Noise when tuning in/through signal

[Edward Cain]

   I just finished recapping my SX-62 and listening with it for the first time (I recapped it before I ever powered it up).
   I notice when tuning in to a signal or through a signal I can hear noise as if I was adjusting the volume with a noisy audio pot. After tuning the signal, the noise disappears and the audio sounds good. I don't remember ever noticing this type of behavior on any of my other receivers.
   Anyone have any ideas what may be causing it?

Thanks,
Ed

[Edward Cain]

I forgot to mention that the audio pot isn't noisy. I can't reproduce the noise by adjusting volume.

[Pete, WA2CWA]

Personally, I never replace any components until I know what problems I'm trying to fix.

Anyway, noise, in AM mode, FM mode, all modes? Did you put a scope on it at any points in the circuitry?

[KA8WTK]

Maybe a dumb question, but did you realine after replacing the caps?

[Edward Cain]

Hi Pete and Bill,
   Bill, I haven't realigned it yet.
   Pete, after reading your questions, I've found that it only occurs in FM. So far the only FM I've received is FM broadcast. I haven't put a scope on it cause I don't know where or what I'm looking for other than what my ears are hearing.
   I recapped it before powering up cause I didn't want to be the next guy to lose his audio xformer due to bad caps.

Thanks,
Ed

[WQ9E]

Ed,

Perhaps the noise is coming from a dirty wiper on the tuning capacitor.  I can't remember how the sections are used on the SX-62 capacitor and whether there is a separate section on the gangs used for the highest range.

There is no AFC circuit in the 62 and the usual discriminator detector defects would also show up after tuning.  But if it is only on FM and not because of the tuning capacitor then I suggest replacing the mica caps in the discriminator can.  I have a SX-42, 3 members of the SX-62 family, and a S-36a and all 5 had leaky mica caps in the discriminator can.

[Pete, WA2CWA]

Check the FM IF alignment along with possible FM discriminator component issues.

[Edward Cain]

Thanks guys. I'll check the discriminator and do the alignment and see what happens.

Ed

[w3jn]

Could be any number of things, but one problem I found on a SX-42 was EVERY silver mica capacitor in the IF cans was bad.  You could have one that's intermittently leaking and causing trouble.

Doing an alignment will tell the story.  The alignment should reveal nice, relatively sharp peaks when you're adjusting each xformer.  If you go a couple of turns on the xformer without a definite peak, the caps in the xformer have gone bad (or you have an intermittent coil).

[KM1H]

Even experienced pros do not shotgun a recap, especially the 44 in a SX-62. Test after about every 3 replacements to be sure you didnt screw something up.  Of course do the Audio and PS first; its pretty hard to mess those up. Those radios also usually need 12-16 resistors replaced.

What is the FM audio like with no signal or a very weak one?

John, early run SX-42's have not had the SMD in my experience.

Carl
KM1H

[w3jn]

Carl, I've gone thru probably 10 or more SX-42s and 62s, and only one had the bad silver micas in the IFs.  I have heard of a couple other cases though.

However I had the same issue with a '43 vintage SX-36.... no peaks in the IFs.

In any event nobody is suggesting this be done without first verifying it's the problem.  It's a big PITA and unless you're careful, you can damage the IF xformers in the process.  I made the mistake of trying to unslobber the micas.  Made a mess and damaged the litz wire... subsequent caps were just clipped out and the replacements soldered to the old pigtail remnants.

[flintstone mop]

IF the scratchy sound is WHILE tuning THEN it could be the variable capacitor that needs to be looked at. The plates might be rubbing together at certain points of rotation or the stator contacts need cleaning.
I assume that this RX uses a variable cap to change freqs and shift the LO during tuning. Not familiar with the circuitry.

Fred

[KM1H]

Ive done around the same number John and havent had a noisy IF yet. I also wonder if Halli went with the deposited silver substrates around 51-52 when it pretty much became an industry standard? They bought from the same suppliers, unlike National, Hammarlund, etc.

