Electrolytic Capacitors in 67/97



#8

I read in these forums where the electrolytic capacitors used in these old calcs, classics etc. have a limited lifespan.

Are the sizes used still available?

What is the likely timeframe when they will give out?

Are they passing AC that will damage other components?

Should they be replaced now?

This was prompted by examing an abit motherboard that just died. Most of the capacitors were like old tin cans that have popped out at the ends.

It would be interesting to have some views on this.

Thanks Malcolm


#9

Electrolytic caps do have a limited life with storage rated at 4 to 8 years, but this is usually extended if they are powered up frequently but well under their max voltage rating. I have used many for 20 years with no problems and the failures will normally be seen as increased leakage and marginally reduced capacitance.

Tantalum electrolytics, the small bead type, can have a shorter life but are more prone to catastrophic failre, with high leakage currents. The failures are usually random, except when a badly manufactured batch has got out. If the circuit uses them at their voltage limit or with spikes going above their limit then the life can be very short (but I dont expect HP calculators to have this sort of design problem!)

I suggest you wait until you have a problem, but then check capacitor leakage if the problem could be associated with a capacitor.

The experience of others who have repaired these calculators will be the most useful info.

Did that help? I'm not sure.....

#10

This might explain your Abit catastrophe:

Leaking Capacitors Muck up Motherboards


#11

That backfired on them!

I doubt there are any as bad as that in the older HP calcs.

#12

Generally, if it ain't broke, don't fix it. Most of the capacitors in HP calculators are under very little stress and/or their characteristics are not critical. The vast mahority of the caps are just fine and will stay that way for a long time to come.

There are two main exceptions. The first is a small blue tantalum cap used in the power supply filter of the HP65/67/97 card reader circuit. It develops a high ESR and quits filtering properly. The reader quits reading, but writes just fine.

The other is the 400 uF (approx) orange filter cap in the HP35/45/55/65/67/70/80 charger. I have seen a couple dozen chargers with it gone bad.

Other than these two caps, I have seen only a couple of random failures in several hundred machines.


#13

David,

So your saying the tantalum capacitors are the ones to worry about?

What are the symptoms of the filter cap in the charger going?

Are these capacitors still readily available?


Thanks Malcolm

#14

I've seen a bunch of failed tantalum capacitors in the Woodstock models. They are used to filter the higher voltages derived by the internal circuit. Fortunately, tantalum capacitors are readily available and even if the same values are no longer made you can simply replace it with a standard one at the next higher voltage and/or capacitance rating. In addition, electrolytics have gotten quite a bit smaller over the past 30 years; they are usually small enough to fit into the same space as the original tantalum ones and in my experience work just fine.


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