Silent, nothing visible

discuss both original and modern hardware for the bbc micro/electron
Colday
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 9:08 am

Thanks for all the guidance there from both of you.

I've done the probe adj lots of times and I can never get a square wave, more of an inverted 'l' (lower case L in case not visible).

I have my probe set at x1.

I think I'm going to have a faff around with that this morning. I know there is a calibration screw on the probe, I'm wondering if I haven't calibrated the probe...

I'll also plug in the other probe I have and see what changes. As well as using a working beeb.
Kazzie wrote:
Sun Aug 02, 2020 10:31 pm
Right, that picture is quite sensible: the timebase knob is working and the scope is showing you a signal, albeit a very small one. If you were to reduce the horizontal timebase (turning clockwise, so you get a line rather than a dot) you should see something that looks like a waveform.
When I extend the timebase I get two lines that stretch out across the full screen, but with a fuzzy mess in between them.

I'm going to have to have a proper look into this.
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Re: Silent, nothing visible

Post by Kazzie » Mon Aug 03, 2020 9:46 am

Colday wrote:
Mon Aug 03, 2020 9:08 am
Thanks for all the guidance there from both of you.

I've done the probe adj lots of times and I can never get a square wave, more of an inverted 'l' (lower case L in case not visible).
That sounds like the triggering isn't configured correctly.

In "auto" triggering mode, if the scope doesn't see a rising/falling edge at the right voltage within a given time, it will just trigger itself and send the beam across. This won't be synchronised to the signal, and will give the same "l" on a long timebase, but a smeared mess on a shorter timebase.

On "normal" triggering, the scope won't send the beam across unless it finds the right condition on its input. In your scope's current setup, I expect that you would see no trace at all with this mode. (The TV-V and TV-H modes can be ignored: they're not relevant here.)

See here for a bit more explanation.

(Internal triggering is correct, as it looks for trigger conditions on the internal amplifier of the probed signal. Line would use the mains (line) frequency, and Ext would use a separate signal on the BNC connector underneath.)

:arrow: The trigger level knob may be set too high or too low at the moment. Try twiddling it while probing a waveform, and see if the waveform stabilises.
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 9:53 am

Ok, I changed the probe over and put it into the probe adj.

This seems to work better.

The trace on the scope does move (obvs not flickering like the vid) and I cannot seem to get a stationary display.

I probed IC43 Pin 6 and got the same as before though.

Two lines filled with fuzziness.

It's hard to get as a pic or video as i don't really have enough hands. I shall persevere though.

This is the calibration video.
IMG_4621 (1).MOV
(2.93 MiB) Downloaded 13 times
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by Kazzie » Mon Aug 03, 2020 10:09 am

Based on what I see in the video, I think it's the triggering that needs tweaking. (See above.)
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 10:16 am

Ok, so, went and did this....
IMG_4625.jpg
And this is the trace I get.
IMG_4626.MOV
(4.83 MiB) Downloaded 9 times
It doesn't matter what I change the timebase to, I still get either a blurry mess or a single line.

I have probed a known good beeb and get exactly the same result.

So, I am going to suggest there is something wrong with the scope?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 10:24 am

When I move the triggering about, it either makes no difference.

If I get to maximum rotation of the dial either way it bugs out a bit.

Even on probe adjust mode the waveform moves.

Seems to be nothing I can do to stop it, which indicates that it is knackered? Right?

Quite annoying as I challenged the guy I bought it from as it didn't seem to work as it should, and he just fobbed me off.

Without any knowledge of these things I had no recourse so had to suck it up.

I'm tempted to get one of those cheapish USB ones and use my laptop. It's not like Beeb's are overly complicated (in the scheme of things) and If we are only looking for divisions of 16mhz only then I'm sure they could manage that.

I see all these digital scopes using single shot triggering and that just seems to work a treat. Lol.

Sigh, oh for a decent disposable income eh?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 10:38 am

I think I may have worked something out....

I don't think my scope is a storage scope.

