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.

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

"Richard Knoppow" wrote in message
m...

"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.


No, the AVC is definitely is NOT negative with reference to the chassis, but
the EMER773 states reference must be to the slider of RV3, where is should
be -1.2v. That is the reading that used to be OK and now is not. And I've
checked and rechecked aagain all the changes I made.

"MoiInAust" wrote in message
...

"Richard Knoppow" wrote in
message
m...

"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.


No, the AVC is definitely is NOT negative with reference
to the chassis, but the EMER773 states reference must be
to the slider of RV3, where is should be -1.2v. That is
the reading that used to be OK and now is not. And I've
checked and rechecked aagain all the changes I made.
I found somewhere on the web a manual for the CR-88. I
no longer remember where but I suspect you can find it. If
not I can send via e-mail, its about 3Mb. It has a
different voltage chart. All voltages are measured to
ground. For instance, using a VTVM the voltage at pin 4 of
the first RF tube is -18 volts with the RF gain control at
minimum and -2 volts wtih the control at maximum. I can't
understand measuring to the slider of the RF gain pot
because the voltage there will depend on the position of the
pot. I suspect this is an error of some sort.
Note that the whole RF gain/AVC sytem gets a negative
voltage from a devider string in the center tap return of
the power transformer. That is, the RF gain control is at a
negative potential with regard to ground on both sides.
Actually, there are two strings from the center tap,
one consists of R-46 (RV-3) and a 6800 ohm resistor called
R-55 on the CR-88 schematic. The other string consists of
R-43, 44, 45 and the taps supply a fixed negative bias to
the audio driver tube and audio output tube. If you are not
getting a negative voltage on the grids of these tubes its a
good indication that something is pulling this point
positive. Almost the only thing there is the first filter
cap which returns to the the top of the negative voltage
deviders rather than to ground. If this is the case you will
also see a positive voltage at both sides (and the slider)
of the RF gain control. The lines from all of the negative
supply points to their respective grids have rather high
value resistors in them so I think its unlikely that
something on that side could pull the entire chain positive.
If you replaced the first filter cap check its wiring and
check to see that it isn't presenting a low resistance path
from positive to the devider chain.


--
Richard Knoppow
Los Angeles
WB6KBL

Richard Knoppow ha scritto:
"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




Hello Alan, Richard and Colin.

Alan, yes, i've done a tenfold error !

Take in good advice what Richard say, about the AVC bus,
he's absolutely correct, *if you are shure of wiring redone*.

The Richard's subsequent reply is *perfect* to pin down the
error(s) or defective component(s), and also the Colin's
reply is of great help.

I can't add anything to those replies.

Good debugging,
cheers, Piero.