Help with older H8-4 Serial Board (#010781)...

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Joe Travis N6YPC

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Oct 7, 2021, 6:18:38 PMOct 7
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Greetings All,

I'm having difficulty troubleshooting an older H8-4 Serial Board (#010781 or #010881).  I need the correct schematic and board X-ray view if possible.  The documentation I downloaded from SEBHC website is for the later version (#050978).

I currently have channels 2 and 3 working while unable to get anything going with channels 0 and 1.  I thought I found the problem when I discovered an open (ground) via located near channel 1 but still having difficulty tracing signals due to the differences in the boards.  IC's 102, 112, 122, 132, 147 and 148 are wired differently than shown in the later version schematic / board X-ray view.

I've swapped IC's between the good and bad channels, the problem remains with the bad channels, no issues with the swapped IC's.

I remember when Heathkit updated the 8250 serial I/O circuit on the H89A CPU board due to some issue(s) and I wonder if they applied the same change to the H8-4?  (Not that that is going to fix my problem but just throwing it out there as a possible reason why this board is different.)

Thanks in advance!

Regards,
Joe Travis n6ypc

glenn.f...@gmail.com

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Oct 7, 2021, 8:35:39 PMOct 7
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Joe Travis N6YPC

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Oct 7, 2021, 10:34:21 PMOct 7
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Thanks Glenn! 

The x-ray view is for the older version however, the schematic is for the later version.  I believe I found the problem.  The older version H8-4 used a CD4007 (U149) which (due to loading) supplied two baud rate clock outputs.  One going to channels 0 and 1 while the other went to channels 3 and 4 which explains why two channels worked and the other two were "dead".  The later version H8-4 uses the channel 0 8250 for the baud rate clock and outputs it to the remaining channels.  At this point I'm debating on whether to buy a CD4007 or modify the circuit.  Thanks again!

Regards,
Joe Travis n6ypc

Les Bird

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Oct 8, 2021, 7:33:41 AMOct 8
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Nice troubleshooting Joe!

Les

Joe Travis N6YPC

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Oct 8, 2021, 10:35:20 AMOct 8
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Thank you Les.  I elected to modify the baud rate clock circuit to match the later version H8-4.  It was simple enough to place a header at U149, add a few jumpers, do a foil cut and it's back in business!
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