Do you have a scope? If so do some signal tracing. \
The only schematic I can find shows the audio output stage to be a
push-pull pentode stage driven via a grid transformer from a 6R7, a dual
diode, triode. The diodes are in parallel and are used as the detector.
The triode is the first audio, it drives the output amp via a
transformer. The audio gain is in the grid of the 6R7. Since the noise
does not vary with the gain control it is coming from the audio stage.
The headphone jack is arranged so that the phones are connected to the
output of the 6R7 through the driver transformer, that is, the grid
drive of the output stage. See if you are getting the noise at the jack.
This is the output of the driver transformer. If not the noise is coming
from the output stage or power supply.
I am not sure from your post if you put 6V6 tubes in the amplifier.
That is what should be in there. A 5881 is a variation of a 6L6 designed
to take higher screen voltage, While they might work in place of 6V6
tubes they might also draw too much current. If you just changed tubes
try another set, one of the replacements might be bad. You can run the
output amp with only one tube briefly to see if the popping is
associated with one of the tubes. Since the 5881 tubes were in there it
probably safe to put them back in to see if they are noise free.
Did you replace the filter caps? In any case I would check them.
Also R-29, the dropping resistor for the driver transformer which is
also part of the B+ filter. There is really not a lot there. Good luck,
its a beautiful receiver.
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On 10/31/2024 7:18 PM, dtssr via groups.io wrote:
G'day y'all.? So the SX-16 was wrapped up, just about, with the bottom
cover put back in place and a final IF/RF alignment done.? Spent a good
part of last evening SWLing and tuning in 20M and 40M SSB rag-chewing.
Next day I fire it up and there is an awful loud cacophony of snaps,
crackles and pops emanating from the speaker.? The level of that csp
does not change with AF or RF gain control, just as loud with both
turned to minimum.
Other than the csp noise all functions operate as expected, even the top
two bands pull in CB'rs (oh joy!) and 6M SSB on just the test bench long
wire.
Yes, a complete electro/wax/mica cap replacement had been done, any
resistor over or approaching 20% out of spec replaced, etc, etc.
Monitoring the B+ it seems a steady 310v DC after a short warm up.
(Variac is at 115vac.)
I swapped the 5881's (that it came with in place of the 6V6G's), swapped
the 6R7 (as it's the first audio amp), no change.
With the 2nd IF amp 6K7 removed the noise is still there confirming it's
after that (or supply related).
I even pulled the cover off of T4 to look at those top mounted trimmers,
very clean and looked fine to me. (Even tho it sort of resembles that
kind of noise, I usually don't suspect an SMD issue with such trimmers
anyways.)
Went after a couple of the remaining ceramic picofarad caps around the
AVC/1st audio section, no change to noise. (The SX-25 developed same
issue after a year's use and that was a mica I had left in that same area.)
Voltage checks are within those listed on the SX-16 chart with the
exception of the screen voltage on the 5881's, pin 4 is at 290v where
the chart shows 275v for a pair of 6V6's.? But the 5881's have a higher
screen voltage rating anyway.
The amount of noise seems to lessen after the receiver is left on awhile
but never lessens in volume or goes away.
I dread it being one of the audio transformers but could it be?
Searched this forum and I found Bill's SX-17 posting with apparently
same issue but could not find any follow up to it.? So, Bill, ever
resolve? (oh please, oh please have a simple fix!)
I'm all ears to any suggestions.
--
Dan
K1DTS
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
