Avo 2 Panel Tester - UK Vintage Radio Repair and Restoration Discussion Forum

Mike-repairman · 1st Aug 2008, 6:12 pm

Hi,

I have an Avo 2 panel tester which consists of the main unit (original) and a made up valve base panel (with wander plugs to select the pins) which works fine.

Last week an old mate (TV engineer) presented me with another complete set, main unit and original valve base panel.

After restoring it I found that it worked well on triodes and double triodes and the results were almost the same as my made up unit.

However when trying to test Pentodes I find that the screen volts are being virtually shorted out, a 100V setting will drop to around 9V. This is the same with the valve removed.

I know that there must be a logical answer to this but I have been pulling out my hair (very little left these days) trying to find out where the short could be. The rotary switch bank is so basic that I can't see a problem with it; and all the connections in it make OK with the valve bases.

I note that the cathode is grounded, selector numbers 0 and 1 are connected on the switch, I assume this to be correct. I have a copy of the circuits but although the print is clear they are not that easy to understand. If I remove the screen connection from the valve base unit at the switch, normal screen voltage returns.

I thought the concept (saw it somewhere) was a grounded grid system with varying cathode voltage to measure ma/v. This appears not to be the case if the cathode is grounded.

Any help would be much appreciated.

Thanks,

Mike.

Mike-repairman · 1st Aug 2008, 7:00 pm

More info...
There are no shorts on the valve base unit with the main plug removed.
There are no shorts between any of the pins on the valve base.

Setting the wheels to numbers 1 2 3 4 5 6 7 8 9 0
restores the screen volts to the panel. Measured between grid or cathode to screen on the valve bases.

Setting wheel 5 to htrs drops volts.
Setting wheel 1 to Screen drops volts and setting it to some of the other positions give varying readings, even D1 and D2 and A2 settings.

I have noticed that htr- and htr+ appear to be reversed when measuring to ground (cathode or grid). Htr ac volts appear on the htr- not htr+ (not sure if that is correct).

The mystery deepens and I wonder if someone has swapped around some wires on the base unit at some time in the past 60 years.

I have a spare main unit and will compare the two, but no spare valve base unit to check the input connections to it.

Again any ideas very welcome.

Brian R Pateman · 1st Aug 2008, 10:05 pm

Hi Mike,

If you haven't done so already have a look in the archive above (Valve Tester section) as we've had a few threads on these useful if basic testers. They do have certain limitations though and they aren't suitable for all types of valves.

Regards,

Mike-repairman · 2nd Aug 2008, 7:18 am

Hi Brian,
I did look through the archive before posting, but there was nothing directly relevant to my problem.
I still have some logical tests to do today, but suspect the problem may lie within the thumbwheel switch assembly which may need to be stripped down.
Mike.

Mike-repairman · 2nd Aug 2008, 11:33 am

Brian,

Success; switch dismantled cleaned adjusted and rewired and all working OK now, no shorts. Not quite sure where the shorts were but the only thing I could think of was the misalignment of the stop positions on the wheels; could have been shorting between two bussbar rods. It certainly needed a clean.

One query; I saw a picture in one of the manuals of the designation of the 3 sockets on the top of the panel that are used for the top cap connections.

Could anyone confirm that they are...
black (left) for screen,
brown (middle) for Grid and
red (right) for anode.

I'm sure that they had been miswired in the past.

By the way, you are correct Brian that these old testers, that were really designed for the large 4,5, 6 pin and octal valves, don't handle some of the newer valves too well; however it tests all the ECC81/2/3 range and the EL34 etc. output valves that I use OK.

Thanks,

Mike.

Mike-repairman · 2nd Aug 2008, 12:02 pm

Just found the diagram; black (left) is anode, middle (brown) is grid and red (right) is screen. Strange configuration of colours? Can get the lid back on now.

