The hell with these things. Everything I've got is >20 years old and they're probably all dried up. I'm trying to restore my cassettes and reel to reels now, which I figured would be mostly mechanical. I got the first deck (a decent consumer cassette) back to running like brand new, got the azimuth almost perfect with a nice loud playback at 10k. 6.3k is off by about a quarter of a dB. 315Hz is equally impressive, then I get to 63Hz and I can barely even hear the S.O.B. This of course has no playback EQ adjustment or anything. I go under the board with an ESR meter and everything looks too high. There's like 70 caps and it isn't even a big board. They're mostly the sub-10uF small ones. I find a pair which looks puffy at the top and replace those plus two nearby caps, suddenly 63Hz is only a couple dB below spec. Those particular caps were just barely past the play head in the schematic, but I know damn good and well the rest of the electrolytic population has to be doing something nasty to some signal somewhere.
Where do you guys draw the line, and do you normally have to sit there and feed something to
every single cap to determine that? I can't imagine pulling everything in this one deck, and the larger rackmount ones look like a solid box of PCBs inside.
Quote:
Originally Posted by amorris  check by running a signal generator through them. usually lose low end check so watch the roll off of all the channels and replace ones that have a higher Lf roll off. then run a 3kish square wave through and view output with scope, bad caps will have poor flatness in the square. (with drastic leading and trailing edges.) |
Thanks for the tips there. So I'm guessing this is all safely done in-circuit? Also, do you have to determine a healthy LF rolloff point for each particular value of cap and/or a specific frequency for the square wave? And what sort of
levels are you talking about running through them? Of course I'd just stick with the ESR meter if I can, but like I said, most everything looks high there and I'm not sure what's too high.
Much Thanks!