Carl

[w3jn]

Symptom wasn't noise crashes in the IF on mine, Carl, there was just no gain in the IF section and the xformers wouldn't peak.  They were discrete mica caps, not those deposited jobs you see in consumer BCB radios.

[WQ9E]

JN,

I found that problem in one of my SX-62 receivers with exactly the symptoms you describe but that was the only regular IF mica I replaced.  But the FM discriminator problem with mica caps was very different in my experience.

I am not sure why the problem seems so severe with the discriminator transformer.  It may be that it is far more sensitive to the drop in Q caused by slight leakage while this same level of leakage in the 10.7 Mhz. IF transformers may slightly drop the gain and broaden the response.  If this effect is minimal with slight leakage it wouldn't be obvious without making measurements.  In any case, every Halli I have in the 42/62 family (including the distant cousin S-36A with 5.25 Mhz. IF) required replacement of leaky caps in the discriminator to achieve reasonable performance.  The S-36A is actually the best performer I have-after 10 minutes warm up there is no discernible drift in the HFO or discriminator tuning on the FM BCB.

[Edward Cain]

Thanks for all the suggestions guys. And I take to heart the advice of a few caps at a time.

Carl asked what the audio was like with no/weak signal. With no signal I have just barely audible background noise. With weak signals I just have weak audio and some background noise but don't notice the same symptom that prompted my question.

I looked at the manual for Hallicrafters' alignment instructions and I'm a bit confused. For the IF alignment it tells me to set my signal generator to approximately 455 kHz and proceed to adjust slugs for maximum output. I assume they're talking about audio output. Am I correct? They make no mention of a modulated 455 kHz signal.

   In any event, I haven't yet aligned it cause I've found that I don't seem to have output from the bfo. I'd like to solve that problem first, since bfo signal is part of the alignment procedure.
   I'll start another topic for that problem.

Thanks,
Ed

[k4kyv]

IF the scratchy sound is WHILE tuning THEN it could be the variable capacitor that needs to be looked at. The plates might be rubbing together at certain points of rotation or the stator contacts need cleaning.

I would say it's the wiper contacts on the tuning capacitor.  I had the same  thing with my NC-173 years ago.  It was only noticeable with strong carriers in AM mode.  It didn't show up in CW mode with the BFO on.

I could spray it with contact cleaner and that would clear up the problem for short while, but it would soon start back again.  Try a small dab of WD-40.  I have found that to work better contact cleaner. Another substance that works is called Electro-clean. It is a degreaser, and the can says it is for cleaning motor brushes and other electrical equipment.  If the contacts are really dirty, maybe try that, then lubricate with WD-40.

I had a noisy AF gain pot on my 75A-4.  Contact cleaner would stop the problem for about 3 days.  Later on, I tried WD-40 and that fixed it for good; the problem never came back again after several years of use.

[w3jn]

THe proper way to clean the contacts on a tuning capacitor is to wet some bond paper with DeOxit, then pry the wiper up VERY gently just enough to slip the paper between the contact and the capacitor rotor.  Move the cap thru its motion as much as you can.

This can also resolve drift problems.

The SX-62's FM IF isn't 455 KHz, it's 10.7 as I recall.  The 455 KHz IF is for the lower bands, and non-FM.  You'll see that the 455 and 10.7 IF transformers are kind of in series; the 10.7 IF has essentially no effect on the 455 IF.  Still, align the 10.7 IF first, then go after the 455.

[w1vtp]

Personally, I never replace any components until I know what problems I'm trying to fix.

Anyway, noise, in AM mode, FM mode, all modes? Did you put a scope on it at any points in the circuitry?

Yep!  Muddies up the water, I think.  I think a good visual for swollen lytics and a thorough check with a Simpson 260 (good analog multimeter)

That said, the guys here will be very helpful, I'm sure

GL, Al

[flintstone mop]

IF the scratchy sound is WHILE tuning THEN it could be the variable capacitor that needs to be looked at. The plates might be rubbing together at certain points of rotation or the stator contacts need cleaning.

I would say it's the wiper contacts on the tuning capacitor.  I had the same  thing with my NC-173 years ago.  It was only noticeable with strong carriers in AM mode.  It didn't show up in CW mode with the BFO on.

