be approximately *minus* 0,3-0,4 volt.

There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3'

Cheers
Alan

"MoiInAust" wrote in message
...
Thanks again Piero.

Interleaved as before

"Piero Soldi" wrote in message
...
OK, make some measures.

With a VTVM or other *high* impedance voltmeter, make a
reading at
juncton between R43 and C97 ( 4uF ) and chassis ground.

Must be approximately *minus* 3-4 volt.

That's the foot of the AVC chain and surely 3-4 Volts is
*much* too low. I get 27-30, which would be expected with
a total current of 110 mA and a total AVC chain of 275
Ohms.

Again, make a measure at junction between pot R46 pin 3
with R55 and chassis.

Must be approximately *minus* 0,3-0,4 volt.

There is no pot R46. Do you mean RV3. Which do you mean by
'pin 3'

Cheers
Alan

The designations are different on the US and UK versions
of the schematic. RV-3 and R-46 are the same, i.e., the RF
gain pot. The voltage table specifies measuring to the pot
but fails to say which contact. Since the pot is variable
(doh) that makes a difference. There is a US version of the
schematic at BAMA which will show the differences in
component lables. I still can't see how the AVC bus could
become positive unless there is something shorting it to the
B+ (HT) or else you are measuring to the wrong reference.



--
Richard Knoppow
Los Angeles
WB6KBL