HRM AVO meter query - UK Vintage Radio Repair and Restoration Discussion Forum

Barnmead · 29th Jan 2015, 7:06 pm

I have recently acquired a model 1 high sensitivity avometer. It had a missing battery cover, a bent pointer, two open circuit resistors and two fried ones.

So far I have found it works fine on DC current and voltage, Low and X100 resistance ranges. I have not tried the AC ranges yet.

The bobbin resistor in series with the 4.5 Volt battery was open circuit but fortunately marked with its resistance value.

I have got the the X1 resistance range working by shunting the burnt out wire wound resistor with 330 ohms but it reads about 10% high.

Can anyone help with a circuit diagram?

pmmunro · 29th Jan 2015, 10:39 pm

Barnmead,

A copy of a print of the ACWEECO drawing for the HR1 (Avometer High Resistance 1) is attached.

If you search back through previous threads you should find more information on the range of meters this model belongs to.

Please enter your meter's serial number in the Avometer Survey if you could.

PMM.

Barnmead · 29th Jan 2015, 11:56 pm

Thank you for that, my trial and error selection of 330 ohms was almost right I see it should be 353 ohms. I will venture out to my workshop over the weekend and see what else may be wrong. I will post the serial number.

Barnmead · 30th Jan 2015, 3:04 pm

The diagram prompted me to check the fried resistor which was not open circuit to find that it had shorted turns. Touching it caused it to rise to its correct value, a coating of shellac varnish hopefully cured that and the addition of selected 330 and 33 ohm resistors in series gave the requisite 353 ohms. So now the meter works as intended including the AC voltage ranges.

The resistors mentioned are this connected between the three left hand tags on the lower component board.

Only after re-assembling the meter did I notice "V.T.Ltd" in the corner of the scale plate. Does this mean that the meter was outsourced and if so by whom or was it specifically made for a particular firm?

Now cleaned and given the AVO recommended application of light machine oil it looks much happier thought the red scale markings have faded over the years.

Barnmead · 30th Jan 2015, 6:37 pm

The instructions on the back plate have been censored! It looks to me as if this was a factory modification. I wonder why?

pmmunro · 30th Jan 2015, 8:22 pm

I don't know the significance of the "VT Ltd" on the scaleplate. It may possibly be the name of the manufacturer of the blank scaleplate but I have thought for a long time that this very unusual design of Avometer may have had some outside influence.

The "censoring" of the scaleplate is probably because there are two basic versions of this meter with different resistance ranges.

I am not at home at present, but I will check these details of the High Resistance Avometers I have and report back.

Has anyone any suggestions for the identity of VT Ltd? If it is not just the name of the scaleplate manufacturer, it would most likely be another instrument manufacturer.

PMM

M0XNA Neil · 1st Feb 2015, 11:26 am

It could be Vosper Thornycroft Ltd.

Neil

Barnmead · 1st Feb 2015, 12:03 pm

I had wondered that, but I think that VT only came about in 1966.

There was a firm called Victoria Instruments Ltd [wound up in 1958] who made quite a range of moving coil meters.

hilitevr · 1st Feb 2015, 5:26 pm

Hi Richard, re the censored instruction plate.

I have two of these HRM AVO's, one of which is a PANCLIMATIC with a metal case.

The non PANCLIMATIC version has the censored plate.

The text that is blacked out reads os follows on the PAN version.

"The intrument as a whole is magnetically screened in a metal case, this is not being connected electrically to the circuit"

They were obviously saving money by not producing two plates.

No VT.Ltd on either of my scaleplates.

Hope this helps.

Barnmead · 1st Feb 2015, 8:47 pm

Thank you Trevor, that is one mystery solved!

Barnmead · 2nd Feb 2015, 4:22 pm

I have looked at my other AVOs and found one with letters GH in a similar position.
It is a model 7 serial 6412-A-749. I wonder if this helps in any way?

Barnmead · 12th Mar 2015, 8:28 pm

I have just posted information about a model 8 194-151 on the main AVO thread which has G&H on the RH corner of the scale plate.

Barnmead · 20th Aug 2015, 8:54 pm

Another model 8 s/n 45836-c-456 with G&H has come my way. There must be many more with this marking. Mystery not yet solved.

