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 » Fri Aug 07, 2020 9:18 am

Good morning.

I popped out to the workshop just this second to check what we had on the queries above on IC1

Pin37 - 2mhz
Pin39 - 2mhz

Pin40 - goes low when I bridge the pads (but not when I press Break)

Pin7 - With the scope there is noise on Pin7 (this is very low and doesn't have a frequency. I have to really push the scope in to see the 'noise') this does change when I bridge the pads (I've got a small reset PTM switch that I'm going to attach to the baord later to make this easier.)

Pin4 - +5v
Pin6 - +5v

All other IC's are in place.

As for the last part of the above, I see noise on Pin7 but I have no idea if this is the 'correct' noise. I can get a pic of the scope if needed (not easy as I only have 2 hands).

Once again, this help is second to none. So pleased. I'm still confident we can get this old girl working. And it is definitely one that is staying in my collection.
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Kazzie
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Re: Silent, nothing visible

Post by Kazzie » Fri Aug 07, 2020 4:26 pm

I'm glad to see that shorting the reset switch is triggering a reset input for the CPU. Having the IRQ and NMI lines high is also good.

The Sync (pin 7) should normally be low (~0V), going high (3~5V) when an instruction is fetched. (I couldn't remember which way round they were meant to be this morning, so that's why I'd been a bit vague.) From your description, it sounds like you've just got a little bit of noise instead.

On that basis, it seems like the CPU is stalling due to bad input. That can happen very quickly after a reset, given that it's running at two million cycles per second. ;) So don't be worried that you don't see anything between resetting and crashing.

The two leading reasons for this to happen would be a failure to read the MOS ROM correctly, or a failure to read/write to the RAM.

For the ROM, one of the best ways of testing it is to blow Tricky's Test OS onto a ROM, and fit that in the MOS socket. Its code is specially designed not to depend on working RAM, and includes a series of test screens to help diagonse a RAM fault. If this Test ROM runs, then your board is able to access the MOS ROM correctly.

An alternative, which doesn't require any extra tools or EPROMs, is to check what values you find on the data bus when the CPU has stalled. On stalling, the 6502's address lines float high (to 3.5V or so), and the read/write line goes high (meaning read), so the Beeb's board should enable the MOS, which will put the contents of location &FFFF on the data bus. You should therefore find that the data bus corresponds to the value &DC in binary.

For the RAM, you can change the position of S25 to disable one bank of RAM (or the other) to see if one has a faulty chip. But there are buffer chips for the address and data buses that could be to blame, which would affect both banks. Using Tricky's ROM could help "exercise" the RAM circuits, and give some clues as to where the fault could be. Alternatively, good old-fashioned legwork going from chip to chip with a continuity tester, and probing inputs and outputs to see if the chips are operating as described, can help get to the root of the problem.
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Re: Silent, nothing visible

Post by Colday » Fri Aug 07, 2020 5:05 pm

Ok, well I have an EPROM programmer, but no EPROMS as yet. (Whereas I'm new at this, I'm REALLY new at EPROM stuff)

The noise is absolutely minimal, I've got the scope set to it's most sensitive and even then it is barely registering.

I will grab a short video of that so you can see the settings etc.

It is a little warm up there at the moment, but I'll brave it in a bit.
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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Colday
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Re: Silent, nothing visible

Post by Colday » Fri Aug 07, 2020 5:15 pm

IMG_4668.MOV
(3.49 MiB) Downloaded 9 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 kfro » Fri Aug 07, 2020 5:24 pm

I think we do not have a reliable reset to the CPU. Shorting the reset has proved that the 555 is working but I think we are not getting the reset from IC33 pin 4. Can we quickly test if this is going low when the reset switch is shorted.

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Re: Silent, nothing visible

Post by Colday » Fri Aug 07, 2020 5:52 pm

So you want me to check IC33 pin4 goes low when reset is bridged?

