(Small) vintage multimeter rectifiers- testing? - UK Vintage Radio Repair and Restoration Discussion Forum

The Philpott · 1st Nov 2019, 12:09 am

Having changed a Copper Oxide rectifier in a (1940's) Universal Avominor due to its somewhat low readings, it occurs to me to ask if there is a good test for such a component other than soldering it in, and testing the meter against a known reference!

Having seen some replacement rectifiers in these meters, which are GEC units about the size of a sultana with 2 green wires, 1 red, 1 black, i am curious as to whether they are silicon, germanium chemistry, or something else..I imagine they are 1960's vintage from the plastic used, and (predictably) they seem to offer better accuracy at low AC voltages.

One more thing- what is the purpose of the bobbin attached to the rear of the rectifier, which is wired across the DC output?

Thankyou.
Dave

pmmunro · 2nd Nov 2019, 10:34 am

Dave,

Do you have a copy of the Westinghouse publication "Westectors - Instructions for Use" which gives technical information on the original copper oxide rcetifiers used in Avometers?

Which rectifier are you referring to which has a bobbin attached to the rear?

PMM

micheal · 2nd Nov 2019, 12:29 pm

The bobbin attached to the back of the rectifier is a shunt resistor across the DC output
of the rectifier, the same as in AVO8 MK2 I do not know the value but if you change the
rectifier for germanium you can adjust the value of the resistor to give FSD on 100 AC
range when compared to a reference,

Mike

The Philpott · 2nd Nov 2019, 6:35 pm

Thanks Mike, Peter,

Yes i do have the Westector document, i think you copied me in on it previously. (and i now note it has a test procedure)
The resistance bobbin behind the CuO Westector is wired to it but they are not 'en bloc'.
It is a loose rather shallow bobbin, separated from the rectifier by a paper washer.

Interesting to know that trimming is possible if a non-standard rectifier is substituted.

Have been reading up on the significance of Chilean copper and the commercial secrecy surrounding whether the superior qualities of rectifiers using it were down merely to purity or to a particular blend of trace impurities. There was also an interesting pause in the manufacture of selenium rectifiers in Germany mid-way through the war, when their characteristics suddenly changed without obvious reason. It was allegedly traced to a different brand of solder containing traces of Thallium. I would like to think that this was sabotage, but..probably just pure chance.

Dave

1st Nov 2019, 12:09 am	#1
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	(Small) vintage multimeter rectifiers- testing? Having changed a Copper Oxide rectifier in a (1940's) Universal Avominor due to its somewhat low readings, it occurs to me to ask if there is a good test for such a component other than soldering it in, and testing the meter against a known reference! Having seen some replacement rectifiers in these meters, which are GEC units about the size of a sultana with 2 green wires, 1 red, 1 black, i am curious as to whether they are silicon, germanium chemistry, or something else..I imagine they are 1960's vintage from the plastic used, and (predictably) they seem to offer better accuracy at low AC voltages. One more thing- what is the purpose of the bobbin attached to the rear of the rectifier, which is wired across the DC output? Thankyou. Dave

2nd Nov 2019, 10:34 am	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: (Small) vintage multimeter rectifiers- testing? Dave, Do you have a copy of the Westinghouse publication "Westectors - Instructions for Use" which gives technical information on the original copper oxide rcetifiers used in Avometers? Which rectifier are you referring to which has a bobbin attached to the rear? PMM

2nd Nov 2019, 12:29 pm	#3
micheal Pentode Join Date: Mar 2007 Location: Croydon, London, UK. Posts: 111	Re: (Small) vintage multimeter rectifiers- testing? The bobbin attached to the back of the rectifier is a shunt resistor across the DC output of the rectifier, the same as in AVO8 MK2 I do not know the value but if you change the rectifier for germanium you can adjust the value of the resistor to give FSD on 100 AC range when compared to a reference, Mike

2nd Nov 2019, 6:35 pm	#4
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: (Small) vintage multimeter rectifiers- testing? Thanks Mike, Peter, Yes i do have the Westector document, i think you copied me in on it previously. (and i now note it has a test procedure) The resistance bobbin behind the CuO Westector is wired to it but they are not 'en bloc'. It is a loose rather shallow bobbin, separated from the rectifier by a paper washer. Interesting to know that trimming is possible if a non-standard rectifier is substituted. Have been reading up on the significance of Chilean copper and the commercial secrecy surrounding whether the superior qualities of rectifiers using it were down merely to purity or to a particular blend of trace impurities. There was also an interesting pause in the manufacture of selenium rectifiers in Germany mid-way through the war, when their characteristics suddenly changed without obvious reason. It was allegedly traced to a different brand of solder containing traces of Thallium. I would like to think that this was sabotage, but..probably just pure chance. Dave Last edited by The Philpott; 2nd Nov 2019 at 6:43 pm.