Question about a non-working ZX Spectrum

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bwinkel67
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Re: Question about a non-working ZX Spectrum

Post by bwinkel67 »

Martin_Head wrote:Any chance that you may have damaged one of the through plated holes in the PCB when removing IC's.

As you have changed the symptoms, maybe you have fixed the initial fault, and just added a new one.
I checked all the traces with multi-meter on the RAM chips before socketing them and the all checked out (it's a pretty set pattern so easy to check).

For the CPU socket I took pictures and that all seems fine. I actually took even closer pictures so anything that looks odd in this composite picture of two images is due to lighting that is cleared up at the closer ones. Of course I also inspected the races up close before taking pictures.
CPU.png


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Pr0f
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Re: Question about a non-working ZX Spectrum

Post by Pr0f »

I would say the issue is more likely on the RAM/ULA side of the address lines or data lines and not so likely to be on the CPU/ROM side of those lines. Between the multiplexer chips (157) and the dram chips - you will notice that the address lines have a resistor between. And in a similar way, the CPU databus and the DRAM chips are also separated by a bank of resistors. It's likely that any short or open circuit is on the DRAM side, but you could test on the CPU side just to double check. Keep in mind that if there is a chip still connected on the board, then the multi-meter will give different readings depending on which way around it is connected - that's a good indication that no open or short is there - as if that was the case - the reading would be the same.


Tests:

For each address line - measure between Muxed A0-A6 from the ULA and the DRAM A0-A6 lines on each DRAM chip, and from the DRAM A0-A6 to Ground and 5v line for any short
Measure from the ULA side of the address resistors from the 157 chips to the same lines on the DRAM chip and from these lines to Ground and 5V
For the data lines - measure between the ULA D0-D7 and the DRAM chips - remember they have both an Data in and a Data out pin - they should be tied together - but check both - also check from these lines to Ground and 5v
Measure from the Databus resistors on the ULA side to the DRAM chips and to Ground and 5V
Measure from the 157 outputs to ground / 5v - for any short there.
Finally measure the Z80 Address lines to Ground and 5v and the databus lines
Check Z80 RD and WR lines to ground and 5v and to the ULA


Nasta
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Re: Question about a non-working ZX Spectrum

Post by Nasta »

bwinkel67 wrote:
Martin_Head wrote:Any chance that you may have damaged one of the through plated holes in the PCB when removing IC's.

As you have changed the symptoms, maybe you have fixed the initial fault, and just added a new one.
I checked all the traces with multi-meter on the RAM chips before socketing them and the all checked out (it's a pretty set pattern so easy to check).

For the CPU socket I took pictures and that all seems fine. I actually took even closer pictures so anything that looks odd in this composite picture of two images is due to lighting that is cleared up at the closer ones. Of course I also inspected the races up close before taking pictures.

CPU.png
There are obviously damaged pads for the CPU pins on that picture (top righthand corner) and possibly even more damaged through plating on holes where pins used to be (which can't be seen in the pic but can be tested with a multimeter).


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bwinkel67
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Re: Question about a non-working ZX Spectrum

Post by bwinkel67 »

Nasta wrote: There are obviously damaged pads for the CPU pins on that picture (top righthand corner) and possibly even more damaged through plating on holes where pins used to be (which can't be seen in the pic but can be tested with a multimeter).
I checked all the pads before socketing it. I also took more magnified pictures where, with better lighting, you can see they look good. Some of it is deceiving in this picture because the white mask melted and covered up part of a corner of some pads making it look like they are incomplete or gone and some is just left over solder playing with lighting. The other pictures get a much better view and there is little damage other than two places on the board that got too much heat and the board substrate chipped ever so slightly (but didn't damage the traces). Plus I have some even better googles to inspect the board at really close up and everything checked out. All the pads make contact as I traced all the lines. The worst of it is on the side that is showing since that's where the tip of the soldering iron heated up the legs of the bad cpu to get sucked out the other side.


Maskenlos
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Re: Question about a non-working ZX Spectrum

Post by Maskenlos »

Hi bwinkel,

may I ask if there is any update about the points asked before:

- check all address lines which I asked in general and Pr0f asked and explained in much more detail. As well as data lines?
- Re-Caping?
- Try to find someone with an ULA to swap?

Are you based in Germany?

Stephan


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bwinkel67
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Re: Question about a non-working ZX Spectrum

Post by bwinkel67 »

Maskenlos wrote:Hi bwinkel,

may I ask if there is any update about the points asked before:

- check all address lines which I asked in general and Pr0f asked and explained in much more detail. As well as data lines?
- Re-Caping?
- Try to find someone with an ULA to swap?

Are you based in Germany?

Stephan
I'm in the US and no update presently. I will recap next and then start the process of checking the address and data lines. I usually only have the weekend to focus on it and have been doing that for the past 3. If I can't find anything then next will try to find a ROM (I saw one on YouTube that had two ROM's on it, with one being diagnostic and you could flip a switch). So it will be slow going but it was nice that last weekend I did debug a bad Z80 so I feel like I made some progress, albeit slowly.


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bwinkel67
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Re: Question about a non-working ZX Spectrum

Post by bwinkel67 »

Ok, update. I recapped the board and replaced the two SN74LS157N since I had access to two of them (date code 1977). The whole board is socketed now as I ended up pulling the RF video chip just to be complete -- Interesting that without it plugged in the composite loses color so I guess it's not just for RF video. I didn't have 22uf caps so I built my own:
caps_20uf.png
I may redo the cap's at a later date and find a recap set with more reasonable sized caps. All the ones I had I needed to be creative to get them to fit.

So I still get the same screen (at least my cap tests were accurate since they found nothing wrong with the original ones). I also went over the board thoroughly for any broken solder joints but it's in really good shape. All the transistors check out as well. I'm guessing it's either a flaky ROM or ULA since CPU and RAM have been separately tested. I'll start with the ROM and do some checking between it and the CPU to see if I can find anything.

Should also look for a replacement ROM. I'm thinking of actually just buying an EPROM programmer and burning my own since that will be more useful moving forward (plus if it's not the ROM then I could stick a diagnostic on there instead). Of course that'll take some time to get. Is AliExpress a good source for EPROM's for the Spectrum (and also for the QL since I may want to play around with it in the future -- I found an empty ROM cartridge I could write stuff to).


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vanpeebles
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Re: Question about a non-working ZX Spectrum

Post by vanpeebles »

That photo is work of art! :lol:


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bwinkel67
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Re: Question about a non-working ZX Spectrum

Post by bwinkel67 »

vanpeebles wrote:That photo is work of art! :lol:
I'm naming it "Opposing Caps" in honor of The Falcon and the Winter Soldier.


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vanpeebles
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Re: Question about a non-working ZX Spectrum

Post by vanpeebles »

When you powered on, did you make this face? >_<


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