I could spray it with contact cleaner and that would clear up the problem for short while, but it would soon start back again.  Try a small dab of WD-40.  I have found that to work better contact cleaner. Another substance that works is called Electro-clean. It is a degreaser, and the can says it is for cleaning motor brushes and other electrical equipment.  If the contacts are really dirty, maybe try that, then lubricate with WD-40.

I had a noisy AF gain pot on my 75A-4.  Contact cleaner would stop the problem for about 3 days.  Later on, I tried WD-40 and that fixed it for good; the problem never came back again after several years of use.

ME 'N' Don seem to be on the same channel for the problem of scratchy whilst tuning. How does the noisy I.F. enter into making noise whilst tuning said tuning condenser??
Fred

[Edward Cain]

Thanks Don and Fred. I'll use WD-40 on the cap wipers and see what happens as soon as I figure out my bfo problem or give up and put it back together.

[W2WDX]

While your experience may tell (or convince) you otherwise ...

WD-40, a rust remover & lubricant, will break down the carbon deposition on carbon comp pots and eventually the pots will fail .. prematurely. On wirewounds it will break down the adhesives (or laquers, shellacs) used to bond the wire and it will unravel. Deoxit will do the same, since it is intended for metal connectors & contacts only and not for controls. Both leave deposits that will attract dust and also gum up eventually, causing dropouts. In the audio trade using these on pots and faders is grounds for dismissal in a recording studio with million dollar consoles! :-)

There are sprays designed specifically for this application that cost the same as WD-40 ... er ... well ... at least Deoxit. Caig R5 followed with an application of Caig MCL (now called Deoxit Fader series) are some that comes to mind (there are many others). There is no petroleum naptha, which is the component in WD-40 & Deoxit that can cause problems. This problem can be even worse if you are using plastic film type pots which will be outright damaged by petroleum products. Lubrication of a pot is OK (even desirable for tactile reasons) but the correct lubricant is essential. The Deoxit Fader series products are one example of a proper lubricant for pots.

Additionally, WD-40 once "dry" is slightly conductive (more so when wet). And to a lesser degree so is Deoxit. Will it change the value of your pot? Yup, albeit only slightly.

Now on contacts or metal surfaces WD-40 and esspecially Deoxit are just fine, albeit messy if just sprayed willy-nilly (what does that mean anyway). On things like variacs, rubbing the contact surfaces with WD-40 on a rag works really well. In fact, some formulations of regular Deoxit can be used in these various applications if something is used to flush it away (like R5), after it is used to remove oxidation and dirt. But on pots ... no sir ...  I just don't like it.

Now ... this is not audiophoolery. It's not voodoo magic. It's chemistry. Besides WD-40 is messy as hell. It's easier to remove nicotine (Windex on a rag) from old boatanchors than old dried up WD-40 rubbery gunk sludge.

Incidentally, Deoxit has gotten a bad rap because people generally do not use in its intended application, and instead use it as a general cleaner. It is not. It is a liquid designed to remove galvanic corrosion and leave a coating that bonds to the surface of metal molecules to help slow this galvanic action, by attaching itself to these free ions on the metallic molecules at the surface only. The carriers also happen to remove dirt and other free materials. But it is not a true cleaner in the pure sense. This action is also time limited because these galvanic reactions, particularly with high current flow, will eventually overcome this coating. So for Deoxit to be effective it must be used with a certain regularity.

You may hear, "Ahhh come on now, I have used WD-40 for 30 years with no problem". However, what you never hear is the pots they have replaced. Nor do these folks know why they really had to do so after using WD-40 over time, and assume it's just aging components. Beware of common "wisdom". Sometimes it doesn't account for chemistry.

 

[flintstone mop]

Everything is going to hell here reading this thread.
These were all of magic tricks!!!!!!!!!!   

Jus keeding

Phred

[W2WDX]

 

[Edward Cain]

Thanks to all who responded. Don and Fred, the WD-40 seems to have done the trick. Sounds good now.

Ed