Beeb Power Up Issue

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KenLowe
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Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 1:39 pm

Hi,

I've got a beeb that will not power up very reliably. 9 times out of 10 I will just get a low constant beep. Pressing Ctrl Break nearly always brings it to life. I've scoped the nRSTA and nRST lines (pins 34 on the 2 VIAs), and see nRSTA on IC3 rising first, followed by nRST on IC69 rising about 125mS later. I've checked the delay between nRSTA and nRST on another machine, and it's about 150mS, so I'm not sure if that indicates a possible problem? On the faulty machine I've swapped the VIAs around, but that hasn't made any difference. Any pointers?

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Re: Beeb Power Up Issue

Post by Kazzie » Sat Sep 14, 2019 1:49 pm

KenLowe wrote:
Sat Sep 14, 2019 1:39 pm
Hi,

I've got a beeb that will not power up very reliably. 9 times out of 10 I will just get a low constant beep. Pressing Ctrl Break nearly always brings it to life. I've scoped the nRSTA and nRST lines (pins 34 on the 2 VIAs), and see nRSTA on IC3 rising first, followed by nRST on IC69 rising about 125mS later. I've checked the delay between nRSTA and nRST on another machine, and it's about 150mS, so I'm not sure if that indicates a possible problem? On the faulty machine I've swapped the VIAs around, but that hasn't made any difference. Any pointers?
The period that nRSTA is low is determined by C10 and R20. If C10 has degraded and lost capacity, that'd make the period shorter, but that would make the delay between nRSTA and nRST longer, not shorter. :?

Is it the case that a Ctrl-Break will usually bring it to life, but just pressing Break alone won't do it?
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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 2:02 pm

Kazzie wrote:
Sat Sep 14, 2019 1:49 pm
Is it the case that a Ctrl-Break will usually bring it to life, but just pressing Break alone won't do it?
Sorry. Just in the habit of pressing Ctrl Break all the time. A Break alone also brings it back to life.

Edit: Another thing that initially confused me was that the scope was only measuring about 2.5v for logic state 1 on the nRSTA line, but I'm assuming that's just down the impedance of the scope resulting in voltage division. I tried the probes in both 1x and 10x mode, but that didn't seem to make any difference. Anyway, that was secondary!
Last edited by KenLowe on Sat Sep 14, 2019 2:08 pm, edited 1 time in total.

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Re: Beeb Power Up Issue

Post by Elminster » Sat Sep 14, 2019 2:25 pm

I think anything over 2volts is a 1 will TTL, you could double check voltage with a multi meter.

Did you check 5v from psu is about right? Could be on the limit, hence sometime works and sometimes doesn’t.

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Re: Beeb Power Up Issue

Post by cmorley » Sat Sep 14, 2019 2:27 pm

I had similar on a machine. IIRC it was the RSTA circuit - one end of the C or R was never soldered from the factory. So it would sometimes boot but always be OK on a BREAK too.

Your 2.5v on RSTA might have been a real measurement. Check for dry/cracked joints on the R & C Kazzie mentioned.

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Re: Beeb Power Up Issue

Post by Kazzie » Sat Sep 14, 2019 2:33 pm

I'd expect nRSTA to get higher than 2.5V, given that IC3 is the only load on the circuit (other than your scope):
BBC-RSTA.png
Circuit for generating RSTA signal
BBC-RSTA.png (6.21 KiB) Viewed 488 times
Take a look at D1, C10 and R20, to check they're present and behaving correctly.
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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 2:37 pm

Thanks for the replies, guys. Board has been out, and everything around the Power Up reset circuit and the 555 timer circuit has been checked and re-soldered. If I measure nRSTA with a multimeter I get about 4.5v, so I'm pretty sure it's an impedance issue with the Scope. I get the same results on a known good beeb.

I've just been re-familiarising myself with the 555 timer circuit, and the measured time delay does seem to match the design (t=1.1 x C x R), where C=100n and R=1M.

Not too sure where to go from here!

Edit: I'm getting a good clean 4.97v at various locations on the board. I'll pull the board again, and double check the R20, C10 & D1.

