AVO Mk.IV Valve tester problems - UK Vintage Radio Repair and Restoration Discussion Forum

dominicbeesley · 7th Mar 2005, 1:55 pm

Sorry in advance if this is the wrong forum, I know its not a wireless!

Yesterday when trying to test some PCL82 triode/pentodes on my AVO I encountered a problem with the cutout kicking in. The valves tested ok on Cold/Hot/Insulation but were kicking off on test, even though it appeared to only be drawing 2mA! Is this something to do with the fact that its a triode/pentode. When testing the triode section should I be doing something special with the screen setting?

After trying a third valve its now died completely, the fuse in the plug and the one in the back of the chasis are ok, so I'm gussing its the cutout itself thats broken. Has anybody got any hints on how to best overhaul this. It all looks a bit delicate to me!

Cheers

dom

Sam · 7th Mar 2005, 2:23 pm

I have use the one at Uni (which I have strongly hinted I would like to have!

). There is a 'normal/unlimited' switch. When testing multiple valves you have to set this to 'normal' or 'unlimited' dependant on the valve.

Hope this helps - I cannot help with the cut-out!

Sam

XTC · 7th Mar 2005, 2:33 pm

Try turning the power off and gently rocking the AVO back and forth a couple of times if the trip goes. They don't always reset themselves.

Pete.

dominicbeesley · 7th Mar 2005, 11:05 pm

Cheers, yes it appears it was stuck open, Ive now poked at the cutout and given it a clean, it was pretty grubby and was not always snapping shut again.

Also, is it supposed to stay set until powered off. Mine just clatters and sparks but doesnt actually lock completely. The pygmy bulb is blown, is that part of the circuit?

My set doesn't have a limited/unlimited switch so maybe its a Mk.III, there's been some confusion in the past as it has a weird set of valve bases on the top. Somebody said it may be some special "military" spec one?

I've had problems in the past testing some big TV pentodes 6P28s where it does this but they looked fairly hefty (and had big currents to boot) so I put it down to them being a bit out of range. But these PCL82 valves are fairly weedy so I wouldn't expect them to bother it. Its fine testing EL84s, 6G6s etc which pull far more current. I'll be a bit miffed if I can't get it going as I've just drilled out a chasis ready to put the valves in for my new HI Fi amp!

Cheers

Dom

Sam · 7th Mar 2005, 11:51 pm

http://www.duncanamps.com/technical/avotesters.html

There are pictures of the different types here. The one at Uni is definitely a Mk IV. The Normal/Unlimited switch is the toggle switch to the right of the ~ set one (I think!)

Sam

XTC · 8th Mar 2005, 1:28 pm

Quote:

Originally Posted by dominicbeesley

Cheers, yes it appears it was stuck open, Ive now poked at the cutout and given it a clean, it was pretty grubby and was not always snapping shut again.

Also, is it supposed to stay set until powered off. Mine just clatters and sparks but doesnt actually lock completely. The pygmy bulb is blown, is that part of the circuit?

Dom,

It sounds as if this is a MkIII and the pygmy bulb is a part of the circuit of the trip. The trip is supposed to stay set until power-off. If you let it chatter and arc it will burn the contacts. The pygmy bulb is a bit odd judging from the manual as it is rated at 200V 15W.

You can download the manual here. http://bama.edebris.com/manuals/avo/

Pete.

dominicbeesley · 8th Mar 2005, 11:48 pm

Cheers to the two previous posts.

From the pictures it definitely looks like a Mk.III (with a few extra sockets).

I'll put the bulb back tomorrow, though don't know where I'll stick it - it used to be attached to the back of the meter assembly but that's all in bits since I put in an op-amp/new meter setup.

I've had a look now at the circuit diagram for the cut-out and /think/ I understand it.

Still not sure why it was kicking in though on these weedy little valves!

Thanks again

Dom

martin4 · 3rd Apr 2005, 8:31 pm

If it is a mark 3, you have to turn the screen volts to zero when checking the triode, as the anode volts on the pentode is still present . the triode has a g1 volts of about 0-2 so the pentode is then seriously over run.
Martin.

