HPAK 8569A Microwave Spectrum Analyzer: front panel repair/re-build

The easy of use by the three-knob operation mentioned before – it has a downside: a very complex front plate assembly. And this might be the reason why more modern units almost exclusively use single-axis rotary encoders, often only one single encoder to operate all functions, per unit. The multi-turn-coupled-limited-synchronized-geared movements, just too complex and too expensive, too heavy, too laborious to maintain.

The front panel controls – as delivered.
8569a front panel rhs as delivered

The 8569A, 8569B and 8565A all use very similar front assemblies – only seem to differ in the print and in the exact shade of the label colors. And these assemblies use plastic parts – knobs, wheels, and so on. After 30+ years, no wonder, aging has visible effect, and parts become brittle. Common defects are broken-off sliding contacts. Not a big thing, can be fixed (re-attach the contact with M1.2 screws).

The attenuator and “0 dB” warning indicator – these are located on the same rotary cam, same contact pair. And, you bet, the corresponding wiper contact – broken off-missing.

Well, having rebuild some of these assemblies for other repair jobs before – the only way to fix this will be to take the assembly apart, almost fully. Actually, if all screws are kept well-sorted, not really such a big effort as is sounds. Some hours, but then it will be working again.

8569a rebuilding the from assembly

In addition to just the repair, all contacts and surfaces were thoroughly cleaned, by various means: DeOxit D5, a good eraser, isopropyl alcohol. For the painted surfaces and parts, use 40-50% isopropyl alcohol only, and a rather soft or medium-hard brush, to avoid damage to the paintwork.

The replacement cam (on the right) has two wiper contacts, the defective part, only one – the plastic fingers holding it down became brittle, and it eventually broke off.

8569a attenuator selector cam

Here, the result: the rebuild assembly. From earlier repairs, the remaining contacts should last – somehow, no all are susceptible to breaking-off; or it is just the amount of use they get – at least, I have some units fixed with small screws holding down the contacts, and there have since been working better than before. Therefore, I am not too worried about more contacts failing in the near future.

8569a rebuild front panel assy

Note: as it turned out, also the 12 dB vernier attenuator encoder had a partially broken off contact – can be fixed with some epoxy glue, but I rather replaced the encoder, with a spare from the parts unit.

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