ZX80 Core - new ZX80 motherboards

[PokeMon]

Well, the orientation of the ZF filter doesn't really matter - too bad you broke some pins.
Did you check for the other things, jumpers and soldering points ?
All IC's properly installed in the sockets ? No pin bended ?
Checking the board with a multimeter is not the best way - a scope would be better.
I will check the voltages later. Maybe I have to write a custom test ROM.
What about the reset pin ?
Can you check that voltage ?

[poglad]

Didn't check bent pins. I'll check all again - and will pick up an oscilloscope Meanwhile I'll have a relaxed weekend and not worry about the broken pins!

EDIT - Just had a look, and apart from the HPS140i there's no way I can afford any of the oscilloscopes right now. Is that one capable of showing the signals to be measured? It claims "up to" 10MHz bandwidth, but presumably works much better on audio frequencies than on video signals...

[1024MAK]

20 MHz is the recommended minimum bandwidth of an analog 'scope for composite video signals.
A 10 MHz 'scope is better than no 'scope...

It may pay to have a look at a second hand (used) 'scope.

Mark

[sirmorris]

This place is utterly _amazing_ (sorry, non-uk friends):

http://www.stewart-of-reading.co.uk/

it's a cave of used test equipment. Andy will back me up on the sheer amazingness of the place

You should call and have a chat ahead of time and let them know what you're after - you can't just turn up (ahem, as we found out It's more of a hobby place than a full on business. I don't think you'll get ripped off there.

C

[poglad]

Thanks for that info guys. This evening I managed to solder some new legs onto the damaged X1 component - so it's back in action! Doesn't look that great, but it'll do until the new one arrives. I checked all the IC legs - no bent pins. I checked for shorts, and found three - all underneath jumpers - so I've cleared those. Checked the jumper positions and they are correct. Unfortunately, the board is still behaving the same as before. There's a bit more to see when you set it for inverse video, lots of little white flecks whizzing around. The LEDs never change from red (even briefly), so I don't think the ROM code is running.

However ----- when I move the ROM selector to position E, the Azmic ROM works! I get a display with the flashing cursor, and I was able to test the keyboard and all the keys work correctly. So I think something is up between the ROM and the switch... definite progress anyhow!!!

[PokeMon]

Great news !
So this will let me think about what's wrong.
So the ASZMIC is working on position "E" - right ?
The ZX80 should be working on position 4 and 5, the ZX81 on position 6.

All positions have the WR line set to high - so wouldn't possible to write anything to the EEPROM.
Normally can not be destroyed because you need a kind of password to write data to it but there are some commands in ROM which make the system to hang while trying to write to ROM (which is normally not possible, some bugs in the firmware from Sinclair). So PRINT is a candidate. So better to leave the EEPROM write protected.

Hmm now I have to thing.
About the oscilloscope - maybe you can borrow one from someone you know for a few days if necessary.
10 MHz is not the best but better than no scope.
For finding problems on the board it would be sufficient as most signals are in the 1 MHz range or below.

If you anyway are interested in an oscilloscope you should try to get one via Ebay.
I got mine 60 MHz dual scope (analog - would be the best) for about 100 EUR, is a Tektronix.

So if you get the video signal in good condition from ASZMIC - the ROM should work at all, same for RAM and the video part.

[PokeMon]

Well - I have here some inexpensive 28C64 EEPROMs (8k) and will provide one for you for free together with your 6.5 MHz filter you ordered with a ZX81 ROM image. So shipment will delay a bit but gives the possibility to test another programmed EEPROM.

[PokeMon]

Can you check following:

Did you solder R100 ?
Can you measure pin 1 of IC2 (ROM) in position 4 or 6 ?
It should be near GND when operating, or you can measure the resistance between pin 1 and GND when not operating (should be 10k).

That's the only thing in my mind when checking schematics.
When choosing ASZMIC (position E of ROM switch) pin 1 is connected to VCC (over a 3k3 resistor) but when R100 is missing this pin is in the air which causes trouble for ROM switch positions < "8".

[poglad]

Thank you so much for your help!

Yes I soldered R100 - a 10K resistor?

Unfortunately, after trying positions 4 and 6 again (and getting a lot of random garbage on the screen), going back to position E there is no longer the Aszmic image running correctly. It just gives a black screen and red LEDs. Position 6 also now gives a black screen, but with green LEDs!

I measured pin 1 of the EEPROM - with the power off, it gives 9.88K - unfortunately with the power on it also gives 9.88K, where you expected GND.

Getting tired so will look again tomorrow evening! Thanks again.

[1024MAK]

Can you measure pin 1 of IC2 (ROM) in position 4 or 6 ?
It should be near GND when operating

He means, with the meter on a voltage range, connect the red lead to IC2 pin 1 and the black lead to any 0V / ground point (for example. The outer metal of the composite video connector).

Mark