SX-42 Audio Static/crackle

[W7SOE]

Hi all,
   My SX-42 produces static crackle when the volume is all the way down.  Here is what I have done to troubleshoot it:

I restuffed the filter cap with modern caps, no change
removed input to 6V6 push-pull amplifier at volume pot, no change
replaced paper caps in audio section no change
removed audio inverter and 6v6's, no change!
replaced line filter caps, no change
the audio transformer CT is connected to the HV before the filter choke, I disconnected the other side, no change

so now the audio transformer primary is disconnected except for the CT (because the 6V6's are pulled).  The secondary is only connected to the speaker.

So, as far as I can tell the primary of the audio transformer is bleeding through to the secondary.  There is ony one cap on the CT, that is a new 30uF cap.  The noise dies down slowly as the caps drain.

Does this make sense?  Anyone seen this? 

Rich

[w3jn]

One - or more - of the IF transformers has a bad silver mica in it.  One way to tell is if it does it only on the bands using the 455 KHz IF (the lower ones) and not the 30-110 MHz bands.

Last one of these I worked on ALL of the silver mica caps were bad, although it didn't manifest itself as crackling - only an inability to obtain a nice sharp peak on the transformers.

This is known as "silver mica disease".

[KM1H]

Did you put a scope on the DC or use another DC source?

If it is transformer breakdown it should show up with a leakage tester.

That transformer will easily get fried due to exccessive leakage in the input coupling caps causing the 6V6's to draw excessive current.

Carl
KM1H

[KM1H]

Even with the audio driver disconnected? I think not John.

When did the SX-42 go to the problem IF's?  Ive read that they didnt come into general use until around 1951.  My early one has the older style and both of my SX-62A's have been OK so far....fingers crossed.

You home for Christmas?

Carl
KM1H

[W7SOE]

I did put a scope on the dc and it looked good, ~1V of sawtooth ripple.

I disconnected both primary wires on the audio transformer and left the center tap.  So there is only the supply voltage on the CT and the speaker on the output and the noise persists.  I have to think that it is the transformer unless there is something on the DC that i am not seeing or understanding.

Thanks for the help.

Rich

[KC4VWU]

I know this is really stretching it, but have you tried another rectifier tube?

Phil

[W7SOE]

I know this is really stretching it, but have you tried another rectifier tube?

Phil

I did!

[W7SOE]

Pulling audio transformer out of parts rig.

(Every SX-42 needs a matching parts rig    )

[KC4VWU]

It's strange. There is hardly anything left in circuit the way you have it now.

That was my next suggestion was to try another outpoot tranny. At least you have another to sub in. I don't have any extra p-p trannys in the treasure chest!

Phil

[W7SOE]

Replacing the audio transformer fixed it.

thanks

Rich

[w3jn]

Even with the audio driver disconnected? I think not John.

When did the SX-42 go to the problem IF's?  Ive read that they didnt come into general use until around 1951.  My early one has the older style and both of my SX-62A's have been OK so far....fingers crossed.

You home for Christmas?

Carl
KM1H

Yeah, I missed the part about the audio being disconnected 

Yep, I'm here until Jan 11, got a good radio fix this afternoon after being off the air for 4 months.

Yeah, the last SX-42 I did I was ready to set it on fire.  EVERY silver mica in every IF xformer was bad.   I've run into a bad one now and again but that was the first one where ALL were bad.  I also did a S-36 a year or so ago and it had the same problem... insufficient IF gain, couldn't get a good peak on the IF trannies -  all the SMs in the IF transformers were leaking.

[WB6NVH]

I just went through that with a rare KAAR KE-23 receiver.  Loud pops and static crashes from the speaker the second the rectifier started conducting, but before the rest of the tubes had warmed up.  Eventually the B+ went away due to a 200 Ohm short to ground.  This was traced to a primary to secondary short in the output transformer, in this case the primary was shorted to the 500 Ohm secondary about 200 DC Ohms up from the grounded end.  I was able to just snip the 500 Ohm secondary set free, because there's also a 4 Ohm secondary for the speaker and I didn't need the 500 for anything. 

I am running into open 5000 Ohm secondaries on Hallicrafters SX-43 (and I think 42's have that also) sets lately - last three I worked on were that way, probably because of electrolysis of the fine wire involved.  Normally not important, since who uses the 5000 Ohm winding anyway, except that Hallicrafters uses the 5000 Ohm secondary as part of the tone control circuit as well.

[N3DRB The Derb]

welcome back to the winter wonderland, Johnny. We got 19 inches up here.

[w3jn]

About a foot out here in McHenry.  Kid sez we got about 2 feet back home in Mt. Airy - he spent a couple hours shoveling after work and barely made a dent in it.

Nice to see some snow though!

[KM1H]

Hey, quit bitchin about a little snow.  My wife is "stuck" in Monterey after going to #2 son's graduation for his 2nd Masters. In some MA Political Science variant of how to be PC in dealing with foreign governments while playing Major in the USAF.

He is heading to Germany again next month to do something he cant talk about and is bring his wife who is Greek. Dont be surprised if they drop in on you

Merry Christmas.

Carl