(four) MK-120's... very strange

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1

I recently bought and assembled four MK-120 kits. One Transmitter/Receiver pair work just fine.
But, three T/R kits don’t… and since I’ve assembled a bunch of much more complicated kits in the past, I’m thinking that there’s got to be something wrong (more wrong, than just me).

Prior to posting this, I read all of the recent posts on the MK-120… and, I’ve used a digital cam as a trouble-shoot aid. I discovered a very strange thing. Three of the transmitter’s dual IR LEDs are working, and can be seen brightly lit, viewing thru the cam. Yet, none of the four receivers respond to any of the transmitters… EXCEPT for the one. (Let me be clear… all four receivers respond to one specific transmitter, but not to any of the others).

I’ve more than ‘triple-checked’ for the correct parts in the correct locations… polarity… and solder-joints. It just makes me wonder… if the LEDs are lit and working… (and the alignment and distance is correct)… why wouldn’t three of the four receivers react properly?

So- Does anyone have any thoughts or suggestions on how to proceed?

2

If all 4 receivers respond to a single transmitter but not to the others, one might think that the oscillating frequency of the 3 non-working transmitters is wrong. Check with scope or counter.

3

Yeah… that’s kinda what I was thinking, but haven’t had the time to check. But a good friend is a master, and his shop has every kind of test gear… I’ll run them by there, and either check them myself… or- he’ll probably wanna do it himself.

Just out of curiosity- what would cause that?.. incorrect or faulty caps? With my limited knowledge, I’m hard pressed to think of any other parts which could (or would) cause this particular symptom… of course, I don’t have what you’d call ‘oodles’ of electronics theory and experience under my belt.

I’ll post my findings to let ya know.

4

Wrong resistor or cap values can cause this problem.