ZX Spectrum stuck before Sinclair copyright message.

[Muttley Black]

I got the board fully socketed. The only parts i replace was capacitors. I did that before anything else cause many problems start just from faulty capacitors. Also i double check the solder side with magnifier and all looks good.

I was just checked the voltages on transistors and i get this results:

What i only found is that the voltages on TR7 are faulty. I read in a pdf repair manual that TR7 must have the following voltages: 0.0v - 0.23v - 0.23v
I got: 0.0v - 0.35v - 9.09v

TR1,TR2,TR3,TR6 have good voltages.

[1024MAK]

I was just checked the voltages on transistors and i get this results:

What i only found is that the voltages on TR7 are faulty. I read in a pdf repair manual that TR7 must have the following voltages: 0.0v - 0.23v - 0.23v
I got: 0.0v - 0.35v - 9.09v

TR1,TR2,TR3,TR6 have good voltages.

TR7 collector is connected to the +9V line (not the +5V as shown in most schematic diagrams), so 9.09V for this transistor lead is correct. It looks like there is also an error in the repair manual.

If all the DIL/DIP chips were already socketed when you got it, then the possibility of damage on the top side of the board is real.

But don’t desolder anything for now. Let me think...

Mark

[1024MAK]

While fault finding, I strongly recommend removing all the chips associated with the upper memory that provide for the optional extra 32K bytes of RAM.

Remove:
IC15 to IC22 (DRAM chips)
IC23 (74LS32)
IC24 (74LS00)
IC25, IC16 (both 74LS157)

Power up, take note of the colour of the border and take a photo of the screen. Power down again. Repeat at least 10 times.
Is the border colour always the same colour? Which colour?

Is the pattern on the screen always the same, or is it different every time?

Mark

[Muttley Black]

We have results.

I did the test 10 times as you said. In every test mostly got new border color. First yellow then cyan, blue, black. The center screen was about the same as always.

Some photos:




At the 6-7 test i got this distortion moving border followed by distortion loud speaker noise.

[1024MAK]

Okay, so now we know that the CPU is not managing to execute the ROM code correctly, but the CPU is doing something. Further, most of the ULA appears to be working.

Even if there is no working RAM, when starting up, one of the first things the ROM code does is set the border to white. Then a memory check is done. But in your machine, none of this is happening.

I suspect either one address line is faulty, or more likely, one data line is faulty.

Do you have a printer that you can use to print out the schematic?

If yes, print it out.
A copy of the schematic is here http://www.retroisle.com/sinclair/zxspe ... matics.gif

Mark

[Muttley Black]

I just printed the schematic. I wish i had A3 printer!

So what i need to do is to check the connectivity from CPU, ULA and ROM address /data lines to each other? Or you suspect just some of this (A) (D) lines?

[1024MAK]

Start with testing each data bus line (D0 to D7) between the CPU and the ROM. You need your multimeter switched to a low resistance (ohms) range, such as the 200 ohms range or equivalent. Or use a continuity mode/range. Mark each line on the schematic as you do it with a tick.

The resistance of each line should be less than 10 ohms.

If no fault is found, do the same with the address bus lines (A0 to A14). Don’t worry about A14 or A15 at the moment.

Mark

[1024MAK]

I forgot to say, you need to test between the legs of the relevant chips. NOT to the solder joints or to the pins of the sockets.

Mark

[Muttley Black]

I forgot to say, you need to test between the legs of the relevant chips. NOT to the solder joints or to the pins of the sockets.

Mark

Of course! To be sure that each line is totally connected to IC Leg. I check the connectivity holding each multimeter probe at the top of ic leg.

By the way. I take the continuity mode (beep sound) option, cause i don't trust so much the accuracy of my multi-meter. Is that ok?

[1024MAK]

After carrying out the continuity tests, the next step is to test for short circuits. This is a bit more complicated.

You need to switch your meter to the 2000 ohm / 2k ohm range.

At the CPU, test each of these lines to each other:
+5V, 0V, D0, D1, D2, D3, D4, D5, D6, D7, A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15.

So for example, first test D0 to +5V, then test D0 to 0V, then test D0 to D1, etc...

I suggest you draw up a grid on a A4 piece of paper, with the above lines as both the columns and rows. Tick as you test each combination.

Report back any results that are less than 400 ohms.

Mark

[1024MAK]

By the way. I take the continuity mode (beep sound) option, cause i don't trust so much the accuracy of my multi-meter. Is that ok?

Yes, as long as you are aware of the limitations. There is no standard as to the resistance value when a test instrument or multimeter should indicate electrical continuity. Similarly there is no standard as to the resistance value when a test instrument or multimeter should indicate an open circuit condition. So you need to read the instruction booklet or manual to find out what it says or specifies for your multimeter.

Mark

[Muttley Black]

Hello! Back with results.

You can see the continuity test on photo below. Lines that marked with red pen are tested and work well.






About short circuits test, none measurements saw me less than 400 ohm. But need to say what different s i saw.

CPU

All (D) with +5v measurement saw me around 895 ohm
All (D) with 0v measurement saw me around 640 ohm
All (D) with all other pins measurement saw me infinite ohms
---------------------------------------------------------------------------------------------

A0 / +5V saw me 712 ohm
A0 / 0v saw me 1681 ohm
A1,A2,A3,A4,A5,A6,A7,A8,A9,A10 / +5V saw me infinite ohms
A1,A2,A3,A4,A5,A6,A7,A8,A9,A10 / 0V saw me infinite ohms
A11,A12,A13,A14,A15 /+5V saw me around 1025 ohm
A11,A12,A13 / 0V saw me 605 ohm
A14,A15 / 0V saw me 691 ohm
All (A) with all other pins measurement saw me infinite ohms
-----------------------------------------------------------------------------------------------

All measurement done with IC4, IC23, IC24, IC25, IC26 and all upper ram removed, as you see in the photo below.
I didn't test ROM IC for short circuits. Tell me if i need to.





