Cromemco dazzler replica project

[nullvalue]

What does the signal actually look like at IC41/1? Can you post a photograph?

Here's IC41/1 (D5=0)


and D5=1

 

[daver2]

Ok, and the trace (in the first shot) is relatively stable?

Can you check that IC43 pins 4, 10 and 13 are (really) connected to +5V.

The same for IC44 pins 1, 4, 10 and 13.

Since we have found the odd issue with little PCB tracks to +5V - we have some more potential issues with this circuit...

Dave

 

[nullvalue]

Some of your traces in post #613 do look a bit suspect.

We are only interested in IC50 pins 12, 9, 8 and 11 and IC51 pin 12 with D5=0.

Do NOT look at any other output pins of IC51 - they deal with a different function...

Dave

ok redoing step 26, i may have been looking at the wrong pin for the final test but most of what I wrote is correct.

IC50/12: With D5=0 I do see 8 pulses. D5=1 I see 16 pulses.
IC50/9: With D5=0 I see 4 pulses, D5=1 I see 8 pulses.
IC50/8: With D5=0 I see 2 pulses, D5=1 I see 4 pulses.
IC50/11: With D5=0 I see 1 pulse, D5=1 I see 2 pulses.

IC51/12: With D5=0 I see no pulses, D5=1 I see 1 pulse.

 

[daver2]

Ah, user error !

That still doesn't account for our ghost pulse though (step 25)...

Dave

 

[nullvalue]

Ok, and the trace (in the first shot) is relatively stable?

Yes it's spot-on.

Can you check that IC43 pins 4, 10 and 13 are (really) connected to +5V.

These are NOT connected to +5V!

The same for IC44 pins 1, 4, 10 and 13.

These pins are connected to +5V. I see on pin 14 there's a bottom-layer trace to +5V (pin 13 is tied to 14). I'm guessing that's missing on IC43 for pin 13.

 

[nullvalue]

Wait a minute though I was looking at the bare board, this was part of the bodge fixes I added to my boards. Let me review.

 

[daver2]

Just checking the PCB layout.

Have you checked them with the power OFF using a multimeter for resistance?

Dave

 

[nullvalue]

Ok sorry about the fire drill - my bodge wires are in place and IC43 pins 4, 10, 13 (and 14) are tied to +5V.

 

[nullvalue]

Just checking the PCB layout.

Have you checked them with the power OFF using a multimeter for resistance?

Dave

Yep just did that, looks good.

 

[daver2]

Ok, you may re-enter the building now...

Dave

 

[nullvalue]

Should I continue with the tests or is there something you want to try to track down that ghost pulse?

 

[daver2]

He he...

Ghost pulse first...

All with D5=0.

Can you check for a good, stable S-100 clock (2 MHz or 4 MHz - whatever your Z80 system is clocked at) on IC44 pins 3 and 11.

Can you check IC43 pin 3 and post the photograph please.

What is on IC43 pin 2? Post a photograph.

What is on IC52 pins 1, 2 and 3.

EDIT: Sorry, I entered IC53 instead of IC52 above. Corrected now.

IC52 pin 1 should be LOW until the first pulse (on IC52 pin 2) has been and gone. Then IC52 pin 1 should go HIGH to let the remaining pulses through (positive pulses on IC52 pin 2) to IC52 pin 3.

Dave

 

[nullvalue]

Can you check for a good, stable S-100 clock (2 MHz or 4 MHz - whatever your Z80 system is clocked at) on IC44 pins 3 and 11.

Perfect 4.00 MHz clock signal on both pins

Can you check IC43 pin 3 and post the photograph please.

What is on IC43 pin 2? Post a photograph.

What is on IC53 pins 1, 2 and 3.

Zooming out to M5.00ms:
Pin 1:


Pin 2: (that's an interesting one!)


Pin 3:

 

[daver2]

Did you read my correction above? I sent you down a rabbit hole. My bad!

It should have been IC52 pins 1, 2 and 3.

Dave

 

[nullvalue]

Did you read my correction above? I sent you down a rabbit hole. My bad!

It should have been IC52 pins 1, 2 and 3.

Dave

Crap nope haha 1 minute

 

[nullvalue]

Ok IC52

Pin 1:


Pin 2:


Pin3 (there's the ghost pulse)

 

[daver2]

That is where the 'ghost in the machine' is coming from...

The enable signal is going high just at the same time as the first pulse. Sometimes it is LOW (and blocking it) other times it is HIGH and letting it through.

Let's work our way back to IC42 pins 3, 6, 8 and 9 and see what is going on there...

Dave

 

[nullvalue]

IC42

Pin 3:


Pin 6:


Pin 8:


Pin 9:

 

[daver2]

I can see 'how' it occurs...

IC43/2 goes HIGH.

The first clock pulse comes along on IC43/3. A LOW to HIGH clock edge on IC43/3 causes the HIGH on IC43/2 to propagate to the Q output (IC43/5).

This will be propagated through IC44 by two clocks of the 4 MHz system clock.

When the HIGH gets to IC44/Q output pin (not connected) IC44 pin 8 (/Q) will transition from HIGH to LOW.

This will cause the S/R flip flop (consisting of the two cross-connected gates of IC42 to enable the signal at IC52 pin 1.

Now, if the original clock input pulse at IC43/3 is sill there - it will propagate through to IC52 pin 3 (hence the ghost).

Likewise, the gates in the flip-flop chain could be operating too fast - thus achieving the same ghost.

What devices have you got in IC41, 42, 43 and 44?

Dave

 

[daver2]

IC42 pin 3 does not look correct to me...

Can you measure it again please?

Dave