A tense moment with the AVO Mk.IV VCM

[Lucien Nunes]

Yesterday I was testing some output triodes - PX25, P27/500, P26/500 and best of all some DA30s. These are the sort of valves, especially NOS M/O ones, where you call someone over to double-check the tester settings before plugging in - I'm not sure what's more valuable; the valves or the tester. Not often does the bias control get turned right up to -100V either.

Anyhow the PX25s turned out to be excellent and all was going well until I put the second DA30 in. It made a few little pings as the heater warmed up, the Ia and Gm came up OK but it carried on pinging and creaking. I decided to turn the HT off and let the filament settle in for a while. As my fingers reached the knob the cutout tripped, so I flicked the mains switch off. I did not see anything happen on the meter.

I pulled the valve out, returned the tester to 'set ~' and switched back on. Red light came on but needle didn't move. Wriggled the switches gently, but not the slightest activity. I could hear from the background hum that the HT transformer was definitely on, Which left me thinking that not only do I have a shorted DA30 but it's killed the meter. I put a good valve back in and clicked the electrode selector round, no reading of heater continuity. Moving onwards, heater lit up, no shorts (not that I expected a reading) and the anode current is... Oh, wait, what if the bias supply is duff now? Get DMM, pull valve out, switch to test, and yes, all the voltages are there. Put valve back, select 100 mA and... up comes the reading. Ahhh.

The fault was half of R19 2x1.48MΩ open circuit. It's the multiplier that sets the FSD from the rectified 150V tap for set~ and leakage tests. That's not even in circuit when the valve is under test for Ia or Gm. So why should it suddenly go open when a valve decides to floor the HT? It has to be pure fluke, or some kind of flashover at the rotary switch. So this morning I made up and fitted two 1.48M resistors, checked the switch carefully, recalibrated the affected settings in the tester, and it works fine. And so does the valve. Creaking has stopped, can't provoke a flashover or any other misbeaviour, readings are steady. I'll keep a close eye on it when it goes into the amp though!

An odd little moment of panic sent to remind me to keep looking after that meter movement (yes the shunt diodes have been checked!). By the way these DA30s aren't going near any weirdo single ended HiFi wannabees, they are in PP driving 6-foot Vitavox horns!

Lucien

[oldticktock]

Hi Lucien

Phew! that's a relief, I don't think my nerves would have remained intact after that little foible, indeed the pained scream of Nooooooooooooo! would have travelled far.

great result and as you say, and serves as a reminder.

[murphyv310]

Personally I would do a quick check on ohms for obvious faults like collapsed grids and heater cathode shorts before plugging a valve into a tester, it could save a bit of grief!
Glad the tester is OK though.

[Lucien Nunes]

I totally agree about the preliminary checks on these mechanically fragile valves - never start testing the characteristics without a visual inspection and checking for shorts first. And of course never turn your back on the tester while the HT is on.

There was one new-looking PX25 that did not make it through the visual and has not been tested. The whole electrode assembly is tilted to one side as though it has been dropped. A pity, but it has to happen sometimes.

Even if I think the valve is good, every test starts at the beginning with leakage cold and hot and HK insulation. I've caught a couple of rogues recently including a KT88 from an amp that was working five minutes beforehand, that had a solid HK short by the time it reached the tester.

One slip I've made before was when testing rectifiers after doing a load of output valves. I had got into the routine of turning the electrode selector round as far as A1, then moving on. One of the rectifiers had a short but I didn't spot it because I didn't go as far as D1 & D2. In other words I tested all the electrodes it didn't have but not the ones it did. Got fireworks as soon as there were volts across it.

Lucien

[Dekatron]

Hi Lucien,

After speaking with a support engingeer, who knew about these resistors since he had worked with them, at Welvyn about the old resistors used in the AVO valve testers I always change all of them. He told me that the reliability of these resistors is very poor after just 10 years, and now these resistors are 40-50 years old. He told me that most of these resistors have a poor coating making air and moisture enter them and corrode the resistance track which will make them susceptible to sudden breakdowns. He was the one who recommended me to change them all after I had asked him about the strange behaviours I had with several of these resistors. It is not uncommon that the black paint is flaking from the heat produced inside these resistors and also from the corrosion.

It is sad that it is hard to keep these testers with their original resistors but they work so much better with newer resistors. And I always recommend people to change all resistors, capacitors and diodes if they are concerned about the valves they test. Sometimes shorted or open circuit resistors will damage the valve really bad, as I know you are aware of.

So if you are not into preserving your tester in original condition I really urge you to change all components for newer types.

[Lucien Nunes]

I'm sure that's good advice. I do like to keep stuff in original condition where possible, but this is a tool and it has to work well. In fact if it is possible to collect two similar items, I like to conserve one with all original parts, non-working usually, and one where I can change as many parts as I like to get it working 100%.

The trouble is that with a popular item such as the AVO tester, which still has a commercial use outside of the vintage world, there won't be many surviving exactly as they were when they left the factory to conserve. Plus I can't really justify buying another one just to have it sitting here as a museum piece, but it ought to be done.

