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HP-81 Capacitors
11-02-2017, 09:49 AM
Post: #2
RE: HP-81 Capacitors
Hi, Duane,

Yes, electrolytic capacitors will degrade with passing time, even when not used for long time. They have several failure modes.

A good capacitor meter would be the ideal tool to measure the capacitance and internal impedance (to do this, I would remove the capacitor from the circuit, or at least disconnect the positive lead in order to have the capacitor in a open circuit).
Or else one just assume the capacitor is defective and replace it.

Despite not knowing that HP equipment, I know about these old capacitors and from the description in the Alex Knight post, I have no doubt that this capacitor is a single section with 3000uF / 30VDC, where the Negative side is the can itself (as usual on these type of capacitors).

The reason to have 3 common tags is to mount the capacitor by twisting them in the chassis or in the PCB, providing a good mechanical fixing, and at the same time providing a good ground connection.

As Alex Knight mentioned, any modern electrolytic capacitor will do; the mentioned 3300uF / 35VDC is a standard these days and will fit.
The capacitance value itself is not that important, as electrolytic capacitors usually exhibit a tolerance in the range of -10% to +50%.

That said, if the capacitor is filtering an switching power supply (where the ripple frequency is on the order of several KHz), then I would use a 105 degree (or a 85 degree as a minimum) rated capacitor in this application, as typically these capacitors exhibit a lower internal impedance and therefore a better filtering and current supply.

On the other hand, if the capacitor is just part of a filter for 50/60&100/120Hz ripple filtering, then a regular capacitor will do, but even here I would use the better graded capacitors (85 degrees minimum).

Mechanical speaking, a Radial type format (like the original one) would be adequate, despite having only two leads.
The negative lead will go to one of the 3 negative points and the other two negative points must be interconnect as well.

If you would like to maintain the original look, then if you are fearless, you could carefully remove the capacitor internal components and save just the can.
Then install the new capacitor inside the original can.
This operation is delicate as the remaining electrolytes inside the old capacitor are contaminants and require special handling.

Tantalum capacitors are not the ideal solution here, and usually they are manufactured for lower values.

Jose Mesquita member

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HP-81 Capacitors - Duane Hess - 11-02-2017, 08:54 AM
RE: HP-81 Capacitors - jebem - 11-02-2017 09:49 AM

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