It is at +4v and when reset is bridged it drops to 0v

As does pin40 of IC1
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)...
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Re: Silent, nothing visible

Post by Kazzie » Fri Aug 07, 2020 6:20 pm

Colday wrote:
Fri Aug 07, 2020 5:05 pm
Ok, well I have an EPROM programmer, but no EPROMS as yet. (Whereas I'm new at this, I'm REALLY new at EPROM stuff)

The noise is absolutely minimal, I've got the scope set to it's most sensitive and even then it is barely registering.

I will grab a short video of that so you can see the settings etc.

It is a little warm up there at the moment, but I'll brave it in a bit.
I've not glanced at the video, but if you need to crank the y scale all the way up to find it, then it's definitely not a TTL logic signal. :D
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Re: Silent, nothing visible

Post by Colday » Fri Aug 07, 2020 6:28 pm

Yeah REALLY low level signal. It may even be background noise from stuff in the workshop.
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)...
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Colday
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Re: Silent, nothing visible

Post by Colday » Fri Aug 07, 2020 9:54 pm

Kazzie wrote:
Fri Aug 07, 2020 6:20 pm
I've not glanced at the video, but if you need to crank the y scale all the way up to find it, then it's definitely not a TTL logic signal. :D
Two quick things...

1. What EPROM do I buy to flash Tricky's test rom to?
2. Where can I download the test rom image from?
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)...
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kfro
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Re: Silent, nothing visible

Post by kfro » Fri Aug 07, 2020 11:13 pm

Read through the last few posts and realised I'd missed you say that the reset to the CPU was good, so sorry about that. I agree, that tricky's ROM sounds like the best option. We could also try a very short looping program.

I also read https://ist.uwaterloo.ca/~schepers/MJK/6502.html#2 that it's possible to implement a single step. But was unable to read the referenced material. After a bit of googling I found that the MOS hardware manual contains a circuit to do this. http://www.6502.org/documents/books/mcs ... manual.pdf, so in theory you should be able to step through until you find the problem, in practice you could be clicking the step button a very long time.

I was also going to ask, do you still have all non-essential chip removed? One good technique of problem solving is to simplify the system to help harrow down the problem area.

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Re: Silent, nothing visible

Post by Kazzie » Sat Aug 08, 2020 7:39 am

Colday wrote:
Fri Aug 07, 2020 9:54 pm
Kazzie wrote:
Fri Aug 07, 2020 6:20 pm
I've not glanced at the video, but if you need to crank the y scale all the way up to find it, then it's definitely not a TTL logic signal. :D
Two quick things...

1. What EPROM do I buy to flash Tricky's test rom to?
2. Where can I download the test rom image from?
The original Test ROM image is available in this post here.

The original MOS uses a 16KB ROM; the EPROM equivalent would be a 27128 or 27C128. They're getting a little rare these days, but an 8KB 2764/27C64 would work just as well for this little Test OS.
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Re: Silent, nothing visible

Post by Colday » Sat Aug 08, 2020 10:03 am

Once again, thanks!!

I messaged Tricky directly and he came back with the thread overnight also.

Just a matter of working out which version to download, the original or the one tweeked for the master... Or should it not matter with a beeb B?

As for a looping program, I would need to be able to type that in or load it first...

Tricky's ROM it is.

I have a few old 27128's I think that came with the big box of ROM that I acquired. I just need to find a UV light to make sure they are 'clean'
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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 Coeus » Sat Aug 08, 2020 3:19 pm

Colday wrote:
Sat Aug 08, 2020 10:03 am
Just a matter of working out which version to download, the original or the one tweeked for the master... Or should it not matter with a beeb B?
That rather depends on whether "tweaked for the master" means "assumes it is running on a master and tries stuff that won't work on a B" or "can do master-specific stuff after it's done the BBC B tests". So if tricky's post doesn't answer that you could either ask him, or stick to the original.

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Re: Silent, nothing visible

Post by Colday » Tue Aug 11, 2020 5:52 pm

Ok dokey.

I now have a working copy of Tricky's Test ROM thanks to the help from the 'Clearing EPROMS/EEPROMS' thread. Learnt a lot!

Now, after testing in a known good beeb (which worked perfectly), I plugged it into the dishwasher beeb.

