AVO Two Panel Valve Tester

[Technomaniac]

Early in the year I bought an AVO two-panel valve tester. It seems that this was their first effort, and it wasn't allocated a model number. I have finally a pressing need to use it to test some Australian Made valves which none of my American made testers have heard of. An instrument of such vintage and versatility deserves the respect of having a decent tidy-up before having power applied, and that is presently under way.
For any fellow-owners of the model, there is something worth inspecting, I have noticed that there are two power transformers, each having a number of white tagstrips attached. I spotted a potential problem, in that several of these tagstrips have warped, and the tags close to their centres are pressed into the steel framework which holds the laminations together. The framework is probably coated with something, but it seems to me that a layer of insulating material needs to be inserted in between, under each warped tagstrip. In my case I cut some strips of Laminex and carefully drilled holes to accomodate the brass hexagonal nuts, this is necessary as the throughbolts have insufficient length otherwise. If a tag or two were to become electrically connected to the core, a transformer may be lost, and thus the instrument written off.
In the notations I downloaded shortly after receiving the instrument, there was mention of possibility of the device going into oscillation when testing some particular valves introduced much later than the date of manufacture of the tester, and in another place I came upon a fellow who was fitting ferrite beads to things. I presume that this was to correct the instability.
So I am wondering if anyone can point me in the direction of a list of modifications. There are two paper capacitors I have replaced, a new power cord has been fitted, I used three-wire cable but not earthing anything at this stage, I will be running it from a 24 to 220 volt inverter which has the output isolated, the tester has a steel cabinet, if I decide to earth it I will probably insert some sheets of insulation between the tranny cores and the case. The power switch measured OC, it is impossible to access, I gave it a squirt of CO which leaves no residue, and worked the toggle to and fro which restored contact for the present. I want to free up the thumbwheel switch bank, and then I might be close to applying power, after a quick run around with an ohmmeter and a Megger. Where do I fit the beads - on the pins of EVERY valve socket?

[Electronpusher0]

Originally Posted by Technomaniac

Where do I fit the beads - on the pins of EVERY valve socket?

Unfortunately yes.


For the roller switches see this thread
https://www.vintage-radio.net/forum/...ghlight=tester

The most important thing to do is to check the meter sensitivity before embarking on other work, the horseshoe magnets are notorious for losing their magnetism. see here for calibration
https://www.vintage-radio.net/forum/...3&postcount=27
and here
https://www.vintage-radio.net/forum/...9&postcount=28



For more information on restoration see this thread
https://www.vintage-radio.net/forum/...d.php?t=167221

Peter

[Technomaniac]

Thanks for the response. I have gathered the information from all of the links for digestion. I seem to have an extra toggle switch mounted above the meter, haven't seen it in any photos, must check that out. The thumbwheel switches number only 9, and the American Compactrons have a few more pins than that, so don't know just yet how or IF that can be easily catered for. There is, I think, a socket for those miniatures used in the German radiograms which have the glass spigot instead of a gap in the pin circle, I think they may also have ten pins, if so, that may be catered for already. At the moment I'm in the process of removing the thumbwheel switch to see if I can free it up, but so far only the panel screws have been removed, I'll read the postings about servicing the switch before proceeding. Thanks again.

Regarding the possibility of weak meter magnet, when I was still at primary school, my late father constructed fairly quickly to help with some task I was involved with, what he called a "Magnet Charger" which was in the form of one he had used many years before, for pepping up magnetos in early motor cars. I believe that device is still in my collection SOMEWHERE..... the laminations of an old radio power transformer with the centre leg cut away, and a winding on each outer leg. Six volt DC was the power source. It was wound with two strands of enamelled wire,(to increase the current to a suitable level) so maybe the ends can be reconfigured for 12 v operation.

[Electronpusher0]

There have been various attempts to re-magnetise the magnet in the 2-panel tester, I have built a purpose made magnetiser but have not had any success, the only solution I found was to replace the magenet with a rare earth magnet and pole pieces. See here:

https://www.vintage-radio.net/forum/...ght=magnetiser

Peter

[Technomaniac]

Hi. One thing you may not have known - The instructions I was given, Place the magnet on the charger (You'd have to determine North and South on both, and whether you'd need like to like or the reverse I'd have to experiment to find out) Magnet on charger, power on the coil, Hit magnet with small hammer - a SHARP tap, Power off, all done. I wonder if you skipped the SHARP TAP step. I certainly wouldn't have thought of it, had I not been told.

