Silent, nothing visible

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Colday
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Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 12:10 pm

Ok, while I await parts for my other beeb, I think it may be time to start work on the next one.

Ok, this is the DISHWASHER model B.

So, it didn't work before, and it doesn't work now.

I've tested the PSU and the correct voltages are getting TO the board, but it doesn't do very much at all.

No beeps, not even the single long beep.

Output wise it goes to a squirly squiggly pattern with no change if I press anything.

The CAPS lock LED is lit however.

Where to start.. I've gone to minimal config with no change and I've changed the S25 to the various places (except inverting) with no change.

Let's get this done.
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 1:16 pm

Have a pic of the screen
IMG_4586.jpg
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Re: Silent, nothing visible

Post by Kazzie » Fri Jul 31, 2020 4:37 pm

Well, that's an interesting display! :?

Are the squiggles static, or do they wobble around?

For the lack of sound, my first suspicion would be the -5V line, but you say you've checked that. The sound chip (IC18) should give a long beep on power-up all on its own, assuming it and the nearby amplifiers (ICs 17, 19) are healthy, and the speaker is connected properly. But whatever the fault in that area, it should have no impact on the ability of the rest of the board to power up, especially if the System VIA (IC3) is currently removed.

I'd suggest disregarding the sound issue, and checking the core areas of the board. Use an oscilloscope or logic probe if you have one, but a DC reading with a voltmeter can also be useful.

:arrow: On IC6 (Video ULA), pins 8 to 4 should have 16, 8, 4, 2, and 1MHz clocks on them. (A voltmeter should give a reading of around 2~2.5V)

:arrow: On IC1 (6502), check that pin 40 (reset) is at a high voltage (3~5V), but goes low when the break key is pressed. (IC3 doesn't need to be present for this.)

:arrow: Pins 37 and 3 of IC1 should have a 2MHz clock signal on them

:arrow: Pin 7 (sync) should show activity every time the CPU fetches an instruction. If it's stuck low, the CPU has probably stalled.
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 4:53 pm

Thanks Kazzie.

I will check all those things a bit later when it gets a bit cooler in the workshop.

I'm not 100% sure my oscilloscope is any good to be fair though so not sure if I can read the clock frequencies.

It's a Goldstar device that did not come with a manual (It's pretty old).

It can measure voltages no problem, but I can never get a clean view of any frequencies, including when I have it in calibration mode.

Now either I am a dolt (possible) and am getting something very wrong, or the old scope is on the fritz.

Now, I do have a very cheap and utterly crap logic probe (One of those cheap kits off FleaBay that I used to practice soldering on) which I can attempt to use...

Multimeter is no probs, I have a decent one of those so can give voltages.

But, watch this space, who knows, with the temp at over 40deg C in the workshop, maybe the old scope will give up and start working properly.

Edit: The squiggles are moving, it's quite freaky. I'll grab a quick video later.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

Colday
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 5:36 pm

Right, did some checks.

Can't get the Oscilloscope to produce a waveform. I'd imagine that's a fair learning curve.

Pins 8-4 on IC6 all have a 4.46v reading (decent multimeter)

IC1 pin 40 - 3.6v with no change on Break

Pin 7 doesn't seem to have much going on at all.

But I am guessing how the logic probe works (I get different readings on different pins of different IC's)
BBC B's... I have 5 again.
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Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » Fri Jul 31, 2020 7:07 pm

Right-o.

We'll get back to the break key not giving a response, but a DC voltage of 4.46V is well out of whack for what should be clock signals alternating between ~0V and ~5V; with a 50% duty cycle you'd expect to have something much closer to 2.5V.

As the 8 to 1MHz clocks are all derived from the master 16MHz clock, I suggest having a closer look at the clock generation area of the circuit.

Pins 11, 12 and 13 of IC40 (74s00), and pin 6 of IC43 (74s04) would be worth probing to see what's going on there. If you don't see any activity there, check components and continuity around the crystal and pins 1-5 of IC43. (Do bear in mind that the capacitance of a probe will affect the resonance of the oscillator circuit, so don't be surprised if you see no clock signal if you probe the area.)
Last edited by Kazzie on Fri Jul 31, 2020 7:23 pm, edited 1 time in total.
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 7:20 pm

Ok, that was a Charlie Brown moment for a lot of that Kazzie.

Let me sit and think about your reply for a while. Maybe a decade or so.

