Dead Elk, any ideas?.

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1024MAK
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Re: Dead Elk, any ideas?.

Post by 1024MAK » Sun May 24, 2020 1:53 pm

Over the years I have acquired various Electrons with various issue boards fitted. Most being eBay ‘untested’ listings (I was after some spare keys or after other bits in a ‘job lot’). None of them have ULA problems (one has a faulty space bar).

So I think part of is just random luck...

If the ULA is socketed, it appears to me that fairly often poor electrical contact between the socket and the chip is responsible for most problems.

The technology used in ULA chips has a strange record on reliability. ZX81 computers if fitted with one of the first two ULA versions often suffer from ULA failure. The later (third version) ULA seems to be a bit better. But the ULAs fitted in ZX Spectrums, especially from issue 3 boards onwards are rather hardy in comparison (although edge-connector abuse can and does kill various chips including the ULA).

In a BBC B, it’s not that often (going on posts on this forum) where the VideoULA is found to be the fault. And the other less complex ULAs in a BBC B also don’t appear to fail very often.

With any microcomputer system, a single malfunctioning signal line (clock, control, address or data bus line) can cause utter chaos. So always make sure that any connectors or sockets that carry such signals are always properly cleaned when you experience any strange symptoms.

Mark

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Re: Dead Elk, any ideas?.

Post by 1024MAK » Sun May 24, 2020 1:54 pm

sP1d3r wrote:
Sun May 24, 2020 1:39 pm
That's interesting, the edge connector on the Pegasus cartridge is clean but as the issue A PCBs that I've seen differ somewhat from each other I think that the arrangement and value of the capacitors on the PCB may be relevant to the particular display.

I've bought the two PCBs in the middle of the picture from Dave H and as I haven't yet got either working, I don't know what works.
Have you read through the thread where some PCB production faults were found with the Pegasus PCBs?

Mark

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Re: Dead Elk, any ideas?.

Post by sP1d3r » Sun May 24, 2020 2:57 pm

1024MAK wrote:
Sun May 24, 2020 1:54 pm
sP1d3r wrote:
Sun May 24, 2020 1:39 pm
That's interesting, the edge connector on the Pegasus cartridge is clean but as the issue A PCBs that I've seen differ somewhat from each other I think that the arrangement and value of the capacitors on the PCB may be relevant to the particular display.

I've bought the two PCBs in the middle of the picture from Dave H and as I haven't yet got either working, I don't know what works.
Have you read through the thread where some PCB production faults were found with the Pegasus PCBs?

Mark
Thanks for the heads up about the PCB faults, I know that some of the PCBs are experimental and a lot of them have via's that 're faulty and I've checked the continuity of the PCB tracks against the circuit diagram in the manual, but I've come up with a blank about the possible fault.
The boards may have faulty chips also but with the chips removed, the Elk should still boot up ok.

Mark71's picture of his spare Elk shows the same kind of display fault as caused by my Pegasus, which isn't due to a defective ULA.
I repaired the faults that I found, there were a lot of them but still no function. :(

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Re: Dead Elk, any ideas?.

Post by sP1d3r » Sun May 24, 2020 4:00 pm

:roll: I think that the issue A Pegasus PCB may need some modifications to work, I noticed some of the data line connections on the circuit diagram are cross-connected but as I don't have much time at the moment for investigating a PCB, it'll have to wait.

Regarding the strange screen display, perhaps Mark71 has a faulty chip or component on his Electron PCB causing a power draw or open circuit.

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Re: Dead Elk, any ideas?.

Post by daveejhitchins » Sun May 24, 2020 4:37 pm

sP1d3r wrote:
Sun May 24, 2020 10:21 am
:^o Hi
I get the same screen as the one that you've posted, but it's only when I put my Slogger Pegasus into my Plus 1.
I was wondering what issue pcb you have in yours, an Issue A by any chance?
If your Pegasus is faulty, please return it and I'll fix. PM sent . . . .

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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Re: Dead Elk, any ideas?.

Post by sP1d3r » Sun May 24, 2020 6:34 pm

daveejhitchins wrote:
Sun May 24, 2020 4:37 pm
sP1d3r wrote:
Sun May 24, 2020 10:21 am
:^o Hi
I get the same screen as the one that you've posted, but it's only when I put my Slogger Pegasus into my Plus 1.
I was wondering what issue pcb you have in yours, an Issue A by any chance?
If your Pegasus is faulty, please return it and I'll fix. PM sent . . . .

Dave H :D
:D No worries Dave, you sold it to me at a discount because it wasn't working, remember?
I'll have to leave it for the moment because I've got too many things to do.

I'd appreciate a picture of a working issue A board, if you have one.

Thanks anyway,

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Re: Dead Elk, any ideas?.

Post by 1024MAK » Sun May 24, 2020 6:48 pm

sP1d3r wrote:
Sun May 24, 2020 4:00 pm
:roll: I think that the issue A Pegasus PCB may need some modifications to work, I noticed some of the data line connections on the circuit diagram are cross-connected but as I don't have much time at the moment for investigating a PCB, it'll have to wait.
If the data lines are “cross-connected” to the SRAM chip, that does not matter. Different story if it’s the EPROM...

Mark

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Re: Dead Elk, any ideas?.

