Heath-Zenith Z-100 series power supply

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

history

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.

Repairs

[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.

- Rich

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

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