Haha.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » Fri Jul 31, 2020 7:31 pm

Colday wrote:
Fri Jul 31, 2020 7:20 pm
Ok, that was a Charlie Brown moment for a lot of that Kazzie.

Let me sit and think about your reply for a while. Maybe a decade or so.

Haha.
No problem: I try to dial the tone of my advice according to how confident or experienced posters sound, but sometimes I need to keep calibrating! :oops:

To paraphrase...

The high voltage readings on all the clock signals indicates that there's a problem with the 16MHz clock. If you don't see a clock signal (or a sensible DC average voltage close to 2.5V) on pin 6 of IC43, then there must be a fault somewhere earlier, in the oscillator circuit.

Image

If pin 6 of IC43 looks good, then suspicion would fall on IC40.
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 7:33 pm

Is that the thing found just above IC59?

The connections looked rubbish so I resoldered that earlier with no change
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » Fri Jul 31, 2020 7:45 pm

Colday wrote:
Fri Jul 31, 2020 7:33 pm
Is that the thing found just above IC59?

The connections looked rubbish so I resoldered that earlier with no change
IC43 is a short distance south (towards the keyboard) from the RS423 socket, and the 16MHz crystal is immediately west of it. You should find the related capacitors and resistors around IC43 too.

The components north of IC59 are related to the video circuitry, so may be related to your video issues. Though with no clock signals anywhere in the machine, we shouldn't expect to see anything sensible on the screen anyway.

(Incidentally, the lack of clock signals would also explain the lack of long-beep: the sound chip expects a 4MHz clock input.)
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Re: Silent, nothing visible

Post by Colday » Fri Jul 31, 2020 8:16 pm

While standing there faffing about with the Oscilloscope ( and downloaded manual) all of a suffen the beeb started making the inital long beep sound.

Break made no difference and when I switched it off and on again, it went quiet on me again.

Just an observation
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » Sat Aug 01, 2020 7:24 am

Perhaps, rather than a faulty component in the clock generation area, you have a poor connection or dry solder joint.

Once you have an idea of how far your 16MHz clock signal is getting, it may be a case of using the continuity function of your multimeter to check that what should be connected is properly connected, and/or reflowing the solder joins in the area.
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Re: Silent, nothing visible

Post by Colday » Sat Aug 01, 2020 8:11 am

Thanks Kazzie.

I slept on your entry and it makes more sense this morning.

It had nothing to do with the few bottles of Corona I had consumed last evening at all.

I'll be back on it today, I tried using the logic probe and although it's really rubbish, it does actually seem to work. Which is a miracle.

I'm also going to read more about the oscilloscope but I don't think it's happy. I may have to buy another one.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Colday » Sat Aug 01, 2020 11:00 am

Right.

Pin 6 of IC43 - 1.68v
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Re: Silent, nothing visible

Post by kfro » Sat Aug 01, 2020 11:21 pm

Hi Colday,

I think you really need to get that scope going as I think it will show you where the problem is. I've never tried to measure a 16MHz clock with the multi-meter before so was not sure what to expect. I did a little test where I set up a 16MHz square wave, 5V peak-to-peak, mark/space ratio 50% on my signal generator. I then measured this with my meter and got 2.34V. I then backed off the duty cycle to 25% to get 1.68V. So, on the face of it, it does seem like you have a dodgy clock, but who knows if your meter behaves in the same way as mine? However this dose seem to be born out by you pic, where the horizontal sync seems to be moving around. Definitely look for dry joints in the area for the clock generator (C37, C40, x1, R91 and R92).

Good luck!

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Re: Silent, nothing visible

Post by kfro » Sat Aug 01, 2020 11:23 pm

oh and IC43

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Re: Silent, nothing visible

Post by Kazzie » Sun Aug 02, 2020 6:33 am

Colday wrote:
Sat Aug 01, 2020 11:00 am
Right.

Pin 6 of IC43 - 1.68v
My Beeb gives a reading of 1.63V on pin 6 of IC43, so you've probably got a 16MHz clock there. :)

(It's not a perfect 50% duty cycle until it's divided down in the Video Processor.)

The next place in the chain to check is IC40, pins 11, 12, 13. Do readings there indicate a clock signal, and does its behaviour make logical sense for a NAND gate?

(If your scope does come back to life, it would of course be useful.)
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Re: Silent, nothing visible

Post by Colday » Sun Aug 02, 2020 10:28 am

Pin 11 - 1.92v
Pin 12 - 1.68v
Pin 13 - 1.68v

I also now have a continuous boop tone today.