No change, no difference whatsoever.

Still has a squiggly screen, still has a constant Booop (no bip) and still makes no difference if I press *Break* or even if I bridge the reset pins (I soldered a reset switch from an old PC to those pads which works good)

So, what's next? For info all the IC's were in place, I could go back to minimal config, but when I did that earlier (with the tricky ROM that didn't work cos of bad programming) there was absolutely no change on the screen at all.

I am happy to pull them again and I suspect that's got to be done, but I expect no change...
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 » Tue Aug 11, 2020 6:27 pm

Don't worry about going back to the minimal configuration; you've establised that removing them didn't resolve the problem, and Tricky's ROM needs the VIA present to do anything useful, anyway.

Just to check: when you say nothing's working, does that include no flashing LEDs or cassette relay clicks?

As we'd had concerns about intermittent problems, I'd suggest checking the CPU pins again with Tricky's ROM fitted. Check that pin 40 is responding to your reset switch, and look at the clock and sync pins in particular to see if the CPU's still doing stuff. (If you have the time and inclination, you could probe all 40 pins and we can pick through the results for you.)
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Re: Silent, nothing visible

Post by Colday » Tue Aug 11, 2020 6:40 pm

Yup, time and inclination doesn't matter, it needs doing if we are going to work this oout.

What would be really useful for me is whether (and what) to expect on each of the 40 pins. I know pin 40 is a voltage for instance, but it would really be a great help to me (and maybe others) if there was an expected good pinout list.

Obviously now I have the new scope that works a treat I can hope fully take screenshots of it as well. Apparently it can hook up to my laptop which would be helpful, however whats the bet that the company in China didn't think to support Linux.

I guess if that is the case then I could reinstall Win10 as it is licensed for it anyway.


Just to check: when you say nothing's working, does that include no flashing LEDs or cassette relay clicks?

It is exactly the same as when it is out and normal OS is present. CAPS lock is on. No clicks, no sounds, no visuals at all. Just squiggly!
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)...
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Kazzie
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Re: Silent, nothing visible

Post by Kazzie » Wed Aug 12, 2020 7:13 am

I can't give an expected values list that's quite as explicit as the 555 reset timer, because the 6502 is (obviously) a much more complicated chip that does more than make a reset pulse. I can give a rundown of what to expect on a 6502 that's in a good, healthy board.

Let's work our way down from pin 40 (in your best Radio 1 DJ voice):

Image
  • Pin 40: Reset line: should be at a high logic level, unless break/reset are being held low
  • Pin 39: Phi2 out: CPU's output of the clock signal received on pin 37. Typically 2MHz square wave (but can be 1MHz when CPU is accessing some slower components on the Beeb)
  • Pin 38: \Set Overflow: An input that is rarely used in any 6502 machine. Just skip this one, there's nothing to measure.
  • Pin 37: Phi0 in: Input clock to the CPU. If there isn't a good TTL square wave here, the CPU won't fetch and execute instructions
  • Pin 36 and 35: Not Connected: nothing to see here
  • Pin 34: Read / Write: How the CPU tells the rest of the computer whether data i sgoing into the CPU, or out. High when reading, low, when writing. A DC voltage reading will vary according to how many reads vs writes are being made, but should typically be somewhere in the 1V~3.5V range (at a guess)
  • Pins 33 to 26: Data Bus: values switching between high and low rapidly as the CPU sends and fetches data. If the CPU has stalled, it will show the contents of address location 0xFFFF (because the R/W line and address bus will be high).
  • Pins 25 to 22, and 20 to 9: Address Bus: switching between high and low like the data bus, but lower address lines (A0, A1) will usually be switching more often than the higher ones (A14, A15). If the CPU is reading a sequence of instructions from ROM, A0 will invert for each location, A1 will invert every 2 locations, whereas A15 will remain high all the time (as ROM is in the upper half of the memory map). If the CPU has stalled, all values will float up to ~3.5V
  • Pin 21: Ground: 0V, or very close to it.
  • Pin 8: VCC: 5V power supply to the chip. As with the PSU, it should be within 4% tolerance, so 4.75V to 5.25V is acceptable
  • Pin 7: Sync: Normally low, goes high when the CPU is fetching an instruction. If it's stuck low (or DC reading near 0V) then the CPU is not fetching instructions
  • Pin 6: /NMI: Non-maskable interrupt. Normally high, goes low when a device (FDC or Econet chips)need an NMI interrupt to be serviced. If stuck low, need to check links and chips in the Econet and FDC areas.
  • Pin 5: Not Connected
  • Pin 4: \IRQ interrupt: Like NMI, should normally be high. If it's low, the CPU will be constantly interrupted, and never get a chance to do its own stuff.
  • Pin 3: Phi 1 out: Alternative clock output, which is effectively an inverse of Phi0 or Phi2.
  • Pin 2: Ready: Input used for pausing CPU execution. Not used on the Beeb, should read a high voltage level
  • Pin 1: Ground: A second ground connection
For more guidance on what what qualifies as "high" or "low", take a look at this thread.

