Rare radio restringing ? ?

[MathWizard]

(edit: the only problem now is the string)

I have a 1980/90/s era multiband radio, more of an amateur or low cost MB radio. When I turn it on, there's no volume or anything, but it blasts static full volume if I adjust the volume knob, then more or less goes away again, more or less silence.

It's 1 of these kind. below. I have MG chemicals contact cleaner, but IDK if it's old or useless, but it leaves a lot of waxy residue, so I don;t trust that at all. Is there any real way to clean those type of pots or just get a new 1?

[wasedadoc]

Been quite a while since I looked at one of those pots but ISTR that they were reasonably well sealed against ingress of dust etc.

[xrunner]

I have a 1980/90/s era multiband radio, more of an amateur or low cost MB radio. When I turn it on, there's no volume or anything, but it blasts static full volume if I adjust the volume knob, then more or less goes away again, more or less silence.

If the radio was set to a frequency where no stations were transmitting (static) then it could be a bad pot, so try cleaning it with an electronics cleaning spray. If it's wavering on it's own between static and no static, it could be some other sort of issue like AGC issues ...

[mikerj]

It's 1 of these kind. below. I have MG chemicals contact cleaner, but IDK if it's old or useless, but it leaves a lot of waxy residue, so I don;t trust that at all. Is there any real way to clean those type of pots or just get a new 1?

Many contact cleaners include a lubricant that remains after the cleaning solvent has evaporated.

[Whitefoot]

The problem could be a bad coupling capacitor letting DC onto the pot wiper.

I use MG Contact Cleaner with Silicone on pots and it works fine if dirt is the problem.

[Kim Christensen]

I've always found that cleaning pots with spray cleaners is usually a temporary fix. I would just replace the pot if you can.

[MathWizard]

Ok the pot might still have a little problem, but the real problem was.......some years ago, I cracked 1 of the PCB's.  So I just had to bridge a few traces. I don't remember that, or why I didn't fix it, it only took 5 minutes.

But now the real problem is, I've barely worked on radios before, and when I took it apart, I unscrewed the posts that the string bobbins were on. I never even thought of that, and now it's fully un-stringed. And IDK how I'm going to figure out the right way to string that.

If I knew more about motor circuits, I could probably motorize the the tuner, better that restringing it.

The tuner also has a fine control on the same shaft, I have no idea how the string was on the shaft.

The problem could be a bad coupling capacitor letting DC onto the pot wiper.

I use MG Contact Cleaner with Silicone on pots and it works fine if dirt is the problem.

I almost went down a capacitor rabbit hole after fixing the cracked PCB. There was still no power to the PCB I fixed. And tracing back the wire, to what looked like it could be a split rail, +/-3V. And one side, that sends power to the receiver PCB I cracked, was that cap and it only had 0.3V on it. But no the polarity of the cap didn't match that theory, and then that cap lead back to a switch, for loud speaker/PA or Radio, and it was on PA.

But I almost pulled that cap, thinking I found severely low voltage.

[MathWizard]

Any advice on restringing the dial ? I can't find any manual for it with this procedure, it was basically a rare Radio Shack model, sold under names like Venturer 2959-2, and SW-100, and not meant to be serviced by the user.

I haven't untied the knot holding the other end in place, and there's no adjustable tension-er, beside the spring, so maybe I should untie the knot ?? But for now, at least I can get an idea of how many turns/loops.

There's 1 main wheel, the tuning knob, and 2 bobbins. The bobbins, have a larger upper, and smaller lower spool, with a little space cut out of the disc that divides (upper/lower), for the string to pass. I can't say for sure it even used upper/lower. I've mainly tried just using the top 1.

I don't have caliper's, so I can't really measure the diameters of the spools, and try to calculate some possible winding combo's.

