Re: 475 questions


Hi All!

I've been working on the scope and digesting information. I am posting this update before going through the posts that have been contributed since I last posted. Everyone has been really helpful and supportive. Thanks for making me feel welcome and for spending your valuable time helping me out!

I took the six large capacitors off the board today. Didn't go too badly, although two of the silver disk traces on the bottom of the board did come loose in the process. All those on the top remain intact. My new ZD-985 solder vacuum pump made the job MUCH easier, as even with solder wick dipped in RA rosin flux, I hardly got any solder out without the ZD-985. Still, I did need significant heat and repeated applications of flux even with the ZD-985. Access was made significantly easier by the removal of the trigger generator and Z-axis logic board (A8), which also went very smoothly.

All I have to measure these with is a cheap, battery powered AVR Mega 328 microcontroller "transistor tester", a GME-236 ESR meter, which has a range of only .47 to 2200 μF, and my Tacklife CM01a DMM for which all of these are out of range (.04 to 100μF). The "transistor tester", on the other hand, by far the least expensive of the lot, does 25pF to 100,000ΩμF along with ESR and "V-loss". I took the measurements using a couple of 4 inch leads inserted into the lever clamp, positions 1 and 3. Here are the readings, out of circuit, with each, although the 236 would not give me a reading for any but the C1412, even though the C1414 was in range. Below, OR indicates "out of range". IC indicates "inconsistent", where the 328 gave readings all over the place, but in the range of just under 0.2 to just over 3μF, so I'm guessing that the C1414 is the worst capacitor here. C1412 doesn't appear to be as bad, but if that ESR reading can be relied upon, it's pretty bad. NR indicates "no reading".

Rating (μF) 328 (μF, ESR Ω, V-loss %) 236 (ESR)

C1412 350 376.1, 6.7, 3.8 4.5 .04 to 100μF

C1414 1000 IC, NR, NR NR

C1442 5500 7411, .54, 1.6 OR

C1452 5000 6542, .61, 1.6 OR

C1462 5500 6499, .15, 1.3 OR

C1472 3000 3911, .19, 0.9 OR

I'm not sure what ESR ohms are acceptable for such large capacitance, but I'm guessing that even though the readings I got were excessive for measured capacitance, such low ESR values for C1442 through C1472 probably indicate that all but the C1412 and C1414 are reasonably good. Since I already have them all out, though, I think I'm going to replace all of them, unless the cost is prohibitive. I'd like not to have to go back in this part of the scope again. Part of the reason I took all of these out is that it's tricky to remove these, and as long as I'm removing one, I want to replace them all at once. Also, I took the advice of a couple of you who recommended them and sent for a set of the small, circular PCB's designed to accommodate more modern capacitors while preserving the job the aluminum cans did in connecting traces. I won't throw the ones that appear to be good out, though, at least until I get a reliable meter to evaluate them. They might still work elsewhere on a project where they will be more accessible and easier to deal with if they do go bad, eventually.

Having identified the bad caps, I'm assuming the next step is to check the rectifiers that feed them. Since these are much more accessible, I'll leave the good ones for now. I'll go back and recheck the advice I've received about checking them. In the meantime, I'm waiting for the circular PCB's and I need to order the large capacitor replacements.

Anything else I should do while I have the timing circuit board (A7) exposed? It appears to have two beige and one orange tantalum caps on it, although in the manual, all of these are identified as ceramics. So maybe my assumption that the brightly colored little blobs are the infamous tantalums is wrong. Lots more to learn! The orange is a 4.7μF and tests fine with my GME-236 ESR meter. Both of the beige 225 (I assume that's μF) ones test bad, although they don't appear to have any problem visually.

I did read a bit online about MLCC's vs tantalums. Apparently each has its advantages and weaknesses relative to the other. Are modern tantalums better? Or if we replace tantalums in these scopes with newer tantalums, do we get higher voltage tolerance ratings than spec to compensate? Sorry if this question has already been answered. It's been a couple of days and I need to reread some of your posts, as well as the unread recent ones.


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