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Old 7th Jul 2018, 1:57 pm   #1
The Philpott
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Default Avometer HRM-rotary selector switches

A clean-up job to rid a High Resistance Avo (1948) of a particularly nasty and persistent tobacco smell, inevitably became a bit more involved, to include grit removal/re-greasing of the selectors and straightening of one of the selector shafts..(the knob was scraping the meter facia due to this kink)

Firstly, it seemed that (much of) the smell was originating from a sticky deposit on the battery lid and the sides of the case. Nothing short of meths and heavy scrubbing with cotton buds would shift it- it was bonded like cornflakes to a breakfast bowl.

This unit has seen heavy use and grit had lodged in the ball/spring/detents so i chose to remove and clean both selectors, taking advantage of the space afforded by the deleted transformer in this model. Driving the pins out of the die cast housing and undoing the cam rotors was fairly easy. The only glitch came when realigning (and removing an arc blob) from one of the radial swiped contacts attached to the facia; the insulating ring got cracked adjacent to one of it's screws(whoops) and had to be reinforced with araldite.

Pics show bent (and then straightened) shaft, and the wear patterns evident on the crackle finish of the facia.

Straightening the shaft involved the section adjacent to the bakelite being grabbed by mole grips, the mole grips being grabbed by the bench vice, and then taps with a pin hammer. The screw was inserted in the end of the shaft to avoid the hole going oval. Before starting i had resolved in my mind that i WOULD be able to mend it if everything went wrong.. (A 'wanted' entry in the forum would not be likely to generate a replacement!)

The little selector springs in the facia sit in a hole which is cone shaped towards the end, this matches the cone shape at one end of the coil spring. (One of mine was in the wrong way round..) The balls are exactly 4mm in diameter. They feel much better with a dab of grease.

I do notice that the 25v and 100v AC are low-reading and i wonder if Barnmead or Pmmunro would oblige with the circuit diagram.. There appear to be two diodes in there for ACV and i wonder if this is where the fault lies. The 250vAC and 1000vAC ranges are accurate on the mains to within a few volts.

Dave
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Old 7th Jul 2018, 2:14 pm   #2
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Default Re: Avometer HRM-rotary selector switches

I have now found the circuit diagram which shows the rectifier, such as it is! (courtesy of Peter and the forum google search)

To be more precise 14.25vAC reads:
12v on 100v range.
10.25v on 25v range.

It's a fairly stable 14.25v source that doesn't suffer a volt drop under this light load.

Last edited by The Philpott; 7th Jul 2018 at 2:19 pm.
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Old 7th Jul 2018, 8:44 pm   #3
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Default Re: Avometer HRM-rotary selector switches

I'll follow this with interest as I've got one of these meters. Good job getting those knobs out, I haven't worked out how you do it with a model 7 (but didn't need to in the end).
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Old 7th Jul 2018, 9:25 pm   #4
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Default Re: Avometer HRM-rotary selector switches

On this meter the switch die castings are asymmetrical, so they can be rotated to opposite positions then viewed, whereupon it appeared to me that the lock pins were tapered. If they are indeed tapered it is only very slight..but enough to dictate the direction of extraction!
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Old 8th Jul 2018, 10:44 pm   #5
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Default Re: Avometer HRM-rotary selector switches

Richard confirms the 'diodes' are Westector rectifiers, could someone advise the serial number of suitable Germanium replacements?

The meter is pretty much original apart from the usual battery compartment mods., so what i will probably do is retain the Westectors but snip their link wire, then wire in the replacements. (which i imagine are very small)

Thankyou for any advice.

Dave
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Old 9th Jul 2018, 11:54 pm   #6
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Default Re: Avometer HRM-rotary selector switches

Dave,

The characteristics of general purpose germanium diodes are very similar to these Westinghouse copper oxide rectifiers. You should find OA85s or OA91s will give accurate results.

If you would like a copy of the Westinghouse data booklet on instrument rectifiers, please send me a private message and I will put a scan of the booklet in a Dropbox for you.

PMM
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Old 10th Jul 2018, 12:02 pm   #7
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Default Re: Avometer HRM-rotary selector switches

Thanks Peter, i have the 'Westectors Instructions for use' MR14 Suppt. 3, 5th Edition (annotated in pen 'June 1956')- I will drop you a PM if you have additional.

I am looking at some 1N34A's, claimed to be similar to OA91. It might even be possible to hide them in the exoskeletons of the old components.

Dave
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Old 13th Jul 2018, 10:40 pm   #8
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Default Re: Avometer HRM-rotary selector switches

Used Germanium diodes, 1N34A's with effort made to shunt the heat away.

Yes, i did wire them in back to front the first time..

I did not go to town testing the westectors as i came to be sure they were faulty.

Accuracy to within 1.7% of reading. Meter still smells- but concentrated in the leather strap. Changing it!

Thankyou Peter and Richard for your help.
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