Edit2:
Kazzie wrote:
Sat Sep 14, 2019 2:33 pm
I'd expect nRSTA to get higher than 2.5V, given that IC3 is the only load on the circuit (other than your scope):
According to the specs for my Rigol ds1054z, the input impedance is 1M, so when in series with the R20 1M resistor, 2.5v sounds about right.
Last edited by KenLowe on Sat Sep 14, 2019 2:59 pm, edited 2 times in total.

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Re: Beeb Power Up Issue

Post by Elminster » Sat Sep 14, 2019 3:03 pm

KenLowe wrote:
Sat Sep 14, 2019 2:37 pm

Edit2:
Kazzie wrote:
Sat Sep 14, 2019 2:33 pm
I'd expect nRSTA to get higher than 2.5V, given that IC3 is the only load on the circuit (other than your scope):
According to the specs for my Rigol ds1054z, the input impedance is 1M, so when in series with the R20 1M resistor, 2.5v sounds about right.
X10 mode should have sorted it. Did you change to x10 on both probe and scope?

I.e. around 4.5v with a 1/10M voltage divider.
Last edited by Elminster on Sat Sep 14, 2019 3:07 pm, edited 1 time in total.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 3:09 pm

Elminster wrote:
Sat Sep 14, 2019 3:03 pm
X10 mode should have sorted it. Did you change to x10 on both probe and scope?
I did, but the result was even odder, so I gave up on that for now. I'm fairly certain the measured voltage is fine. Here's a couple of screen captures from the scope. The yellow trace is nRSTA and the blue trace is nRST:

Edit: It's worth pointing out that the first trace looks the same on both a good power up and a failed power up...
DS1Z_QuickPrint4.png
Power Up
DS1Z_QuickPrint5.png
Short dab on Break
DS1Z_QuickPrint6.png
Longer dab on Break
Edit2: I'm thinking there's probably nothing wrong with the reset circuitry, and my guess is that some other component is taking too long to initialise on power up. I wonder if I should extend the duration of the 555 timer reset pulse to mask the issue.
Last edited by KenLowe on Sat Sep 14, 2019 3:21 pm, edited 3 times in total.

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Re: Beeb Power Up Issue

Post by Elminster » Sat Sep 14, 2019 3:18 pm

Could be global warming. I might have a play later, but might not have time. Not that it sounds an issue as meter giving expected result.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 3:33 pm

Elminster wrote:
Sat Sep 14, 2019 3:18 pm
Could be global warming. I might have a play later, but might not have time. Not that it sounds an issue as meter giving expected result.
So, the global warming issue was just in my house! 10x did work. I just didn't realise the scaling on the scope changed from 1v/division to 10v/division when I set the scope input to 10x. Now measuring about 4.5v.

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Re: Beeb Power Up Issue

Post by 1024MAK » Sat Sep 14, 2019 3:43 pm

We’ve see something similar before with a misbehaving Beeb, but nothing wrong in the reset circuits.

Is the NMI line high (correct) or is something pulling it low?
Also test the IRQ line with your scope, just to check that that is normal.

Also check / search the hardware thread, as I say, this kind of problem has come up before.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 4:09 pm

1024MAK wrote:
Sat Sep 14, 2019 3:43 pm
We’ve see something similar before with a misbehaving Beeb, but nothing wrong in the reset circuits.

Is the NMI line high (correct) or is something pulling it low?
Also test the IRQ line with your scope, just to check that that is normal.

Also check / search the hardware thread, as I say, this kind of problem has come up before.

Mark
I've just replaced the 1M resistors R13 & R21 with 5.6M resistors, and the delay is now in the order of 800mS, but the power up issue still exists, so checking the interrupts seems a logical next step.

Edit1: So, the nNMI line goes high as soon as the machine is powered up, and remains high. Same behaviour when the machine powers up successfully, and when the machine fails to power up.