29th Jan 2015, 7:06 pm	#1
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	HRM AVO meter query I have recently acquired a model 1 high sensitivity avometer. It had a missing battery cover, a bent pointer, two open circuit resistors and two fried ones. So far I have found it works fine on DC current and voltage, Low and X100 resistance ranges. I have not tried the AC ranges yet. The bobbin resistor in series with the 4.5 Volt battery was open circuit but fortunately marked with its resistance value. I have got the the X1 resistance range working by shunting the burnt out wire wound resistor with 330 ohms but it reads about 10% high. Can anyone help with a circuit diagram? __________________ Richard

29th Jan 2015, 11:56 pm	#3
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query Thank you for that, my trial and error selection of 330 ohms was almost right I see it should be 353 ohms. I will venture out to my workshop over the weekend and see what else may be wrong. I will post the serial number. __________________ Richard

30th Jan 2015, 3:04 pm	#4
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query The diagram prompted me to check the fried resistor which was not open circuit to find that it had shorted turns. Touching it caused it to rise to its correct value, a coating of shellac varnish hopefully cured that and the addition of selected 330 and 33 ohm resistors in series gave the requisite 353 ohms. So now the meter works as intended including the AC voltage ranges. The resistors mentioned are this connected between the three left hand tags on the lower component board. Only after re-assembling the meter did I notice "V.T.Ltd" in the corner of the scale plate. Does this mean that the meter was outsourced and if so by whom or was it specifically made for a particular firm? Now cleaned and given the AVO recommended application of light machine oil it looks much happier thought the red scale markings have faded over the years. Attached Thumbnails __________________ Richard

30th Jan 2015, 6:37 pm	#5
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query The instructions on the back plate have been censored! It looks to me as if this was a factory modification. I wonder why? Attached Thumbnails __________________ Richard

30th Jan 2015, 8:22 pm	#6
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: HRM AVO meter query I don't know the significance of the "VT Ltd" on the scaleplate. It may possibly be the name of the manufacturer of the blank scaleplate but I have thought for a long time that this very unusual design of Avometer may have had some outside influence. The "censoring" of the scaleplate is probably because there are two basic versions of this meter with different resistance ranges. I am not at home at present, but I will check these details of the High Resistance Avometers I have and report back. Has anyone any suggestions for the identity of VT Ltd? If it is not just the name of the scaleplate manufacturer, it would most likely be another instrument manufacturer. PMM

1st Feb 2015, 11:26 am	#7
M0XNA Neil Tetrode Join Date: Apr 2009 Location: Towcester, Northamptonshire, UK. Posts: 92	Re: HRM AVO meter query It could be Vosper Thornycroft Ltd. Neil __________________ An engineer makes for a penny what a scientist makes for a pound!

1st Feb 2015, 12:03 pm	#8
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query I had wondered that, but I think that VT only came about in 1966. There was a firm called Victoria Instruments Ltd [wound up in 1958] who made quite a range of moving coil meters. __________________ Richard

1st Feb 2015, 5:26 pm	#9
hilitevr Pentode Join Date: Jul 2006 Location: Sidcup, Greater London, UK. Posts: 122	Re: HRM AVO meter query Hi Richard, re the censored instruction plate. I have two of these HRM AVO's, one of which is a PANCLIMATIC with a metal case. The non PANCLIMATIC version has the censored plate. The text that is blacked out reads os follows on the PAN version. "The intrument as a whole is magnetically screened in a metal case, this is not being connected electrically to the circuit" They were obviously saving money by not producing two plates. No VT.Ltd on either of my scaleplates. Hope this helps. __________________ Regards Trevor BVWS member Last edited by hilitevr; 1st Feb 2015 at 5:35 pm.

1st Feb 2015, 8:47 pm	#10
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query Thank you Trevor, that is one mystery solved! __________________ Richard

2nd Feb 2015, 4:22 pm	#11
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query I have looked at my other AVOs and found one with letters GH in a similar position. It is a model 7 serial 6412-A-749. I wonder if this helps in any way? __________________ Richard

12th Mar 2015, 8:28 pm	#12
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query I have just posted information about a model 8 194-151 on the main AVO thread which has G&H on the RH corner of the scale plate. __________________ Richard

20th Aug 2015, 8:54 pm	#13
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: HRM AVO meter query Another model 8 s/n 45836-c-456 with G&H has come my way. There must be many more with this marking. Mystery not yet solved. __________________ Richard