Issue 3 Spectrum 48k no display no boot nothing

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ziggy123
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Joined: Mon Jul 15, 2019 5:42 pm

Issue 3 Spectrum 48k no display no boot nothing

Post by ziggy123 » Mon Jul 15, 2019 10:12 pm

Hi all,

I picked up a untested Spectrum from ebay, hoping to relive some of my youth, I had a ZX81 back in the day and was always envious of my classmates when they got the speccy,
I would appreciate if you guys could help me bring it back to life :-)

Before I turned it on I followed JoulesperCoulomb youtube video 'Initial Tests'
all tests seemed to be ok, transistors and diodes tested ok in circuit and resistances looked ok.

Im using a DC-DC converter set at 9v.
When I powered it up (around 450MA draw) there was no display. I went ahead and done the composite mod with capacitor, to rule out TV, same thing no display , the voltages at the DRAMS 4116 -5V +12V +5V are ok.

I changed all the capacitors and had a good look all over the board for damage or breaks and shorts, the board is actually in really good condition for its age, all good there.
So basically no video, no audio on key presses, I also shorted lines of KB1 and KB2 to test input.

Is there anything else to test before I start looking for ULA, Dram or CPU. I know the ULA is hard to get hold of, is it worth buying another spectrum just for the parts.

I bought a cheap USB 24MHz Logic Analyser, though I only used it once on an arduino so Im not used to it, would this help find my problem?

Is it worth testing all transistors out of circuit?
after 5 mins the VOLT REG, CPU, ULA, ROM and lower RAM gets a little warm around 30 degree, the upper ram stays bascically the same tho

Ive exhausted my ideas - any pointers where I should go next would be great thx
Attachments
full board.jpg
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Issue 3 Spectrum 48k no display no boot nothing

Post by 1024MAK » Tue Jul 16, 2019 10:31 am

Hello Ziggy and welcome :D
ziggy123 wrote:
Mon Jul 15, 2019 10:12 pm
Im using a DC-DC converter set at 9v.
When I powered it up (around 450MA draw) there was no display. I went ahead and done the composite mod with capacitor, to rule out TV, same thing no display , the voltages at the DRAMS 4116 -5V +12V +5V are ok.
I trust that your power adapter/PSU is set to the correct polarity?

What are the actual voltages on the +5V, +12V and -5V rails?
ziggy123 wrote:
Mon Jul 15, 2019 10:12 pm
So basically no video, no audio on key presses, I also shorted lines of KB1 and KB2 to test input.
That’s not good. The ULA will generate a picture even if the Z80, the ROM and all the DRAM are missing or faulty.
If the ULA is generating the clock for the CPU, if it is running and the ROM, RAM and the address, control and data busses are fine, pressing a key should get you a key click...
ziggy123 wrote:
Mon Jul 15, 2019 10:12 pm
I bought a cheap USB 24MHz Logic Analyser, though I only used it once on an arduino so Im not used to it, would this help find my problem?
Yes!
You obviously have a multimeter. A logic probe, or an oscilloscope are the other useful items of test equipment.

So, taken what you have said, a lot of items are very unlikely to be single point of failures, so for the time being can be put to one side in terms of fault finding.

First, let’s look at the +5V and +12V supplies to the video section.
Test pin 14 on the ULA (IC1) for the +5V supply.
Test pins 14, 15 and 16 on IC14 (LM1889) for +12V.
Test either side of resistor R64 (15Ω) for +5V.

If all these are okay, next we need to know if the CPU is getting a clock signal from the ULA.

This test needs either an oscilloscope, a logic probe or a logic analyser. It is a 3.5MHz square wave. So first check if your test gear is capable of accurately handling this signal (keep in mind that you need a bandwidth of at least 20MHz).

Now test on pin 6 of the Z80 MPU (IC2).

Report back on your findings.

Mark

ziggy123
Posts: 3
Joined: Mon Jul 15, 2019 5:42 pm

Re: Issue 3 Spectrum 48k no display no boot nothing

Post by ziggy123 » Tue Jul 16, 2019 2:41 pm

Thank you Mark for your help :-)

I trust that your power adapter/PSU is set to the correct polarity? yes

What are the actual voltages on the +5V +12V and -5V rails? +4.96V +11.98V -4.82V

First, let’s look at the +5V and +12V supplies to the video section.

