Avo Model 9 Mk II assembly glitch - UK Vintage Radio Repair and Restoration Discussion Forum

The Philpott · 7th Jun 2018, 11:40 am

I have two of these, and discovered the same fault in both, which is too much of a coincidence.

Symptoms: Intermittent low readings and sudden drop in readings, sometimes spontaneous but sometimes initiated, (then corrected,) by tapping the facia.
When the fault is found i was surprised the meter was working at all, and it would have been all too easy to be befuddled by it and blame a dry joint or leaf switch trouble.

Cause: The scale plate/cast A frame/movement assemblies are all held to the magnet and concentrator by two screws, which in the picture below are situated just above my blue dots...They were very loose, and this was immediately apparent before removing the assemblies, simply by getting a fingernail between the mirror and scale-plate and lifting. (insert 'clang' sound effect here)

Things that make you go Hmm: The screwdriver was drawn onto these screw heads quite strongly, they appear nickel plated but are they steel under the nickel? A bronze screwdriver would have been better, but i got away with it without losing any magnetic flux (that i could detect.) The screwdriver was an insulated one so the steel tip is relatively short.

Anyway, there it is. Quite a simple fix.

Dave

pmmunro · 7th Jun 2018, 12:42 pm

Dave,

I've met this problem before, although I can't say for sure what models it was on.

You are right about the strong advisability of using non-magnetic screwdrivers around movement magnets. A recovered cistern ball valve stem makes a good large size and heavy brazing rods can be used for smaller ones. Now that stainless steel is readily available, a length of round bar in a non- magnetic grade would be a good source.

I'm sure you'll know to hollow grid the tip and file handles will complete the job.

PMM

The Philpott · 7th Jun 2018, 5:32 pm

By hollow grind do you mean a concave edge on either side to make the extreme end parallel? Thanks Peter, if i can't find them i may make some. I have a feeling a brazing rod would be harder than ordinary brass..but easy to modify compared to s/s.

7th Jun 2018, 11:40 am	#1
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,108	Avo Model 9 Mk II assembly glitch I have two of these, and discovered the same fault in both, which is too much of a coincidence. Symptoms: Intermittent low readings and sudden drop in readings, sometimes spontaneous but sometimes initiated, (then corrected,) by tapping the facia. When the fault is found i was surprised the meter was working at all, and it would have been all too easy to be befuddled by it and blame a dry joint or leaf switch trouble. Cause: The scale plate/cast A frame/movement assemblies are all held to the magnet and concentrator by two screws, which in the picture below are situated just above my blue dots...They were very loose, and this was immediately apparent before removing the assemblies, simply by getting a fingernail between the mirror and scale-plate and lifting. (insert 'clang' sound effect here) Things that make you go Hmm: The screwdriver was drawn onto these screw heads quite strongly, they appear nickel plated but are they steel under the nickel? A bronze screwdriver would have been better, but i got away with it without losing any magnetic flux (that i could detect.) The screwdriver was an insulated one so the steel tip is relatively short. Anyway, there it is. Quite a simple fix. Dave Attached Thumbnails

7th Jun 2018, 12:42 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avo Model 9 Mk II assembly glitch Dave, I've met this problem before, although I can't say for sure what models it was on. You are right about the strong advisability of using non-magnetic screwdrivers around movement magnets. A recovered cistern ball valve stem makes a good large size and heavy brazing rods can be used for smaller ones. Now that stainless steel is readily available, a length of round bar in a non- magnetic grade would be a good source. I'm sure you'll know to hollow grid the tip and file handles will complete the job. PMM

7th Jun 2018, 5:32 pm	#3
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,108	Re: Avo Model 9 Mk II assembly glitch By hollow grind do you mean a concave edge on either side to make the extreme end parallel? Thanks Peter, if i can't find them i may make some. I have a feeling a brazing rod would be harder than ordinary brass..but easy to modify compared to s/s.