7th Mar 2005, 1:55 pm	#1
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	AVO Mk.IV Valve tester problems Sorry in advance if this is the wrong forum, I know its not a wireless! Yesterday when trying to test some PCL82 triode/pentodes on my AVO I encountered a problem with the cutout kicking in. The valves tested ok on Cold/Hot/Insulation but were kicking off on test, even though it appeared to only be drawing 2mA! Is this something to do with the fact that its a triode/pentode. When testing the triode section should I be doing something special with the screen setting? After trying a third valve its now died completely, the fuse in the plug and the one in the back of the chasis are ok, so I'm gussing its the cutout itself thats broken. Has anybody got any hints on how to best overhaul this. It all looks a bit delicate to me! Cheers dom

7th Mar 2005, 2:23 pm	#2
Sam Hexode Join Date: Apr 2003 Location: Higham-on-the-Hill, Leicestershire, UK. Posts: 338	Re: AVO Mk.IV Valve tester problems I have use the one at Uni (which I have strongly hinted I would like to have! ). There is a 'normal/unlimited' switch. When testing multiple valves you have to set this to 'normal' or 'unlimited' dependant on the valve. Hope this helps - I cannot help with the cut-out! Sam __________________ Can he lead a Normal life, Doctor? No, he will be ... an Engineer.

7th Mar 2005, 2:33 pm	#3
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: AVO Mk.IV Valve tester problems Try turning the power off and gently rocking the AVO back and forth a couple of times if the trip goes. They don't always reset themselves. Pete.

7th Mar 2005, 11:05 pm	#4
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: AVO Mk.IV Valve tester problems Cheers, yes it appears it was stuck open, Ive now poked at the cutout and given it a clean, it was pretty grubby and was not always snapping shut again. Also, is it supposed to stay set until powered off. Mine just clatters and sparks but doesnt actually lock completely. The pygmy bulb is blown, is that part of the circuit? My set doesn't have a limited/unlimited switch so maybe its a Mk.III, there's been some confusion in the past as it has a weird set of valve bases on the top. Somebody said it may be some special "military" spec one? I've had problems in the past testing some big TV pentodes 6P28s where it does this but they looked fairly hefty (and had big currents to boot) so I put it down to them being a bit out of range. But these PCL82 valves are fairly weedy so I wouldn't expect them to bother it. Its fine testing EL84s, 6G6s etc which pull far more current. I'll be a bit miffed if I can't get it going as I've just drilled out a chasis ready to put the valves in for my new HI Fi amp! Cheers Dom

7th Mar 2005, 11:51 pm	#5
Sam Hexode Join Date: Apr 2003 Location: Higham-on-the-Hill, Leicestershire, UK. Posts: 338	Re: AVO Mk.IV Valve tester problems http://www.duncanamps.com/technical/avotesters.html There are pictures of the different types here. The one at Uni is definitely a Mk IV. The Normal/Unlimited switch is the toggle switch to the right of the ~ set one (I think!) Sam __________________ Can he lead a Normal life, Doctor? No, he will be ... an Engineer.

8th Mar 2005, 11:48 pm	#7
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: AVO Mk.IV Valve tester problems Cheers to the two previous posts. From the pictures it definitely looks like a Mk.III (with a few extra sockets). I'll put the bulb back tomorrow, though don't know where I'll stick it - it used to be attached to the back of the meter assembly but that's all in bits since I put in an op-amp/new meter setup. I've had a look now at the circuit diagram for the cut-out and /think/ I understand it. Still not sure why it was kicking in though on these weedy little valves! Thanks again Dom

3rd Apr 2005, 8:31 pm	#8
martin4 Retired Dormant Member Join Date: Mar 2005 Location: Devon, UK. Posts: 33	Re: AVO Mk.IV Valve tester problems If it is a mark 3, you have to turn the screen volts to zero when checking the triode, as the anode volts on the pentode is still present . the triode has a g1 volts of about 0-2 so the pentode is then seriously over run. Martin.