HP 2648 Power Supply Repair

Martin Hepperle

Recently I obtained two 2648A terminals with 2647 keyboards.

The first one is working well. At first the image was rather dark and adjusting the pots did not help, but after using it a few times contrast improved - maybe some of the larger capacitors were recovering?
I have removed the screen mold and have attached the front glass pane with four narrow self adhesive foam pad spacers and sealed it with silicone rubber around the edges.
Looks good but produces a somewhat blurry image due to the etching of the inner surface.  Maybe I should leave the glass off or replace with a flat, smoke acrylic sheet. Or try to apply some clear lacquer to the inner etched surface?.
The second terminal has a problem with the PSU.
There was no output voltage, neither 5V, +/-12V nor -42V.
I found the 5V regulator LM 309K on the main PCB to be defective and replaced it temporarily with an 7805.
Now I obtain 2.95V where 5V are expected and some components (including the 7805) become hot quickly (I tested a few times for only about 30 seconds).
So I suspect that another component (probably one of the diodes) behind the 5V regulator is broken and has overloaded the original regulator.
The small digital control board seems to work - I verified the main clock signal and the 1:100 divider circuit.
Before I dig deeper and remove more components I would like to know whether someone has experience with 2640 power supplies and whether there are known typical problems.
I did not yet yet dare to swap the PSU from the good terminal in fear of destroying it too.
But this would be one of my next steps (after removing all unnecessary boards (like the CTU, additional RAM, datacom)
In the meantime I created some items for 264x terminals which might be of interest:
- a PDF with redrawn keyboard overlays for the 2647A and 2648A which can be printed, laminated and cut.
  Thiese can be placed on top of ugly yellow, scratched or broken keyboard labels.
- a 3D-printable hood for the serial interface (or HP-IB interface).
  This allowed me to make a proper cable with a standard DB-25 connector.
- a 3D-printable cover for the interface settings switch box on the keyboard.
  These covers are often missing because people were always curious to know what was hiding below.
If there is interest I can place these files in the Files section or elsewhere.


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