On Jan 28, 2:21 pm, "Richard Knoppow" wrote:
"N4JOY" wrote in message

...
On Jan 28, 2:26 pm, Don Bowey wrote:



On 1/28/08 9:37 AM, in article
,
"N4JOY"

wrote:
Hi -- I recently completed some repair work on a
National NC-57. I
did the typical replacement of electrolytics, out of
tolerance
resistors, etc. It was functioning reasonably well
beforehand but was
in need of some TLC. Following my repair work, I turned
the unit on
and am only receiving a hum with no RX. I conducted some
resistance
measurements at all tube sockets and every measurement
was nominal
(according to my NC-57 manual) except for pin 6 of tube
7 (6V6GT).
The manual indicates a resistance reading of 380K ohms
and I'm
receiving around 50K ohms (same as pin 3 and pin 4). To
be honest, I
can't see how pin 6 should be 380K ohms. The output
transformer
primary shunt resistor is 22K ohms and everything is
wired correctly
at the tube socket. I double checked my work and
carefully reviewed
the schematics -- everything looks good. I'm really
stumped here...

Any help would be greatly appreciated!

Chris, N4JOY

There is either a wiring error or a defective component.
Pin 6 of the tube
socket is not used by the tube, so it is obviously used as
a tie-point.
Trace in both directions from pin 6 and you should find
the problem.

Hmmmmm... Are you certain that each tube is in it's
correct socket?- Hide quoted text -

- Show quoted text -

Thank you for your reply. I just double checked each tube
socket and
all tubes are installed correctly. You are right about pin
6 of 6V6GT
(tube 7) being used as a tie point. One end of a .1uf cap
and 22K
resistor are soldered to pin 6 (the other end of the .1uf
cap goes to
the tone switch). Pin 6 is also jumpered to pin 3, which
then
connects to the speaker (output) transformer. Pin 4 has the
other end
of the 22K resistor soldered and another lead to the speaker
transformer. There are also two wires leaving pin 4 of
6V6GT: one
goes to the reception switch and the other to the voltage
regulator
(pin 4). I left the wiring alone during my repairs so
either the
value of 380K ohms in the NC-57 chart is a typo (unlikely)
or perhaps
the 6V6GT is defective?

I did try to inject a signal via the antenna connectors with
my RF
signal generator... still no luck. I just have a slight hum
and no
RX.

Chris, N4JOY

The junction of the 0.1uf cap and 22K ohm resistor is at
the plate of the tube. I hate to be obvious but are you
counting the pins in the right direction?
The 22K resistor goes between plate and screen grid of
the 6V6.
See if your hum changes with the volume control. The
control is on the grid of the first audio tube and from that
plate to the grid of the 6V6. If there is no change in hum
level the problem is probably after the first audio, or
nothing is getting to the grid. If the hum does change the
problem is probably in the detector circuit somewhere. If
you can try signal tracing the audio path.
My guess is that your eye keeps missing some miswired
connection. That happens all the time. Perhaps you can get
someone else to go over the stuff you replaced. Sometimes
another person will spot a moved wire or short right away
when you have struggled with it for hours.

--
---
Richard Knoppow
Los Angeles, CA, USA


Since posting this I've discovered that there are two versions of
the NC-57, the 57 and the 57B. They are not quite alike. The NC-57
that appears in the Sams Photofact booklet is the B version. Since
that has socket resistance values and the National handbooks do not I
assume that is what you have. The handbooks for both the NC-57 and the
57B are available on the BAMA site. One difference is the location of
the volume control. In the B version the control is as I described,
that is, at the grid of the first audio stage. In the first version it
is at the grid of the output stage. If you have an original NC-57 and
have been following the schematic in the Sams Photofact folder you may
have been mislead in your wiring. Its certainly worth checking.

--
Richard Knoppow
Los Angeles, CA, USA