It is much cooler in here today so could be heat related. I will check again later when it warms up.

I'm working with the scope, going through the service manual for it and so far it all checks out.
It could well be user error.

What settings Volt/div time/div do you have your scope set at?
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by kfro » Sun Aug 02, 2020 11:40 am

Colday wrote:
Sun Aug 02, 2020 10:28 am
Pin 11 - 1.92v
Pin 12 - 1.68v
Pin 13 - 1.68v

I also now have a continuous boop tone today.

It is much cooler in here today so could be heat related. I will check again later when it warms up.

I'm working with the scope, going through the service manual for it and so far it all checks out.
It could well be user error.

What settings Volt/div time/div do you have your scope set at?
Based on what we know so far, that's probably good, the signal on pin 13 will be slightly delayed by the RC network R104 and C42, which will in turn change the make/space ratio at the output on pin 11. I'm guessing this is done to clean up the signal a bit. Sorry I do not have a working machine ATM, I'm in the middle of re-building the PSU.

As to scope setup, You'll want 1V/dev and a time-base of around 20 microseconds/dev and then tweak from there till you can see a couple of cycles or so. You'd normally set your probe to x10. On my scope I tell it the attenuation setting and it does the maths, but on older scopes you may need to work it out in your head. I like to have the trigger set to "normal" and then tweak the level myself but I would think auto will work just fine for this sort of thing.

Hope this help!

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Re: Silent, nothing visible

Post by Colday » Sun Aug 02, 2020 11:56 am

Thanks for the help with the scope settings.

It is seeing the voltage at the pins and measures that fine, I just cannot get it to produce a waveform.

I'll have more of a play later.

I presume it needs to be measuring DC rather than AC? Although would you ever get a waveform measuring DC?

Please be aware mine is an old analogue scope. 20mhz Goldstar (latterly LG) 2 channel jobby. Think it's the 9020 model.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » Sun Aug 02, 2020 1:47 pm

That scope should do fine for a Beeb.

For DC vs AC, that just indicates whether the scope should use a capacitor to filter out the DC element of the incoming signal.

Take, for example, a nice TTL clock signal going between 0V and 5V at a 50% duty cycle. That will have a DC element (average) of 2.5V, and an AC element that alternates 2.5V above or below that DC value. If you set the vertical/Y position for ground/gnd in the middle of the screen, with AC coupling you'll see half the waveform above the midline, and half below. If you use DC coupling, the waveform will be in the upper half of the screen, possibly going off the top. You can always adjust the Y position to compensate.

For the messy waveform you have onscreen, it sounds to me like you need to twiddle with the "trigger" switches on your scope. This controls when the scope starts drawing across the screen, which it can be doing tens of times a second. If it doesn't trigger at the same "point" in the waveform each time, the successive traces won't line up, and you'll have a mess on-screen.

As you're only using one channel of your scope (i presume?) you want to make sure that the trigger functions are set to the same channel. There's no use asking channel 2 for timing information if it's not connected to anything! There may also be controls for whether it triggers on a rising or falling edge, and at what value, but those are less of an issue than selecting the correct channel.

If you'd care to take a quick photo of your scope's controls, we could have a look and advise you if anything needs changing.
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Re: Silent, nothing visible

Post by kfro » Sun Aug 02, 2020 2:50 pm

Is it the 9020P you have, only there seem to be a whole series of that scope dating over some time.

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Re: Silent, nothing visible

Post by Colday » Sun Aug 02, 2020 4:23 pm

Thanks for the help with the scope settings.

It is seeing the voltage at the pins and measures that fine, I just cannot get it to produce a waveform.

Yes, it is the 9020P from Goldstar.

See below the readouts I get when I probe PIN 4 of the 6502.
IMG_4590.jpg
Without probing
IMG_4591.jpg
Probing Pin 4 of IC1
So basically the line has just moved up the same as the voltage as measured by the meter.

I also tried probing Pin 6 of IC43 with much the same result.

I guess I really should try probing a working beeb.

Edit: I still have the constant boop noise, which is a bonus.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by kfro » Sun Aug 02, 2020 6:11 pm

Ok, you have your time base (horizontal) set to 1 millisecond/div, you need to wind that clockwise 5 steps to 20 microseconds.

