Electronics > Repair

Fluke 8502A - fuse blown

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Rax:
Hi all,
Going through some of the other units I got - see my other thread(s) - there's also a Fluke 8502A that worked for a minute, then blew its 1/2A fuse.

I've been going through the PS with one of those universal testers, and most electrolytics check OK, except C7 (which gave weird, either non functional, or resistive readings). Pulled it out and measures OK out of circuit. I will likely simply replace all electrolytics soon, but in troubleshooting this there's also a C10 right across C7, just very near the regulator's legs (of course). This is a tantalum 5.6uF (and I'm generally deeply suspicious of tantalums...). Due to where it's placed, checking it and/or taking it out will likely mean replacing it - what have others used for this? I am tempted to go for a 4.7uF film cap (maybe even further bypass it with a .1uF or so). Datasheet says 25uF for an electrolytic if not going tantalum, but I'd rather go film there.

Not at all sure this may cause my fuse meltdown (I'd say not likely), but other things like the safety caps seem to check OK. Any suggestions for what to check or look for are very welcome!

RichardM:
Hi, we are on a similar journey, see me recent post on the 8502a.

I replaced the three tantalums near the back heat sink and regulators with 10uf tantalum caps and I seem to have the correct voltages now. After powering the unit on the first time, 2 of those old 5.6uf caps let out the magic smoke. The negative voltage regulator was also dying and these are socketed and easy to replace.

Rax:
Richard,
Yes, I was just reading your notes... :) As soon as I posted my thread, I went back to the listing of all topics, and for a second I wasn't sure which thread was mine... ;)

I am tempted to do an all-recap, but only after I conclude it's worth going through the expense. So far so good, as during the minute or so it was working, I didn't see much obvious signs of trouble. Not a lengthy examination, for sure, but at least I get good heart to keep at it.

On those tantalums - I never stock any, as almost all my work is in audio or FM, and dread their reliability record, but I'd be tempted to replace them with film caps to keep the esr low. I have WIMA caps that are very miniature for their values - essentially, almost the overall size of the electrolytics for given value, and the same typical lead spacing - and may use here. I am not yet sure these are blown, but I'll try to correct things I suspect or measure out of whack, as hopefully they correct whatever blows its fuse, as I don't have the comfort to fire it up and measure for issues at my leisure. I got a variac and a "dim-bulb" fixture, but not sure if I should try it here.

Looking forward to your progress!
Radu.

RichardM:
Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard

Rax:

--- Quote from: RichardM on November 22, 2021, 11:32:20 pm ---Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard

--- End quote ---

Richard,
If you could drop a quick indication of how you gained access to those parts (7912, tantalum caps). Does the heatsink pop out? Been looking at this many different ways, but before I start disassembling too many parts, some of which I may not need to touch, I though I'd ask you as you've just done this.

Thanks!

[meanwhile: loudly slaps forehead, repeatedly, and hides in a corner for a while]

Jeez, I guess reading the SM (which I finally could print today, after getting the darn toner order) was too much to ask of me... It slides right out from the back!

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