Volume knob, Roberts radio, intermittent failure

[ElectronicTenderfoot]

We have a Roberts R250 analog radio. It works very well, intermittently. The volume knob turns it on and off, but sometimes it adjusts volume and we hear something and sometimes we hear nothing. By twiddling the knob (up, down, sideways, wiggle a lot or a little) it works. I have taken it apart and by holding the wee PCB it is mounted on I can replicate the fault by tiny movements of the shaft.

Reaching back a long way for physics 101 etc., it is a potentiometer? Is it had-it or is there something I can do?

I am wondering about the solder on the washer under the nut holding it to the PCB. Maybe it is not connecting properly?

The purported circuit diagram is attached. I am not sure because there are more connections to the knob than shown here.

 

[duke37]

Obviously good connections are necessary for it to work. Try to find the problem area. A picture of the volume control would help, it may be soldered to a printed circuit track which is cracked.
There will be three connections for the volume control and more for the on/off switch
If the volume control is dirty, it can be washed with switch cleaner or isopropyl alcohol as long as you can get at the track.

 

[ElectronicTenderfoot]

Thanks for your reply. Pictures attached. I neglected to mention it also crackles sometimes when turning the knob. I have a can of "Lectroclean" (trichloroethylene + tetratrichloroethylene) but that will take all lubrication out of the switch. I suppose that is a first attempt to see if a clean will help?

 

[Bluejets]

Normal pot one can squirt a bit of crc down inside where the 3 terminals come out BUT this miniature one has a different arrangement. (riveted to the back casing) might be difficult to get anything in there ..

 

[Alec_t]

might be difficult to get anything in there ..

I'd be tempted to run a fine drill through one of those hollow rivets, to get some switch cleaner inside.

 

[KJ6EAD]

This is a typical failure for a well-used radio. The cleaning may help temporarily but a replacement will be the long-term solution. You'll need all of the dimensions, the value and the taper type to source it.

 

[duke37]

I would check if the rivets connect to the legs and solder if necessary.

 

[davenn]

I'd be tempted to run a fine drill through one of those hollow rivets, to get some switch cleaner inside.

Nooooooooooooooooo that will instantly destroy the pot

I would check if the rivets connect to the legs and solder if necessary.

they are rivet press fit .... you cannot solder to a carbon track


unsolder the pot from that board
you can see 3 lugs on the back of it, gently bend these vertical and you will have access to the carbon track and wiper for cleaning


Dave

 

[ElectronicTenderfoot]

Thank you for all your replies.

I bought a can of CRC 2-26 (in big letters on the can it makes the ridiculous claim "IMPROVES ELECTRICAL PROPERTIES" (!) whatever that means) and managed to get a few drops to wick their way inside it at the interface between the black plastic and the metal back, as well as up the shaft from the bottom. Now it works apparently perfectly and my wife is happy, which means I am too!

Inside the "hollow" rivets I see something black - plastic or carbon perhaps. It doesn't move when the shaft turns.

 

[davenn]

I bought a can of CRC 2-26 (in big letters on the can it makes the ridiculous claim "IMPROVES ELECTRICAL PROPERTIES" (!) whatever that means) and managed to get a few drops to wick their way inside it at the interface between the black plastic and the metal back, as well as up the shaft from the bottom. Now it works apparently perfectly and my wife is happy, which means I am too!

yup, that stuff does wonders .... you will find it wont be a permanent cure, but will keep you out of trouble for a good while

 

[Mike F]

Thank you for all your replies.

I bought a can of CRC 2-26 (in big letters on the can it makes the ridiculous claim "IMPROVES ELECTRICAL PROPERTIES" (!) whatever that means) and managed to get a few drops to wick their way inside it at the interface between the black plastic and the metal back, as well as up the shaft from the bottom. Now it works apparently perfectly and my wife is happy, which means I am too!

Inside the "hollow" rivets I see something black - plastic or carbon perhaps. It doesn't move when the shaft turns.

 

[Mike F]

Perhaps is conductive plastic resistive material.

 

[73's de Edd]

While you are at it also note all of the BLACK SILVER OXIDE on the terminals.
BUT mainly the two little knife edges that make the switch action when the
phenolic eccentric cam moves away from the off position.
A very-very-very light scraping of those contact areas would be in order,just
enough to make them silver again would be in order .
A silicone oil or grease drop (NOT SILICONE RUBBER !) would allay further oxidation again.

73s de Edd