Open frame H8

[Ronald West]

I tried to send this last night but had too many pictures in it.

Selected 1 picture of the processor board (8080). I did some modifications on the ROM socket to allow a 4k ROM with a combination of the PAM and H17 ROMs copied to their respective spots in the EPROM.

Currently i have a 32K EEPROM in there and addressing may be set up for 8K. There is a 28 pin socket plugged into the 24 pin socket so that the larger ROM can hang off the top and wires routed accordingly. That chip is a microchip 28C256. Currently looking to make sure i didn't torpedo myself here assuming that 10 years later it would all just work.

The big Green wires on there are part of the XCON8 mod as well. The 30ga wires i did when i was a bench tech.  I guess i have a bit of cleanup to do before i can get this bad boy working

I put the PAM8 image on the Microchip EEPROM and gonna fix the addressing to enable the chip only in the 1k range. also tried the memory at 0k and 8k and no difference. assume that should be set for 8k once i get the addressing fixed up. i assume with the PAM8 i can leave out the I/O board and it should work?

FYI,  the processor board had the interrupt chips missing and some kind of lash up plugged into one of them. Not sure what that was. Restored the chips in (ic217 & ic218) those two sockets as in the schematic. Not sure what i was doing with that mod. Yikes!

Ron

[Glenn Roberts]

Certainly an “interesting” board! My first instinct is to reverse a lot of those mod’s. Norberto made a nice little adapter board to let you use an EEPROM in the ROM socket. That’s a much cleaner approach. I can post pictures of my 8080 board later. I will also read your message more carefully when I’m at my computer later this evening.

Sent from my iPad

On Jun 13, 2026, at 3:59 PM, Ronald West <west...@gmail.com> wrote:



<20260612_205859.jpg>

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[norberto.collado koyado.com]

I see IC 202 missing. You will need to add the equivalent as the 8080A cannot take +/-18v directly. Not sure if the 8080A is already damaged.

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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of Glenn Roberts <glenn.f...@gmail.com>
Sent: Saturday, 13 June 2026 14:02:41
To: se...@googlegroups.com <se...@googlegroups.com>
Cc: se...@googlegroups.com <se...@googlegroups.com>
Subject: Re: [sebhc] Open frame H8

 

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

Here’s my 8080 board.

https://photos.app.goo.gl/NSaPSJhMPTeCMXKKA

 

this uses Norberto’s 28-to-24 pin converter to house the EEPROM in the 24-pin socket

H8 8080A 28C256 EEPROM Adapter

 

It looks like your board has a 28-pin socket soldered into the 24-pin location. Hard to tell what’s going on with all the spaghetti jumper wires.

 

I share Norberto’s concern about shorting in and out for IC202 – would seem to be applying +18V to the 8080? Was this part of an attempt to modify the board for use with an ATX power supply?

 

With the original PAM 8 image the first RAM location should be at 8K (040.000)

 

The arrangement on my board allows up to 8K of addressable ROM space. That means both A11 and A12 need to be picked off from board traces (connected to S2 and T2) so both pins 2 and 3 need to be lifted on the ROM decoder (IC207). The EEPROM can be programmed with four different 8K PAM images, selectable via the jumpers on the adapter board.

 

If it were me, I’d certainly bring the board back to original condition. For the 12V regulator you’ll need to use something like a 7812 I presume

[norberto.collado koyado.com]

It can use the 79L12 or 78L12 instead. Not sure if IC202 is positive or negative regulator.

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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of glenn.f...@gmail.com <glenn.f...@gmail.com>
Sent: Saturday, 13 June 2026 17:45:41
To: se...@googlegroups.com <se...@googlegroups.com>
Subject: RE: [sebhc] Open frame H8

 

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

IC202 is for the +12V line so 78L12…

 

From: se...@googlegroups.com <se...@googlegroups.com> On Behalf Of norberto.collado koyado.com
Sent: Saturday, June 13, 2026 9:27 PM
To: se...@googlegroups.com
Subject: Re: [sebhc] Open frame H8

 

It can use the 79L12 or 78L12 instead. Not sure if IC202 is positive or negative regulator.

To view this discussion visit https://groups.google.com/d/msgid/sebhc/SA3PR01MB79183268C275B4F846769348F7E72%40SA3PR01MB7918.prod.exchangelabs.com.

[Ronald West]

Ref IC202. I removed that and shorted out the 7805's as i have a little power supply (not shown) that gives +5v, +12 and -12. i left ic203 in there to regulate the -5 volts. I checked the power supply and board voltages and they look fine. 

strange thing is this worked fine before. I did find a couple wires that could be better routed so working on that. 

Also found the 74LS30 to in IC207 to have a broken (missing) pin besides the one that was lifted for the XCON mod. put in a new chip and connected pin 2 and made some other changes so it will respond as a 1k rom. 

Thanks for the suggestions i will keep reading.I may do a quick Arduino project to test the memory board to be sure that isn't the cause of all this.

has anyone done an 8085 board for the H8? that would be cool as well. 6Mhz would be fun.

Ron

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[norberto.collado koyado.com]

Yes, we have an 8085A H8 board designed by Scott. Also, we have a new 8080A board as well with the support for the H37 board. The 8080A supports the H37 natively. 

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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of Ronald West <west...@gmail.com>
Sent: Saturday, 13 June 2026 20:12:53
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[glenn.f...@gmail.com]

Here’s scott’s page & video on the 8085

An 8085 CPU Board for the Heathkit H8 Computer – Dr. Scott M. Baker

 

Here’s Norberto’s 8080 CPU board

H8 8080A/EEPROM/RAM/UART/HA8-8

To view this discussion visit https://groups.google.com/d/msgid/sebhc/SA3PR01MB7918945BA5B688E3BD8294AEF7E72%40SA3PR01MB7918.prod.exchangelabs.com.

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

For high speed most of us are using z80-based boards, e.g.:

https://koyado.com/heathkit/New-H8-Website/h8-z80-v4.html

 

this board runs fine up to 10Mhz and can push to 16Mhz with addition of a wait state generator

 

Les Bird has also developed some nice Z80 boards

Home · sebhc/sebhc Wiki · GitHub

 

40-pin DIP/20Mhz Z80s can be hard to find but we have an adapter that lets you use PLCC version which may be easier…

 

• Glenn

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

Good point that if the 8K RAM board is not working the monitor probably won’t come up. If you’d like to build a modern RAM board this one’s great:

https://koyado.com/heathkit/New-H8-Website/h8-8080a-64kb-board.html

 

Todd goodman has the blank boards

boardinventory [Retro PCBs Wiki]

 

 

From: se...@googlegroups.com <se...@googlegroups.com> On Behalf Of Ronald West

Sent: Saturday, June 13, 2026 11:13 PM

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