I'm watching https://www.youtube.com/watch?v=6WFlbHi2xnE and it seems I am missing the storage mentioned at about 8m.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Kazzie
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Re: Silent, nothing visible

Post by Kazzie » Mon Aug 03, 2020 11:39 am

Your scope isn't a storage scope, but then neither is my analogue HM400 scope. It's the kind of functionality that's standard with a digital scope, but an optional extra in the analogue market.

The behaviour seen around the 7~8 minute mark only works on that scope because it's storing a record of its recent measurements, and redrawing them repeatedly on the screen.

If your scope were triggering correctly, then it would display periodic waveforms correctly, as mine does. (Though long timebases would still be an oscillating dot progressing across the screen, because it can't store and redraw measurements.)

I'm not sure that I can do much more to help with sorting that scope out remotely: it seems that it needs some repair itself. :(

Cheap little scopes can be useful to probe around many parts of a Beeb, but take note of their analogue bandwidth before you buy anything: an analogue bandwidth of 2MHz would be enough to detect and show a 2MHz sine wave, but the higher harmonics of a 2MHz square wave would be lost, and it's look like a 2MHz sine wave. A 200kHz scope, for example, really won't cut it. A scope with sufficient bandwidth to show the 16MHz master clock signal will probably have a fair price tag attached.

Having said all that, we can infer the correct operation (or not) of the master clock from the lower frequency clocks that are derived from it. Continuity tests between components, measuring component values, and selective replacement of suspect passive components and chips is another approach that we can follow.

Plus, even working with a DC voltmeter and a cheap logic probe, you can compare readings between your working Beeb and your faulty Beeb, to get an idea of what sort of results you should expect to see.

Have a think about how you'd like to progress, and let us know. :)


(P.S. It seems this thread title could apply to your Beeb and your scope! :wink: )
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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 12:08 pm

sorry done it again, at least we agree :-)

I don't think there is much wrong with your scope to be honest. I think the main problem is that trying to look at a 16MHz square wave is right at the limit of what it can do, even when it was brand new. Here's the problem...

A guy called Fourier managed to work out that any waveform is just a bunch of sine waves added together at various amplitudes. For a square wave you have the fundamental frequency and then all its old harmonics, in theory up to infinity (so we won't worry about that bit) So you are trying to look at:

Fundamental = 16MHz
1st odd harmonic 3 x 16 = 48MHz
2nd odd harmonic 5 x 16 = 80MHz

Your scope has a bandwidth of 20MHz so you will only ever be able to see the fundamental, which means the best you'll ever be able to see is something that looks like a sine wave. To see this you'll need to turn the time base up to it's maximum and probably also switch on the x10 magnification. Even then it will be very difficult to get it to trigger nicely. oh, I forget to say that you will probably also need to turn the intensity right up too.

But, have your goal in mind, we're not really needing to see the wave in all it glory, we just need to see that it's oscillating and it is!

The fact that the scope does not read voltage correctly is a bit concerning, I would say it probably needs to be calibrated. No idea how much that would cost but it might be an idea to get a quote before you buy something new. If you can get to the point of mastering this scope, you will be in a much better position to make an informed purchase if you decided you want something better. Modern digital scopes have a a huge number of feature and it's only with experience that you can sort the bull from the useful.

Just had another thought... give channel 2 a try, that might be working fine. Don't forget to switch over your trigger source too.

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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 12:17 pm

Thanks for all your help here. It is very much appreciated.

I was under the impression this scope does measure voltage perfectly fine.

The line moves up the correct amount (per v/div) when I probe any voltage source.

I can measure the 1.68v mentioned earlier with no issue, just don't get to see the waveform at all.

Except when doing the probe adv. Then I see a square waveform (albeit the vertical lines are almost invisible).

I feel the scope is past it's best. I was just thinking of getting one of those cheap PC based things, something like

https://www.ebay.co.uk/itm/Hantek-6082B ... SwfvRfIcNu

What are the thoughts on these?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 12:23 pm

...and one other thing the little pot on the probe that you mentioned is not for vertical deflection (voltage) calibration it's for compensation. The idea is that you connected the prob to the "prob adj" test point and tweak the pot so that you have a nice square wave.

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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 12:25 pm

ok, we are expecting to see 5 volts peak to peak. You get a lower reading on your meter because it's averaging, sorry should have explained that.

