Re: Tek 2236 and the CTM Board


 

I got the 2236 reassembled, but I had to unmount the CTM board in order to get the power supply cover back in place (the CTM board and its back hinge interfered with getting the power supply cover back in, and this was only made more difficult by the fact that the alt sweep board is not removable, at least not without unsoldering a couple dozen pins). The fan sounds a little better than it did, but I've ordered a replacement anyway, because I figure its going to fail eventually, and the replacement was only $5.

After buttoning everything up and playing around with all the toys (it's awfully nice to have a multi-meter/counter-timer that can read what you've got on the screen!) I realized that the x10 mag wasn't working. At all. So I unbuttoned everything (except the power supply) and had a good slow wander through the service manual again. It turns out that the x10 mag function is supported by a handful of passive components hanging off of one IC, and you can pretty much test them all because the x10 mag switch breaks the loop in the circuit. I found that the trim pot was reading 3x what its maximum value should be (it's a 100K pot, but was reading almost 300K). I figured that there was no harm in fiddling with it to see if it could be brought into trim, because I'd have to adjust it anyhow if I replaced it, but when I twiddled it just a little bit it came back into trim. I ginned up a signal that spanned 10 cycles across the reticule, pulled the 10x mag, and trimmed it to show exactly one cycle. Good enough for government work.

I also cross checked the counter-timer against two of my other DMMs that have a Hz measurement feature, and everyone agrees to within 1% so I'm feeling pretty confident in the 2236 CTM features.

So now I've got a 100MHz scope with multi-meter that I can use to check the 475 and 475A, which both still need some attention.

I really feel like I'm getting my sea legs now. I think I'm beginning to really understand a bunch of things that were only the vaguest theory to me before this (transistor circuits are just coming into focus, for example), and I'm getting a lot more confident in both my ability to diagnose problems, and manually perform the work to fix them. I'm also feeling confident enough in my abilities to tackle the two scopes that belonged to my dad without fear that I'll do something stupid and destroy one of them. It's a good feeling.

-- Jeff Dutky

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