Hi all, curious to find out if anyone else has experienced a carbon type volume control changing its value over time...increasing in value.
Certainly come across many many conventional carbon resistors changing, usually in the upward direction over the years, but never a variable resistor...
It’s the volume control in a Hotpoint bandmaster 198DE radio featuring push button motorised tuning with 8 presets or “memories” if you like. (Mechanically set via adjustable brass bands) Not bad at all for 1939....
The circuit specifies a 1Meg tapped pot part 4785 which was manufactured by Noyes Bros of Sydney
Sadly no value for the tap is specified so I’m not sure what to replace it with....Anyone out there with info on it would be gratefully received!
The one fitted into the unit has the correct number printed on its case but the value is now just under 2Mg
The pot is noisy & crackly & although I can probably move the contact pad slightly across to a less worn area & possibly gain some relief it’ll still be way out of spec
The carbon track area is very wide at about 7mm so maybe it’s a 2W or even greater
Any help on what the tap should be & sourcing a replacement greatly appreciated
It measures from the centre connection 470k- 1.7Mg depending on which end of travel the wiper is at.

Regards Paul

Sadly no value for the tap is specified so I’m not sure what to replace it with

Rule of thumb: The tap is often one third of the resistance between the counter clockwise end and the centre of the carbon track, but many don't bother with the tap if its function is to cut treble or boost bass -- just install a regular pot and bypass/ignore the components associated with the tap.

Thanks GTC, a look at the diagram....probably should have done that first! Shows you’re spot on...appears it’s tied into the music/speech switch. Sometimes a prod in the right direction is all that is needed!

Photo uploaded.

Just to update, thanks for uploading the pic Brad, ‘tis a beautiful looking unit which hopefully I can preserve in a working condition for future generations to enjoy.
Managed to scrounge two NOS IRC Australian made 1Mg pots, both tapped, one at 100k & one at 400k.
Went with the 400k, no idea of the curve original or new.
Change is amazing, no more unusual squeals & speaker traumatising crackles when increasing the volume at the low end of the scale and instead of soft to loud being less than a 10 degree rotation I can actually use most of its range of travel now.
Should’ve looked into this a long time ago, makes me suspicious of the vol controls in a few other of my radios.
Upon a closer inspection the contact pad of the original pot looks like a silver disk that’s removable, the contact face was badly pitted & burnt looking, reversible, but even so the resistance was still double, the carbon track appeared to be bonded to thin paper & in poor condition.
Anyway treated the radio to new & more suited speaker cloth so it’s looking & performing great.
This time stuck at home is great for all the “one of these days” type jobs.....
Take care all.

Sorry for being completely off topic but Duconbuster can you tell me about what your profile pic is going on about?
I’m a bit of a fan of Ducon myself

Hi there BurntOutElectronics,
Its actually a Ducon advertisment from AORSM Vol 14 page 28 (1955)
It certainly grabbed my attention!....... rockets possibly guided by leaky ducons
I guess the whole space race rocket age era was evolving & Ducon (simplex) jumped on the band wagon as well!
I've sent an image to Brad for posting
Regards Paul

Photo uploaded to Post 7.

Yes, just as bad as all the other brands of paper capacitors!!

Yes their paper caps are total garbage but Ducon’s later plastic film caps and even some electrolytics are pretty good in my experience. Although I always change the electrolytic caps I like to test them and I find that later ducon electrolytics stand the test of time quite well, probably because of the addition of PCB’s as a stabiliser

For late 50s and later TVs with Ducon cans, I always attempt reforming if there is no sign of physical leakage. Never disappointed so far.

Ducon made 3 main types of polyester film caps. The first ones were sealed in plastic tubes and were light blue. Later this changed to dark olive green. Then they made them dipped, like the Philips mustard caps, sometimes in maroon, sometimes in light yellow.

All these caps can be trusted. Leave them alone.

Agree with Ian about re-using Ducon electrolytic caps.
My experience is mainly with chassis mount types.
I have used a lot of these in my builds.
They have usually come off TV chassis or are NOS.
They draw current in 10's milliamps for around a minute when voltage is applied.
I start at low voltage, say 100.
The current dies away and becomes less than a milliamp after 10 minutes or so.
I then ramp the voltage up to 300 or so.
They dont heat up and draw a tiny current current there after.
When checked for capacitance they are then within spec.
If the end seal is buckled or mucky or a tit showing they just get hot and die.
Usually that is the small frame single units.
All Ducon WAX paper capacitors leak like sieves, some are just resisitors and some are like little batteries with a small voltage developed across the terminals. True! Some unholy chemical reaction is going on inside those!
Fred.

Yes Fred I pulled a chassis full of wax paper caps (mix of Ducon and UCC) from an AWA C209 TV chassis some years back.

This set had been in hibernation for about 20 years, not powered in that time, and had been in a garage that had flooded at some stage.

I checked the polarity of all those little batteries and soldered them in series.

I got 37 volts! I wish I'd taken a picture.

Paper is acidic and hygroscopic so I guess it's not surprising.

The best example of paper caps behaving like batteries I've ever seen was in a Pye T18, back in the late 60s. I ended up re-capping it in the customer's home, correcting one fault after another!

Now if ever there was a design that could NOT tolerate leaky caps it would be the T18! Look at that vertical circuit and surmise what would happen....

I definitely don’t always replace old Ducon electrolytics, But I only keep them in my own gear and I always visually inspect them before trying to reform them. For instance the big filter Ducons in my Kriesler TV used in the power supply (two 100mfd at 200v) are in perfect shape! I monitored them when I first turned the TV on and I could barely feel any warmth and after a while they went stone cold again! Didn’t need to change one electro in that TV as they were all good ducons!

However if I fix a radio for a friend I always change them anyway just from a liability standpoint but I’ve kept every good electrolytic I’ve pulled out and reformed on my home made cap reformer that’s basically an isolation transformer, full bridge rectifier, and a 600v MOSFET to make a variable voltage from 12v to ~370vdc and I put a 100 micro-amp meter on it with different calibrated shunts I can switch in and out across it which I can switch in and out for different current ranges.

I have no idea what I’ll do with all the recovered electrolytics and I don’t want to throw them away as there’s nothing wrong with them.

There arent many other brands I would trust of that age. Especially the jap-crap-caps of that era. I find that the later UCC red plastic wrapped electrolytics are ok but I’ve never left one in circuit to find out.

Nothing mentioned in this thread so far will ever encourage me to leave old electrolytic or paper condensers in service. A Kriesler 11-20 almost brought the curtains down on my collecting future once due to an overheated power transformer caused by defective filter caps. I always replace them. For a few lousy dollars for replacements, there is no incentive to keep old ones in service and in some cases it is dangerous - just as dangerous as leaving a valve radio running without supervision, and I don't do that anymore either.