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Heathkit H8 weird issue

neosunrise

Experienced Member
Joined
Jan 17, 2019
Messages
132
Location
Chicago, IL
My Heathkit H8 did not work correctly when i received it. I noticed that many chips had oxidized pins, so I spent some good hours cleaning the pins and “deoxiting” the sockets. It actually worked fine for a few days. Today when I fired it up, it acted really weird: there were missing segments on the LEDs, and after a few seconds, the display went to a single digit. I shot a video and I would really appreciate it if anyone can share some idea here.
 

hmb

Experienced Member
Joined
Aug 8, 2019
Messages
129
Location
Canada
My Heathkit H8 did not work correctly when i received it. I noticed that many chips had oxidized pins, so I spent some good hours cleaning the pins and “deoxiting” the sockets. It actually worked fine for a few days. Today when I fired it up, it acted really weird: there were missing segments on the LEDs, and after a few seconds, the display went to a single digit. I shot a video and I would really appreciate it if anyone can share some idea here.
Heathkit's assembly and troubleshooting manuals for the H8 are easily available on the internet. They provide detailed instructions for diagnosing virtually everything that can go wrong with an H8, typically requiring little more than a multimeter and your own visual observations.

Most likely problem though... still more oxidized pins with marginal connections either at IC sockets or at cable/board interconnects. Possibly poor "cold solder" joints, since most of these H8s were hand-assembled from kits by people without a large amount of experience. Least likely but possible... marginally functional ICs driving the LEDs on the display/control board. An inexpensive logic IC tester can be very helpful in sorting out that issue.

If this is an older model H8 with the tin-plated backplane pins - these pins are yet another common cause of intermittent problems. You can try "deoxiting" them as well, but inevitably the problems return. If you have one of these backplanes, the only good long-term solution is to replace these backplane connector pins with gold-plated ones.
 

Bruce Tomlin

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Jan 6, 2010
Messages
159
Location
San Antonio, TX
I ended up with an ET3040 where some chips in sockets had gotten so oxidized that the tips of the pins actually fell off when I tried to reseat them. I had to solder on some donor pins from old PAL chips.

and after a few seconds, the display went to a single digit
The H8 uses a scanning display controlled by the CPU, and that's exactly what would happen when it crashes.
 

daver2

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Jun 19, 2012
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UK - Worcester
Yep, probably more of the same I am afraid.

Welcome to the problems of owning old computers...

Dave
 

neosunrise

Experienced Member
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Jan 17, 2019
Messages
132
Location
Chicago, IL
Thanks all for your comments. I fiddled with the boards and some chips. The computer now works as it should. I know for sure there are some contact issues but I don't know where they are. Guess I'll find some time and clean the sockets and pins again, oh well.
 

daver2

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What you might find useful is to sacrifice a TTL chip and attach a single pin removed from a 'gash' IC to a stiff piece of copper wire and use that as a 'probe' to test the force exerted by each pin of the socket on the test IC pin.

Remove the IC and go around each socket pin in turn with the probe and see if there are some sockets that have less force than others. These could be candidate problem areas.

Also, check which parts of the IC leg are being 'pinched' by the IC sockets. These areas of the IC pins deserve special attention to cleaning more than others.

Dave
 

Bruce Tomlin

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For the sacrifice chip, I prefer to use PAL chips from assorted junk boards, since they're usually socketed and have perfect pins. Being OTP makes them completely useless otherwise.
 

durgadas311

Veteran Member
Joined
Mar 13, 2011
Messages
1,232
Location
Minnesota
You might check the bus connector pins. If they are "tinned" (instead of gold plated/flashed) then you will very likely experience intermittent issues. Replacing them can be a task, but many have done it to great success.
 

neosunrise

Experienced Member
Joined
Jan 17, 2019
Messages
132
Location
Chicago, IL
What you might find useful is to sacrifice a TTL chip and attach a single pin removed from a 'gash' IC to a stiff piece of copper wire and use that as a 'probe' to test the force exerted by each pin of the socket on the test IC pin.

Remove the IC and go around each socket pin in turn with the probe and see if there are some sockets that have less force than others. These could be candidate problem areas.

Also, check which parts of the IC leg are being 'pinched' by the IC sockets. These areas of the IC pins deserve special attention to cleaning more than others.

Dave
Good point! I actually found that the 8k static ram card has very oxidized socket. Several pins were actually covered by rosin or some other sticky stuff. I inserted a thick resistor leg into each hole. Then sprayed some deoxit and moved the pin back and forth through each hole. It definitely made a big difference. The computer works like is should now. I really wish I could replace all the sockets but that’s a bigger project.
 

neosunrise

Experienced Member
Joined
Jan 17, 2019
Messages
132
Location
Chicago, IL
You might check the bus connector pins. If they are "tinned" (instead of gold plated/flashed) then you will very likely experience intermittent issues. Replacing them can be a task, but many have done it to great success.
I might do that. Any good sources for the gold plated pin headers?
 

hmb

Experienced Member
Joined
Aug 8, 2019
Messages
129
Location
Canada
I might do that. Any good sources for the gold plated pin headers?
The backplane headers require pins that are a bit longer than those typically used for jumper blocks on PC boards. I used Amphenol part number 77311-822-25LF, available from Mouser as part number 649-77311-822-25LF.

I have a powered-suction desoldering tool (the Hakko FR-301) which made the process fast and painless. Without such a tool the recommended (and far more laborious) method is to pull each pin out individually from the front of the board as you heat the solder side of the pin on the back followed by a cleanup of each hole with flux and solder wick.
 
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