Voltages wondering about the Oric-1 - Just to be sure before I toast it!

hi guys, I have bought a loose Oric-1 a few weeks ago for only 50.00 € , and since I had some free time today, I wanted to give it a quick look.


Here is the beast, nothing fancy, just the usual Oric-1, no PSU, no video cable.

I have a multiple voltage center positive PSU, but I have no video cable (and no TV set that accepts the RF signal), I just wanted to PING, ZAP, SHOOT and EXPLODE it before I build (or buy, I don't know yet) an RGB SCART cable.

I first did a little mistake, by applying 15V (instead of the 9V required) for a couple of seconds before I see my error. Since the voltage is regulated by a 7905 + radiator, and since no magic smoke or poping noise came from the Oric, I thought my error was not a big deal.

I then applied +9V and blindly typed "ZAP" followed by ENTER several times, but the Oric remained silent. Now I just can hear a scratching sound when I plug in the PSU (not concerning), and a constant faint humming sound from the speaker...
My first thought was: the 7905 is fried.

I decided to have a look inside. The inside is clean, with no signs of rust or oxydation, nothing special to report. I have discovered an issue 3 motherboard. Notice the IC10 & IC11 are not present. Most of the chips were manufactured in 1983 at the exception of the sound chip from 1982. This one was changed at some point, I can clearly see some rework was done on its solder joints.


The main board, issue 3 mfd. in 1983 excepted IC4 replaced by a 1982 chip version. I have desoldered the 7905.

I have measured the output voltage from the 7905 and it gave me... +3.8V Isn't it supposed to be -5V? I measured some various points on the motherboard and still, 3.8V, and some points at +8.8V are we supposed to find some 9V on the board? Or thinking of it, it is just because I measured the voltage before the 7905(?)...

I have desoldered the 7905 to test it apart from the board, it is not shorted. With my bench power supply it still gives +3.9V instead of the usual -5V.

I already have ordered a new 7905.
What do you think? Is there something else I should do?

If I would like to power the Oric with my bench power supply, how should I plug it? The 7905 puzzles me, I am more used to deal with positive voltage 7805s. I found that the 7905 leftmost pin (IN) is considered as the + line, whereas the center (GND) and rightmost (OUT) are considered as Ground? Can I safely power +5V to the leftmost and GND to the rightmost pin holes?

Let me know, skilled Oricians!!

 

Richie will give you all the correct voltage readings at the various points, Manu.
I've no idea if those 15V applied will have fried anything - and of course it may have been damaged before if it has arrived untested. I wish I had known about you having this and I would have sent you a RGB/Peritel cable I had for the past 2months or so - I sold it last week!
As I say, Richie will help and all I can confirm is the correct voltage to be supplied is 9v centre positive, mate.
I do hope we soon have this little beauty up and running real soon.
I have some notes on the voltage regulator somewhere and if I find them before Richie replies, I will post them, buddy.
BON CHANCE, MON AMI!

 

Hi guys,I have received and installed the new 7905, and now I have the correct +5V all around the board.

But it's still a no go.

I have noticed with my scope that some address lines seems ok, but also that :

• The CPU doesn't resets itself, although the reset line is tested ok,
• I have no bus activity (address lines always high and data lines always low) between the CPU, the VIA, the sound chip and the ULA,
• I have some good looking address bus activity between the ULA and the RAM, through IC8 and IC20, but no data I/O on the RAM,
• I have a good looking 64µs video sync signal from the ULA but no R, G & B signal (always low).

I tried to read the ROM with my programmer, it is not listed as compatible but the closest I could find was the HN27128AP, and it says "Read error".

Any idea of what I should test next?

 

This is so much in Richie's territory it hurts!

Sorry to hear that despite all your best efforts so far you have a dead Oric-1 there, Manu.

Here's hoping Rich can kickstart the recovery, mate.

He's always performed miracles with my micros in the past so hopes are high.

