I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a massive
raw AC hum on the output.
By way of diagnostics I downloaded the manual and schematic from
the BAMA site.
I changed out the 3 section filter electrolytic and the cathode
bypass on the audio out tube with new, tested caps. No help.
I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer with
the best of three available. Still hummed.
Recapped all caps in primary 117 vac to dc supply and audio output
stage. Still hums massively.
I've already got more in time and parts than an S-38 is worth in
this project. Anything obvious I should have thought to look at before
I give up ??

William Mutch wrote:

I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer with
the best of three available. Still hummed.

You might try subbing the AF output tube. Tube testers occasionally
miss some H-K problems.

-Bill

William Mutch wrote:

I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a massive
raw AC hum on the output.
By way of diagnostics I downloaded the manual and schematic from
the BAMA site.
I changed out the 3 section filter electrolytic and the cathode
bypass on the audio out tube with new, tested caps. No help.
I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer with
the best of three available. Still hummed.
Recapped all caps in primary 117 vac to dc supply and audio output
stage. Still hums massively.
I've already got more in time and parts than an S-38 is worth in
this project. Anything obvious I should have thought to look at before
I give up ??

Does the loudness of the hum vary with the volumn control?

In article , says...
William Mutch wrote:

I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a massive
raw AC hum on the output.
By way of diagnostics I downloaded the manual and schematic from
the BAMA site.
I changed out the 3 section filter electrolytic and the cathode
bypass on the audio out tube with new, tested caps. No help.
I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer with
the best of three available. Still hummed.
Recapped all caps in primary 117 vac to dc supply and audio output
stage. Still hums massively.
I've already got more in time and parts than an S-38 is worth in
this project. Anything obvious I should have thought to look at before
I give up ??

Does the loudness of the hum vary with the volumn control?


only slightly. the main hum injection seems to be after the
pot... still there even with audio gain all the way down.

William Mutch wrote:

Does the loudness of the hum vary with the volumn control?

only slightly. the main hum injection seems to be after the
pot... still there even with audio gain all the way down.

Before doing anything else, substitute the audio output tube.
As I recall, that's a hot chassis radio with a 50C5 output tube, so
also check the power line filter caps and the the rubber isolation mounts
that keep the chassis isolated from the case.

I can't seem to get the schematic off of BAMA... it tells me that I don't
have permission. So I am not sure if the audio gain pot tap goes right
to the grid of the 50C5 or not... but it might well.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

William Mutch wrote:
I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a massive
raw AC hum on the output.

Does the hum persist when you turn the audio gain all the way down?
Or does it go away?
---scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

William Mutch wrote:
I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a
massive raw AC hum on the output.
By way of diagnostics I downloaded the manual and schematic from
the BAMA site.
I changed out the 3 section filter electrolytic and the cathode
bypass on the audio out tube with new, tested caps. No help.
I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer
with the best of three available. Still hummed.
Recapped all caps in primary 117 vac to dc supply and audio output
stage. Still hums massively.
I've already got more in time and parts than an S-38 is worth in
this project. Anything obvious I should have thought to look at
before I give up ??

So...did you get it fixed? Enquiring minds want to know....

jak

In article ,
says...
William Mutch wrote:
I was listening to my most modest boatanchor, an S-38E, at the
ktichen table when it suddenly, in mid broadcast, came on with a
massive raw AC hum on the output.
By way of diagnostics I downloaded the manual and schematic from
the BAMA site.
I changed out the 3 section filter electrolytic and the cathode
bypass on the audio out tube with new, tested caps. No help.
I checked the rectifier (35W4) and all other tubes for heater to
cathode shorts on a good Hickok tube tester. Replaced the rectifer
with the best of three available. Still hummed.
Recapped all caps in primary 117 vac to dc supply and audio output
stage. Still hums massively.
I've already got more in time and parts than an S-38 is worth in
this project. Anything obvious I should have thought to look at
before I give up ??

So...did you get it fixed? Enquiring minds want to know....

jak



I've shelved the project for now. Next step would be to re-cap
the audio driver, but at least for now I'm tired of looking at
it...rather put my effort into an NC-183...more to do for full
restoration, but more radio when working properly.