Repairing my HP 67 Twins

[murphy9]

I've got both running and the red LED plastic filter polished on the inside using Autosol Acrylic Polish (we have this in Bunnings (Home Depot equiv) here in Australia but I also remember it in the UK) - used a cotton bud and cotton wool to remove the cloudy finish and buff it up to new. Both calcs displays are now like new and the previous cloudiness has gone.

Both are charging the Nicad.

The next hurdle is the Card reader. Both had gummy wheel repair a few years ago, both feed the cards in but...

"Twin one" will feed original cards through but displays an 'error' every time.

"Twin two" can store programs on a new card and retrieve that program, but won't read original HP provided program cards which came with the HP-67. My HP-41c card reader can read both the new and the originals.

The motor speeds 'sound' slightly different for each of the Twins, but no indication of clutch issues etc.

At this point I'll need think about deeper visual inspection probing with a multimeter, and then possibly use this as an excuse to invest in an oscilloscope after the christmas break.

Question - is it worth just replacing the capacitors first as these are normally the things that go after 50 years ;-) and if so what are the best replacements ? I know the values etc, but there are too many versions of each (and sizes etc). There's no sign of electrolyte leakage, but these things are close to Tutankhamun's age.

Finally - I have bought all the latest CD/DVD/USB from the [tremendous] community over the years, but only seem to have the HP97 service manual in pdf - can someone point me to one for the 67?

Thanks again !