HMV 501 Dual Ganged Volume Control

[PsychMan]

Hi Folks,

I have a lovely old HMV 501 in my workshop at the moment. I bought it a few years ago very cheaply. The dual ganged volume control had been removed and used as a donor for another unit by the previous owner, and along with the bits the chap gave me were a few pots.

I had hoped the spare dual gang unit would be the "correct" and faulty one from his other unit I could refurbish. Sadly not. While physically it is correct, the pot values are way off so it is not suitable to use.

VR1 controls the radio volume, and VR2 controls the gramophone pickup volume. VR1 should be 50k, and VR2 should be 25k.

On the spare part I have VR1 is 100k, and VR2 is 200 ohms...

So I see limited options here unless anyone else has some ideas.

1) Locate correct part (not hopeful).

2) Have correct part fabricated, could be expensive.

3) Fabricate new part somehow using the one I have

Any advice would be appreciated. Ive attached some pictures showing how it is mounted in the chassis.

I do have a single 50k pot that would fit, but would then need to somehow attach another volume control for the pickup, which I don't feel appropriate for such a nice piece as this.


Cheers
Adam

[Gridiron]

Hi Adam,
PM sent, Mike.

[Herald1360]

The pots look like a common wirewound type, though the resistance values suggest lots of fine wire. The power dissipation rating may be needed for the volume control which does interesting things in the screen grid circuit of the RF amplifier and is at least in part the reaction control. The pickup volume control is conventional and could use any log track pot that fits.

You could perhaps fabricate a suitable dual pot by piggy backing a small modern 25k pot on the back of a suitable wirewound volume/reaction pot. It should be possible to get at the back end of the wirewound pot spindle by removing the paxolin rear cover to allow some judicious drilling and tapping to enable the piggy back pot's spindle to be coupled to the main pot's one.

Could be fun!

(Of course if Gridiron's PM obviates the need for this life would be easier!)

[PsychMan]

Unfortunately Gridiron didn't have the correct spare part, thanks very much for checking though!

Herald1360, Its tempting, but I don't have the proper gear to make a good job of that. I might contact Blore Edwards to see how much a custom wirewound would cost

[jonnybear]

I was just wondering weather you need the PU volume control? unless of course you have record player that comes with it. Other than that you could dismantle your unit and replace just the ww track with suitable doner.I have just seen this one which would probably work OK,https://www.ebay.co.uk/itm/vintage-2...8AAOSw1OBd9maX.
John

[PsychMan]

Just had a reply from Blore Edwards, lets just say having a custom wirewound of the spec I desire is beyond economic sense!

Is the concern with carbon due to current capacity or high voltages involved? Ive always assumed current, but not thought much about it before. Maybe a better specced carbon could be doable.

And yes, this is a table radiogram. I imagine the magnetic pickup is very high output and is attenuated as required using the volume control.

Its a good idea about switching wirewound tracks, and would be doable if the pots concerned had the tags screwed on. Ive not had much success removing the rivets cleanly in the past...

Anyway thought I may as well add a few pictures. Its a hulking beast of a chassis. Quite clever how so much was fitted into a small space, the speaker recessing into the chassis is a new one for me.

[TowerRadio]

Is this a working radio or an unknown quantity that needs a control plus any other amount of attention? I have 20k wire wound of this sort,using the hardware you have you could probably make something that would do.Les.

[PsychMan]

Hi Les,

It will need full restoration, But some positive signs so far.

Here's where I am:

Initial power up via the lamp limiter looked good, so I said sod it and gave the chassis full mains .

-No life from the radio yet, though I only had a 100k pot to try there and a short bit of wire for an antenna, not too good for TRFs... Audio stage is working and I fed an ipod signal into the pickup input. Quite low volume but then I believe the pickup would have delivered a more powerful signal.

-C20 (iirc) got quite hot, so I powered off. Applied the mains a few more times for short durations, and C20 no longer gets warm, perhaps it has reformed. In general most capacitors will need checking, as will resistors, but there are a lot of micas there so maybe not too many cap changes. The pitch block caps are likely suspect.

-The electric motor mechanically seemed ok, just in need of oil. Again applied the mains to it, motor spun up and run fine. A little too fast but someone has ******** about with the governor adjustment so that will want looking at, as will the mechanics that control the motor switch.

-Wood work structurally good, no wood worm, but cabinet needs cleaning and rejuvenating. New speaker cloth needed. Some bits of ply on the motor board and lid have delaminated.

-Mechanics around wave change switch and on off switch want stripping, cleaning and relubricating. Along with general cleaning of muck from the chassis.

- Some of the wiring will want replacing, the insulation has hardened and has become brittle. How much I replace Im not quite sure yet. A lot of it is cloth and varnish covered so will stay ok providing its not moved. Ive seen much worse.

-Knobs present, mostly missing lids and need refinishing as they are wooden.

All in all, for a 1931 radiogram that has been kept in atrocious conditions for 35 years plus, I think its very good and fully restorable.

Its gone on the back burner for now till I source a solution for the pots.

Cheers
Adam