[sebhc] Re: H8 data record format

Lee Hart leeahart at earthlink.net
Fri May 7 14:47:07 CDT 2004


> Anyone have experience reviving old gear? Mine's been boxed
> for over a decade. Any precautions I should take before I try
> to power up another unit?

Patrick Rigney had some good ideas. Here is my contribution.

The main offenders are old batteries (but the H8 doesn't have any unless
you have added a real time clock board), electrolytic capacitors, and
tantalum capacitors, and high-current connectors. These parts all age
(get much worse over time), and can fail in dramatic, dangerous ways.

Batteries

Any batteries over 5-10 years old should automatically be replaced. On
some equipment, they will be little cylindrical or coin cells hiding on
a clock board somewhere. They will leak, and short or corrode traces to
cause all sorts of bizarre circuit behavior.

Electrolytic capacitors

Electrolytics have two main problems; they dry out, and they lose their
dielectric forming.

Dryout means they have lost water, which increases internal resistance
and reduces capacitance. Dryout is proportional to *operating* time; the
amount of time the equipment has been turned on and working. High
voltage and high temperature drastically increase the rate! For
instance, a 25vdc 85 deg.C rated electrolytic will last 10 years at
16vdc 45 deg.C -- but only 2 months at its full ratings; 25vdc 85 deg.C!
(Now you can see why you shouldn't use electrolytics at their full rated
voltage or temperature).

An electrolytic that has dried out isn't really working as a capacitor.
It allows excessive noise and ripple in the circuit that will cause
other parts to malfunction or fail. It will also get hot. If the heating
gets bad enough, it will leak or explode! So, if the equipment has spent
a lot of time turned on and tends to run hot, replace the electrolytics.

An aluminum electrolytic consists of two aluminum plates in a caustic
water-based electrolyte. During manufacture, they apply a DC voltage
until a small current flows. Like a battery, this current "reduces" one
plate (makes is pure aluminum) and "oxidizes" the other (coats it with
aluminum oxide). This is called "forming" the capacitor. Since aluminum
oxide is an insulator, the current stops when the plate is completely
coated, and you have a capacitor.

If the capacitor is not powered for a long time (years), this oxide
coating gradually goes away. When you later apply voltage, it isn't a
capacitor; it's a *resistor*! It will get hot, and if hot enough to boil
the water inside, it will vent or even explode!

So, if the equipment has not been used for several years, power it up
slowly, with a light bulb or variac or light dimmer in the primary.
Start at about half voltage, and crank it up slowly over a 24-hour
period. This allows time to re-form the oxide layer so it will work
again.

Tantalum capacitors

Tantalum capacitors are similar to electrolytics, but with tantalum
oxide forming the dielectric. Their problem is that their breakdown
voltage gradually falls as they age, usually from high temperatures or
tiny cracks or damage to their case during assembly. When the breakdown
voltage gets down to the applied voltage, they suddenly fail shorted. If
they happen to be connected as filter capacitors right across a
high-current power supply, they explode with a violent bang!

There's no way to fix them, and they are usually reliable enough that
it's not worth blindly replacing them. But be prepared for a sudden
"bang!" shortly after you put an old piece of equipment back in
operation. The good news is that tantalums are tiny, and rarely do any
real damage unless your eyeball is in the path (running with the boards
exposed. Then, you have to find the place where the tantalum capacitor
is now missing from a board, and replace it (easy with Heathkits, thanks
to their magnificent documentation).

Connectors

Equipment stored in basements, garages, attics, and other "dirty" areas
will suffer from corrosion damage to connectors. The effect is much
worse if you live in a city or other area with high air pollution
levels.

Connector problems mainly cause intermittents and flaky operation. But a
high-current connector can melt down or even burn up. As a preventative,
I generally unplug and re-plug all the connectors in an old piece of
equipment; the friction helps clean them up.

It's generally *not* a good idea to do this with cheap IC sockets,
though. Many of them that Heath used are really garbage-grade, and
you'll create more intermittents than you fix if you plug/unplug ICs in
these sockets more than a few times.

There are contact lubricants and greases that help prevent these
problems, as have been mentioned previously. But they don't do much good
once the connection is already bad. You may have to just mechanically
clean or replace any connectors that get hot or are discolored or
corroded.
-- 
"Never doubt that a small group of committed people can change the
world. Indeed, it's the only thing that ever has!" -- Margaret Meade
--
Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net


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