Here's the radio, expect mine was sold as a Venturer 2959-2 by Radio Shack
https://radiojayallen.com/radio-shack-sw-100-am-fm-sw-cb-tv-air-weather-radio/

[Kim Christensen]

Best to post a clear picture of the "main wheel, the tuning knob, and 2 bobbins"...

[coppercone2]

I kind of wonder how serviceable those sealed pots are if you take a jewelers saw and cut them in half carefully and build some kind of bracket with alignment pins to put it back together.

[Kim Christensen]

Just search Google Images for "radio tuning string" and find one that looks like it has a similar config of pulleys and wheels.
Note that each end of the string usually ends up terminating on the tuning capacitor "pulley". Think about how it will work: When you tune the tuning knob clockwise, the needle on the dial is going to move right (up in frequency), and the tuning capacitor is going to rotate towards less capacitance (higher frequencies).

[Gyro]

I kind of wonder how serviceable those sealed pots are if you take a jewelers saw and cut them in half carefully and build some kind of bracket with alignment pins to put it back together.

It's not a sealed pot, there's a screw that secures the thumbwheel.

[coppercone2]

in that case, grease. I just did a carbon pot that was the cal offset for a HP meter with deoxit grease (260 DL) and it works wonderful, before cleaning and greasing it was jumping around like crazy when I tried to get a zero. As always ultrasonic clean with simple green, then bristle brush to get any crud out, rinse, scrub gently the carbon with a q-tip on alcohol (it abbrades the carbon but I think it polishes it a little bit, but the thing to know is that no matter how many times you rub it, the q-tip will be dirty after, a quick dip with clean water in ultrasonic, followed by drying by compressed air, followed by alcohol spray and more compressed air, and grease smeared on the carbon and carefully applied to the contacts. And then careful resoldering making sure that flux does not ingress the potentiometer.

I think the problem the user had in this thread is not that the pot was not cleaned, but that the pot did not get sufficient grease after cleaning. The penetrating lubricants might just not have enough staying power to keep that lubed. I got fader lube recently to try on more potentiometers I need to restore.

If you take a potentiometer apart that is old you will see crud in there, like tar almost. And what you want on the potentiometer surface is like bacon grease that you let solidify in the pan.

and cross sections of cut open sealed potentiometers are welcome BTW, they are common and a serious cost that comes with restoring old equipment, making them worthwhile to cut apart and service if possible. I had replacement potentiometer cost dwarf equipment cost before,

I think the.. lack of success people have with sprays is because sometimes you have too much oxidized/caked/resinified grease left inside of the potentiometer that interferes with the wiper traverse.

And FYI while not recommended, while I was lazy, I did dip entire units that have semi-sealed potentiometers into a ultrasonic bath, and after baking and spray lubricant (attempting to have it flow down into it along the shaft), they worked fine, but its not as nice as a proper regrease. I am guessing the crud settled to the void in the potentiometer where it did not cause a problem. But this was the potentiometers that you could take apart, I never tried it with a fully plastic sealed one.

[MathWizard]

Yeah looked up some old radios, but I don;t see anything like it yet. So I just went ahead and detached the spring, it was just glued in place. I also desoldered the RX PCB, so no more worries about breaking it all up.

The good news is, the string was glued to the dial pointer, with red glue. So I can see when my winding attempts match. I'm not sure if they use the upper and lower halves of the bobbins, at either end of the dial.

They have a gap/channel cut in the disc, between upper and lower halves of the bobbins. And it's not very smooth, so I can't see the string lasting if it's wearing on those edges. But if's it tight enough, and wound right, I think the string is not supposed to slide, so it would be allowable to use that.

But will I over stretch the old string, or break it. Or end up it with not covering the whole range, I'll see.

I'll see if I can get a clear picture with my webcam.

I would really like to motorize it tho, with an ardiuno type thing. I have printer motors with graduated/marked see through discs, for optical sensors. But I'd need to have a solid way of mounting the motor. I could still use the string, to turn the main wheel. Maybe that would make a bunch of electrical noise tho too.