Finishing the repair
For finishing everything I have replaced the temporary cap with the exact replacement which is luckily available. The replacement made a calibration of the screen necessary, the picture was not really centred anymore
- With the pots “1” you can change the vertical height and the vertical position of the picture in a very fine way,
- With the pot “2” you can set the intensity.
The 4957A leverages the same self test menu as its predecessor which you can use for position alignment and the right beam intensity (the patterns are made for this).
However my CRT was also shifted horizontally, which is a bit more of a pain.
You will have to change this by manually moving the magnet rings show in the picture above. The one in the front is changing the horizontal position, there is one in the back for vertical (which is far more rough than the pot calibration for vertical).
Super-carefully remove the hot glue (the rings are super-briddle) and then change the position which the CRT is powered. Super scary as the are 450V around at the neck board.
NOTE-1: Since this CRT is monochrome and only has one beam cannon, there is no issue with a shadow mask or color convergence. But don’t do this on a color CRT or TV set.
NOTE-2: The horizontal width can be changed by a variable coil on the board near the big white capacitor – but I did not have to do this.
NOTE-3: Molex 396 connectors cannot be used for a replacement of the connectors on the boards. The connectors are the same in size and shape, but the ones used in the 4957A have round pins, where as the modern ones have rectangular pins. The holes on the PCBs are too small.
The Result
That’s how it should look like when you’re done (the different brightness is a photo effect).
And just for fun I used my 1991 HP 4957A to open a serial terminal on my 2024 Mac – of course using the 80×25 VT-100 terminal emulator which is build in.
And here we go…again couple of years apart, but both very cute.
What’s up next?
I still struggle with copying disks for the 4951C.
My 4957A came with an original X25 emulator HP disk and I was able to copy this via Greaseweasle – no big deal. I also used the 4957A to copy the disk (the ROM contains a disk copy program) and also with this I was able to load this app from the copied disk.
But also formatting a disk on the 4957A and then writing a TD0 image with an application until track 77 results in an “application denied” error. My next hope and last resort is to use real DS-SD disks (I used DS-DD disks with a closed identification hole) – maybe the old drive is a bit picky.