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erg
January 11th, 2008, 07:01 AM
Well I have been reading all sorts of good stuff from everyone but I'm just not upto helping anyone with so many cluey members so I though I would write a short missive on what I am upto so I can get out of just "lurking".

Well in my side of the world it's hot!! 35-42 degree celcius temps for days so not much gets done and I finally have a chance to sit down and get a cpu board running that I have been trying to get going for many months.

The cpu board is 1982 vintage and I was given it in "unknown condition" so I pulled out an 11 volt 1amp plugpak and wired it to the 8 volt rail of a small 4 slot motherboard I have. I measured the 5 volt regulator on pin 24 of the spare empty socket for boot roms and it was near zero volts.

The next step was get the magnifier out and have a look around the CPU board and I didn't find anything like a track s/c so I proceeded to pull out the regulator (78H05) and refit it with a known working reg. Powered up the CPU board and still no 5 volts.

I then started to pull the chips out 2 and 3 at a time and power the board, still nothing. By the time I finished I had all the chips out. This obviously left the board itself or the "odds and ends" circuitry as the cause of the problem.

I put the magnifier back on the board and found a ceramic cap that looked suspect, pulled it out and still no 5 volt supply.....mmmmmm

Anyway I was thinking for a short time and the regulator current must be going somewhere, then I ran my finger over part of the Power-On-Clear circuitry and low and behold I found this tantalum capacitor that was very warm.

Removed tantalum capacitor and voilia!! ....5 volts supply rail again.

Moral to a 1 hour story.....I hate tantalum capaciors...it's replaced by a 1 uF 63 volt electrolytic.

Well at this stage I had a desktop covered in IC's and my CPU board so I though I might use this point to my advantage. I dragged out a device I bought when I was more affluent and it's called an ALL-01 or specifically it plugs into an IBM 8088 or 286 and can program eproms, 8050 somethings and has the ability to signature test 74 and CMOS series IC's. If you can find one they can be a godsend for making custom system eproms and finding faults.

At this stage my yourgest son came into my study so I was selecting the ic's and I showed him how to identify a particular IC, program the tester and give the IC a couple of tests to see if it was OK. Once he understood what I was doing and why I was doing it he had a marvelous experience helping. Anyway we chewed our way through all the CPU board's ICs and found a 74LS139 that failed but all the other 40 something IC's were OK. Moral to this episode, check everything!!....that could have been a hair tearing episode trying to find that faulty 74LS139 with the board powered up and not operating properly.

Anyway I restuffed the board, tested it's boot eprom was OK and it's waiting to be powered up. To be honest one thing worries me is just how reliable the 30 year old sockets are going to be but that...I will leave to another day.

How does everyone do this stuff ?? Any thoughts on getting S100 boards running ??

Erg

billdeg
January 11th, 2008, 05:55 PM
Sounds like you've got the right idea!

Dwight Elvey
January 13th, 2008, 08:21 AM
---snip---
How does everyone do this stuff ?? Any thoughts on getting S100 boards running ??

Erg

Hi
After removing shorts, the next thing I do is look for addressing problems
( I'd find the 74139 doing this ). The 8080 for instance does pushes to the
stack when looking at blank memory ( reading 0FFH ). This will cycle
through memory, allowing one to monitor various address decodes.
Once this is done, I see if the processor is running. I try the code
that is on ROM first but may blow experimental EPROMs to do simple
test( it is hard to tell what is happening when some BIOS is doing
20 things and one doesn't work ).
I rarely pull all the parts from a board. I have no chip tester and find
it relatively easy to test on the board with the use of a good scope.
As a piece of test gear, I would recommend a scope over a meter.
Of course, scopes do cost more than most meters but still, it is much
more useful than knowing that the 5 volt rail is 4.95 or 5.05. One just
sets the scale to 2 v/div and if it deflects 2.5 division, it is 5 volts.
Any ripple will show as well.
Other than that, it is just using the knowledge of knowing what the
signals should look like. One learns to recognize a floating wire or
shorted signal. One needs to understand fetch and store cycles of
the processor.
The scope should be a 100MHz analog scope with delayed sweep
and two input channels. I don't recommend anything less. It doesn't
have to be a Tek but they do have a reputation for good trigger
circuit. I've found HP and Leader to be good scopes as well. I personally
have a Leader LBO-518.
Digital scopes have their advantages but being samplers can lie to one
and confuse the trouble shooting. Logic analysers will show a lot of info
at one time but take a lot more effort when simply looking at many
things in series.
Dwight

Floppies_only
May 28th, 2008, 03:19 AM
...then I ran my finger over part of the Power-On-Clear circuitry and low and behold I found this tantalum capacitor that was very warm.

Removed tantalum capacitor and voilia!! ....5 volts supply rail again.

[...]

How does everyone do this stuff ?? Any thoughts on getting S100 boards running ??

Erg

I've never actually done this, but I've heard that you can spray freon onto components to see if they start working again when they cool down. It was in Scott Meuler's book, "Upgrading And Repairing PCs".

Sean, the original appliance operator

billdeg
May 28th, 2008, 06:14 AM
Check the 8705 regulators too. Every s-100 board has one and they can go bad. Check the voltage to be close to 5v.

bd

MikeS
May 28th, 2008, 08:12 AM
Check the 8705 resistors too. Every s-100 board has one and they can go bad. Check the voltage to be close to 5v.

bd
What's an 8705 resistor? Measured in volts instead of ohms?
m

billdeg
May 28th, 2008, 08:23 AM
ha ha ...I meant 7805 regulator