1 millisecond = 0.001 seconds
1 microsecond = 0.000,001 seconds

I think your scope is working fine :D

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Re: Silent, nothing visible

Post by Kazzie » Sun Aug 02, 2020 7:03 pm

I'm not so sure: turning the horizontal timebase to a smaller interval (clockwise) is effectively the same as "zooming in" horizontally. My first impression was that the timebase was too short already, so we were only seeing a small portion of the waveform: before it drops low again. If we were too zoomed out, we'd see lots of waves squished together, possibly looking like a blurred rectangle eventually.

BUT.... Pin 4 of IC1 is the /IRQ pin. It only goes low when another chip requests an interrupt; it's normal for it to stay high most of the time.

Pin 6 of IC43, on the other hand, should show plenty of evidence of going up and down at a 1ms timebase. 1ms is equivalent to 1kHz, so at that time base we'd see one full wave onscreen. There'd be 1000 periods of a 1MHz wave, or 16,000 periods of a 16MHz wave. Obviously we couldn't see them all, it'd look more like a rectangle.

(There must be a 16MHz output there for the sound chip to make any noise.)

Try twiddling the timebase knob. If you've got any doubts as to whether it's actually doing anything, turn it all the way counter-clockwise to the 1s or 2s positions. This time, the trace will move slowly enough across the screen that you should be able to see the dot move across the screen, rather than a line.
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Re: Silent, nothing visible

Post by kfro » Sun Aug 02, 2020 7:46 pm

Ah sorry, I completely missed that we where looking at the the IRQ and thought we where still looking at the 16MHz clock. I'm glad someone is awake! :oops:

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Re: Silent, nothing visible

Post by Colday » Sun Aug 02, 2020 9:19 pm

Ok, here we go again.

With the settings as you see them below, and as Kazzie suggested I see the dot go slowly across the screen.
IMG_4614.jpg
Then if I probe IC43 on Pin6 I get the following.
IMG_4615.jpg
Notice the V/div. If I don't have it set so low, the dot only expands a tiny tiny bit.

That 'line' goes across the screen exactly as you see it.

I think tomorrow I am going to fire up a working Beeb and see what happens when I probe.
BBC B's... I have 5 again.
2x not working, 3x working, 1 is unmolested.
Lots of disk drives, some working, some not. Lots to repair.

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Re: Silent, nothing visible

Post by Kazzie » 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.

0.1V peak-to-peak is very small for the clock signal on IC43 pin 6 though; I'd expect something much closer to 4 or 5V.

If you haven't done so already, you could attach your probe to the "probe adj" terminal at the bottom right of the scope, with the ground clip attached to the adjacent earth. That's a 0.5V peak-to-peak square wave generated by the scope itself, so you can check the accuracy of what it's showing on-screen. Do note that as well as the large knobs for selecting Volts/Div, there is a smaller 'variable' knob in the centre that can be used for fine-tuning and adjustment. There may be a detent (click) when it's in the zero-adjustment position, if you're lucky.

I definitely agree that getting comfortable using your scope on a known-good Beeb is a good move. :)
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Re: Silent, nothing visible

Post by kfro » Sun Aug 02, 2020 10:44 pm

hehe just typed all this an then realised that Kazzie had already answered, but thought you may as well read it anyway...

Ok, so you clearly have a 16MHz clock. There's a couple of things to check re the voltage readings you are getting. Your probe will have built in attenuation. Have a look at the probe. You should see a little switch marked x1 and x10 (normally anyway). Usually this would be set to x10 as this helps prevent loading the circuit you are trying to measure too much. This means you need to multiply the set volts/div by 10, so 0.1 x10 = 1 volt in this case.

The other thing is, see the little knob in the middle of the bigger selector where you are setting volts/div? Make sure this is fully clockwise to the "cal" position.

You should probably turn the time base up a bit now, so you don't burn-in the screen. Now that you have a somewhat reliable way to check clocks I suggest that you check pin 5 on IC6 and then pin 37 on IC1. You should see 2MHz clocks at both locations. Also check IC1 pin 40, which should be high, i.e. your trace should move up. Might also be a good idea to check your 5 volts supply with the scope just to confirm if it's reading the correct voltage or not, I think you already checked this with you meter, so this should confirm what the scope is telling you.
Last edited by kfro on Mon Aug 03, 2020 8:23 am, edited 1 time in total.

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Re: Silent, nothing visible

Post by Kazzie » Mon Aug 03, 2020 6:36 am

kfro wrote:
Sun Aug 02, 2020 10:44 pm
hehe just typed all this an then realised that Kazzie had already answered, but thought you may as well read it anyway...
Don't worry: you've covered some details that I left out, and having two separate explanations can help a lot. 8)
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