H-8-4 detective work

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

Had an interesting challenge repairing an H-8-4 board. Channel 0 was inoperative. After some sleuthing with the scope I determined that serial out data (UART pin 11) was never being received by the 1488 chip. Here’s what I found when I removed the socket for U103:

 

 

There had been a previous repair attempt (I removed the remnants thereof before I took this photo) but it was a bit slipshod.  There are two broken connections (pin 3 and 9) and a missing foil (on reverse side) that would have jumpered pins 9 & 10.  The board is now back up and fully functional…

 

• Glenn

 

[Joseph Travis]

I feel your pain... Great job finding that one!

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

Tx joe. Pain and joy. As you know it’s quite satisfying to repair these old babys and bring them back to life!

 

• Glenn

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[smb...@gmail.com]

It's too bad there's no written history of prior repairs when you inherit a piece of vintage equipment.

I still remember fixing a GC-1000 clock where the primary problem was that a prior owner had wrecked a trace to a CPU reset pin while doing some recapping. After that, my policy is any repair starts by looking at what was repaired before! :)

Scott

[Glenn Roberts]

Good strategy. In this case there was no visible sign of a prior repair. It wasn’t obvious until I removed the ic socket.

Another annoyance is boards with obvious mods and bodge wires. These may be useful and even necessary, but my first instinct is to focus there. It can be hard to resist the urge to undo these and start fresh. Trionyx boards frequently needed these tweaks.

Sent from my iPad

On Sep 26, 2023, at 2:59 PM, smb...@gmail.com <smb...@gmail.com> wrote:

It's too bad there's no written history of prior repairs when you inherit a piece of vintage equipment.

To view this discussion on the web visit https://groups.google.com/d/msgid/sebhc/644e02a6-9b87-4918-b5cb-11793a98982en%40googlegroups.com.

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

On a second H-8-4 board I found the H8 wouldn’t come up when the board was inserted. Classic shorted tantalum symptom (always shut down immediately if you get no front panel response!)

 

To my surprise I realized on inspection that Heath used only 20 volt rated tantalums.  That’s really close to the limit on the +/- 18 volt supply side (which is the one that shorted)!  Replaced with 25-volt ones (could have used 35 if I had any) and board comes up and tests fine…

 

• Glenn

[Glenn Roberts]

Come to think of it, I guess it was somewhat fortunate that I was using an 8080 CPU board for the test. I believe the z80 boards don’t use the +/-12V supplies (?) and so the cpu would have run just fine with a shorted tantalum on the 18v input side? Of course smoke and fireworks would have followed shortly thereafter…

Sent from my iPad

On Sep 27, 2023, at 9:16 PM, glenn.f...@gmail.com wrote:



On a second H-8-4 board I found the H8 wouldn’t come up when the board was inserted. Classic shorted tantalum symptom (always shut down immediately if you get no front panel response!)

 

To my surprise I realized on inspection that Heath used only 20 volt rated tantalums.  That’s really close to the limit on the +/- 18 volt supply side (which is the one that shorted)!  Replaced with 25-volt ones (could have used 35 if I had any) and board comes up and tests fine…

 

• Glenn

 

 

From: glenn.f...@gmail.com <glenn.f...@gmail.com>
Sent: Monday, September 25, 2023 9:19 PM
To: se...@googlegroups.com
Subject: H-8-4 detective work

 

Had an interesting challenge repairing an H-8-4 board. Channel 0 was inoperative. After some sleuthing with the scope I determined that serial out data (UART pin 11) was never being received by the 1488 chip. Here’s what I found when I removed the socket for U103:

 

<image001.jpg>

[Joseph Travis]

Well, there's one way to find out...   ;)

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[norberto.collado koyado.com]

On power on the tantalum suffers a lot. So, do several power on/off to stress the caps. Wait around a minute or two between power on. It will burn if voltage is out of specs on power on.

Get Outlook for iOS

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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of Joseph Travis <jtravi...@gmail.com>
Sent: Wednesday, September 27, 2023 7:40:09 PM
To: se...@googlegroups.com <se...@googlegroups.com>
Subject: Re: [sebhc] Re: H-8-4 detective work

 

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