[Electronpusher0]

That's interesting, no I did not give it a tap, definately food for thought.

As I said in post #2 the first thing is to check the meter sensitivity. Not all of them have lost magnetism, I have had 3 through my hands and 2 had lost magnetism. I acquired the latest one with the intention of replacing the meter with a digital panel meter as an experiment (the valve panel is incomplete, missing the roller switch) but on checking, the meter is in calibration so I cannot bring myself to strip it.

Peter

[Electronpusher0]

The operators manual is available for this tester but I have a very good copy of the original manual so I have scanned it to pdf.
If you print this double sided (flip on the long side) you can reassemble it as a booklet to the same size as the original.

Peter

[Technomaniac]

Thanks for all of the help. The project has just been removed from mothballs and plugged into power through a 60 watt lamp to dry things out properly. I still don't know much about the "Navy Version", I DO have an extra toggle switch mounted just below the meter, I am led to believe that there may be a lamp that shines through the meter glass when an overload condition occurs, but I have seen a photo of a very substantial relay which is purported to be in the Navy Version, I don't seem to have one of those, so there may be a couple of different "Navy Versions". I found reference on this forum somewhere about a diode that had been replaced, and connected backwards, causing the meter to go left past zero unless the "set zero" or "set mA/V" (I can't remember which) was set at max clockwise. Mine is doing exactly that, but I think the diode is correct to schematic. I've been confused before by the old diodes that are marked + and - instead of showing the stripe for cathode, but I've had a modern diode test DMM across it and it looks correct to me, that's if it is shown correctly in the schematic. I haven't fathomed how it all works so could easily have confused myself. I was concerned when I found a pencilled X on one power Tx tagstrip thinking I may have been stuck with a crook tranny but I've had power applied through the lamp for an hour and no smoke or excess heat so far. I haven't yet plugged in the socket/pinswitch panel. Also have yet to run around the resistors, the caps I have changed, and power cable. Had to do a bit of work on the tagstrips of the power trannies, they had buckled, putting their centre tags flat against the outer laminations or mounting brackets. I made Laminex strips which in most cases had big holes at the ends to go over the hexagonal spacer nuts, I cheated on one or two where it wasn't possible and made the strips shorter. I don't remember what secures them but I must have used something. If anyone has any more info about that diode, polarity wise, it'd be appreciated. I prefer not to turn it around on spec. We've had terrible weather here, perspiration running down the arms while you're working indoors, and you can feel a real BITE when outside, the UV must be really high. Really discourages one from doing anything but the simplest of jobs.

[Technomaniac]

IS the diode SYMBOL correctly shown in the schematic? I think it may be reversed. My unit agrees with the schematic. The diode that is fitted has + and - markings and no symbol. But using a modern diode test function on a DMM confirms that it is fitted in agreement with the schematic.
It seems to me that the diode should be reversed, and fitted in the reverse direction to that shown by the symbol on the schematic.

[Station X]

Diode markings can be confusing. I don't know if it applies here, but when used in a simple rectifier circuit (transformer secondary, diode and smoothing capacitor), the -ve end of the diode will be the positive side of the supply. Modern diodes have a ring on the -ve end which will be positive when used in a PSU.

[ms660]

Originally Posted by Technomaniac

IS the diode SYMBOL correctly shown in the schematic? I think it may be reversed. My unit agrees with the schematic. The diode that is fitted has + and - markings and no symbol. But using a modern diode test function on a DMM confirms that it is fitted in agreement with the schematic.
It seems to me that the diode should be reversed, and fitted in the reverse direction to that shown by the symbol on the schematic.

The diode symbol is shown correct in the schematic, it supplies the current that opposes the standing anode current that flows through the meter (set zero) The +ve output from that diode supply goes to the meter -ve via the set mA/V control etc and the -ve output from that supply goes to the meter +ve, enclosed is a simplified drawing I did some years ago showing what's what if it's any help.

Lawrence.

[retailer]

My early MKII VCM was fitted with diodes (black with metal end caps) that were painted red at the cathode end, at first I mistook this for the anode and assumed they had been installed incorrectly.

[Technomaniac]

Thanks, MS660 and retailer. My diode looks like a Philips cap with + and - marked. I guess the next step is run through the test procedure, then to plug in a bottle and see if I have a reading that's on the scale.