Edit2: Here's some captures from the scope showing what happens with the nIRQ line. I've still got the bigger resistors in the 555 timer circuits, so the delays are still a bit longer than normal. As before, yellow trace is nRSTA, blue is nRST, and magenta is nIRQ:
DS1Z_QuickPrint7.png
Good power up
DS1Z_QuickPrint8.png
Failed power up
DS1Z_QuickPrint9.png
Reset after failed power up
Last edited by KenLowe on Sat Sep 14, 2019 4:31 pm, edited 3 times in total.

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Re: Beeb Power Up Issue

Post by Elminster » Sat Sep 14, 2019 4:27 pm

KenLowe wrote:
Sat Sep 14, 2019 3:33 pm
Elminster wrote:
Sat Sep 14, 2019 3:18 pm
Could be global warming. I might have a play later, but might not have time. Not that it sounds an issue as meter giving expected result.
So, the global warming issue was just in my house! 10x did work. I just didn't realise the scaling on the scope changed from 1v/division to 10v/division when I set the scope input to 10x. Now measuring about 4.5v.
Mystery solved.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 5:00 pm

Looking at the IRQ traces, I notice that on a good start the IRQ line is being held low by something. On a failed start, nothing is holding it low. Time to start swapping out some chips. A quick look at the circuit diagram I note the following all drive the nIRQ line:

IC3 - 6522
IC4 - 6850
IC69 - 6522
1MHz Interface
Tube Interface

Have I missed anything? I don't have anything plugged into the 1MHz or Tube interfaces, so looks like it might be one of the ICs. I'm hoping it's not the 6850, since it's soldered in...

Edit: Hmmm. Also seeing a good power up with IRQ line sitting high on initial startup:
DS1Z_QuickPrint10.png
Good power up
Last edited by KenLowe on Sat Sep 14, 2019 5:05 pm, edited 2 times in total.

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Re: Beeb Power Up Issue

Post by 1024MAK » Sat Sep 14, 2019 5:33 pm

IIRC, the 6502 ignores maskable interrupts at power-up. However early on in the OS code, the OS briefly enables interrupts.

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Re: Beeb Power Up Issue

Post by Kazzie » Sat Sep 14, 2019 5:51 pm

If we're playing with interrupts, there's also the NMI to consider. At least that's just two sources to consider (IC27 pin 3 for the Econet, and IC27 in 8 for the FDC).
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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 6:25 pm

Kazzie wrote:
Sat Sep 14, 2019 5:51 pm
If we're playing with interrupts, there's also the NMI to consider. At least that's just two sources to consider (IC27 pin 3 for the Econet, and IC27 in 8 for the FDC).
The NMI line remains high at all times, so I don't think that's the issue.

I've also swapped out the 6522s for known good ones, and still seeing the same effect; although I have now noticed that it sometimes takes a couple of Breaks to kick it into life.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 7:00 pm

Right. I think this might actually be a RAM issue. Moving S25 from North to South gets it booting reliably in 16k mode. Removing the jumper, boot up is still unreliable. All RAM is soldered in except IC53, which has been replaced at some point in the past (I think I did this). As a quick test, I pulled IC53, and it would no longer boot with S25 south. It still unreliably boots with S25 removed. Since IC53 is driven by CAS1, I'm guessing the issue is with one of the RAM chips driven by CAS0 (ie IC61 thru IC68). I'm hoping it might just be a bad connection. I'll try resoldering first, before swapping out. I'll also return R13 & R21 on the 555 timer circuit back to 1M, since I've got the board out again.

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Re: Beeb Power Up Issue

Post by KenLowe » Sat Sep 14, 2019 10:17 pm

Final post, just to close out this thread.

I tried just re-soldering the RAM first, but that didn't make any difference. In fact, I made things worse - I started to get strange sparkles in Mode 7. Turns out that was down to me not re-soldering one of the RAM pins properly. Panic over.

Next, I removed RAM ICs 61 thru 64, installed sockets, and installed new RAM. That seems to have fixed the problem. So far I've done about 10 power ups, and not one hang up. I wouldn't have got that far previously.

I actually managed to remove the RAM ICs non destructively, so I might try and narrow down the specific IC that failed, so I can re-use the others.

