View Full Version : Zenith Z-90 repair log and schematic request

March 5th, 2016, 02:39 PM
My son recently acquired a Z-90 that would start-up, but only displays a cursor, no H: prompt. Seems like the CPU board is not starting. A quick tear down and right away here's a problem, blown tantalum cap and lifted traces all the way back to the power input on the +12V area of the CPU board.


Looks like this guy had the same problem.


Maybe the traces are undersized, heat up, lift, and then short to ground? And/or those 20V tantalum's on an 18V line are cutting it a little close (should be derated more)? Anyways, new parts ordered, cut away the old traces, nice beefy wire patch on the backside, and both regulators are now showing +12V.


Still no boot though. Checking the -12V regulator (U565), it's zero volts on both the input and output. Pull the plug between the terminal and CPU boards, and -12V and GND are shorted on the terminal board. Trace it back through a couple of caps and it looks like C411 is shorted internally. More parts ordered, now we wait again.


I have the service manual, but not the schematics. Anyone have a copy of these?

Edit: Found of complete set of schematics tucked away in one of the other application manuals that came with computer.

March 5th, 2016, 09:17 PM
There is a mail-list just for Heathkit computer, you may be interested in joining - https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!forum/sebhc There are quite a few people on there with extensive knowledge of the hardware.

Are you sure the tantalum capacitors are only rated at 20V? I thought they were 25V. I had the same issue with the trace and tantalum capacitors, the tantalum shorted out, which caused much higher current through the traces which also burnt/peeled off. On that one, I replaced the trace with a wire, and put in new capacitors, and the system came back up.

March 6th, 2016, 09:59 AM
Thanks for the link.

All my logic board tants that I replaced look like 2.2uf 20V. The ones on the terminal board on the same voltage rails (-18V, +18V) are 4.7uf 50V, including the one that is shorted (but did not blow like the logic board).


March 9th, 2016, 09:48 AM
Technical manual (minus schematics) can be found here: http://maben.homeip.net/static/S100/HeathZenith/systems/Z-89/Zenith%20Z-89%2090%20Computer%20Service%20Manual.pdf

Schematics can be found here: http://www.pestingers.net/PDFs/Heathkit_computers/Z90_sch.pdf

Going 'bang' like that invariably caused a high current on the PCB tracks and hence the damage. Make sure you replace them with components with double the voltage rating of the rail they are on. If you have some gone - consider replacing them all in one go - as others will invariably be from the same batch/supplier and therefore prone to going bang as well!


March 19th, 2016, 08:45 AM
Success! After replacing the shorted 4.7uf tantalum on the terminal board and repairing another burnt and lifted trace (the -18V supply between pins 1 on P514 and P515) I've got an H: prompt and working memory test. After swapping the jumper shunts between my two drives (I've got another burnt tantalum in the original DS0 drive), it even booted an old CPM disk.


Now to find some sort of external drive enclosure for two 5.25" full height drives. The original owner had the factory enclosure, but threw it out after the power supply stopped working, shame.