Help with two Orics

If you want to ask questions about how the machine works, peculiar details, the differences between models, here it is !
How to program the oric hardware (VIA, FDC, ...) is also welcome.
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Chema
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Re: Help with two Orics

Post by Chema »

Great! Good work! Congratulations!

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mikeb
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Re: Help with two Orics

Post by mikeb »

protek wrote:
Sat May 30, 2020 10:16 am
Turns out it was the multiplexer that was soldered onto the motherboard all along.
Excellent -- I must admit that is an odd symptom to end up with, but I guess it will be filed away for future reference :)

The multiplexer is supposed to swap between the low (A0-A7) part of the address output from the CPU, or delegate to the multiple-multiplexers inside the ULA to provide the high (A8-A15) of CPU, or one of 4 internal partial-addresses for the ULA's access.

I'm trying to think how a failure to do that correctly would lead to the symptom you saw. It's almost like the multiplexer was mostly working correctly, except it was a bit lazy/slow to switch between CPU (first part of cycle) and ULA (2nd part of cycle, ASCII lookup, failed, but then 3rd part bit-pattern lookup was OK).

How that translates to looking up the correct address, but garbling one bit of the DATA (b3 forced high) -- I don't know, as all DRAM chips would get the same wrong addressing at the same time, in parallel.

Now you've socketted the new multiplexer, does putting the old (faulty) multiplexer back return the fault? If so, it was the chip. If not, and the old chip seems to work too, you had a board fault which you've cleared in removing/refitting :)

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protek
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Re: Help with two Orics

Post by protek »

Unfortunately, I had to resort to clipping the legs of the old chip as trying to desolder it was causing too much wear on the pads. Therefore, it’s not really usable. I really need to invest into a better desoldering station.

Interesting side note about the two motherboards that I have is that the other, still faulty one seems to be of much better quality manufacturing-wise. The pads feel more solid and overall, it doesn’t look like that it’ll lift a trace if you sneeze at it.

I managed to find correct-sized screws for the motherboard and the case, and I did some rudimentary cleaning.

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iss
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Re: Help with two Orics

Post by iss »

iss wrote:
Thu May 07, 2020 6:48 pm
Else there still IC20 which can be put on socket and changed :).
I'm quoting my message 'just for the record' ;). Congrats for the good work and many happy Oric-hours!
Can you update us: what's exactly the current condition of the second faulty board? What's seams to work and what not? Which tests you've done?

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protek
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Re: Help with two Orics

Post by protek »

I’ve tried your diag ROM and I can faintly hear the busy tone. I’ve tried swapping the CPU, ULA, ROM and VIA from the other board, but there seems not to be any difference. I haven’t done more thorough testing yet. I’m waiting to get a hex inverter and new multiplexers as I don’t have those at the moment.

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protek
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Re: Help with two Orics

Post by protek »

Image

Here’s how board #2 looks like with mikeb’s diagnostic ROM. I borrowed the hex inverter and multiplexers from board #1.

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protek
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Re: Help with two Orics

Post by protek »

Image
Here’s board #2 with 1.1 on eprom. Also using a 74HC04 hex inverter as the local store didn’t have LS.

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protek
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Re: Help with two Orics

Post by protek »

I probed the data lines from all the chips and the RAM chips. Those seemed to be okay and the clock on the ULA too. I noticed that the CPU was getting quite hot. It doesn’t burn, though. The VIA was getting warm too. The other chips stayed cool.

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protek
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Re: Help with two Orics

Post by protek »

I tried the CPU in board #1. It worked fine, so that’s not the issue.

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HigashiJun
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Re: Help with two Orics

Post by HigashiJun »

One of my Atmos had several dead chips.

I guess the previous owner used a wrong polarity PSU or a higher rated voltage one...

It's quite frustrating, because you replace the chip you suspect to be defective but your machine still doesn't work, then you have to replace another one until everything goes fine.
HigashiJun

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protek
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Re: Help with two Orics

Post by protek »

I guess I’ll check also the ROM, ULA and VIA in my working board once more, just to make sure. If all of those work, I’ll check the RAM too just in case the Oric board just doesn’t like KM manufactured RAM.

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protek
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Re: Help with two Orics

Post by protek »

CPU, EPROM, VIA, ULA, 74HC04 hex inverter and a batch of KM branded RAM have now all been verified to be working on board #1. Both multiplexers are new, so I don’t really suspect those.

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HigashiJun
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Re: Help with two Orics

Post by HigashiJun »

protek wrote:
Tue Jun 02, 2020 10:40 am
CPU, EPROM, VIA, ULA, 74HC04 hex inverter and a batch of KM branded RAM have now all been verified to be working on board #1. Both multiplexers are new, so I don’t really suspect those.
Did you check if all the chips have the right voltage supplied ?
HigashiJun

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protek
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Re: Help with two Orics

Post by protek »

Not yet. I did check that the regulator supplies 4.96V, though.

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mikeb
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Re: Help with two Orics

Post by mikeb »

I may have suggested this, or may have forgotten, but once it is powered up, what happens if you briefly short (with a wire) CPU pin 1 (VSS) and 40 (Reset)?

This forces a more enthusiastic cold reset, once power is up and stable.

Sometimes, an Oric won't start because the onboard reset just didn't hold the 6502 down for long enough for the power to stabilize.

If this helps it start up, replace C21 1uF/10V, small electrolytic.

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