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Old 10th Jun 2021, 2:47 pm   #1
retailer
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Default AVO two panel tester strange fault

I've had the control panel of a 2 panel tester for some years waiting for a socket panel to turn up at a reasonable price - looks like it isn't going to happen so I may make a banana plug and patch lead type of valve socket panel, however I hadn't ever powered up the control panel so I pulled it out of it's box (the same one it arrived in from Ebay newspaper packing is dated 2003) I hooked up a 6V6 to see if it worked or had issues, the 40 ohm meter swamp resistor was OC so I fitted a temp 40 ohm in it's place, it does seem to work but on checking voltages I found the anode voltage right up at 364V, some more probing and I found most of the anode voltages are over voltage by around 65%-70%. The anode/screen transformer has the correct 232V on the primary, all of the screen voltages are within 5%-7% it is only the anode voltages that are affected. The 12V and 30V are ok and within 10%, the voltages I measured are:-
80V is 109V 37% over
100V is 154V 64% over
125V is 177V 70% over
150V is 208V 72% over
200V is 292V 68% over
250V is 364V 68% over

Leaving it on for 30 min, the transformer did not even get slightly warm, I thought that maybe the transformer was OC between the taps and my DMM was getting some sort of capacitive coupling causing strange readings but a quick check with an Avo 7 found the voltages matched those I measured with the DMM. The circuit shows one continuous winding with taps, but as some of the voltages are duplicated it looks as if there maybe two sets of windings one for screen and one for anode, just wondering if anyone has any ideas.
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Old 10th Jun 2021, 3:01 pm   #2
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Default Re: AVO two panel tester strange fault

The AC anode supply voltages are nominally 1.4 times the DC anode voltage selected +- a tolerance, your voltages are within tolerance according to the manual:

80 volts DC =106.5 to 117.5 volts AC
100 vots DC = 133 to 147 volts AC
125 volts DC = 166 to 183 volts AC
150 volts DC = 199.5 to 220.5 volts AC
200 volts DC = 266 to 294 volts AC
250 volts DC = 332.5 to 367.5 volts AC

Anode and screen use the same transformer winding.

Lawrence.

Last edited by ms660; 10th Jun 2021 at 3:14 pm. Reason: extra info
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Old 10th Jun 2021, 7:03 pm   #3
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Default Re: AVO two panel tester strange fault

Ref your need to move on and use a patch lead solution instead of a "proper" valve base panel with a rotary switch.
I have done this and it works well. I have also made up a matrix of brass strips, two rows at right angles to each other with insulation between rows and between the two sets of rows and used ordinary 4mm "banana" plugs to short between the cross points to program the connections.
see the pictures.

Peter
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Old 11th Jun 2021, 3:36 am   #4
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Default Re: AVO two panel tester strange fault

I was assuming that the 2 panel tester operated on a similar principal to the MKI, MKII, MKIII etc, without going back and checking the documentation, I recall the procedure for calibrating these starts out by checking that the relationship between the marked anode voltages and measured voltages is 1.1, so when the anode voltage is set to 200V the measured voltage will be 220V perhaps the 2 panel tester is not like this, I have an Avo setup/test document for the 2 panel that states that when measured with an Avometer the anode voltages will about 20%-30% higher than that marked on the control panel, my voltages are clearly way over this. It would make sense to use the same windings for anode and screen, but the transformer on my 2 panel has separate sets of terminals for the anode and screen voltages, the 250V anode terminal measures 368V and the 250V screen terminal measures 265V, in addition there is around 125 ohms resistance between the anode 250V terminal and the screen 250V terminal, to me this would indicate separate windings, I still think something is not right, just wondering if any forum members with 2 panel testers would mind checking the 250V anode voltage.

Electronpusher0 your patch panel looks good and also is relatable to the data book roller switch settings, I was planning on something simpler and was wondering how I could make it relatable to the data book - I'll have to give it some thought.
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Old 11th Jun 2021, 5:39 am   #5
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Default Re: AVO two panel tester strange fault

Looking through my PDF Avo documents I found one titled AVO EMER, 86 pages, the service info on the 2 panel gives permissible limits for the anode voltages and mine seems to be ok if I ignore the odd volt or two over, so nothing to be concerned about, so now it is just the meter to concern myself with, it is a bit sticky and needs a some good taps to help the needle along.
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Old 11th Jun 2021, 5:44 am   #6
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Default Re: AVO two panel tester strange fault

The two panel tester predates the operating principle of the later single-box AVO valve testers, and works differently.

Original 1938 patent attached.