Keep the faith, buddy.

 

@Klyde Hey mate, can you test the resistance between the 5v and ground on some of the logic circuits on the PCB and report back.
Have you got a copy of the test ROM?

 

@Vyper68 Yes, good idea! I will.

 

Putting 15v into an Oric is not in itself a disaster as long as it didn’t go beyond the 7905. The electrolytic capacitor at C8 should have exploded as they are normally quite a low voltage ( 10v or 12v ) so it wouldn’t be able to cope with more volts for long. The 7905 can take up to 30v input but it would get insanely hot trying to get the surplus power dissipated as heat.
All that being said, if you found 8.8v ( 9v in reality ) and 3.8v elsewhere something has gone very wrong. 9v on a Oric will kill everything.
So quick and dirty check on the resistance on some of the logic IC’s which should be around 270 Ohms if it’s low like 160 Ohms then desolder a couple and check them in your EPROM programmer.
Also check the RAM and see if they heat up when the Oric is powered on.
Finally bud if you want it fixed and don’t have the spares then you are welcome to send it to my workshop. I have a couple of ULA’s and 6502’s and 6522’s and an AY chip. I have a 65c02P4 as well that I use for testing so I have all the bits bar the PROM for the RF output.

 

Brilliant, Rich.
Top man for all that assistance.
I hear my pal Johnny is as happy as a pig in mud now with his working Atmos and I have been invited down for an outside meet-up for tea n scones when he will be happy to pose for a photoshoot for the lads to show his lurve for Oric...thread to follow!
(Yes, he is categORICally mad).

 

I have tested the resistance from all the chips including the 6502, 6522, AY, ROM, RAM etc.
They all give 180ish Ohms. It would mean they are all fried? I don't think so, since I have a good looking activity between some of them with my scope, at least for the Address bus.

There is no overheating, I have tested all the chips with an IR thermometer gun.

So, for the moment I have no hints of who the culprit could be. I'll focus on the ROM chip since it cannot be read with the programmer. Can it be?

Thanks for you help Rich, I already have ordered (from what appears to be a good source here in France, not from China) a 6502, a 6522 and a 27C512 to program a test ROM and/or BASIC ROM, and to do some swapping with the main chips.

I'll keep you updated.

 

Well the Ohms test isn’t 100% it was just something I picked up on the Acorn Stardot forums. If you have a logic analyser or scope and can see activity it would be handy if you could check the video output and see what is presented on screen.
I have a couple of Oric test PCB’s left if you wanted one to build one.

Plus, just a gentle reminder you need the 2Mhz version of the 6502 and 6522 so that’s a 6502A and 6522A in case you weren’t aware.

 

The resistances are consistent, I guess it is ok.

I don't have a logic analyser, but various multimeters, logic probes, and a medium quality but handy 25Mhz 100Msa scope.
About the video output, the 64uS sync signal is present on the ULA but no RGB signal. Do you want me to test the RF out?

Yes, I'd like to have a test PCB please. I'll send you my details.

I have ordered Rockwell 6502AP & 6522AP, basically the same as in the Oric.

 

If you are getting no signal on the RGB outputs of the ULA that might mean it’s toast.

So if you have 5v all around the PCB...
So can you check you are getting a 12Mhz clock from the crystal from Pin 2 of IC21 to pin 7 of the ULA
1 MHz from pin 14 of ULA to pin 37 of 6502,
1 MHz from pin 3 of 6502 to pin 15 of AY chip
1 MHz from pin 39 of 6502 to pin 25 of 6522 and pin 5 IC21
1 MHz from pin 6 IC21 to pin 6 of IC09 and IC10

A missing clock from one of these might help narrow down where the problem is.

 

I forgot to mention that I found various clocks are present accros the board , but I'll recheck thoroughly.

Many thanks for your help.

 

@Klyde here is a link for the service manual with the waveforms you should see.

http://www.48katmos.freeuk.com/servman.pdf

 

Thank you richie! I printed it!

