I’m just jumping right in –
I acquired the following from Ray (WA1FFT) in NJ.
- Heathkit 89A
- Complete set of spare boards from H89A (different mb)
- H17 Dual hard sectored 5 1/4” floppy drive
- H77 Dual soft sectored 5 1/4” floppy drive
- Box of hard & soft sectored disks.
I was really just after the H17 to go along with my H8, but $30 for all that? No brainer.
Ray warned me that two caps popped on the main board, and indeed C508 & C515, 2.2u tantalum caps, failed violently. We’ve got to see what other damage they did; tantalum capacitors are like the suicide bombers of the electronics world.
They were replaced with 10u caps I had in inventory (close enough):
I found Herb Johnson’s 2016 adventures
in Heathkit H89 rebuilding while surfing around, and his page is a great reference. As I got a spare set of boards as well, I figured I’d first just swap them out. No go. The boards are slightly different, and that difference includes the power supply connectors. According to Lee Hart,
the key’d power supply molex connector indicates a late-model H89A. That would be the 2549 board
I’m assuming that also took out U585, a 79M12 and U565, (only have a heath part), as there’s no -12V & +12V on the output of those regulators. I also have a spare, earlier model board to compare with (and possibly steal parts from if I’m impatient), but I’m noticing more and more inconsistencies between the two boards, and with the documentation. Here’s where it gets (more) annoying:
U515 is a 71M12C, your run-of-the-mill -12V regulator. OK, terrific. Same on the old board as well, and consistent in all docs I have.
U567 only has a heath part#, 442-674. It’s in parallel with U568, they both feed separate +12V supplies. Here’s where it gets squirrelly:
- In the docs, it’s called out as part 442-663
- The pinout labeling is different (common pin center, whereas board image & earlier board call common pin as the left one)
Here’s the old board (85-2208-1) incorrectly marked pin-outs, but check out that sexy ceramic ram chip:
Here’s the documentation for the newer board, but the pin-outs are still mis-marked:
Here’s what I’m left to deduce, and can confirm after some metering:
- The earlier board silkscreen and artwork in my manual (for the later board) are both wrong for U567 & U568
- U515, a negative regulator, indeed has it’s common (gnd) on the left; it’s input and output are swapped from positive regulators.
- U567 & U568 are almost certainly 7812s (+12V regulators), as they are on the earlier board. They are only silkscreened correctly on my newer board. The new board artwork illustration, and the old board silkscreen are both incorrect.
OK, well U515 seems to be working well out-of-circuit, but there’s almost no resistance between ground and it’s output in the circuit. Suspecting C509 & 511 next. Lifted both, no dice. I’ve got to be careful with the desoldering gun, the traces are easy to lift.
So what else is connected to the -12V line? Besides the 2 -5V regulators, and the card bus, I’d imagine it’d be the built-in UART used to communicate with the terminal board, since for whatever reason, it’s full blown RS-232 between the two. Yep, sure enough there’s a quad EIA-232 driver. Nope, there’s still a short with that removed.
So it’s the -5V regulators? Nope. Forgot about C513. Yep, it’s shorted. It’s always the last place you look…
Next up – replace the regulators and attempt to power up again.