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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 13th Jul 2016, 7:06 pm   #1
Stockden
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Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
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Default AVO 8 Mk2 Resistance Range

This is the 8 Mk2 referred to in this thread and the symptoms were as stated.

Following Phil's advice on the original thread I checked for 1.5v on the meter terminals and found that it was present on ohms/100 but vanished when the switch was moved to "ohms". I then had a look at the cam switches, which proved to be working correctly, so my attention turned to the circuit diagram. As far as I could see the only difference between the 2 ranges was that contacts h&j are closed on ohms/100 but open on the higher range, so my suspicion fell on the 1230ohm resistor (marked "w" on the circuit in the Model 8 Avometer Marks 1 & 2 repair manual).

Sure enough the resistor proved to be open circuit and when it was replaced by a carbon composition one of almost exactly the right value the ohms range came back to life. At some point I'll replace it with something more suitable but for the moment it can stay where it is.

So there we have it, for once it wasn't the switch contacts!

Hugh
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Old 14th Jul 2016, 2:12 pm   #2
IanG3XYV
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Default Re: AVO 8 Mk2 Resistance Range

I was very interested in reading the original thread because my Model 8 Mk2 had exactly the same problem in that the "ohms" range wasn't working but the others were.

Spurred into action I opened mine up and with the circuit diagram traced out the connections. I found this quite hard and also had a good look at the cams which seemed to be working properly. Eventually, and with the use of a DMM on resistance, found that my 1230 ohm wirewound resistor was open circuit too. I note it is very fine gauge so obviously susceptible to this failure mode. With some difficulty I managed to solder in a 1.2K resistor and it now works on all three ranges. I've got some pretty good calibration equipment and after checks, it appears to be within specification on all of the ranges I checked so I'm in no hurry to replace the replacement.

It's great to have a 56 year old piece of test equipment working on all ranges and many thanks to the original thread contributors and suggestions made.

Ian
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Old 14th Jul 2016, 10:52 pm   #3
Phil G4SPZ
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Default Re: AVO 8 Mk2 Resistance Range

Well diagnosed, Hugh!
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