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Old 31st May 2018, 10:05 pm   #1
fenland787
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Default EICO 667 Valve Tester Grid Pot Repair

I believe it is well known that these Grid Volts setting pots can be fried by faulty instrument set-up/valves.
Because they are relativity accurate (5%), high resolution (ie large diameter), high(ish) power at 4W and also incorporate a switch, replacements are hard to come by. I've found many posts out there bemoaning this!

Turns out my new-to-me 667 had suffered this fate in the past and been 'fixed' by shorting out the fried portion of the track with a copper bridge. This was a snare and delusion as, of course, it meant that over the range of the bridge the grid voltage was not actually changing - but with no independent voltage monitor I was blissfully unaware of the fact!

As a reasonable fix I installed a 5K 5W switched pot from Digikey - CT3025-ND - but as it was 20% and less than half the diameter of the original I was aware that the resolution/calibration could be a bit off. After some hunting I found several un-switched versions of the proper part (MALLORY M5MPK) on eBay and snagged one for about 15 + 14 P&P USD with the intention of using bits of it to replace the burnt out track/over-heated wiper.

To my joy, on opening the back I found that the rotor assembly was actually the same as the switched version, just missing the little pin that actuated the switch. 10 minutes later a small brass pin approx 2.6mm dia (1/10"?) by 4mm long was push-fitted and epoxied into the waiting hole, the switched rear plate transferred from the fried pot and life was good again.

I found a good guide to the 666/667 testers on the net with recommendations for mods to prevent these otherwise nice testers from self-destructing - stick a small ~150mA lamp bulb in series with the grid pot wiper and some back-to-back diodes on the meter movement, which I did!

Hope this may help anyone else who has this issue.

Adrian.
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Old 2nd Jun 2018, 8:32 am   #2
Diabolical Artificer
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Default Re: EICO 667 Valve Tester Grid Pot Repair

Glad you were able to fettle a replacement, luckily a lot of this old gear used the same pots from the same maker so it's possible to fix the issue.

Not sure I like the htr supplies on old valve testers, every valve has a different htr/filiment resistance and as such the voltage on the htr's at test changes, most of the time within the 5% or so as on the valve spec sheets, so for a good/bad type tester not a big issue. It can be an issue though if you have a borderline valve or are testing to more stringent specs.

Andy.
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Old 23rd Jul 2018, 12:50 pm   #3
Otari5050
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Default Re: EICO 667 Valve Tester Grid Pot Repair

We learn very quickly with this old gear!

In my enthusiasm to have a tube tester, I jumped on an Eico 666 (the beast) on that site, because it was a quarter of the price of any other of that model or of the 667 due to it 'not working, meter does not move'.
It then cost me close to three times what I paid for it to get it shipped down here to the bottom of the world.
While waiting for it to arrive I started reading up on them and learned that the meter movements can often be open circuit and there's no replacement for them!
My heart sank - as you can imagine.
It finally arrived and I was relieved to find the movement was intact!
After a few hours doing the requisite mods (small lamp in series with the grid pot, diodes across the meter etc), replacing a few out of spec parts, cleaning and lubing all the switches and just generally tidying it up I had a nice, fully working tester.
It's great to know that a very close match pot can still be had.
My panic about the inability to source a replacement meter was unnecessary as I found this site (http://www.alltubetesters.com/parts.htm) which stocks new replacement meters not that I should ever need one now that the mods have been done - but it may be of use to someone else reading this who owns a 666 or 667.
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Old 26th Jul 2018, 1:35 am   #4
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Default Re: EICO 667 Valve Tester Grid Pot Repair

I have a a 667 minus it's meter and kept an eye on the bay for 4 yrs waiting for a replacement to turn up with no luck so I looked through my stock of used meters and found one close in spec and same size, drew and laser printed the meter face - looks like the original, I just had to fit a shunt resistor to bring the electrical specs into line. I checked out the alltubetesters site but at $125 plus I think I'll stick with my home brew replacement.

The Eico valve testers could be purchased as either a kit or fully built, my first valve tester was a non working Eico 666 back when shipping could be had for AU$30 via surface mail, with internet copies of the user and construction manual I spent many weeks of lunch times trying to get the tester working with no luck. Then during one of my fault finding sessions I was measuring voltages with the front of the tester facing me and was trying to workout which switch connections to put the meter probes on while looking at the a diagram which showed the tester from the rear and it suddenly dawned on me that the switch was wired in a mirror image of itself, the tester had most probably been built from a kit - the switch incorrectly wired and had never worked from new, 5 min with the soldering iron and it all was all working. I must admit I do have trouble at times when measuring valve voltages from the top of the valve holder, not sure which hole is pin 1 I usually have to resort to turing the valve over noting pin 1 then turning it back the other way to see which hole lines up with pin 1, I guess I must be dyslexic.
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Old 26th Jul 2018, 1:21 pm   #5
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Default Re: EICO 667 Valve Tester Grid Pot Repair

I've heard similar story's of Eico and Heathkit gear having faults from when built, there must be quite a few out there like this if you think about it.

"I must admit I do have trouble at times when measuring valve voltages from the top of the valve holder, not sure which hole is pin 1" Yes, that's a tricky one to get your head round sometimes, I have trouble too. Once you've done it a few times it becomes easier.

A.
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Old 29th Jul 2018, 5:05 am   #6
FrankB
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Default Re: EICO 667 Valve Tester Grid Pot Repair

I have at least one 666 & 667.
Pretty fair testers.
Did you know they made an "add on" deck adapter for the 666, so it would test the more "modern "tubes? (Actually with the correct setup data, it would work with almost any old tester. Note: The I177 Military T/T had a "breakout box" type adapter with pin plugs you could "program" to different configurations to test tubes the I177 didn't have sockets for.

You can make the same type of adapter for a tester by wiring up the needed sockets pins in parallel, and then setting up a rotary switch bank to select which pin hooks to which element.
You can use a 14 pin or greater plug to connect to the socket pins of your tester. Some adapters used the grid/plate cap as a conductor also.

Quite likely you can find a schematic of one of the adapters online and bodge it together.

I did find some 24 position single pile rotary switches from either Moouser or Digikey some years back. Also scrapping out some makes of old parallel port selector boxes for computers had the same switches.
A bit hard to find, but Hickok and Heath also made them, as well as Coletronics.
The Coletronics and Eico ones turn up at times at hamfairs for about 15-25.00 USD.
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