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Another PET 2001 motherboard needs your help...

 
MikeS said:
Hey, hey, I've got a 6550 riding on the '93 being the source of the problem! What's your reasoning?
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Don't worry Mike, I've got a couple of 6550's with your name on them, regardless of wether you diagnose the problem correctly or not. Maybe we should put down some bets on what component is the problem ;-)
I've also promised I'd send a few 6550's through to Ethan Dicks who got the power section working on this board.

Also, for those who want to see my work bench/environment, look here:

http://web.me.com/lord_philip/other_computers/20012.html#103
 
MikeS said:
Hey, hey, I've got a 6550 riding on the '93 being the source of the problem! What's your reasoning?
Click to expand...

Mike,
From looking at Phil's excellent scope photos of the 8 MHZ vs the 1 MHz, it shows a nominal 40 nS propagation delay. Well within the spec of the 30 year old part.

From looking at the latest batch of photos, it is clear to me that the real culprit is the D8 NAND gate. The reset pulse output from D8-8 is activating low, well before the enabling 8 MHZ clock goes high. Basically, the NAND gate is acting like an inverter of the flip flop Q output and feeding it right back to the flip flop reset, creating a glitchy signal on C8-3 instead of a normal 43 nS pulse.

EDIT: This may also explain why hanging a capacitor on the flip flop reset made the circuit work. It delayed the reset just long enough to get a slightly better C8-3 clock and make everything work. As to why Phil's magic jumper from the Q output to the loads worked, I don't know, so Phil will have to answer that!
 
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Well, assuming we're really looking at D8 pins 8 & 9, I have to grudgingly agree that they should never be low simultaneously, so that does indeed look suspicious...

Grrr... ;-)

(But it coulda been C9, mumble grumble...)
 
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dave_m said:
Phil,
You put your finger on the problem. While there seems to be no issue with the LS93 counter, it seems obvious to me that the C8-3 output of the flip-flop is starting to reset BEFORE the rise of the 8 MHz clock. This can not be. The problem must be around the premature reset of the flip flop. Either the C8 flip flop is bad (and one has been replaced already) or the D8 NAND gate is bad.
Click to expand...

Starting from here on the timeline with catching up ... yes, the diagrams look as if they could not be. I am baffled....
 
What I find interesting comparing http://web.me.com/lord_philip/other_computers/20012.html#93 is that the '93 actually seems to be too fast... The rising edge on C8-3 on the broken board, which has gone through the '93 to be divided by 8 and then the C8 Flipflop is earlier compared to the working one.

But what is really strange is, is that the falling edge of C8-3, which comes from the D8-9 8MHz clock via D8, appears when the clock is still low. As D8 generates the reset pulse only when its both inputs are high, this should be impossible. However, http://web.me.com/lord_philip/other_computers/20012.html#96 proves otherwise...

Comparing http://web.me.com/lord_philip/other_computers/20012.html#90 and http://web.me.com/lord_philip/other_computers/20012.html#89 which directly compare the 8MHz clock and the counted 1MHz result, the only difference I can see that on one side - the working PET actually! - the '96 output has a weaker rising edge, where it may trigger the C8 flip flop a bit later than the other one. Still have to think about what that may mean though...
André
 
Andre: Did you get my email about your page on 6502.org?

Philip: Just curious: Why are there four ROMs in there if everything's on the adapter board?
And what's that funny green stuff around the power diodes?
Finally, which video adapter is that?
 
Hi André,
I'll quickly recheck these test on the non working board just to make sure they are accurate and I haven't made any errors.

I won't replace any parts unless there is a consensus between Mike, Dave and yourself.

UPDATE:
To the best of my knowledge I can confirm that this is correct:
http://web.me.com/lord_philip/other_computers/20012.html#93

With the test that compares the 8MHz clock and the counted 1MHz I have posted a new composite image here:
http://web.me.com/lord_philip/other_computers/20012.html#104

I have tried to align the 8MHz trigger as best I can. Seems the broken boards 1MHz pulse may start a fraction earlier than the working boards pulse, I don't know if this is significant or due to my funky scope. However I am assuming it's correct.
 
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MikeS said:
Philip: Just curious: Why are there four ROMs in there if everything's on the adapter board?
And what's that funny green stuff around the power diodes?
Finally, which video adapter is that?
Click to expand...

The 3 EPROMs with green stickers on top are from the BASIC 2 set that I burnt, and the one with the yellow sticker is Ruud's test ROM (the 4th BASIC 2 EPROM is removed at the moment).
The funny green stuff around the diodes is electrical tape so that I dont accidentally short anything out in this area.
As for the video adaptor, I'm using one of these:
http://home.comcast.net/~medasaro/6540rom.com/petvideo.html
works really well for me.
 
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Nama said:
I won't replace any parts unless there is a consensus between Mike, Dave and yourself.
Click to expand...

