[sebhc] Re: H8 data record format
CarrollWaddell at sc.rr.com
Fri May 7 16:38:37 CDT 2004
I heard someone talk about Deoxit. What is this, what does it do, and
where do you get it?
Lee Hart wrote:
>>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.
>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.
>Electrolytics have two main problems; they dry out, and they lose their
>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
>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).
>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
>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
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