We made two good HP 4957A with two bad ones!

You saw us unbox my HP 4957A in a previous video, and as expected, it did not even power up. This was a cheap non-working machine I got for parts. This was to help getting my other  "good" HP 4957A working. 

The "good" one was a clean machine, complete with all the application disks, documentation, and cables. And it worked almost perfectly. 

Except that the feature I really bought it for did not work: the high resolution, 80 character per line VT100 terminal emulation had a corrupted display. This is only available on the 4957A: the earlier 4951 and 4952 only had the lower resolution video with 24 characters per line. However the tiny hi-res characters are malformed in a strange way which keeps changing as you type more characters, and it renders some of them unreadable. All the other modes besides the hi-res VT100 emulation use the normal resolution and worked fine. 

But the big downside of the 4957 compared to its earlier 4951 and 4952 brethern is that it is not documented and full of Xilinx FPGAs.  Too much work to reverse engineer, hence the parts machine for a board swap.

However the parts machine came in in much better shape than expected, so I decided to give it a proper chance first. We repaired its switching power supply, which had a combo of bad solder joints and a bad startup capacitor. 

And when it finally came up, it's VT100 emulation worked. But to my immense surprise, it was in the former 4951 low resolution mode, which this machine is not supposed to have!

Since the hardware looked identical, maybe it was an early demo machine, and had pre-production firmware without the hi-res feature? We decided to steal the 5 newer ROMs from the good machine and put them in the old machine. And yes, with the other machines' ROMs, the parts machine was upgraded to a hi-res VT100, which worked flawlessly.

We "downgraded" the other one, and since the low res video worked, it was problem-free with the earlier ROMs. The VT100 emulation has less characters, but is now perfectly usable as it was in a 4951 or 4952. Which is fine, as long as you don't use it for an application that requires a full VT100 screen display.

So here we go, we've made two working machines out of two bad ones. How lucky is that?

Marc