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Kaypro PIO puzzle

Wa-hoo! Solved it!

Wa-hoo! Solved it!

Good news team - found the problem. Ye haa!!! :D

It was the *other* PIO in the Kaypro - the SYSPIO (U72). This generates the BANK signal which has to go Hi to enable the Boot ROM. This BANK (A) [A means this signal goes to schematic sheet 'A'] signal was stuck low and didn't pulse on Reset. At that moment I knew I had the smoking gun. So I pulled that PIO just to observe the signal & low & behold - a boot screen appeared! EPROM was fine after all - just wasn't being enabled. Yet, observing the /CE on the EPROM with a probe showed it was active for a burst at Reset, but this must have been a side effect of Reset. I interpreted that as ok, but now I see /CE is enabled (pulsing) *the whole time* while the "Insert Disk" screen is displayed. Renewing *that* PIO got the disk to boot up to "A>" prompt.

Now what about that suspect Z80 CPU? Sure enough, when it was put back in - screen flashed garbage, no boot. So it was *two* big i/cs that failed - PIO & CPU. Nasty, because the symptom (screen flashing garbage, no boot) was the same for either one of the failures. Come to think of it thou - this symptom is typical in 8 bit micros that won't boot & can mean a multitude of causes.

I will write up a full blog to go on Tezza's site, when I get my head around more of this. I will include some Troubleshooting Tips that will assist future Kaypro fixers regarding no-boot scenarios.

Thanks team - we got there in the end.

Philip
 
Last edited:
Tez,

It seems to me that you are pretty handy with a scope to troubleshoot a Kaypro. I am wondering what kind of scope you use.
I thinking about buying a scope and like to get some insight.

Doug
 
On the subject of 'scopes, I highly recommend a used Tektronix. I bought mine off eBay (for $150!), and it's great! It's a Tek 465 with DMM. In my opinion, the oscilloscope is the most practical piece of electronic test equipment!

See what your local hamfests have too. There's one guy that travels to most of them in the SE that always has a bunch of high quality restored/refurbished test equipment at pretty reasonable prices. (You're not in the SE, though!)

Kyle
 
Dougtronics said:
Tez,

It seems to me that you are pretty handy with a scope to troubleshoot a Kaypro. I am wondering what kind of scope you use.
I thinking about buying a scope and like to get some insight.

Doug
Click to expand...

Me? Handy with a scope?? LOL!!

No, I bumble around with it trying hard to figure out what I'm doing and not short-circuiting things! I'm getting there though.

You'd be better to address the question to Philip Avery (pavery). It was his scope and in fact he's reclaimed it now he has more computers to fix (damm!). I can't actually remember what brand it is?

Tez
 
Dougtronics said:
Tez,

It seems to me that you are pretty handy with a scope to troubleshoot a Kaypro. I am wondering what kind of scope you use.
I thinking about buying a scope and like to get some insight.

Doug
Click to expand...

Well, for the most part I used/use a logic probe. I only used the scope in this case to check the power supply was free of ripple & the clock signals were correct. I find a probe so much quicker & easier to use - you don't have to adjust its settings in order to see meaningful waveforms. Plus, the probe has 'memory', so it can detect if one (or more) short pulses occurred. Only a flash/expensive scope will do that.

Have you a probe? IMHO a logic probe will help locate *nearly all* logic faults.

Philip
 
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