Faulty Avo VCM Mk2 Zero Pot - UK Vintage Radio Repair and Restoration Discussion Forum

Pamphonica · 14th Oct 2008, 3:29 pm

Not sure if this is the right place for this (possibly hints and tips?) but here goes.
I am restoring my first valve tester, a recently acquired. Avo VCM Mk2.
It has had a hard life, but it still seems to work.
The intriguing thing I noticed when I got the lid off was that the Zero pot (2 section) had only the front section rotating! The universal joint in between was just spinning, despite the collet bolts being well tightened. The UJ had failed.
I removed the pot, unscrewed the long joining bolts between the sections and got to work. The rear pot was locked solid, hence the UJ failure.
- having taken the lid off the back section it was clear that the pot was completely glued up, with gunk on the track, but most particularly on the spindle. Removing the little circlip allowed me to ease out the shaft, clean thoroughly, lubricate and return it. A wipe round the rest with a meths-soaked rag completed the cleanup.
- the UJ looked impossible to replace - it's tiny, serving 1/8" spindles. But I carefully introduced a punch and tapped down the hollow rivets that hold the collets to the UJ section. Job done!
So the moral seems to be: Check inside the tester and see if BOTH sections of the Zero pot are moving. If not, the tester accuracy must be suspect and a repair is needed.
Hope this helps someone. Photo attached.
Jeremy

14th Oct 2008, 3:29 pm	#1
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Faulty Avo VCM Mk2 Zero Pot Not sure if this is the right place for this (possibly hints and tips?) but here goes. I am restoring my first valve tester, a recently acquired. Avo VCM Mk2. It has had a hard life, but it still seems to work. The intriguing thing I noticed when I got the lid off was that the Zero pot (2 section) had only the front section rotating! The universal joint in between was just spinning, despite the collet bolts being well tightened. The UJ had failed. I removed the pot, unscrewed the long joining bolts between the sections and got to work. The rear pot was locked solid, hence the UJ failure. - having taken the lid off the back section it was clear that the pot was completely glued up, with gunk on the track, but most particularly on the spindle. Removing the little circlip allowed me to ease out the shaft, clean thoroughly, lubricate and return it. A wipe round the rest with a meths-soaked rag completed the cleanup. - the UJ looked impossible to replace - it's tiny, serving 1/8" spindles. But I carefully introduced a punch and tapped down the hollow rivets that hold the collets to the UJ section. Job done! So the moral seems to be: Check inside the tester and see if BOTH sections of the Zero pot are moving. If not, the tester accuracy must be suspect and a repair is needed. Hope this helps someone. Photo attached. Jeremy Attached Thumbnails