DAC90A with a faulty volume control – no surprise!

[John G8MWF]

I have a DAC90A with a faulty volume control – no surprise!

Replacements are getting much harder to find and more expensive to replace but I have one in my stock which only has a ¾ inch shaft, that is too short, and a 1¼ inch shaft is needed, – see enclosed picture 20190731_192809

One way of fixing this is to use a shaft coupler but most of these are typically ¾ inch long and will be very tight and not quite work with the lengths needed.

Do you know of another way to overcome this problem or do you have, say, a ½ inch long ¼ inch shaft coupler and would this work without wobbling?

Cheers

John

[Herald1360]

Drill and tap pot shaft and extender M3 or similar then join with some studding and threadlock? Might need some careful shimming to get flat lined up.

['LIVEWIRE?']

Blore Edwards stock a wide range of pots, including those with metal shafts for those wishing to use them. I have just ordered a 470k Log Pot. with D.P. switch for a DAC90A from them. Admittedly total cost including postage & VAT is about £13, but if other items are ordered at the same time, the postage per item works out less. They currently have a sale in progress, with many items reduced (Usual disclaimer - I have no connection with the firm other than as a customer)

[John G8MWF]

I didn't know that you could still get new ones!

So the Blore Edwards solution is going to be the answer - just a shame they have such high postage rates for such a small item.

Thanks for your help.

John

[David G4EBT]

You can get 470K switched log pots from Bowood Electronics, but they'll be the modern style with a plastic control shaft and won't have a circular metal body as did the original pot. They cost £2.45 each, and £2.85 P&P. (No VAT is payable).

https://www.bowood-electronics.co.uk...switched-p-652

Bowood is a well established 'man and his wife firm' and sell a useful range of other items of interest, including 1 Meg switched log pots which are also not so easy to find nowadays. Back in the 1980s, Wilf used to work for Bardwells in Sheffield, who some forum members will recall.

Hope that helps.

[crackle]

You can buy small bore brass and aluminium tube on ebay, this can be used to join an extension 1/4 inch shaft. Probably 8mm would be about right, but best to ask the internal diameter. Drill through the tube and shaft and pin.
Mike

[G6Tanuki]

What about:

https://www.cricklewoodelectronics.com/MLG470K.html

"Rotary Pot with DPST Mains Switch 470K Log"

£3.50 +VAT.

[Half a Mullard]

I tried one of the Cricklewood pots. They are quite small, the shaft is round (plastic) not the 'D' profile. Measures 5mm in diameter. The bush is smaller too. They should work OK but would need to be used with screw fixing knobs rather than push on types. Due to the smaller diameter shaft the control knob would be slightly eccentric if that makes sense. Good value though.

[crackle]

I have never heard of a 5mm shaft, are you sure it was not 6mm. The DAC90A knobs have a locking screw which you access through a hole in the base of the cabinet. So I would file a small flat on the shaft, in the correct place, and fit the knob.

Mike

[David G4EBT]

Ordinarily, modern pot shafts are 6mm, older ones are 6.4mm (1/4").

[Half a Mullard]

Apologies, the shaft is slightly tapered at the top which threw me, yes it would be 6mm

Some DAC 90's do have push on knobs though, the gold tuning scale ones which have the knobs with the clear plastic skirt.

[John G8MWF]

Thank you all for your help. I finally managed to get new replacements from Blore Edwards and fitted one but it did NOT cure the problem!!!!

With the volume turned to minimum I still get a reasonably loud signal from the speaker.

The radio has had all of the capacitors replaced as part of the restoration.

So I read the other messages about replacing the bypass IF capacitor, which I believe to be C13 100pf, and replaced it with exactly the same results.
And before you ask - yes I have changed the UBC41 and checked the resistors.

There is 0 ohms between the wiper and feed so I cannot figure out how audio is bleeding through - yet.

The original pot also displayed exactly the same symptoms so I don't think that it is a POT shielding issue and the earthing strap is in place.

Any practical ideas on this one?

[HamishBoxer]

There has to be something high resistance here,if not a res than a joint? Check R 6 and R9. C16

[Graham G3ZVT]

Originally Posted by John G8MWF

Thank you all for your help. I finally managed to get new replacements from Blore Edwards and fitted one but it did NOT cure the problem!!!!

With the volume turned to minimum I still get a reasonably loud signal from the speaker.

That's not necessarily a symptom of a faulty pot. I would first suspect that the cathode resistor of V2 (UF41) has gone high. R6, 220ohm on the Bush sheet.

[HamishBoxer]

As I thought,see above.

[ms660]

The common cathode resistor is 330 Ohms according to what I'm looking at (Trader sheet)

Lawrence.

[David G4EBT]

It's also shown as 330 Ohms on the Bush data, where it's referred to as 'R8' (rather than 'R6' shown on Trader Sheet 1161).

[ms660]

Originally Posted by David G4EBT

It's also shown as 330 Ohms on the Bush data, where it's referred to as 'R8' (rather than 'R6' shown on Trader Sheet 1161).

I posted that for clarity in response to the value given in Post#14.

In my original post I did mention R8 from the manufactures data (which I have a copy of au fait) but decided to edit and delete that bit so's to avoid any confusion.

Hope that helps.

Lawrence.

[Silicon]

The metal case of the volume potentiometer is 'strapped' to the chassis.

However the anti-clockwise terminal of the potentiometer should not be connected to the chassis in this circuit.

In this design it should be connected to the cathode of V3.

[HamishBoxer]

Strange I had R6 on my sheet, not sure which one.