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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 12:51 pm

When I started doing electronics again about two years ago after about a 35 year hiatus I looked at a bunch of different scopes. I ended up rejecting the PC based ones mainly because I wanted a dedicated instrument, that I could pick up and move around. In the end I bought the RIGOL DS1054Z. It's not perfect, but I was comfortable with the price/performance. Yes, sometimes I wish I'd got something better, but it does 95% of the stuff I want just fine. At the time I mainly wanted to do audio stuff, so the 50 meg bandwidth was fine (huge), now it's a bit of a limitation, especially with some of the FPGA stuff I've been playing with. But the issue is where do you stop, 95% seems ok to me :-)
Last edited by kfro on Mon Aug 03, 2020 1:20 pm, edited 1 time in total.

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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 1:05 pm

Yes, that Rigol looks very nice, but at nearly £400 it is way beyond my budget.

(Unless I sell some of my vintage computers... Amstrads, Spectrum, ZX81, Commodore and some Beebs.... No, no no no. What am I thinking)

I'm torn as I really want to get the dishwasher board working, The fact that we get some output to me says it can be fixed, but without a decent(ish) scope that can measure and display a waveform I'm at a bit of a loss.

I would send it to mark over at RC, but I am, if anything, a stubborn sod and I'll be buggered if I'll let some inanimate bits of plastic and metal from the 80's beat me. I'm older and therefore wiser... oh wait..

I think I will swap back to getting the other faulty machine (the other thread that's running) sweetened up for now. With that daughter board it's quite an interesting one, that's for sure.

I also need to work out why the original PSU on this machine is poorly. Maybe start another thread for that one. Lol.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 1:12 pm

This looks interesting.

The reviews are very good.

https://www.amazon.co.uk/DOS1102-oscill ... 939&sr=8-4
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 2:04 pm

Colday wrote:
Mon Aug 03, 2020 1:12 pm
This looks interesting.

The reviews are very good.

https://www.amazon.co.uk/DOS1102-oscill ... 939&sr=8-4
Yes looks ok at a glance, except for the mains cable issue, though that would be easy to fix. My mantra is always, "if it looks to good to be true, it probably is". Check out the scope reviews on EEVBlog.

Did you manage to check the 2MHz clocks and the reset signal BTW?

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Re: Silent, nothing visible

Post by Colday » Mon Aug 03, 2020 2:19 pm

I've seen quite a lot of Daves stuff on EEVblog, but I do find him overcritical 'Muriel' on a lot of aspects as to what he expects.

That scope arrives tomorrow. And I have MANY 'kettle' leads to replace the 'One Hung Lo' that comes with it.

I'll be back with you all on this thread once that scope arrives and I get it set up. Then we can start looking at clocks and reset circuits.

By the way, no long booop this morning. Seems quite random and a likely dry joint somewhere I suspect.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
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Re: Silent, nothing visible

Post by kfro » Mon Aug 03, 2020 10:19 pm

Wow that was quick, I take ages to make up my mind of stuff like that. It will be interesting to see how it does. Yes a scan with a good magnifying glass is would probably be in order.

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 4:30 pm

Ok, here we go....

I checked Pin 7 of IC43 and I have a spot on 16mhz frequency as read by my very pleasing new scope.

However, on checking anywhere else, whatever PIN of whatever IC I get no signal whatsoever.

So using the below...

Does IC1 PIN40 get a nice /RESET pulse at power-up and when Break is pressed? Nope

What signal is at IC1 PIN3 (PHI1OUT) – 0

What signal is at IC1 PIN4 (/IRQ) – 0

What signal is at IC1 PIN6 (/NMI) –0

What signal is at IC1 PIN7 (SYNC) –0

What signal is at IC1 PIN34 (R/W) – 0

What signal is at IC1 PIN37 (PHI1IN) - 0

What are the Video ULA Frequencies at the following PINS?
PIN7 (Should be 8Mhz) = 0
PIN6 (Should be 4Mhz) = 0
PIN5 (Should be 2Mhz) = 0
PIN4 (Should be 1Mhz) = 0

What signal is at IC33 PIN 3? Is there a pulse at RESET? Nowt
What signal is at IC33 PIN 4? Is there a pulse at RESET? Not a sausage

What is the signal at all pins of IC16?
PIN1 = 0
PIN2 = 0
PIN3 = 0
PIN4 = 0
PIN5 = 0
PIN6 = 0
PIN7 = 0
PIN8 = 0


So, I'm guessing the failure is really early in the circuit?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by kfro » Tue Aug 04, 2020 5:34 pm

Just on my way out, but make sure you have 16MHz on pin 8 of the video ULA.