Craig
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Old 11th Jun 2021, 11:44 am   #7
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Default Re: AVO two panel tester strange fault

Quote:
Originally Posted by retailer View Post
It would make sense to use the same windings for anode and screen, but the transformer on my 2 panel has separate sets of terminals for the anode and screen voltages, the 250V anode terminal measures 368V and the 250V screen terminal measures 265V, in addition there is around 125 ohms resistance between the anode 250V terminal and the screen 250V terminal, to me this would indicate separate windings, I still think something is not right, just wondering if any forum members with 2 panel testers would mind checking the 250V anode voltage.
It should become clear if you look at the schematic in the AVO EMER manual in the link below:

https://frank.pocnet.net/instruments/AVO/index.html

The 250 volts g2 is from a separate tap to the 250 volts anode supply, that's because although the voltage for the anode is given in "DC" terms the actual voltage is AC and it's nominally 1.4 times greater than the indicated "DC" voltage on the anode voltage selector switch, hence your resistance reading on the 250 volts settings between those two selected voltages of the same "DC" magnitude.

Lawrence.

Last edited by ms660; 11th Jun 2021 at 11:50 am. Reason: extra info
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Old 14th Jun 2021, 1:17 am   #8
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Default Re: AVO two panel tester strange fault

I read the patent document, it is quite interesting - I can see how Avo managed to do away with a grid bias control, it is quite clever, quite simpler and easy to use provided one is only looking to test for 'goodness' - the shorts/leakage test though does look to be an afterthought.

On the subject of the meter - I spent an hour or so on the meter and managed to get the tiny bits of metal stuck between the pole pieces out using a thin strip of masking tape wrapped around a bit of brass shim - the pointer now consistently stops on zero without resorting to tapping the case, the bits of metal look very much like strands of steel wool, I initially tried to coax them to one side where they wouldn't get in the way but they seem to have a mind of their own and work their way back.

Looks like the magnet has lost some power over the years as FSD is now 700uA, resistance is 26 ohms, I played around with an online meter shunt calculator (https://www.calculatoratoz.com/en/sh...lator/Calc-483) to see the affect of changing the swamp resistor value, if I reduce the swamp resistor from 40 ohms to 17 ohms the effective meter resistance will be 43 ohms (swamp R + meter R), the 90, 9 and 1 ohm shunts will all work ok - however the Avo service/setup document states that the swamp resistor value should be kept as high as possible to minimise temperature variations affecting the calibration, I can't see how this could be, 700uA through a 17ohm resistor is only microwatts of power, am I missing something here ?
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Old 14th Jun 2021, 7:58 am   #9
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Default Re: AVO two panel tester strange fault

https://electricalbaba.com/why-swamp...ce-in-ammeter/
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Old 14th Jun 2021, 10:27 am   #10
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Default Re: AVO two panel tester strange fault

Thanks for the link for the how and why of swamping resistors and the effects of ambient temp change. I did try it though and ignoring the effects of temp change found it was not a 100% cure for a low reading meter, at 100mA and 10mA my meter read within 5% so that was fine - at 1mA however if read around 15% low, attempting to bring the 1mA scale into line will affect the other 2 also the 90 ohm shunt is not easily changed as it is a variable resistor, so rather than mess around with shunts etc it's clear I need to take a different approach.
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Old 14th Jun 2021, 6:30 pm   #11
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Default Re: AVO two panel tester strange fault

Take a look at this thread:

https://www.vintage-radio.net/forum/...d.php?t=162366

In the first post I attached a schemetic of the meter circuit redrawn to make it clearer what each component does.
I also attached an excel spread sheet that allows you to modify the elements, including the meter sensitivity, to see what the effects will be.

Peter
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Old 14th Jun 2021, 11:39 pm   #12
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Default Re: AVO two panel tester strange fault

I recall reading your post back when you made it and just re-read it - your meter issues were the same as I'm having now, my meter has 26ohms resistance and has a FSD of 700uA, also my pot is 90ohms, my swamp resistor has corroded through at the midway point so for now I'm just using regular metal film resistors, I've ordered some magnets but have not yet decided if I'll try to remagnetise the movement or make up a new magnet assembly much like you have done. I still need to make up a socket panel so once the control box is fixed/working it will be a while before I have a complete instrument.
I'm just finishing up an Eico666 - there are those that think Avo data manual errors are annoying - they are nothing compared to Eico666 errors.
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Old 15th Jun 2021, 5:34 am   #13
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Default Re: AVO two panel tester strange fault

I am not sure If I have mentioned this before so forgive me if I have.....

There is some debate as to whether the AVO 2 panel used the same movement as the AVO 7.
I have looked at this and have concluded that the answer is a qualified yes, at least with the early Avo 7s with the horseshoe magnet.

It is somewhat confusing as the AVO 7 is 1mA fsd and the 2-panel meter is 0.6mA.
BUT the AVO 7 meter subtends a far greater angle from 0 to fsd.
The 2 Panel uses the same movement but the angle from 0 to fsd is only 0.6 that of the AVO 7, hence 600uA fsd.

The complication comes in that the 2 panel has a longer pointer so that the reduced angle still sweeps a decent scale length.

To use an AVO 7 movement in a 2-panel valve tester you would have to find a way to extend the pointer and re-balance it by adding weight to the other end.

Peter
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