Looking at volume pot replacement, I realise that the Philips circuit uses a potentiometer type that has a 4th terminal - this terminal seems to carry the inverse of the resistance on the ouput terminals.

From what I have found online, it seems that this is what is known as a "tapped potentiometer". The extra tap feeds a circuit that maybe acts as some kind of "loudness" control that changes the tone bias as volume is increased. Maybe has a function in mixing the inputs from audio stage and Pick-Up input?

Anway, that type of pot is not available locally and doesn't appear in any catalogs I've looked at so far.

Question is - can I work around that circuit somehow with a standard single-gang pot, or maybe try to improvise something with a double-gang potl?

Or does somebody know where a suitable 500k tapped potentiometer can be obtained?

Maven

There is a discussion on tapped pots here:

http://vintage-radio.com.au/default.asp?f=2&th=131#767.

Thanks for that pointer. It seems the only recourse is to modify a new, standard pot to create the extra tap.

That raises the question - how to determine where to connect that new tap to the resistance arc of the new pot. I'm not sure how critical that is. At worst, I could open up the old troublesome pot and just approximate that by eye.

Or how would I measure the value to try to give the extra tap - I don't think it is "centre", and the resistance scale is logarithmic?

Maven

Sometimes the circuit diagram for the radio will have the value at the tap.
I guess if the values can't be read with a meter at the tabs, you will need to dismantle the pot and measure the values directly from the resistive strip. If this is cooked, hopefully someone will have another radio of this type to look at.

Sometimes the tapping value is on the pot, or still measurable it can be 40K on a 1Meg pot, does vary.

Mark Oppat in the USA does rebuild pots.

It's a Philips what?

Marc

The radio is Radioplayer 123. The original part number for the pot is CZ.029.129, 0.5megohm tapped, but no value given for the tap itself.

The part I am replacing seems not to be original - the shaft is about 1/4" shorter than the others and construction is brass instead of steel.

Now that I understand what is going on, I think I have worked out that the tap is at about the 40k mark on a 500k range, on the end of the pot that goes straight to the speaker (via resistors) bypassing the speaker transformer.

One of the posts referenced through the earlier thread here http://vintage-radio.com.au/default.asp?f=2&th=131#767 describes a method for adding a tap by drawing a trace, using a trace repair pen, from the right point on the resistance arc, then leading it out the back of the pot with a new wire. I don't think that is beyond me, and common pots are only a couple of dollars, so I might as well have a go at that.

Maven

I've successfully added a 40k tap to a standard 500k potentiometer.

Sorry about the picture being a bit fuzzy, but I have added captions to clarify.

The pot was easy to disassemble. The case is only held on by 4 bent tabs. The sweep disc is held to the shaft purely by friction, as it is pushed through a slot in the plastic sweep disk mount. It could be levered off carefully without damage.

The point to make a 40k tap connection is easily determined using a multimeter on the 200k resistance scale. I marked it with a pen mark on the phenolic base.

A "trace pen" dispenses a trickle of liquid polymer containing metallic power. It sets to a polymer film that is fully conductive. Since the pen needs to be pumped as you draw, it is not easy to make a precise line. However, once the trace has set to a dry film, it can be trimmed back by careful scraping with a sharp point such as a scalpel or sharp knife. The best thing is that it is fully reversible - it took me several goes to get it right.

The new drawn trace needs to have about 1mm clearance from the edge of the pot, to avoid any chance of shorting to the case (which is grounded). About 1mm clearance on both sides of the trace is advisable and doable.

The polymer trace cannot be soldered, so you have to create a new metal terminal. Because of tight clearances within the pot case, it's best to avoid any solder joint inside the case (I tried it and it was too troublesome). My solution was to bore a 1mm hole in the base plate and push a piece of 1mm copper wire into it, pre-bent at 90degrees, then super-glued in place from the outer side. The wire inside the pot is insulated with spaghetti tube, as it needs to pass through a short slot cut in the pot case. That slot also stabilises the terminal wire. The internal electrical connection is made by painting the polymer trace fluid onto the wire. Even when fully dry, the polymer retains a small amount of resilience which should help maintain the connection if the terminal wire is bumped from outside the pot case.

Here is the photo of the reassembled pot with the new tap connection poking out the back.



Replacing this tapped pot seems to have finally solved the volume fade problem that has been bugging me for months. However, along the way I have replaced so many components that I can't rule out several components contributing to the problem.

Anyway, I think it is good news that tapped pots can be replaced even when they are no longer sold.

Maven

Very interesting and inventive, I just hope I never need to do it!

Yes, it's probably not something I'd undertake, but I'm looking forward to seeing the photos for future reference.

Postscript:
Once I was sure the new pot was working correctly in the radio, I opened up the old one to see what was faulty.

Turns out there was nothing wrong with the sweep contacts or the resistance surface. In fact they look like better materials and better made than the new cheapo one.

The problem seems to have been that the terminals of the old pot are fixed to the base plate with small copper rivets. Two of those rivets had worked a little loose, possibly from work being done on the tags, or small movements in the pot body during normal use. They must have been moving slightly as the radio armed up, enough to introduce slightly more resistance in the volume control circuit.

It was very easy to make the rivets tight again by tapping the open end of the rivet with a suitable blunt punch.

Now I have a new tapped pot in the radio and the old one as a fully working spare.

Maven