21 Apr
2009
21 Apr
'09
7:04 p.m.
While we're giving electrolytic caps another go-round, here's another angle on
(modern) electrolytic caps:
I was repairing an HP 9815 (desktop programmable calc) a month ago. While
tracing it out to make the schematic, I noticed that a 680 uF, 25V electrolytic
I am wondering why you traced out the schematic of a 9815, when you can
download both the official service manual (boardswapper guide) and
unofficial full schematics (covering both main versions of the CPU board)
Anyway... the 9815 is a somewhat odd machine. It's the only HP desktop
computer before about 1980 that used a standard CPU. Other machines
either used a bit-serial processor built from TTL or HP custom
chips/hybrid modules. But the 9815 uses a 6800. The memory map (and thus
the address decoding circuitry) is a bit strange, mainly because they
wanted to put the 6821 PIA at location 0 (so they could use zero-page
addressing to access it), they wanted RAM in the rest of page 0, etc.
The PSU is, of course, a switching regulator run from a mains-frequency
step-down transformer. Strangely there's no crowbar circuit. If that
chopper transistor shorts, the 5V line leaps to about 30V with fatal
effects on all the chips. I don't know why HP cut corners in this way.
Can you tell I've worked on these before...
cap for the +15V supply filter had been installed
backwards (reversed
polarity). This was not a replacement or earlier repair, it came out of the
factory this way in 1975 (even HP screws up on occasion).
I've seen this too. In fact I've seen it twice rcently. The most recent
case was in that Hp120 I've been working on. The decouping capacitor for
the +12V line on one of the boards (connected from +12V to ground) was
fitted backwards. And the PCB was laid out backwards. HP, you see, put a
square pad for the 'indicated' lead of a component -- pin 1 of a
connector or IC, the +ve end of an electrolytic capacitor, the cathode
(banded end) of a diode, the emitter of a transistor, etc. And in this
case, the square pad was ground.
Of course I replaced the capacitor (10uF). I didn't keep the old one, I
have no idea if it had reformed backwards (i.e. with the oxide film on
the wrong plate), but I susepct it wasn't going to meet it's spec.
Granted that it's on the output of the regulator
so it's not as critical as if
it were the main smoothing cap right after the rectifiers, but it was
nonetheless interesting that the calculator had been running just fine with it
this way since inception (it had nothing to do with the problem I was looking for).
What was the fault?
I made note of it in a repair log for the unit, but left it as is. Someday it
night be interesting to take it out to measure it's characteristics.
Alas I didn't keep the one I replaced in the HP120, so I can't check that.
-tony