1st Aug 2008, 6:12 pm	#1
Mike-repairman Heptode Join Date: Jun 2008 Location: Cambridgeshire, UK. Posts: 613	Avo 2 Panel Tester Hi, I have an Avo 2 panel tester which consists of the main unit (original) and a made up valve base panel (with wander plugs to select the pins) which works fine. Last week an old mate (TV engineer) presented me with another complete set, main unit and original valve base panel. After restoring it I found that it worked well on triodes and double triodes and the results were almost the same as my made up unit. However when trying to test Pentodes I find that the screen volts are being virtually shorted out, a 100V setting will drop to around 9V. This is the same with the valve removed. I know that there must be a logical answer to this but I have been pulling out my hair (very little left these days) trying to find out where the short could be. The rotary switch bank is so basic that I can't see a problem with it; and all the connections in it make OK with the valve bases. I note that the cathode is grounded, selector numbers 0 and 1 are connected on the switch, I assume this to be correct. I have a copy of the circuits but although the print is clear they are not that easy to understand. If I remove the screen connection from the valve base unit at the switch, normal screen voltage returns. I thought the concept (saw it somewhere) was a grounded grid system with varying cathode voltage to measure ma/v. This appears not to be the case if the cathode is grounded. Any help would be much appreciated. Thanks, Mike.

1st Aug 2008, 7:00 pm	#2
Mike-repairman Heptode Join Date: Jun 2008 Location: Cambridgeshire, UK. Posts: 613	Re: Avo 2 Panel Tester More info... There are no shorts on the valve base unit with the main plug removed. There are no shorts between any of the pins on the valve base. Setting the wheels to numbers 1 2 3 4 5 6 7 8 9 0 restores the screen volts to the panel. Measured between grid or cathode to screen on the valve bases. Setting wheel 5 to htrs drops volts. Setting wheel 1 to Screen drops volts and setting it to some of the other positions give varying readings, even D1 and D2 and A2 settings. I have noticed that htr- and htr+ appear to be reversed when measuring to ground (cathode or grid). Htr ac volts appear on the htr- not htr+ (not sure if that is correct). The mystery deepens and I wonder if someone has swapped around some wires on the base unit at some time in the past 60 years. I have a spare main unit and will compare the two, but no spare valve base unit to check the input connections to it. Again any ideas very welcome.

1st Aug 2008, 10:05 pm	#3
Brian R Pateman Nonode Join Date: Jun 2005 Location: Western Lake District, Cumbria (CA20) - UK Posts: 2,136	Re: Avo 2 Panel Tester Hi Mike, If you haven't done so already have a look in the archive above (Valve Tester section) as we've had a few threads on these useful if basic testers. They do have certain limitations though and they aren't suitable for all types of valves. Regards, __________________ Brian

2nd Aug 2008, 7:18 am	#4
Mike-repairman Heptode Join Date: Jun 2008 Location: Cambridgeshire, UK. Posts: 613	Re: Avo 2 Panel Tester Hi Brian, I did look through the archive before posting, but there was nothing directly relevant to my problem. I still have some logical tests to do today, but suspect the problem may lie within the thumbwheel switch assembly which may need to be stripped down. Mike.

2nd Aug 2008, 11:33 am	#5
Mike-repairman Heptode Join Date: Jun 2008 Location: Cambridgeshire, UK. Posts: 613	Re: Avo 2 Panel Tester Brian, Success; switch dismantled cleaned adjusted and rewired and all working OK now, no shorts. Not quite sure where the shorts were but the only thing I could think of was the misalignment of the stop positions on the wheels; could have been shorting between two bussbar rods. It certainly needed a clean. One query; I saw a picture in one of the manuals of the designation of the 3 sockets on the top of the panel that are used for the top cap connections. Could anyone confirm that they are... black (left) for screen, brown (middle) for Grid and red (right) for anode. I'm sure that they had been miswired in the past. By the way, you are correct Brian that these old testers, that were really designed for the large 4,5, 6 pin and octal valves, don't handle some of the newer valves too well; however it tests all the ECC81/2/3 range and the EL34 etc. output valves that I use OK. Thanks, Mike.

2nd Aug 2008, 12:02 pm	#6
Mike-repairman Heptode Join Date: Jun 2008 Location: Cambridgeshire, UK. Posts: 613	Re: Avo 2 Panel Tester Just found the diagram; black (left) is anode, middle (brown) is grid and red (right) is screen. Strange configuration of colours? Can get the lid back on now.