kit failure
As you might imagine there are times when things just don't work out with a new kit idea. Here is an example: solid state replacements for the tubes in a VTVM. In the 1980s I bought a Heathkit IM-18 VTVM from Advance Schools in the Chicago area. They had run a classified ad in the Chicago Tribune for kits, I also bought the RF signal generator IG-102.
Fast forward roughly 40 years and I notice on the internet the idea of using a voltage regulator circuit using an LM317 in place of the C cell in a VTVM. This was a pretty easy modification to make using an inexpensive module from ebay. However I managed to crack the plastic meter cover of the IM-18 during the process of adding the mod. Perhaps that was a bad omen ? I also changed the incandescent bulb to an LED bulb. And I changed the high voltage electrolytic caps as well.
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| mods on the left: the LED lamp socket provided ground and 6.3VAC to the LM317 module |
Then I remembered that Heathkit used to sell an accessory to add solid state "tubes" for use in a VTVM (note that the IM-18 was $47.95 at that time so the solid state tubes were an expensive add-on.
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| from December 1978, price inflates to $85.20 in 2022 dollars |
After learning from our fellow travelers, the tube audio enthusiasts, that a pair of LND150 depletion mode, high voltage, n-channel MOSFETs might be usable to replace the dual triode in the VTVM (and the IG-102 as well?), I thought maybe a no-touch kit is in order ?
So I designed two circuit boards (actually used some high-school geometry to calculate the tube pin coordinates), one with two diodes for the 6AL5 and the other with two LND150 MOSFETs for the dual triode. I also found some nice gold plated pins from Mill-max and away I went.
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| 6AL5 replacement on the left, dual triode replacement on the right |
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| assembled versions, note the reversed diode, top left |
But then things started to go wrong. I tried the dual "triode" board in the Heathkit IG-102 signal generator. It did work except on the top two bands. In addition, there was a 6AN8 tube in the signal generator which is a triode-pentode - no easy way to create a pentode. Also while the stability was a little better than the tube it wasn't super great. Anyway, STRIKE ONE !
I tried the board in the Heathkit IM-18 VTVM. The DC and AC measurements worked but not AC. I finally realized that one of the diodes was reversed on the 6AL5 dual rectifier replacement board. Reversed the diode and AC worked but call this STRIKE TWO ! (two components on the board and I got the placement of 50% of them wrong :(
Finally I sent the solid state replacement kits to my beta tester. He had some problems with AC measurements and damaged the 6AL5 rectifier replacement. I tried duplicating the problem with my VTVM and promptly tripped a circuit breaker. That was it: STRIKE THREE ! (I don't want customers tripping circuit breakers in their houses or damaging their kits!)
Since these are solid state devices, what about using them with batteries ? NOPE, tried that and it didn't work. I know that others have converted old Heathkit tube gear to solid state and battery operation but it's not a "touchless" conversion, the circuit has to be modified. And the JFET through hole transistors used in those conversion circuits are hard to find in 2022.
Also, the square boards were awkward to deal with, round might have been better ? The original replacements had cylindrical covers that were easy to grab for insertion and removal.
So I ditched the idea, there are other kits that never made it onto the web site but this was one of the more recent ones. It's not that it couldn't have been made to work, it was that I didn't think it was worth the hassle. I put the IM-18 and the IG-102 into plastic bags and stored them away.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm