ZXmore - questions and answers (FAQ)

[rune]

Hi, I've now completed the ZXMore and am in bother.

This is whats on screen, The brighter bands are actually just one white and grey band. The vertical band to the right is bad pixels on the screen.



The multicoloured LED doesnt come on when the reset button is pressed. But it did come on a couple of times when I was checking voltages.

The three pin 6.5 MHz crystal is oscillating, but the 12 Mhz doesnt seem to do much.

I havent soldered the capacitor to the bottom of the board yet.

I had problems getting a 1UH inductor. Finally found this one which seems a lot bigger than the one shown in the manual - I did resolder the top and get 5V at either leg.



Spec is 1uH 10% 4.45A 200Mhz - Farnell 2475028

I checked that there was 5V at the top-right pins on the ICs. All around 4.98V The Z80 pin 6 shows a frequency and Pin 34? shows 5V which drops and comes back up on reset.

Any advice or ideas about what has gone wrong will, as always, be gratefully received.

Meanwhile, I'll go over the board to check soldering etc.

Dave

[PokeMon]

Hi Dave,

the LED usually is not active in instance 0 or after startup (dark/out).
The capacitor on the backside is important (47pF). Also the connection of pin 1 and 2 of 74HCT74. And C34 and C33 with 150pF and 470pF - they are not marked at the board but I did post a picture here:

http://sinclairzxworld.com/viewtopic.ph ... 095#p26095

The address jumper should be set to A3 and probably NTSC jumper set for your setting.
The 12 MHz is not easy to measure with standard probes. Anyway not important as the USB chip is not required for startup.
Most issues are forgotten solder joints and this happens sometimes even to me.
Probably when someone is ringing on the phone during soldering ...

By the way - Reset signal is at pin 26 of CPU.

[rune]

Thanks, I think I counted wrong way on Z80, 6 up from the bottom.

OK, I'll put the cap on the back and try again and check Pin 1 & 2.

I did spot your message about C33/34.

Thought I had finished for the night Ah well back out to the shed.

[rune]

I've bridged Pin 1 & 2, added the capacitor on the back and still no difference.

I've went over (almost) all IC pins with the soldering iron, nothing changed.

Could I use a 4Mhz Z80 to test?

Any other ideas appreciated. Meantime Im going to stop for tonight or I'll end up wrecking the board.

[PokeMon]

Did you check if IC15 is a 74HCT165 ?
I accidently used a 74HCT166 shortly and not sure if I did sent anybody with a kit.
In general it should startup with 3.25 MHz.
You can measure the frequency at the CLK pin 6 of CPU.
Did you check the orientation of the resistor arrays - one has other direction pin 1.

[rune]

Hello Karl

IC15 is a 165,

but the resistor arrays...

R2, R3 and R5 all have Pin 1 facing the top of the board. R1 and R4 is facing the bottom according to the silk screen markings. I'll reread the build document.

Dave

[PokeMon]

Yes - R1 and R4 have a different orientation.
Pin 1 is marked with a special square about it.

[rune]

Hi Karl,

I'm tracing the video circuit back from the centre pin on the Cinch.

I've got to AC86 OK and see two inputs go through R101 and R102. Im not sure what that means, I thought maybe the Resistor Network R1 with 01& 02 being pins. But the schematic shows different resistances.

Normal resistors R1 & R2 are next to the IC but none of the inputs go to those.

Can you tell me what R101 & R102 are?

Also there is an input marked VidPix. Whats the source of that?

Dave

[PokeMon]

R101 and R102 are smd resistors added to the solder side of the board by myself.
There have been some timing mismatches with the video signal (inverted characters) which are corrected this way.
R100 and D100 are also smd components added this way.

The problem is not in the video output circuit. When you see those stripes there is some other problem, probably the program execution. What about the resistor arrays - are the two soldered wrong way ?

[rune]

Hi Karl

The resistor arrays are as follows:

R1, R4 with the dot in the P1 position - towards the keyboard
R2, R3, R5 with the dot in the P1 position - towards the edge connector.

I think these are correct.

Im still working my way through and ticking off the connections between ICs, this will at least prove that the soldering is OK.
Resistor values are being checked. So far all is good.

What I cant check is the capacitors. Two I am concerned about are C33 and C34. I didnt have any bead caps so used the brown discs.
C33 has 471 on it and is about half the size of C34 which has 150 on it. If any of these were bad would it cause the problems I have?

My other issue with caps was the lack of numbering on the board. I think I got it right, but it would be good to check where the 12 decoupling caps go.

I also had a look at the data in the flash ROM. I'm not sure about the following, but I though they had to have a FF where there was no data. This is what I found:

ZXMaster starts at 70000H and has FF where there is no data
ZX80 ROM (4K) at 00000H with 00 where there is no data
ZX81 ROMs (8K) at 10000H, 20000H, 30000H, 40000H, 50000H, 60000H with 00 where there is no data