Thank you!

[Ast A. Moore]

Remove all RAM and reset the machine a few times. What do you see on the screen? Is the border always white? If not, with all the RAM still removed, replace the CPU, ROM, and ULA with known good ones (one at a time at first), and retest.

[1024MAK]

Some resistance values above 600 ohms are expected, because various lines have resistors connected to them. In addition, the test current from the multimeter will flow into or out of the power supply rails and via the internal resistive paths in some of the chips.

Mark

[Muttley Black]

Remove all RAM and reset the machine a few times. What do you see on the screen? Is the border always white? If not, with all the RAM still removed, replace the CPU, ROM, and ULA with known good ones (one at a time at first), and retest.

I will do. We did about the same test with 1024MAK but all lower ram was pluged. So next test follows with all ram off the board in a sort of time. Want me to keep IC4, IC23, IC24, IC25, IC26 out of the board or socketed?

[Muttley Black]

Some resistance values above 600 ohms are expected, because various lines have resistors connected to them. In addition, the test current from the multimeter will flow into or out of the power supply rails and via the internal resistive paths in some of the chips.

Mark

I see. Thank you for the info

[Ast A. Moore]

Remove all RAM and reset the machine a few times. What do you see on the screen? Is the border always white? If not, with all the RAM still removed, replace the CPU, ROM, and ULA with known good ones (one at a time at first), and retest.

Want me to keep IC4, IC23, IC24, IC25, IC26 out of the board or socketed?

IC4 should remain on the board. In other words, leave everything as pictured in your last photo, except for the actual lower RAM ICs.

[Muttley Black]

I just did the tests.

Steps

a) with all ram ics removed from the board and with original ula,cpu & rom
b) with all ram ics removed from the board but swapped ula.
c)with all ram ics removed from the board but swapped cpu.
d)with all ram ics removed from the board but swapped rom.
e)with all ram ics removed from the board but swapped ula, cpu &rom.

In all steps and with many restarts, i get exactly the same results. White boarder with the same graphics in the center.

Here is the video:

[media]https://drive.google.com/file/d/1ydeFbQ ... sp=sharing[/media]

[Ast A. Moore]

The screen seems to show a bit too much activity for my liking. I’d expect there to be almost none without any RAM present. It’s possible the ULA is picking up random floating signals on the data bus, however.

Well, the border stays white, which means the ULA can do at least that much, and the CPU can read and execute a few bytes from the ROM, including a jump.

Barring any passive component failures, solder bridges, or cold solder joints, looks like it’s one of the multiplexors; most likely IC3.

Replace the lower RAM ICs but swap IC3 and IC4 around. See if swapping them makes any difference to what appears on the screen.

[1024MAK]

Or try using the spare 74LS157 multiplexer chips from the upper RAM...

Mark

[Muttley Black]

I did that test yesterday with new SN74LS157N. Same results. I order some new lower and upper ram IC's to give a try to. When i got some free time i am gonna desolder ALL sockets to check the traces under them. After that i put new round pin sockets and do the same tests.

I have to say that painful and weird repairs like this one, are extremely interesting!

[Ast A. Moore]

Indeed. Well, keep us posted!

[1024MAK]

When i got some free time i am gonna desolder ALL sockets to check the traces under them. After that i put new round pin sockets and do the same tests.

Before you do that, can you try these tests:

With the upper RAM chips and the associated 74LSxxx chips not fitted, try running with only one of the 4116 (or equivalent) DRAM chips fitted at a time. First with a DRAM chip fitted in position IC6. Then with a DRAM chip fitted in IC7, etc... After fitting each chip, power up, note the colour of the border. Power down and back up, again note the colour of the border. Do this at least four times, before trying with the next DRAM chip.

We are looking for the border to be white EVERY time. As this indicates that the Z80 has managed to execute the the code in the ROM before it gets to the point that working RAM is needed. If at any stage you find the border is not white. Note which socket it is that you are trying DRAM in. Try a different DRAM chip and see if the result is the same.

If a DRAM chip works in every socket, repeat the above process with two DRAM chips, keep one fitted in IC6. Continue until the border is not white...

Mark

[1024MAK]

One other thing. Did you say earlier that you had some or all of the 4116 (or equivalent) DRAM chips in double sockets? That is, DRAM chip in a turned pin socket, which in turn was in the stamped pin socket mounted on the board?

I can’t remember if the /RAS, /CAS and /WE pins on 4116 (or equivalent) DRAM chips float high if not connected, or if instead are high impedance and hence get influenced by any nearby electric field from other circuits. But if there is/was poor connections on any of these pins, it would seriously mess up the operation of the DRAM, including possibly outputting data on the data bus when it should not be. So check the continuity of these chip pins to the source of these control signals, which is the relevant pins of the ULA chip.

Mark

[Muttley Black]

I did the test and i get always white border. Yes i fit round sockets on slide sockets to be sure about the connectivity, cause slide sockets are not in good shape. In some sockets i can see big gap that for sure an ic leg can't touch the connection point. With double socket fitted, i check the connectivity and it is good. All this made for the testing progress. After that, i put the new round pin sockets sodered on the pcb for sure.