We want to be in a position where we know for sure that the CPU is getting a reset pulse and a clock signal with Tricky's ROM fitted before we cast any aspersions on it.

Incidentally, if you do find the CPU to be stalled, the readings off the data bus can help show us whether the CPU is getting access to the ROM correctly.
Last edited by Kazzie on Wed Aug 12, 2020 4:42 pm, edited 2 times in total.
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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 9:28 am

Ok, something interesting.

Most stuff seems to check out ok.

pin40 +4.43v drops to 0v when the reset switch is pressed

Pin37 and Pin39 - Yellow trace is Pin39, blue is Pin37
IMG_4690.jpg
The video is triggered from Pin39 and reading Pin35
IMG_4691.MOV
(4.39 MiB) Downloaded 13 times
Pin34 +3.84v

Now here is the interesting one - If Kazzie's blurb is correct, Pin20 should be ground. My Pin20 is showing +4.0v

This DOES NOT change when I press the reset switch. It stays at the same voltage. +4v almost exactly. It's actually +4.003v with a little fluctuation

Pin8 - +5.03v

Pin2 - +5v

Pin1 - 0v - Ground
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Re: Silent, nothing visible

Post by Kazzie » Wed Aug 12, 2020 11:02 am

Apologies: I wrote two descriptions for pin 20, and none for pin 21. Pin 20 is address line A11, pin 21 is ground. #-o

I'll correct the post above for any future readers.
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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 11:10 am

Damn. I was hoping we had got somewhere then!
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Re: Silent, nothing visible

Post by Kazzie » Wed Aug 12, 2020 12:19 pm

Have a look though the rest of the pins, and tell us what you see.
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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 12:24 pm

What did you make of the little vid with the trace moving up and down like that?
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Re: Silent, nothing visible

Post by Kazzie » Wed Aug 12, 2020 12:46 pm

Colday wrote:
Wed Aug 12, 2020 12:24 pm
What did you make of the little vid with the trace moving up and down like that?
I hadn't looked at that (my phone's browser doesn't download forum attachments properly, I have to do some workarounds).

If I'm reading your annotations correctly, in the video the blue trace is pin 35, at a vertical scale of 200mV per division on the screen. Given that pin 35 is meant to be left unconnected, that signal could be noise from the nearby clock, or through the ground plane (as both your probes have grunding clips that will be connected electrically). The signal jumping up and down will be another noise source, most likely.

The fact that the signal is only 200mV peak-to-peak (0.2V!) means it's unlikely to be a cause for concern: TTL logic chips have a wide range for valid inputs specifically to accomodate potentially noisy signals. It's no smoking gun, so I'd recommend moving on to check the other pins.
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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 12:59 pm

The blue trace is Pin37, the yellow trace Pin39.

The input and output clocks for the 6502?
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Re: Silent, nothing visible

Post by Kazzie » Wed Aug 12, 2020 1:42 pm

Colday wrote:
Wed Aug 12, 2020 12:59 pm
The blue trace is Pin37, the yellow trace Pin39.

