Avominor Rectifier replacement/s - UK Vintage Radio Repair and Restoration Discussion Forum

The Philpott · 20th Jan 2017, 6:18 pm

On a 1955 Universal Avominor Model 2, having tweaked the magnet shunt through 45 degrees to get good accuracy on DC ranges, The ACV ranges are under-reading by between 9 and 25% (50 cycles), depending on which voltage is applied and which range is used.

Having looked inside, the nut and screw on the rectifier look rather corroded, and other components close by have a thin coating of grot that has the appearance of mould or grey soot. Having isolated where the problem is, may i ask:

1. I assume this is a westinghouse copper oxide rectifier..?
2. Do the contents escape with age and does it sound like this is what has happened here?
3. They appear to have been originally supplied with their own wire 'tails' attached; is this so that soldering heat doesn't damage them?
3. Is the power rating quoted for these units a maximum, or do they have an operating range that must be closely matched to the instrument they are used in?
4. Will a rectifier from a basic Model 1 Avominor suffice bearing in mind the meter consumption is different by a factor of 10?
5. Would i be better off buying one off the shelf-- or maybe making a bridge rectifier from diodes?
6. why is there a set screw on these rectifiers- is it an adjuster!?

Thankyou.

avocollector · 21st Jan 2017, 2:15 pm

If this is a copper oxide rectifier it consists of discs with the oxide coating on one side stacked together with a lead at the top bottom and other places. The set screw holds them all together - I took several of these to bits in the mid 60's as a kid scrounged form old valve radios etc. Never tried putting them together again to see if they worked - some also I recall had a thin sort of grease between the discs. I understand these give up the ghost relatively easily (common in older oscilloscopes) in response to spikes etc. so a solid state replacement hits me as the best bet - I'd experiment/calculate until I got the right bridge - if I recall correctly the power rating of these oxide rectifiers was very low - in the 100's of milliamps. Hope that helps.

Herald1360 · 21st Jan 2017, 2:28 pm

If the rectifier is on the secondary of a current transformer, almost anything will do. If there's no such transformer, you may need to experiment a bit with Ge/Si/Sch and some resistors to get a similar response to the original at low levels.

The Philpott · 22nd Jan 2017, 12:46 am

Thankyou people.

There is no transformer (it only measures ACV, not current)

I have a spare (more modern looking) unit fitted into a broken Model 1 meter so will experiment with that.

The Philpott · 23rd Jan 2017, 9:52 pm

On second thoughts- have found an identical (looking) period replacement for the original. Both the Mod1 and Mod2 APPEAR to use the same rectifier.

Thus far i have only ever had one failure of this component so chances are this shinier replacement will be fine. I am pleasantly surprised at the accuracy in other meters using this component (as long as it's seeing a sine wave)

The Philpott · 29th Jan 2017, 11:40 pm

More! The Model 1 unit has a small flat bobbin inserted under the main rectifier, which is connected across the + and -

On the Model 2 this bobbin is absent and the space is taken up by a tubular spacer.

A trimming bobbin, then.

20th Jan 2017, 6:18 pm	#1
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Avominor Rectifier replacement/s On a 1955 Universal Avominor Model 2, having tweaked the magnet shunt through 45 degrees to get good accuracy on DC ranges, The ACV ranges are under-reading by between 9 and 25% (50 cycles), depending on which voltage is applied and which range is used. Having looked inside, the nut and screw on the rectifier look rather corroded, and other components close by have a thin coating of grot that has the appearance of mould or grey soot. Having isolated where the problem is, may i ask: 1. I assume this is a westinghouse copper oxide rectifier..? 2. Do the contents escape with age and does it sound like this is what has happened here? 3. They appear to have been originally supplied with their own wire 'tails' attached; is this so that soldering heat doesn't damage them? 3. Is the power rating quoted for these units a maximum, or do they have an operating range that must be closely matched to the instrument they are used in? 4. Will a rectifier from a basic Model 1 Avominor suffice bearing in mind the meter consumption is different by a factor of 10? 5. Would i be better off buying one off the shelf-- or maybe making a bridge rectifier from diodes? 6. why is there a set screw on these rectifiers- is it an adjuster!? Thankyou.

21st Jan 2017, 2:15 pm	#2
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Re: Avominor Rectifier replacement/s If this is a copper oxide rectifier it consists of discs with the oxide coating on one side stacked together with a lead at the top bottom and other places. The set screw holds them all together - I took several of these to bits in the mid 60's as a kid scrounged form old valve radios etc. Never tried putting them together again to see if they worked - some also I recall had a thin sort of grease between the discs. I understand these give up the ghost relatively easily (common in older oscilloscopes) in response to spikes etc. so a solid state replacement hits me as the best bet - I'd experiment/calculate until I got the right bridge - if I recall correctly the power rating of these oxide rectifiers was very low - in the 100's of milliamps. Hope that helps.

21st Jan 2017, 2:28 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Avominor Rectifier replacement/s If the rectifier is on the secondary of a current transformer, almost anything will do. If there's no such transformer, you may need to experiment a bit with Ge/Si/Sch and some resistors to get a similar response to the original at low levels. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

22nd Jan 2017, 12:46 am	#4
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: Avominor Rectifier replacement/s Thankyou people. There is no transformer (it only measures ACV, not current) I have a spare (more modern looking) unit fitted into a broken Model 1 meter so will experiment with that.

23rd Jan 2017, 9:52 pm	#5
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: Avominor Rectifier replacement/s On second thoughts- have found an identical (looking) period replacement for the original. Both the Mod1 and Mod2 APPEAR to use the same rectifier. Thus far i have only ever had one failure of this component so chances are this shinier replacement will be fine. I am pleasantly surprised at the accuracy in other meters using this component (as long as it's seeing a sine wave)

29th Jan 2017, 11:40 pm	#6
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: Avominor Rectifier replacement/s More! The Model 1 unit has a small flat bobbin inserted under the main rectifier, which is connected across the + and - On the Model 2 this bobbin is absent and the space is taken up by a tubular spacer. A trimming bobbin, then.