Vintage Radio Volume Control Problem

[aglawrence]

Hi guys,

I have a Radiomobile 1085 in my car, and the volume control does not work as it should.

Most of the adjustment takes place at the lower end of the volume pot's travel, followed by a significant 'dead patch' where no noticeable increase in volume occurs, followed by further adjustment towards the upper end of the pot's travel.

Is this something which can be fixed, or should the volume control be replaced? Is it likely that the part would be available?

[boxdoctor]

Hi and welcome to the forum.
I would suggest that you look to see if the volume pot. is original. If it looks to have been replaced, then possibly someone has fitted a linear pot instead if the required logrithmic type. The symptoms you describe are those which result after wrongly fitting a linear pot as the volume control. Tony.

[aglawrence]

Thanks Tony,

It does look original. Do you think I am likely to be able to obtain a replacement part?

If I Upload the circuit diagram, might anyone be able to advise as to what exactly I need?

[Herald1360]

A good photo would be more help- sourcing an electrically similar part is easy enough, but if it's special mechanically (like part of a dual concentric setup, perhaps) then it's a different ball game......

[aglawrence]

I'll try and post a photo tomorrow.

Earlier this evening I pulled the Radiomobile and swapped out the volume pot with one from a non-functional Radiomobile of the same model. The same fault still exists though. Any ideas??

['LIVEWIRE?']

The volume pot. in the Radiomobile 1085 (and all other RDM/Motorola sets in the 1070/1080/1085/1095 ranges) was a dual contorl, the Volume part being 220k Log(at the rear), and a 100k Lin Tone control at the front. AS you have already swapped pots. with a known good one, the fault is most likely due to external components. Does the tone control (basically a 'top cut') function correctly? Is the cscreened lead between the v/c and the PCB OK and correctly connected? (on a 1085X to hand, the blue inner core is connected to the detector diode via, I think, a 100nf cap. at the PCB and to the 'hot' side of the v/c, whilst the red inner connects to the v/c slider, and the 1st. AF stage) BTW The service data is available from VRSD via the link 'up top' The control, BTW, will not be available as it was a specialist part made for Radiomobile back in the 1970s, and is long obsolete!

[Herald1360]

Hmm. If it does need replacement, at least you have a spare S/H part. Otherwise, I suspect that forgoing the tone control (no great loss) and fitting a standard part would be the only way to go.

[aglawrence]

Thansk Livewire, very helpful post. The tone control does function correctly. I will report back re the v/c to PCB lead - I visually inspected it yesterday but didn't test.

['LIVEWIRE?']

Originally Posted by Herald1360

Hmm. If it does need replacement, at least you have a spare S/H part. Otherwise, I suspect that forgoing the tone control (no great loss) and fitting a standard part would be the only way to go.

Possibly, Chris, but, IMHO, it would be almost impossible to find a replacement pot. having (a) the correct length & diameter thread - approx. 0.75" x 0.4", without measuring it!), and (b) the correct diameter and length shaft to accept the original volume knob. Again, without measuring it, the shaft protrudes ca. 0.6"" out of the threaded bush, and is smaller in diameter than the standard 0.25".