I've been renovating a Malden Mass Monster...NC183D. It sortta
works, but the B- is supposed to be -21 volts and 500 ohms from chassis
ground acting as the bias for the audio power amps and the negative end
of the RF gain pot.
On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?



tnx de kc2lvq

William Mutch wrote:
I've been renovating a Malden Mass Monster...NC183D. It sortta
works, but the B- is supposed to be -21 volts and 500 ohms from chassis
ground acting as the bias for the audio power amps and the negative end
of the RF gain pot.
On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?

Without digging up the schematic is one of the filter caps NOT supposed
to be connected to chassis ground?
-BM

"William Mutch" wrote in message
ell.edu...

On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?



tnx de kc2lvq

This happens all the time when somebody replaces the filter caps with the
typical modern ones that have the case connected to the negative lead. The
original capacitor probably had two leads, neither one grounded to the case.

You may need to insulate the capacitor case from chassis ground.

This happens all the time when somebody replaces the filter caps with the
typical modern ones that have the case connected to the negative lead. The
original capacitor probably had two leads, neither one grounded to the case.

You may need to insulate the capacitor case from chassis ground.


This filter cap is an actual new old stock in the old style case
which sits in an octal socket isolated from ground. I can try pulling
the cap and see if the B- line goes up to 500 ohms above ground.

William,

I'd check C-65, pin 8 on AC Jumper/Battery Soc. (X-1), R-36 and R-37,
everything at S-1A, and in general, all components associated with the AVC
Amp (V-16), the Phase Inverter (V-12) and the Audio Output section (V-13,14)
and anything else which might in any way connect to these sections. Perhaps
this is elementary and you have already run down these trails. R-36 and 37
would be my first looking place. Any of these could easily pull your line
down (actually, I guess that would be up).
Good luck,

Roger Brown, KL7Q

William Mutch wrote in message
ell.edu...
I've been renovating a Malden Mass Monster...NC183D. It sortta
works, but the B- is supposed to be -21 volts and 500 ohms from chassis
ground acting as the bias for the audio power amps and the negative end
of the RF gain pot.
On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?



tnx de kc2lvq

In article . edu, wcm1
@NOSPAM.cornell.edu says...
I've been renovating a Malden Mass Monster...NC183D. It sortta
works, but the B- is supposed to be -21 volts and 500 ohms from chassis
ground acting as the bias for the audio power amps and the negative end
of the RF gain pot.
On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?


Thanks to all those who responded. I found it. After pulling the
main p.s. electrolytic and effecting no constructive change I
undsoldered and checked the *one* electrolytic on the line I hadn't
previously replaced...C19. It looked so new I'd figured the previous
owner had replaced it...but it was shorted. I replaced it and now have
normal bias and RF gain control operation...and signal thru-put on all
bands. There is still a lot of work to do to restore full original
performance, but it's "over the hump"

tnx de kc2lvq

Just goes to prove, you can NOT "ASSUME" anything. You always end up wasting
time. It takes a minute to test an item to make sure, rather than waste an
hour or more to only have to come back to it and end up kicking yourself
because of it. Been there done that, I don't assume anything in a circuit
anymore. Even if it does look new. I've had hunches about certain parts
being bad, but assumed they were good only to find out I should have
followed through on my hunch. So, I learned to check anything which could be
suspect, especially initial hunches. It pays off and wastes little time. MNS

"William Mutch" wrote in message
ell.edu...
In article . edu, wcm1
@NOSPAM.cornell.edu says...
I've been renovating a Malden Mass Monster...NC183D. It sortta
works, but the B- is supposed to be -21 volts and 500 ohms from chassis
ground acting as the bias for the audio power amps and the negative end
of the RF gain pot.
On my rig the line measures only millivolts negative and 0.4 ohms
to ground. I thought first I had a shorted electrolytic and replaced
all the caps on the line. No joy. Then I thought the transformer
center tap might be shorted to the case, so I unsoldered it and measured
it. The Xformer is very healthy but the line still measures 0.4 ohms to
ground.
Any suggestions what I should look at next?


Thanks to all those who responded. I found it. After pulling the
main p.s. electrolytic and effecting no constructive change I
undsoldered and checked the *one* electrolytic on the line I hadn't
previously replaced...C19. It looked so new I'd figured the previous
owner had replaced it...but it was shorted. I replaced it and now have
normal bias and RF gain control operation...and signal thru-put on all
bands. There is still a lot of work to do to restore full original
performance, but it's "over the hump"

tnx de kc2lvq