Faulty 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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Igotafro
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Faulty orics

Post by Igotafro »

I got two orics and neither works. I have borrowed a third one which kinda works, this one is getting a picture but it's not really stable. I've measured the supply voltage with an oscilloscope and I can see that there's a ripple in the 5V with a frequency of about 100 Hz. This corresponds to the two blacks bars I get on the display.
One of my other orics have the same ripple in the supply voltage. What can cause this ripple? Faulty RAM-chips?
Seven of my RAM-chips is already in sockets, if I remove the seven chips and also the ULA, the ripple is almost gone, there's only a little at power-on but it disappears after a while.

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Dbug
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Re: Faulty orics

Post by Dbug »

Try to switch on the machine and put your finger on the various ram chips, if one or more got warm fast it's most probably faulty.

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kenneth
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Re: Faulty orics

Post by kenneth »

good evening Igotafro.
The ripple appears when a chip uses more energy than typical one.
Like said Dbug, feeling the over heating surface is the best way to find the faulty chip.
Try the Ram chip one by one until the ripple appears. :wink:

highwayman
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Re: Faulty orics

Post by highwayman »

open the power supply and replace the capacitor in it.

if the supply looks sealed - it's not.
some had plastic caps pushed in over the screws.
drill into them or put a soldering iron through them - then screw a wood screw into them & pull them out.
or try to hook them out with a small screwdriver.

highwayman
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Re: Faulty orics

Post by highwayman »

just to add a bit of tech info.

the oric psu is a linear type (transformer) on 50hz mains,
it's output is fullwave rectified - giving you a chopped-up 100hz dc output.
then it's smoothed by a cheap rubish cap.

SO THAT'S YOUR 100hz SOURCE. 8)

once in the oric, it goes through a 7905 regulator and another small cap.
the internal cap is just to protect the regulator from noise from the chips - it cant help you if the psu isnt smooth.

so replace the junk cap in the psu with a good 105' rated one from Panasonic or Rubycon.
(NOT FROM EBAY)
.

highwayman
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Re: Faulty orics

Post by highwayman »

did you fix them yet?
:D

Igotafro
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Re: Faulty orics

Post by Igotafro »

It wasn't the cap, because I don't have the original PSU for the Oric.
But you gave me the right idea, because it was indeed my PSUs fault.
It got a forth Oric now which works fine, and I got another PSU with it, so then I found out that the one I borrowed was also working.
Still got two faulty ones though, I'm working on one of them now.
All clocks is working fine, so now I'm about to burn a diagnostic ROM.
I'm using this nice repair guide: http://oric.signal11.org.uk/html/repairguide.htm

Igotafro
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Re: Faulty orics

Post by Igotafro »

Yay, one Oric repaired! 7 of 8 RAM chips were in sockets, the one not in a socket was broken.
The picture quality is a little worse than the other ones, but I'm only using RF for now.
One other thing is that the other orics have black and white stripes at boot, but this one has a little green and random stuff on screen.
It all disappears when I get the prompt.

highwayman
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Re: Faulty orics

Post by highwayman »

the crap you see before the prompt is just random data in ram.

what's wrong with the last one?

Igotafro
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Re: Faulty orics

Post by Igotafro »

highwayman wrote:the crap you see before the prompt is just random data in ram.
Ah, ok, maybe it's because the RAM-chips are of different types, since I replaced some.
highwayman wrote:what's wrong with the last one?
Well, I have just had time to check with my oscilloscope and found two RAM-chips that seems to be broken (no activity on pin 2 and 14).
I will replace them tomorrow and hopefully that one will work too.

Igotafro
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Re: Faulty orics

Post by Igotafro »

I have replaced the RAM-chips, but the computer still doesn't work. The address lines A0-A7 is stuck high, so it probably the processor or perhaps IC8/IC20 (74LS257).

highwayman
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Re: Faulty orics

Post by highwayman »

scope the pins on the cpu.
check the clock, rst, irq and address & data pins in that order.

Igotafro
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Re: Faulty orics

Post by Igotafro »

The clock seems fine, but rst is not stable. I've replaced C21.

Igotafro
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Re: Faulty orics

Post by Igotafro »

rst IS stable, I just made an error with my oscilloscope. The data pins seems alright, but A0-A7 constant high and A8-A15 constant low.

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