![[Zenith Z100 system]](z100_frontpanel.jpg)
![[Z-100 top off]](z100_notop.jpg)
Last update Aug 13 2009 by Herb Johnson
In August 2009 I pulled a few Heath/Zenith Z-110 systems from storage, to test and repair. Here's some of the repairs I performed, and some of the features of the systems I have. - Herb Johnson
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To go to my Heath-Zenith Web page follow this link.
To go to my S-100 Home page follow this link.
These Z-110 series systems are referred to as "low profile", because the Z-120 is the same system with an encased CRT (video tube) mounted above the motherboard, replacing the top of the Z-110.
Here's a Z-110 with front bezel detatched but still in front.
Here's another Z-110 with the top removed, exposing two half-height floppy drives
in a black fiberglass sub-frame. Note on the far left of that frame, the date "Jun 12 1983". The switching power supply in metal is to the left under the frame. Behind the drives is the 6-slot S-100 card cage, with a Zenith floppy controller card in the cage. With the drive assembly removed, you can see part of the video card as mounted on the motherboard underneath it. The Z-110 with lower case removed and keyboard lifted out of the way, the motherboard and the entire video board is exposed. Note the bottom of the keyboard; it's simply an array of key closures which is scanned by the motherboard and is cabled to it.
Other features of the Z-100's I examined include this 8087 daughterboard which fits in the 8088 socket. A hard-drive based Z-110 used this CDR317 SCSI controller S-100 card in addition to a SCSI/MFM adapter card mounted above the hard drive. All Z-100's include the Heath floppy controller card. NOte the card has both 44 pin and 50 pin connectors, for 5.25-inch and 8-inch floppy drives respectively. Other Zenith cards, and cards from other manufacturers, are shown in later sections.
A useful part of the Zenith Z-100 system is the diagnostic software. From this photo of the Heath diagnostic screen you can see diagnostics for RAM memory, video display, I/O devices, and floppy. There is also a diagnostic for the Znith "Winchester" hard drive controller.
One problem with old computers, is that the soft rubber padding turns to mud or dust.In this photo I'm testing Z-110 with floppy resting casually on the system. Most of these floppy drives dated from the early 1980's with these systems. In this floppy drive photo, note the former foam padding on the front edge (to the left) as a gummy, lumpy residue. Alcohol wiped away most of the substance. It was originally applied with double-sided tape; often I could peel away the tape itself.
A common problem with floppy drives is a consequence of old diskette media becoming brittle and scraping away. The iron oxide coating actually strips away, as its binders become brittle. Other media problems include mold, growing on the plastic surface. Crud on the floppy head requires cleaning with Q-tip in alcohol. you can see in this photo, a Q-tip with oxide from a floppy head. When the head accumulates even a little iron oxide, it acts as a scraper, and picks up more material. A dirty or damaged read/write head, produces these scratches on media. A sharp ear can hear the difference in sound, when the drive head starts to drag or even scrape the diskette. It can cause damage not just to the "bad" diskette with fragile coatings, but to subsequent diskettes read on that drive.
Shorted capacitors will actually ignite. One of my Z-110 systems refused to power up more than a fraction of a second. Experience told me, a tantalum power capacitor may be shorting out some DC power voltage. Fortunately the Z-100 series switching supplies simply shut down instead of failing or blowing a fuse. I pulled al the S-100 cards, and the system came up. When I put the floppy controller card it, the system failed again. I used an ohmmeter to check the DC lines, and found one shorted line. Disconnecting and measuring the capacitor on that line as shorted, identified the component failure. In this photo, a red cap is in place near the 78M12 regulator; the shorted small cap is loose above.
On another floppy controller, the capacitor drew so much current, it ignited. In fact, bright red smoke curled above the board, as the short did not open but continued to draw current. Identifying the failed component was no challenge, it was blackened. I cleaned the board around the component, replaced it with a similar capacitor, and the board functioned correctly. Here's some photos of burned tantalum capacitors.
Floppy drive problems are often mechanical. I had a full-height floppy drive on one system, which would not accept a diskette. Turned out, the floppy drive's write protect sensor, was sticky. In the center of the photo, it's a small black triangle at the end of a slot. This triangle is part of a lever assembly which controls a microswitch underneath it. Unsticking the lever, allowed the disk to ride over the triangle instead of being stopped by it.
Hard drives eventually fail, and they fail hard. Here's a Z-110 with a MFM drive, a SCSI/MFM adapter card, and in the S-100 cage the CDR SCSI card. Here's a closeup photo of the SCSI/MFM adapter card mounted above the MFM hard drive. The drive did not spin up with power. So, I pulled the drive out of the system, and gave it a few manual jerks of my hand to make the interior platters spin briefly to "break" the "stiction" that is often said to be the cause of spin problems. Unfortunately, this broke the drive. If you look at the hard drive interior photo, note the sand-like crud, AND a missing head on lower arm. Where was the head? If you look at this fuzzy photo, the loose head is stuck to platter. ahead of the drive arm. OTher spots on the platters suggest the other heads were "stuck" to the platters.
Video displays are based on RAM, and RAM will fail. the screen shows dots where video RAM is damaged.
diagnostics show one of the bad VRAM locations.
Zenith Z-100 video card, with RAM on the right. the diagnostics showed two bad RAM chips by exact location. Replacing these 4164's (64K by 1) removed the "dots" from the screen.
Some hard drives continue to work, even after decades.Z-110 with the Zenith hard drive controller and data seperator, on a 11MB MFM drive.
Zenith S-100 hard drive controller (left), and data seperator card (right).
a MS-DOS "dir" command on the MEM drive.
Power supplies also fail, often with fireworks. In the course of working on the system with the "good MFM drive", I was trying to cram the drive assembly back into the system. But there was not enough space for all these DC power wires behind the hard drive. As I tried to place the drive and the wires, apparently I causes something to short out, even with the AC power switch off! The power supply started to smell and smoke, in a fashion that suggested a large capacitor was burning. Long story short, I pulled the AC cord out, and replaced the AC supply completely. There was no obvious damage to the motherboard, after one false start it came up OK with the replacement supply.
Power supplies can be repaired sometimes. When another AC supply failed with a flash of light inside, I decided to open it up on the hunch it was just a brief short that blew the fuse. Here's the interior of the Heath/Zenith switching power supply. and Here's the back of the PC board. There's not much documentation on it, apparently no schematics. I replaced the 6A fuse and tested the supply using a hard drive as a "load". Never operate a switching power supply without a load, you can cook it. But it blew the fuse immediately, here's a well-blown fuse. Checking from the AC side where the fuse is, I saw low resistance across the AC power jack, maybe 12 ohms - far too low and different from measurements on a good supply. Walking through the circuit, I saw that the full-wave rectifier was shorted on two of the four diodes.
So, I needed a 6A and at least 200V or much better rectifier of the right physical size; and some 6A fuses. Wonder of wonders, Radio Shack had just the right parts!. Replacing the rectifier, and of course the fuse, did the trick!
Herb Johnson
Copyright © 2009 Herb Johnson
New Jersey, USA
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