Oh. And I replaced the X1 crystal too. It's nice to see my monitors displaying a clean display, without the stupid HDMI adaptor OSD popping up every few minutes.

Edit: I've subsequently been able to verify that 3 of the 4 chips I pulled were ok. These have been put back in the board. Computer has been powered up multiple 10s of times now without any further problems. The remaining faulty one will be packaged up and sent to 1024MAK for analysis.
Last edited by KenLowe on Sun Sep 15, 2019 5:46 pm, edited 2 times in total.

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Re: Beeb Power Up Issue

Post by 1024MAK » Sat Sep 14, 2019 11:02 pm

👍 :D

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Re: Beeb Power Up Issue

Post by Kazzie » Sun Sep 15, 2019 5:24 am

And a further =D>
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Re: Beeb Power Up Issue

Post by trixster » Mon Sep 16, 2019 10:23 am

Please forgive my ignorance, but why would pressing break allow the beeb to continue booting normally if one of the ram chips is faulty?
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Re: Beeb Power Up Issue

Post by KenLowe » Mon Sep 16, 2019 11:13 am

trixster wrote:
Mon Sep 16, 2019 10:23 am
Please forgive my ignorance, but why would pressing break allow the beeb to continue booting normally if one of the ram chips is faulty?
Boot up was a bit intermittent. Occasionally it would take more than one press of Break to get it to boot. Also, it would not always fail to boot on power up. Sometimes it would boot ok without pressing Break. Hence my initial thoughts that this might have been a marginal timing issue between nRTSA and nRST.

What didn't help with the initial diagnosis is the fact that my IntegraB expansion board replaced the 'BBC Computer 32k' startup message with its own message. When I changed this to show the normal startup message, I started to see the Beeb reporting 'BBC Computer 16k' on some power ups (or follow a Break if the computer hung on power up), even with S25 in the north position. However, other times it would report the normal 32k.

It may be my imagination, but the boot up process seems to be much quicker now that I've replaced the faulty DRAM. I'm observing this by the speed at which the SHIFT lock LED flickers before the CAP lock LED comes on steady.

Not sure what failure mode in the DRAM chip could cause these intermittent symptoms. It didn't seem to be temperature related. Others with more knowledge may be able to explain.

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Re: Beeb Power Up Issue

Post by trixster » Mon Sep 16, 2019 11:36 am

The reason is ask is because my beeb has being doing the low burrrp noise and will only continue to boot if I press break. It’s being doing this for the last 30 years! My dad tried to fix it but never managed to diagnose the problem, so I’m interested to read what you’ve done to sort your issue :)
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Re: Beeb Power Up Issue

Post by Kazzie » Mon Sep 16, 2019 12:30 pm

trixster wrote:
Mon Sep 16, 2019 11:36 am
The reason is ask is because my beeb has being doing the low burrrp noise and will only continue to boot if I press break. It’s being doing this for the last 30 years! My dad tried to fix it but never managed to diagnose the problem, so I’m interested to read what you’ve done to sort your issue :)
Never booting unless you press break sounds like it could have a different cause. But if you'd like our collective minds to try to pick up where your dad left off, feel free to start a new thread for your machine. :)
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Re: Beeb Power Up Issue

Post by KenLowe » Mon Sep 16, 2019 12:45 pm

If your beeb is working reliably after you hit the Break, then it's unlikely to be a RAM issue. As Kazzie has indicated, it might be better starting a new thread, but a couple of quick pointers:
  • I'd definitely check that nRSTA and nRST lines are working as expected. If you've got a scope, then they should look similar to what I posted earlier.
  • cmorley indicated earlier in this thread that he had a machine that wouldn't boot on power up, but would work on reset. He traced the fault to an un-soldered wire on the R20 / C10 components of the power up reset circuit.
  • I had a very similar problem on another beeb, where the lead on the vertically mounted resistor (R13, I think) on the 555 timer circuit had snapped at the point where the body of the resistor touched the PCB. it was not obvious to see on inspection, but a quick check with a multimeter revealed the issue.
Last edited by KenLowe on Mon Sep 16, 2019 12:47 pm, edited 1 time in total.

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