Hallicrafters S-38B audio static when tuning

[pintopete]

I have been messing with an old Hallicrafters S-38B receiver.  With a good antenna it receives quite well except for the lower half of the tuning range and the lower half of the badwidth adjustment.  When tuning in this range you hear only static much like you would hear through a bad or dirty volume control.  The same symptoms occur when adjusting the bandwidth.  I have checked the tuning condenser and it looks pretty good, free from significant dust and although the blades come very close to each other they don't appear to be touching at all.  I sprayed the switches and other contact controls with contact cleaner and it did not change this condition.  I have not yet tested the tubes, but I am not sure that weak tubes could create this condition.  Anyone got any ideas?

[w3jn]

Most likely the plates of the variable capacitor are shorting together.  May not look like it but that's probably what's happening.  Other problem could be a bad converter tube (12SA7 in this radio perhaps) so it quits oscillating at the lower end of the band.

Does it happen on all bands or just one, or what?

Be careful, this radio has one side of its chassis connected to 110VAC so unplug it before you work on it.

[pintopete]

The static noise occurs in the entire lower half (approximately) of the tuning scale and also on the lower half of the bandwidth adjustment.  The static starts somewhat gradually and then gets louder or worse the further you go up to a point that occurs fairly quickly and then you hear nothing but static.  I will likely get a new (NOS) set of tubes.  One of the metal can types (12SA7) has a label on it that reads "weak".  I think that's a clue.  If you have any other ideas I would appreciate it.  If I remember I will post the solution if I find one.  Thanks, Peter.

[KB2WIG]

Your traveling down a long road.... The heartbreak of the S38 series..  But, a man has gotta know his limitations... This is not a bad little radio to work on and gain some skills....   I use my s38 as the bedside radio, as its not to big and the wife doesn't mind

I think you mean bandspread, not bandwidth.  I think the noise is coming from the tuning condenser. Try moving the body of the cap when your in the lower range of tuning.. Maybee u have some loseness of something..

On another note, you may want to put an isolation xfermer in the powersupply. These things are AC/DC and may be a shock hazard..A few things I suggest.. make sure the power cord "hot" side is the lead that is  switched on/off, replace the .25 cap that goes from the switch side to ground. Use a Accross the Line type capacitor , Replace the rubber gromnets if needed, replace the 3part cap off the 35zg7 with something more modern.  ---You can basically rebuild the radio with more modern components--- You have the schematic?? its on the "Nostalgia" site, they have some nice articals r/e the older stuff...  I dont know if its on the BAMA site...      klc

[WU2D]

Bedside OK, Poolside or Bathtub -side Not OK

How do you ground a waterbed?

Mike WU2D

[flintstone mop]

Yup John may have hit the problem right on.
The plates of the variables are bent and scraping against each other. Dirt on the sliding ground connection can be showing itself too.
It doesn't seem like a tube problem. It sounds like a mechanical problem.
Fred

[W1GFH]

Had a similar problem when I was restoring an SX-62 last year: static noise when tuning. Tapping on the top of the chassis near the tuning condensers would also result in static. After much poking around the underchassis with a strong light and a dental mirror, I found a resistor lead that was routed behind a steel partition. The spaghetti insulation on the lead was pushed back so that the circuit was making intermittent contact with the partition (and chassis ground).

[W2JBL]

   over the years i have had quite a number of S38's pass through my collection. the bent bandspread tuning condenser plates is a familiar and annoying sound. they get bent easily when the radio is out of it's cabinet. also as somebody mentioned earlier the rotor contacts get noisy, which is a whole lot easier to fix.

[KB2WIG]

May be of interest.........    http://www.stanwatkins.com/hals38.htm

[KI4OWV]

To find out if the tuning or bandspread cap is shorted unsolder it from the circuit. One side of the cap works to ground. Take a volt/ohm meter with one lead on ground and one lead where it was hooked into the circuit and run the cap from minimum to maximum and watch for shorts on the meter. (Radio unplugged) ANY CONTINUNITY IS MAJOR MALFUNCTION and you have found your noise source. No continunity? No shorts! Check this and let us know - this way no guess - no problem. Then we go to the next step...

[pintopete]

I promised I would post the cure!  It was as many stated.  It was not easy to find or see.  One segment of one blade of the tuning condenser was touching.  You may cringe when I tell you how I found it, but here goes.  In my lack of knowledge about radios, I got the bright idea that dust or other particles between the plates could be the source of the scratchy staticky noise.  So I cut a piece of file folder about 1/2" wide and proceeded to methodically and carefully slide it between the plates of the tuning condenser.  It slid through all effortlessly except one.  Bingo, there was the problem in the front set.  After I found it you could see that it had been touching for a long time.  Tonight a buddy with lots of experience came to visit and we heard from all over including faint but audible transmissions from the Netherlands on my 100' +- longwire antenna.  This little old radio works great and has real nice tone.  Thanks everyone for your help.  Peter    KC2RBX

[w3jn]

That's the way to do it!  No reason to cringe, believe me 

[W2JBL]

Pete- don't know if i mentioned the other day when i saw you, but i just came up with a kickin' audio mod for the S38. i mananged to loop inverse feedback around the audio amp. it cleaned up a ton and now gives really nice smooth audio, better than a lot of much more high end radios. now that i actually have schematic (thank you for those!) i'll draw it up. does your boss know you are blowing off work to play with boatanchors? such behavior leads me to beleive you will make a fine AM'er... also i gotta warn you- if that same boss sits down at your station to operate after you put up a better antenna than he has for HF you may as well give him a room there, cause he ain't gonna leave!

[WQ9E]

This is the isolation transformer I am using for several of my little AC/DC "canoe anchors" : 
http://www.fairradio.com/catalog.php?mode=viewitem&item=1585

Works fine and it also drops the line voltage a bit to compensate for voltage creep that has happened over the years.  I have a much bigger isolation transformer for the bench that I got from Surplus Sales years ago when they attended hamfests AND had low prices.

Rodger WQ9E

[W2JBL]

don't know if you can anchor a canoe with an S38 but i once  (actually more than once) used a dead plate transformer to anchor a Dyer Dhow!