 

Hi Rich, the 12Mhz and 1Mhz clocks are present and have the appropriate shape, like the ones shown in the service manual, excepted for IC9 pin 6 (no clock) where I find a constant +3.6V. I have no IC10 installed (27128 chip ROM version).

Are you sure about IC9? According to the schematics, pin 6 seems to be an address line...

 

Sorry I made a mistake it should be pin 6 IC21 to pin 22 IC9

 

I have a good looking 1Mhz clock on IC9 pin 22.

It is a mystery. I continue to investigate... Next step will be to swap the chips I should receive in a couple of days now.

Many thanks for helping me mate!

 

There is a guy in Bulgaria selling the ULA's ( most Oric spare chips come from there now ) They are NOS from when they made the Pravetz 8D clone of the Atmos. I have bought one from there previously and they are genuine chips. I have a couple spare but it might be easier to get one from Bulgaria now we are outside the EU and Customs Union. All the other chips ( save for the PROM used to generate the RF output ) are standard and easy to find. The PROM is impossible to find unless you strip one from another Oric. They are less important really as most people use the RGB now. In fact you can live without all the Chips in the top quarter of the PCB under the Modulator as they and the second crystal are all to generate the RF signal.

 

I forgot to say, I'll be posting the Oric Test & Joystick PCB's via Royal Mail Airmail tonight or tomorrow morning so you should have them by middle of next week
If you are waiting for the chips to arrive have you spent any time looking at the rear of the PCB yet? sometimes there are track breaks and deterioration on these PCB's down by the AY and 6522A and the RAM. If all your tests on your chips prove inconclusive then it would seem like you will need to check all the tracks one by one

 

Yes I was already watching it. Thank you for confirmation of genuinity! I am going to tackle this Oric thanks to you.

 

@Klyde PCB’s are on their way mate. You can fit a 64KB EEPROM in this so you can have V1.0, V1.1 and V1.22 BASIC as well as the test ROM. Where you can only fit a 16KB ROM inside unless you make some modifications.
Have you managed to connect it to a display yet? I have been thinking about this Oric for a while and if you have all the clocks where they should be but it won’t start then I think the RAM might be the problem as the ROM does a dirty RAM initialisation before it boots to BASIC.
The last couple I’ve fixed that had IC faults were the RAM and none were getting hot and the resistance check didn’t flag any, so I replaced all the 4164’s. You can normally pick up 8 for £8/€9 on feeBay. Don’t forget to check for the service modification on the AY chip. The ROM basically kills the AY chip over time on Oric-1’s and it needs a service mod to fix this or fit a V1.1 ( Atmos ) ROM which fixed,the problem. The details are in the service manual.

 

As ever, brilliant advice there from Richie - massively appreciated and all being well, Manu will be able to get his Oric-1 flying in the end. Good luck, Manu!

 

Thank you Richie, you are really helpful!

I found an Oric-1 SCART cable for sale on leboncoin for 15.00€, but the seller never replied, so I built one myself. I only have a black screen (the ULA outputs a sync signal but no R, G or B).

I have ordered the full Oric chips set (CPU, 6522, ULA, RAM) except the AY one. The ULA and the RAM are still on their way. I also have a 28C512 EEPROM.

I'll do some chip swapping as soon as I receive them all, and I'll let you know what I find. Very interesting info about the RAM being faulty with no obvious signs... Thank you for sharing your experience.

 

Richie, I don't have received your parcel yet, but it should arrive in a couple of days now.

I either don't have the ULA and RAM chips yet, although the ULA is announced as "delivered" by the tracking system I hope the parcel was not lost...

I wanted to thank you again for helping me so kindly, that's why you will receive a gift from me in a few days. You'll know when you see it, hopefully it doesn't break during transportation.

Of course I'll let you know as soon as I have some news with the Oric.

Cheerio!