Test pin 14 on the ULA (IC1) for the +5V supply. 4.96V
Test pins 14, 15 and 16 on IC14 (LM1889) for +12V. 11.62V
Test either side of resistor R64 (15Ω) for +5V. 4.81V and 4.96V

Now test on pin 6 of the Z80 MPU (IC2)
Not getting any reading at all, hope im using the analyzer properly. to test I connected it to a SSD1306 OLED with a arduino, I set PulseView using 1G samples and 24MHz and got output see pic, I connected to ULA pin6 and ground to the tab of 7805 I then turned on speccy it just shows a straight line :-(
I need to go out I will be on later ;-) thx again
Attachments
Session 1 - PulseView.jpg
ouput1

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Issue 3 Spectrum 48k no display no boot nothing

Post by 1024MAK » Tue Jul 16, 2019 7:18 pm

Hi Ziggy

All those voltages are good :D

Now test on pin 32 of the ULA (IC1). This signal, if present will be a lower amplitude signal and the waveform is not very sharp. So I hope that your logic analyser can pick it up. Again, it is a 3.5MHz frequency. Ideally an oscilloscope is better for this test. But I think a logic analyser should work (I’ve not tried a logic analyser on this signal before).

If you have no signal here, then it is looking like the ULA is faulty. But do carry out some DC voltage tests with your multimeter. That is ULA pin 32 and then Z80 pin 6. Report back your results.

Using a magnifying glass, carefully check to see if all the chips legs are correctly fitting in the socket (that is none are bent under the chip). Also inspect all the solder joints for all the pins on the underside. You are looking for broken tracks, short circuits and poorly made, or dry solder joints. A dry joint may look dull, there may be fine cracks, it may look distorted instead of a nice volcano shape.

If you don’t find any problems so far, gently remove the ULA from its socket. Use either a blunt rounded knife, or a flat bladed screwdriver, to gently ease up one end at a time, only applying pressure between the plastic of the socket and the plastic case of the chip. Don’t lever against the PCB you may damage some of the fine PCB tracks. Work at one end, then the other, alternatively. Keep a finger on top just in case the chip gives suddenly. If the chip comes out too fast, it will rotate as it comes free and you will end up with bent legs :-(

Carefully inspect the chip legs and the contacts in the socket, looking for damage and corrosion.

If after cleaning the legs (if needed), you don’t find anything wrong. Try plugging it in again.

If still no joy, then it is looking like you either need another ZX Spectrum to swap ULAs around, or to get a ULA replacement module.

But you may also want to inspect the track connecting to ULA pin 39, crystal X1 (14MHz), capacitor C73 and their tracks/solder joints.

Mark

ziggy123
Posts: 3
Joined: Mon Jul 15, 2019 5:42 pm

Re: Issue 3 Spectrum 48k no display no boot nothing

Post by ziggy123 » Tue Jul 16, 2019 10:53 pm

Hi Mark, im back again...

test signal on pin 32 of the ULA..
no signal, im just getting a high signal, the line goes to top then drops when i turn it off.
I also tested using Saleae logic program but im getting an error on the 24MHz setting saying that it cant keep up with the data,(ive changed USB ports etc..) I had problems with the PulseView aswell, the device disconnecting and weird results, the signal could be there but the analyser couldnt handle it maybe? clutching at straws lol

DC voltage tests....
ULA pin32 2.89V.
Z80 pin6 around 100Mv basically zero, the Z80 gets a little warm tho

I checked for signs of breaks, shorts etc.. nothing found.
I removed the ULA It looked ok aswell all pins straight, I gave it a clean to be sure.

But you may also want to inspect the track connecting to ULA pin 39, crystal X1 (14MHz), capacitor C73 and their tracks/solder joints....
I traced the track from pin 39 to the crystal, then to C73 but I have a resistor there but no capacitor and its reading open on my meter :?
Edit: I didnt know there was capacitors like that ;-)
Attachments
resistor.jpg
resisitor
2019-07-16 19_38_47-Saleae Logic Software.jpg
Logic

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