Sony HB-F900 repair

Symptoms: black screen, no sound, nothing.

There was some NiCad battery leakage. Nothing compared to what I saw on the Sony HB-T7, and I was able to clean it up quickly.

There was no oscillating signal on the VDP’s XTAL 1/XTAL 2 (pins 63 and 64). Unlike all other retro computers I have seen thus far, this signal is generated by a 74LS628 IC on the analog board. However, it took me a while to figure out that that is the case. In fact, I did not realize this until I decided to take a look at the service manual for a very similar computer, the Sony HB-G900P, linked from the bottom of the page at This service manual mentioned the 74LS628 IC, and how to adjust it.

However, this IC wasn’t even getting 5V, and it turns out that there’s a 5V supply separate from the 5V supply used to supply power to all the other logic chips on the two boards. The IC gets its 5V through two linear regulators, first a 7809 turning 12V into 9V, and a 7805 turning 9V into 5V. The 7809 was broken with the following failure mode: up to about 10V, it output input minus 1-2V, and beyond that, it output 0.5-2V.

Input: 10V (display is inaccurate, actually closer to 9.7V), output: 8.93V
Input: 12V (actually 11.8V), output: 1.8V.
Running the machine with the regulator removed and my trusty USB power supply (set to 9V, display is imprecise) attached instead.

Replacing the 7809 fixed the machine. However, it appears that the floppy drives may be somewhat broken. I’ll look into that soon.

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