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