The input and output clocks for the 6502?

That's odd, then.

The photo you posted has the blue trace at a scale of 20V per division (presumably with the probe set to 10x), and a peak-to-peak measurement of roughly 4V, which is perfectly reasonable. The shape and frequency is pretty good, too.

The blue trace in the video, on the other hand, is 200mV per division, 100 times smaller than in the photo. That's more than can be explained with a flick of the 10x switch on the probe, which (combined with the mention of pin 35) made me think it was a different signal. :?

Can you double-check those measurements, or think of anything that may have changed between photo and video? (Might the probe have fallen off the pin before you took the video?)

If you still have a similar signal, with the trace wavering up and down, try selecting channel 2 for the trigger source, and/or extending the timebase so we see the high and low portions of the bounce onscreen at the same time.
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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 2:30 pm

I will go back and check again...

It is quite hard taking the two measurements at once and I'm still learning with the scope so it may well be a mistake, although I noticed it first, then went back and reproduced it to take the video.

I kind of need a tripod to record or stream this as I do it. Lol.
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Re: Silent, nothing visible

Post by hoglet » Wed Aug 12, 2020 2:54 pm

There are two traces that would be useful to see, as they indicate what's happening to the processor just after reset:
- Channel 1 = pin 7 (Sync) ; Channel 2 = 40 (nRST), trigger off the rising edge of channel 2, timebase about 1us per division
- Channel 1 = pin 34 (RnW) ; Channel 2 = 40 (nRST), trigger off the rising edge of channel 2, timebase about 1us per division

The first few instructions after a reset are:

Code: Select all

D9CD	LDA #&40    ;set NMI first instruction to RTI
D9CF	STA &0D00   ;NMI ram start
D9D2	SEI         ;disable interrupts just in case
D9D3	CLD         ;clear decimal flag
If this code is executing, there is a very distinctive pattern on Sync and RnW.

Make sure to put the original OS ROM back in.

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Re: Silent, nothing visible

Post by 1024MAK » Wed Aug 12, 2020 3:19 pm

What is the condition of the 6502 socket?
What is the condition of the socket for IC51 (the OS ROM)?

Do you know if this machine has ever had any ROM, RAM/ROM or any other internal expansions fitted?

If you think that the socket contacts are okay, or especially if you think there may be problems, then one thing that can b3 done, is to use a multimeter to continuity test between the relevant chip pins and another chip pin that is connected to the same line.

I mention this, as poor / bent / damaged socket contacts is a known problem with Beebs.

Mark

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Re: Silent, nothing visible

Post by Colday » Wed Aug 12, 2020 3:22 pm

Mark.

I think we have done a lot of the continuity testing earlier in this thread.

hoglet, I shall give those a go a bit later.
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!

Colday
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Re: Silent, nothing visible

Post by Colday » Sun Aug 16, 2020 2:04 pm

hoglet wrote:
Wed Aug 12, 2020 2:54 pm
There are two traces that would be useful to see, as they indicate what's happening to the processor just after reset:
- Channel 1 = pin 7 (Sync) ; Channel 2 = 40 (nRST), trigger off the rising edge of channel 2, timebase about 1us per division
- Channel 1 = pin 34 (RnW) ; Channel 2 = 40 (nRST), trigger off the rising edge of channel 2, timebase about 1us per division

The first few instructions after a reset are:

Code: Select all

D9CD	LDA #&40    ;set NMI first instruction to RTI
D9CF	STA &0D00   ;NMI ram start
D9D2	SEI         ;disable interrupts just in case
D9D3	CLD         ;clear decimal flag
If this code is executing, there is a very distinctive pattern on Sync and RnW.

Make sure to put the original OS ROM back in.

Dave
Ok, finally found time to get these checks done.

Interestingly very different results.
IMG_4720.MOV
(4.67 MiB) Downloaded 9 times
The above is the first set of measurements
IMG_4722 (1).MOV
(4.31 MiB) Downloaded 5 times
This is the second. Looks like no output on Pin34?
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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