I started working on my Heathkit H-89A

[high...@lightspeed.net]

I started working on my Heathkit H-89A. When powering on I only get one beep and on one floppy drive the light comes on and motor starts spinning. The terminal board appears to be working fine. The power supply voltages look okay. I removed the CPU board (85-2549-1), Serial I/O board (85-2550-1), Floppy Disk Controller board (85-2597-1) and 16K Memory Board (85-2539) for inspection. I cleaned all the connections with Deoxit and reassembled. Nothing changed, still no H prompt. I did power on with the accessory boards removed too. I am likely going to replace the tantalum caps on the CPU board and see what happens. I do have the original full height hard sector floppy drive (Siemens 100-5B) but not the controller for it. My computer has the soft sector floppy controller and two Teac 14733770-00 drives. My primary goal is to use this computer as a terminal for an older 8-Bit machine. Any suggestions appreciated.

[Glenn Roberts]

From your description it sounds like the machine is configured for auto boot on power up so it wouldn’t put you into the monitor (hence no H: prompt). The DIP switches are obscured in the picture so I can’t tell how things are set. Suggest you disable the auto boot setting.

Even though it’s a single beep the cpu is clearly running since it tries to boot (is OFF LINE key depressed?).

If you choose to replace tantalum’s the ones most likely to cause problems are on the 12v regulators. Heath used 25v parts which can exceed that during power up. Replace with 35v ones.

Sent from my iPad

On Oct 2, 2025, at 1:51 AM, high...@lightspeed.net wrote:

 I started working on my Heathkit H-89A. When powering on I only get one beep and on one floppy drive the light comes on and motor starts spinning. The terminal board appears to be working fine. The power supply voltages look okay. I removed the CPU board (85-2549-1), Serial I/O board (85-2550-1), Floppy Disk Controller board (85-2597-1) and 16K Memory Board (85-2539) for inspection. I cleaned all the connections with Deoxit and reassembled. Nothing changed, still no H prompt. I did power on with the accessory boards removed too. I am likely going to replace the tantalum caps on the CPU board and see what happens. I do have the original full height hard sector floppy drive (Siemens 100-5B) but not the controller for it. My computer has the soft sector floppy controller and two Teac 14733770-00 drives. My primary goal is to use this computer as a terminal for an older 8-Bit machine. Any suggestions appreciated.

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[high...@lightspeed.net]

I found a short on a 7912 voltage regulator and traced it back to a tantalum cap. Several tantalum caps were replaced as well as the 7912 regulator. I now get two beeps on boot, but I have another problem. I no longer have video display. A lead on U569 was broken and it was replaced with an equivalent transistor (but a different form.) U566 was removed, tested and re-soldered. Here are images of the work and the dip switch settings. Any suggestions appreciated.

[glenn.f...@gmail.com]

DIP settings look fine.  Having switch 4 in the “ON” position is a little unusual, but it means that by default the system will try to boot from the controller at port 170Q (which in your case is the H37 soft sector board) so you’re good.  Once we get your monitor running you can just type “B” and it should boot from the floppy.

We need to figure out what happened to the video.  Do you have glow in the CRT filament?  If not there are several possibilities (we can follow up with more suggestions if we go down that road).

Another frequent problem is people inadvertently turn down the brightness control!  Check that.

When you depress OFF LINE you should be able to type characters and see them on the screen (and hear audible click, unless that’s turned off via a DIP setting).

FYI if all you want is to use this as a terminal you may want to consider essentially turning it into an H19.  It only involves joining the two connectors on the left side of the CPU board, removing CPU power connector and attaching the appropriate DB-25 wire harness to the terminal board serial output. You can even remove the whole CPU board if you like.

At any rate, we should first make sure the video and terminal board are working again.  Then you can decide whether to revert the box to an H19 or boot an OS and run terminal software. There are also some ROM options that implement a “terminal” command in the monitor (I’m thinking of the Magnolia Microsystems ROM). That just passes the terminal functions straight through to the “modem” port (330Q) on the serial board.

• Glenn

 

 

From: se...@googlegroups.com <se...@googlegroups.com> On Behalf Of high...@lightspeed.net
Sent: Thursday, October 2, 2025 6:40 PM
To: SEBHC <se...@googlegroups.com>
Subject: Re: [sebhc] I started working on my Heathkit H-89A

 

I found a short on a 7912 voltage regulator and traced it back to a tantalum cap. Several tantalum caps were replaced as well as the 7912 regulator. I now get two beeps on boot, but I have another problem. I no longer have video display. A lead on U569 was broken and it was replaced with an equivalent transistor (but a different form.) U566 was removed, tested and re-soldered. Here are images of the work and the dip switch settings. Any suggestions appreciated.

 

 

To view this discussion visit https://groups.google.com/d/msgid/sebhc/b9e4a581-c751-41ed-81ef-72a9e1240e89n%40googlegroups.com.

[high...@lightspeed.net]

There is no longer is glow in the CRT filament. When I picked this computer a few months back the brightness knob did need to be adjusted, but sadly this is not the case today. When I depress the OFF LINE key I can hear the sound that I used to when using the keyboard. When the OFF LINE key is down when I power it on, I get one beep. The terminal board seems to be fine (although I can't see anything.) While I did purchase this computer primarily for use as a terminal, I would very much enjoy using it as a computer. I find the styling elements on the H89 to be some of the best ever on a computer. I have the original User and Assembly manuals as well as all of the pictorials. I have a PDF of the service manual. I have several boxes of other related paperwork. 

[glenn.f...@gmail.com]

I guess the first thing to check is to make sure you haven’t forgotten to plug something back in or somehow changed the configuration. Any time you take a system apart it’s easy to forget a step.

 

Assuming it’s not something simple like that, two things come to mind.  First if you have any way to determine if there’s high voltage to the CRT that would be useful information.  No high voltage points to possibly one or both of the NPN power transistors that drive the horizontal circuit. We’ve heard from a few folks who’ve had these go bad.  I recently thought I had this problem, removed both transistors (a bit of a pain to do) and they both tested good. I later decided it was just a poor socket connection. The system is now operational.

 

The second problem we’ve seen (I’ve had it on three different systems) is a break in the connection that feeds the filament voltage. The power is derived from a small tap wrapped around the flyback transformer but that voltage is then fed through a number of soldered connections on the video board. These connections can weaken over time. The solution is to remove the video board and re-flow all solder connections related to the filament voltage feed. The video board can be removed out the side of the machine once you’ve removed the 4 screws holding it in.

 

You could also just check for a clean connection at the CRT itself, maybe physically remove an insert the connector to abrade the connection or put some deoxit contact cleaner on.

 

Let us know what you find…

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[high...@lightspeed.net]

For a minute I thought it was back. I had the CRT glowing and then poof. Some small component on the video circuit board went up in flames.

[high...@lightspeed.net]

It looks like three tantalum caps on the video circuit board decided to ignite independently of the motherboard repair. I don't think the failure of these caps was a result of anything to do other than age and the voltage applied during testing. I'll put in some mil spec replacements (with higher voltage) and I think the H89 will be alive again. I did make sure the anode cap was discharged before I removed the board. I'd appreciate your thoughts and comments.  Ken

On Thursday, October 2, 2025 at 6:05:57 PM UTC-7 glenn.f...@gmail.com wrote:

[Joseph Travis]

Those are not tantalums, they were resistors.  I suspect one or more of the nearby transistors are shorted.

Joe

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[high...@lightspeed.net]

It would help if I looked at the schematic (which I have.) Thanks, Ken