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Re: Silent, nothing visible

Post by Coeus » Tue Aug 04, 2020 6:09 pm

kfro wrote:
Mon Aug 03, 2020 12:51 pm
When I started doing electronics again about two years ago after about a 35 year hiatus I looked at a bunch of different scopes. I ended up rejecting the PC based ones...
I assume by PC-based you mean scopes that are actually just an A/D in a box, connect to a PC with, for example, USB, and rely on installing software on the PC to view the results?

I mention that because one of the Rigol scopes I saw at an ABUG meeting was clearly based around a PC but rather than an add-on to an existing PC, it has the PC within it in much the same was an an iMac.

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 6:31 pm

Yes, I was considering one that plugs into the USB port.

However went for the Hanmatek one mentioned earlier.

Seems very good so far.

So, back to IC43, it seems that I have 16mhz on pins 2, 3, 4, 5 and 6.
Does that seem right?

Pin 8 of the ULA also has nothing on it
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by kfro » Tue Aug 04, 2020 7:03 pm

That's good, it means it's probably not the ULA! What have you got on pins 11, 12 and 13 on IC40.

Yes I did mean connected to a PC via USB

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 7:28 pm

Ok, each of those pins has 16mhz.

Although the waveform on 13 was different to that of 11 and 12.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by kfro » Tue Aug 04, 2020 7:56 pm

Yes, 13 will be difference because it's fed from an RC network. So that means there's a break somewhere between IC40 pin 11 and pin 8 on the ULA. Look for a dry joint, broken trace or if the chips are in sockets dirty pins or sockets.

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 8:14 pm

RC network?

I shall go and have a look between those pins right now.

I'll check continuity first. ;-)
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 8:46 pm

Ok, so there is no continuity between those pins but I have got continuity between IC7 pin8 and IC73 pin15.

So I know IC7 Pin 8 is good.

I resoldered the whole of IC40 with no change.

Does pin11 of IC40 go elsewhere so I can check it?

If it goes directly to pin8 of IC7 can i put a patch wire in? That would be my next thought
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

kfro
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Re: Silent, nothing visible

Post by kfro » Tue Aug 04, 2020 9:10 pm

Yes this become the 16MHZ signal which goes to IC42 pin 2

See the resistor R102 and the capacitor C42. When the clock goes high, the cap will be charged up via the resistor. The time it takes the cap to charge is related to the capacitance times the resistance. The rule of thumb for the time it takes for the cap to charge to the supply is about 3RC. But the gate will switch long before then (at 2 volts apparently). For short we just use the term RC. I do not think this is particularly critical, I think it's just there to remove noise (signal that we do not want) from the clock. The cap charge time will have the effect of delaying the signal on pin 13 and the NAND gate will not switch low until both pin 12 and 13 are high. When the clock goes low pin 11 will switch straight away, So I guess if you compare pin 12 to pin 11 you should be able to see this difference. If you need any more excuse to play with the new scope that is :-)

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Re: Silent, nothing visible

Post by Colday » Tue Aug 04, 2020 9:16 pm

kfro wrote:
Tue Aug 04, 2020 9:10 pm
Yes this become the 16MHZ signal which goes to IC42 pin 2
So is that a yes for putting the patch cable between IC7 pin8 and IC40 pin11?

I'm pretty tired tonight, sorry. Been one of those days.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: Silent, nothing visible

Post by kfro » Tue Aug 04, 2020 10:05 pm

Sorry, been in and out tonight. Yes try adding a link between IC40 pin 11 and pin 8 on the ULA.

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