This is a good plan. Let everyone mull over the data and we can proceed from there.

It is hard enough replacing one part. Let's not have to replace two.

Phil,
You have done a great job with the photos which has really helped the investagation. When I take a scope photo, all I get is glare.
-Dave
 
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Nama said:
Hi André,
I'll quickly recheck these test on the non working board just to make sure they are accurate and I haven't made any errors.

I won't replace any parts unless there is a consensus between Mike, Dave and yourself.
Click to expand...
Well, I have to go along with Dave that you can't have a NAND gate's input and output simultaneously low as they appear to be in the display showing D8 pins 8 and 9. Also, now that I see the clock relative to C9-11 in your latest image I have to also admit that the 'LS93 looks OK after all.

So yeah, it looks like D8's pin 9 input is not working correctly.
 
Nama said:
The 3 EPROMs with green stickers on top are from the BASIC 2 set that I burnt, and the one with the yellow sticker is Ruud's test ROM (the 4th BASIC 2 EPROM is removed at the moment).
Click to expand...
Oh, OK, so you're just parking them there.
As for the video adaptor, I'm using one of these:
http://home.comcast.net/~medasaro/6540rom.com/petvideo.html
works really well for me.
Click to expand...
That's what I thought; I think his is the only version that mounts vertically right on the internal connector. But it's either/or, i.e. there's no connection for the internal monitor, right?
 
MikeS said:
But it's either/or, i.e. there's no connection for the internal monitor, right?
Click to expand...

Yes, unfortunately so. I was thinking to rig something so both the internal monitor and an external one could be run simultaneously...not sure what that would do for the monitor levels though.
 
Nama said:
Yes, unfortunately so. I was thinking to rig something so both the internal monitor and an external one could be run simultaneously...not sure what that would do for the monitor levels though.
Click to expand...

On your monitor, did you have to fiddle with the horizontal hold or did it sync right away?

Is the PET 9" video signal totally NTSC compatible?
 
dave_m said:
On your monitor, did you have to fiddle with the horizontal hold or did it sync right away?
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on the small 7" monitor I use I didn't have to do anything, perfect image first time. But I have uses another monitor that produces some tearing, especially up the top.

dave_m said:
Is the PET 9" video signal totally NTSC compatible?
Click to expand...

Hummm...no idea actually. I know Anders has tried to build a video adaptor before, maybe he'll have more insight. I just wanted a simple plug and play (no brainer) option for this troubleshooting.
 
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Nama said:
Yes, unfortunately so. I was thinking to rig something so both the internal monitor and an external one could be run simultaneously...not sure what that would do for the monitor levels though.
Click to expand...
Shouldn't be a problem. They're digital signals after all and a little extra load shouldn't bother them; that circuit was usually connected to the user port anyway, in parallel with the internal connector.

Too bad he didn't just put a row of 7 pins right beside and in parallel with the 7-pin header.
 
Actually, I think André helped me with the schematic for the video adapter. As far as I remember, there were two different old school designs, of which he had tried both and updated one with slightly more modern components. I didn't have the exact same parts available so I fibbed a bit and got an adapter that outputs composite video which is out of sync on 40 column PETs. When used with 80 column PETs, only pixel garbage was present on the 1084 monitor it was connected to. In total there probably is a handful different designs on how to adapt PET video signals to be used on an external monitor, some designs more flexible and compatible than others. When it comes to standard 60 Hz NTSC composite video, it usually displays fine on my 50 Hz PAL monitors, except colour information is lost. Any signal out of sync probably is due to other factors than not being compatible with the video mode in use.

Furthermore, I got ahold of an old PET to TV adapter, i.e. a RF adapter that connects to the user port. To make a long story short, it doesn't output a meaningful picture at all, no matter which PET or which RF channel I tune in on the TV. But it looks nice in its package and might be collectable for a dedicated PET collector. It even comes in a nondescript cardboard box!

With the risk of getting off-topic, I notice Matthew on the web site mentions the adapter might work with other contemporary computers like TRS-80. It makes me wonder how many video standards the computer industry used in the early years. I've heard about the 5-pin DIN "American standard" output with ground on pin 2, sound on pin 3 and (colour) composite video on pin 4. It is a pinout that works with VIC-20, C64, Atari 800, 600XL, supposedly also Spectravideo SVI-318/328, even NTSC TI-99/4A (the one that outputs composite video rather than component R-Y, B-Y, Y).

There is also the CBM 610 pinout: Luminance, Ground, V Sync, Comp Vid, H Sync which I by chance noticed also seems to be used on the Philips P2000c portable CP/M computer. When two relatively different computers use the same pinout, it suggests to me there was a standard behind, but I don't know what it is called or which other computers would use it. Also I don't know if the vertical and horizontal syncs would be different for each computer even if the pinout itself is the same.

Maybe I should open a new topic about this in particular, video pinout standards prior to the IBM PC compatibles.
 
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