Post by 1024MAK » Sun May 24, 2020 6:49 pm

I’m thinking about splitting out the Pegasus posts into a new topic.

Mark

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Re: Dead Elk, any ideas?.

Post by sP1d3r » Sun May 24, 2020 7:45 pm

:-s
1024MAK wrote:
Sun May 24, 2020 6:48 pm
sP1d3r wrote:
Sun May 24, 2020 4:00 pm
:roll: I think that the issue A Pegasus PCB may need some modifications to work, I noticed some of the data line connections on the circuit diagram are cross-connected but as I don't have much time at the moment for investigating a PCB, it'll have to wait.
If the data lines are “cross-connected” to the SRAM chip, that does not matter. Different story if it’s the EPROM...

Mark
I meant that IC5 looks unusual, if you'll look on the picture the data lines are swapped on the chip. The power lines have 5 100nf ceramic capacitors on the diagram but all the issue A boards, labelled as 'prototypes' in the post 'Slogger Prototypes, PCBs and Roms' have varying numbers of capacitors of different values in different positions on the PCB.
Attachments
WP_20200225_10_10_54_Pro.jpg
WP_20200216_13_52_05_Pro (4).jpg

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Re: Dead Elk, any ideas?.

Post by 1024MAK » Sun May 24, 2020 8:19 pm

sP1d3r wrote:
Sun May 24, 2020 7:45 pm
I meant that IC5 looks unusual, if you'll look on the picture the data lines are swapped on the chip. The power lines have 5 100nf ceramic capacitors on the diagram but all the issue A boards, labelled as 'prototypes' in the post 'Slogger Prototypes, PCBs and Roms' have varying numbers of capacitors of different values in different positions on the PCB.
With a 74LS174, the labelling of the inputs/outputs is arbitrary. As long as the input pin function matches the output pin function, it does not matter which section of the chip you use.

Or have you spotted a different problem?

The value of power rail decoupling capacitors is not critical, although these days 100nF has become common, various values were used in the past. The BBC A/B uses mostly much smaller values. If you feel that the values are too small, fit 100nF ceramic capacitors.

With no chips fitted, the value of the power rail decoupling capacitors becomes irrelevant anyway.

Mark

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Re: Dead Elk, any ideas?.

Post by mark71 » Sun May 24, 2020 9:47 pm

sP1d3r wrote:
Sun May 24, 2020 10:21 am
:^o Hi
I get the same screen as the one that you've posted, but it's only when I put my Slogger Pegasus into my Plus 1.
I was wondering what issue pcb you have in yours, an Issue A by any chance?
All Issue 4 PCB. I did try a good working ULA in it and no joy, thinking it could be the 6502 in both cases, tried the ULA in the non working one in the good Electron but no no life, bit of a head scratcher, other than of the Issue 4 machines I have the manufacture was in 3 different places, Hong Kong/Malaysia and the UK.

Will have another try in the morning.

Mark

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Re: Dead Elk, any ideas?.

Post by sP1d3r » Mon May 25, 2020 8:16 am

mark71 wrote:
Sun May 24, 2020 9:47 pm
sP1d3r wrote:
Sun May 24, 2020 10:21 am
:^o Hi
I get the same screen as the one that you've posted, but it's only when I put my Slogger Pegasus into my Plus 1.
I was wondering what issue pcb you have in yours, an Issue A by any chance?
All Issue 4 PCB. I did try a good working ULA in it and no joy, thinking it could be the 6502 in both cases, tried the ULA in the non working one in the good Electron but no no life, bit of a head scratcher, other than of the Issue 4 machines I have the manufacture was in 3 different places, Hong Kong/Malaysia and the UK.

Will have another try in the morning.

Mark
I meant the Slogger Pegasus PCB revision, not the Electron PCB - perhaps you weren't using it for the Flatron screen display.

It must mean that the ULA causes the same problem as my Pegasus board.
1024MAK wrote:
Sun May 24, 2020 8:19 pm
sP1d3r wrote:
Sun May 24, 2020 7:45 pm
I meant that IC5 looks unusual, if you'll look on the picture the data lines are swapped on the chip. The power lines have 5 100nf ceramic capacitors on the diagram but all the issue A boards, labelled as 'prototypes' in the post 'Slogger Prototypes, PCBs and Roms' have varying numbers of capacitors of different values in different positions on the PCB.
With a 74LS174, the labelling of the inputs/outputs is arbitrary. As long as the input pin function matches the output pin function, it does not matter which section of the chip you use.

Or have you spotted a different problem?

The value of power rail decoupling capacitors is not critical, although these days 100nF has become common, various values were used in the past. The BBC A/B uses mostly much smaller values. If you feel that the values are too small, fit 100nF ceramic capacitors.

With no chips fitted, the value of the power rail decoupling capacitors becomes irrelevant anyway.

Mark
I've only checked the continuity so far, I'll try the spare PCB without any components and see if the result is the same.
Attachments
WP_20200525_001.jpg
Pegasus issue A PCB

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Re: Dead Elk, any ideas?.

Post by 1024MAK » Mon May 25, 2020 11:59 am

The problems that were noticed and pointed out in the thread about building Pegasus boards, was that some of the PCB tracks on some versions are very close together and due to production problems, may actually be in contact with adjacent tracks.

Mark

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