This Web page last updated date Mar 4 2022.
Rich Lentz contacted me in Aug 2010, to get a Heath Z-100 power supply repaired and replaced. We correpsonded about the repairs he accomplished, and he was kind enough to annotate the Zenith repair guide and find some docs for a critical component. With his permission I've added this information to my Web site.
For more Z-100 repairs, check my own repair notes on my Web page.
For more Z-100 stuff from me, check my Z-100 and other Heath computer Web pages.
- Herb Johnson
August 2010, from Rich Lentz: I am a retired Navy Electronics technician and could (can) fix most electronics equipment, some without a schematic. But I just don't understand the [Heath/Zenith Z-100] switching P/S.
Got a Z100 in excellent physical condition - with a dead P/S. Popped the fuse when turned on. Found the Zenith Z100 Service Manual on the internet and dug in. [Here's the Service Manual section on the power supply. - Herb]
Determined that CR1 was shorted, thought that would fix it but no luck. With CR1 replaced, C5 & C6 are good and I have 330 volts Dc across the two ( in series). I also have 12 VDC feeding the switcher oscillator (Q3 - Q8) at C37. Don't know if these are correct, but all other output voltages are about 1/2 to 1/3 of what they should be.
If you don't have a P/S do you have a schematic that has expected voltage/resistance so I can troubleshoot further? I assume that I can trouble shoot the P/S out of the PC and out of the metal enclosure, if not is that the problem? I heard that you can't power up the newer PC P/S unless they are hooked up to the mother board. I assumed this was because there are sense lines going back to the P/S. I don't see anything that looks like sense lines unless some of the 5 volt lines need tied together.
[I sent him a power supply, a schematic I had obtained, and notes from a Heath test and repair guide for the Z-100 series. Rich agreed to summarize his situtation and results so I could put them on my Web site. Here's what he sent. - Herb]
I found a Z-100 on eBay that had a minimum bid of 49.99. The description stated "Floppies spun up but would not read, then nothing worked. May have bad power supply." I figured it was worth risking $50 if only the floppy drives worked, and if not there had to be spares that could be used in my Z-100. Many do not realize that the Z-100 will not work with PC-Dos and the only "compatible" monitor output is the composite video out. Maybe they do not know what they have. Or it could just be a shorted capacitor in the power supply. Large electrolytic capacitors will short out when power is applied to them after years of nonuse. Maybe it's is just a bad cap. So I bid $50, And got it.
Upon arrival, I disconnected the P/S from all the boards, tested it and got 0.0 on all outputs. Read the Z-100 Service manual (available on Herb's web-page) and dug into fixing it. After 3-4 evenings of getting nowhere, and only determining that every cap in the P/S was good, I contacted Herb and ordered one of his Z-100 P/S.
While waiting I searched around on the internet and found out the real part number for the switching-regulator chip and the data sheet for it. After reading it I had a few more ideas to test and dug in again. Eventually, I discovered that one of the two switching transistors was bad, but after replacing them - I had output, but it seemed like there were problems on all but the 12 volt power supply. Eventually, I discovered that you must have a load on the 5 volt supply to make the P/S work correctly. I had a park lamp from my old 6 volt VW Bug, and wired up a "dummy-load" to put some load on the P/S. And the P/S worked.
I have inserted into the description from the Heath Z-100 Service Manual Power Supply section, some notes that explain some things to look for and some more explanations on how the thing works. [If the RTF document is not readable, try this PDF version. - Herb]
This is not a step-by-step procedure/instructions. I assume you know how to repair the typical analog power supply, otherwise, don't attempt it. (The Z-100 Service Manual is available via my Heath/Zenith Web page. - Herb)
I have also included the data sheet for the SG2524 - IC1 - the heart of the power supply. The Principles of Operation section should help you understand how switching power supplies work.
If you have any corrections to the above information I'd appreciate hearing them: please include appropriate references so I can confirm that info. I am not responsible for errors or omissions in the above info, use it at your own risk. - Herb Johnson
Copyright © 2022 Herb Johnson
New Jersey, USA
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Copyright © 2022 Herb Johnson