Success story Radford LDO3 and warning. - UK Vintage Radio Repair and Restoration Discussion Forum

chriswood1900 · 28th Mar 2016, 8:37 pm

In the round-tuit pile I have had 2 broken Radford low distortion oscillators for a couple of years, one missing several case parts and the other with a broken shaft to the 3 ganged pots. The shaft is made of Paxolin or similar and it clearly had quite a bang shearing it off just where it enters the pot.
I powered up the one still with a complete shaft and it worked by the frequency range was limited and it was prone to vary the amplitude. The other powered up and gave a stable output but I could not vary the frequency. I thought simple just unscrew the 6 terminal used on the pot and swap them over. Big mistake it turned out the screws (one circled in red) go into non captive nuts which fall off inside the pot when unscrewed. The pot is bonded together with no access to inside.
This is now where I spent about 3 hours trying various solutions, none worked until I managed to shake the pot so that I could see the nut through the hole with a small pointed screwdriver position it over the hole on the inside. I clamped the pot in a vice so I could see the nut still through the hole whilst lying on my back, I tried to pass the screw through hole and pick up the nut without success. I then repositioned the nut for another go but this time I sharpened the end of the screw to a point and cleaned up the thread, after about 5 more goes the screw caught on the nut and by keeping a downward force the screw went in and tightened up. Concerned that it would not work I connected it up to an Avo and checked that the pot was still working which it was, clearly I must have collected up any wires or terminals that the nut tightened down on.
So having refitted the pot with a shaft in the second unit I was pleased to discover that it all worked. I set up the PSU as per the manual then checked the output ranges, set the frequency calibration against the dial, the meter calibration, finally the distortion on an HP distortion meter proved everything was on spec. Phew off for a cuppa.
You have been warned!
Chris

Phil G4SPZ · 29th Mar 2016, 5:01 pm

What a fiendish design. Obviously the screws were inserted and tightened down onto the solder tags, with the wires ready attached, before the pots were assembled. Anyway, well done for sorting it out! And thanks for the warning.

mark pirate · 29th Mar 2016, 5:53 pm

I have had a similar experience with these types of pots, luckily they were single types with a paxolin cover held in place with glue. It was a wire wound type with the nut clamping down the end of the track. I am pretty sure they were made by Colvern.

Mark

chriswood1900 · 29th Mar 2016, 6:04 pm

Mark, you are correct it was made by Colvern but it was 3 bonded together only the rear one was accessible through the Paxolin cover and of course I undid the front one first! At least I twigged on the first screw.
BTW the photo doesn't show the size they were huge at around 3" in diameter.
Chris

mark pirate · 29th Mar 2016, 6:16 pm

Hi Chris, The ones I have had dealings with are about 2" dia and a common fitment to early 50's Bush TV's. Looking at your photo, it appears the outer cases are glued together. I am curious as to how the shaft is fitted?

I have certainly learned to unsolder the tags rather than remove the screws!

Mark

chriswood1900 · 29th Mar 2016, 7:51 pm

Mark. Unfortunately I have not been able to work out how the shaft works it is larger than normal at about 3/8" you would probably need to destroy the pots to get inside, so the mystery must remain awhile longer!
Chris

28th Mar 2016, 8:37 pm	#1
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,479	Success story Radford LDO3 and warning. In the round-tuit pile I have had 2 broken Radford low distortion oscillators for a couple of years, one missing several case parts and the other with a broken shaft to the 3 ganged pots. The shaft is made of Paxolin or similar and it clearly had quite a bang shearing it off just where it enters the pot. I powered up the one still with a complete shaft and it worked by the frequency range was limited and it was prone to vary the amplitude. The other powered up and gave a stable output but I could not vary the frequency. I thought simple just unscrew the 6 terminal used on the pot and swap them over. Big mistake it turned out the screws (one circled in red) go into non captive nuts which fall off inside the pot when unscrewed. The pot is bonded together with no access to inside. This is now where I spent about 3 hours trying various solutions, none worked until I managed to shake the pot so that I could see the nut through the hole with a small pointed screwdriver position it over the hole on the inside. I clamped the pot in a vice so I could see the nut still through the hole whilst lying on my back, I tried to pass the screw through hole and pick up the nut without success. I then repositioned the nut for another go but this time I sharpened the end of the screw to a point and cleaned up the thread, after about 5 more goes the screw caught on the nut and by keeping a downward force the screw went in and tightened up. Concerned that it would not work I connected it up to an Avo and checked that the pot was still working which it was, clearly I must have collected up any wires or terminals that the nut tightened down on. So having refitted the pot with a shaft in the second unit I was pleased to discover that it all worked. I set up the PSU as per the manual then checked the output ranges, set the frequency calibration against the dial, the meter calibration, finally the distortion on an HP distortion meter proved everything was on spec. Phew off for a cuppa. You have been warned! Chris Attached Thumbnails

29th Mar 2016, 5:01 pm	#2
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Success story Radford LDO3 and warning. What a fiendish design. Obviously the screws were inserted and tightened down onto the solder tags, with the wires ready attached, before the pots were assembled. Anyway, well done for sorting it out! And thanks for the warning. __________________ Phil Optimist [n]: One who is not in possession of the full facts

29th Mar 2016, 5:53 pm	#3
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Success story Radford LDO3 and warning. I have had a similar experience with these types of pots, luckily they were single types with a paxolin cover held in place with glue. It was a wire wound type with the nut clamping down the end of the track. I am pretty sure they were made by Colvern. Mark

29th Mar 2016, 6:04 pm	#4
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,479	Re: Success story Radford LDO3 and warning. Mark, you are correct it was made by Colvern but it was 3 bonded together only the rear one was accessible through the Paxolin cover and of course I undid the front one first! At least I twigged on the first screw. BTW the photo doesn't show the size they were huge at around 3" in diameter. Chris

29th Mar 2016, 6:16 pm	#5
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Success story Radford LDO3 and warning. Hi Chris, The ones I have had dealings with are about 2" dia and a common fitment to early 50's Bush TV's. Looking at your photo, it appears the outer cases are glued together. I am curious as to how the shaft is fitted? I have certainly learned to unsolder the tags rather than remove the screws! Mark

29th Mar 2016, 7:51 pm	#6
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,479	Re: Success story Radford LDO3 and warning. Mark. Unfortunately I have not been able to work out how the shaft works it is larger than normal at about 3/8" you would probably need to destroy the pots to get inside, so the mystery must remain awhile longer! Chris