"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