I might start collecting together a complete set of resistors, although the tester can't be pulled apart quite yet - must crack on with getting some music out of the amplifiers...

Lucien

[Dekatron]

I agree fully with your comment that it is nice to have an original instrument no matter what instrument it is, but if you really need a work horse it is always good to follow the recommendations from the manufacturers of the components (if you can still get any of those recommendations). I was lucky in finding someone at Welvyn who had worked with these resistors and knew what how they degraded over time,unfortunately that person is no longer with Wlevyn and I have not had any contact with him for a few years time now.

I tend to only buy equipment that is beyond the condition where it is possible to restore it, or keep it, in original condition as I usually use them as work horses. I leave the really nice examples to others who are more into collecting items than me.

In this link: https://www.vintage-radio.net/forum/...=avo+resistors on the forum I have included two files with lists with resistor values that can are combined into the necessary resistances in the AVO Mk IV and AVO Mk III which I have used several times. These resistor values are made up from series connected values from the E12 up to E96 values of metal film resistors. I have also checked these values so they can withstand the power and the voltage levels (something which some people forget to check when they buy resistors today as cheaper resistors have lower voltage ratings) for 200V rating on each resistor! many metal film resistors have voltage ratings of only 150V or 200V of the cheaper brands (many cheap brands sold on eBay have these lower ratings) but 350V on the more expensive brands like Vishay and others. I also use clear PTFE shrink tubing to seal them in to protect for shorts to components nearby as PTFE withstands a hot soldering iron very well when it is short on space when soldering!

[Guitarist28]

Hi Martin,
having read this thread it has made me a little concerned about my Mk1 VCM which does appears to be working well but I would imagine that its very much a 'stock' tester although it has be refurbished by one of our very competent and respected forum members.

I have tested a large number of valves using this and the results that i have so far have been suprising good and I would say that they have been within the 10% tolerence (I might be over optimistic here!!) of the Avo data manuals.

I would be loathe to change anything within the unit and then find that i have a problem al la 'if it ain't broke don't fix it!'

best Regards

Rob

[Dekatron]

Rob,

The AVO Mk I & Mk II does not contain many of the black Welvyn resistors that I speak of, only a handful of them and they are of an older type that seems to be holding out much better than the types in the AVO Mk III & Mk IV. Since there are so fewresistors of this type it is quite easy to exchange them if necessary, just remember what power and voltage they need to withstand. There are also a few white ceramic resistors in the AVO Mk I & II that needs to be checked, especially those under the valve panel. So if you want a work horse I can recommend you to check those resistors and replace all that are outside of their specifications.

If an AVO Valve tester has been stored properly and not been subjected to too much moisture over the years it will probably work quite well with the original resistors in place. But if it has been stored in cold and damp places it would probably need an overhaul and replacing the resistors is a good start. As with all testing equipment they will work for many years after their expected lifetime if they are treated properly, but unfortunately there are not many of those around yet, and most that I see that are sold are not in as good condition as the sellers think. As with all components there are also good and poor batches of components and the resistors I was discussing with the Welvyn employee were only those used in the AVO Mk III, Mk IV and the later VCM163.

[Lucien Nunes]

'if it ain't broke don't fix it!'

My preference too, unless I am deliberately overhauling something. But how do you know when it's broke? In this case it was obvious, the tester suddenly stopped working. Had it just been a case of slow drift then I might have tested loads of valves before realising it was giving marginally incorrect results. OK, you can do a quick check on the basic calibration at the beginning of a session, I sometimes do this with a pair of valves kept just for that purpose, but to prove that all ranges and settings are giving equally accurate results takes time that could be better invested in making it inherently more stable and reliable.

Lucien

[Dekatron]

I agree to that expression too, 'if it ain't broke don't fix it!', but only sometimes!

Instead of endangering one of my very very rare valves I choose to exchange all old components in those instruments I use for new ones when I have become aware of problems with for instance a particular brand or type of components. And unfortunately most old components degrade and especially those used under heavy load or those that has been stored poorly.

So I save time and money by replacing components when I get a new (old) instrument which I would otherwise have to spend later in faultfinding or when it wrecks a rare valve.

When I got my first AVO Mk IV I spent a good deal of money on getting hold of original components, silicon diodes, resistors and capacitors which took a considerable amount of time and money. When I got my second AVO Mk IV it was in such poor shape that I had to strip it completely of components putting new ones in. When I compared these to the last one was giving more accurate and more reliable results, the same result every time. I then decided to refurbish the first one with new components and then it became as good as the second one. Now I started to test the resistors and it was also at this time I contacted Welvyn. After exchanging a few emails with their support engineer where he explained how these old resistors behaved after all these years I understood that it was the right move for me who wanted a working tester, without the risk of blowing up my valves, to exchange them all.

So for me it is really a decision on why I want my instruments, as fully reliable working instruments or for collecting them and using them and at the same time being aware of the risk they pose every time I use them.