ZX spectrum 48k repair

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Thu Jun 28, 2018 6:24 am

I don't have a spare edge connector :(
I'll go the socket way...
I'll keep you updated !

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Thu Jun 28, 2018 11:46 am

Okay, another way is to find resistor R33. One leg connects to pin 34 on the ULA.

The other leg goes to edge-connector pin 25A and to one of the ROM configuration links (marked N and H). It is this side of the resistor that you need. So use a multimeter to confirm you are on this side and NOT the ULA side. Then connect this resistor leg to any convenient +5V supply point.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Thu Jun 28, 2018 2:23 pm

Ok, thanks for your help !

I also saw that it was possible to start my smart card v2 in diag mode with the help of a switch. Then it will boot on a special diag ROM which is inside of the 16 ROMs slots in the cartridge memory. It should help me diagnose what is wrong with my speccy.

I'll come back here afterwards :)

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Thu Jun 28, 2018 7:56 pm

For the non working speccy, smart card diag tells me that ic6,ic11 and ic17 are probably bad.
I'll try to replace them.
Stay tuned :)

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Thu Jun 28, 2018 8:05 pm

If you do disable the ROM and there are no other faults on a ZX Spectrum 16k or 48k, what you get on screen is shown in this post on WoSf here.

Good luck ;)

Mark

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Thu Jun 28, 2018 8:09 pm

tyrell wrote:
Thu Jun 28, 2018 7:56 pm
For the non working speccy, smart card diag tells me that ic6,ic11 and ic17 are probably bad.
I'll try to replace them.
Stay tuned :)
Wise to renew/replace IC6 and IC11 first. As long as IC17 is not shorting out the data bus, the Spectrum may start in 16k mode.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Fri Jun 29, 2018 6:34 am

After replacement of ic6, 11 & 17, spectrum is up and running. Ram chips from the c64 (4164's) are 100% working ! I was fortunate to have some spares ! Almost all the original 4116 chips were bad.
For those who would like to use the ram chips from a c64, this mod is required: cut pin 1, bend pin 8, solder a wire between pin 8 & pin 9.

2 problems left :(1) no sound ! I have to trace using the oscilloscope because d9 & tr7 seem ok (issue 3b).
And (2) the colors are totally off (colors are washed out...). I replaced c25 and c65 but this didn't have any effects. Maybe TR2 is bad... I'll unsolder it and will check it out of circuit.

So this one is near completion ;)
I'll also install an UBEC module which is far more efficient than the 7805 regulator. I already did that in my Timex 2048.

The other board (issue 6a) is still not properly working.
Even though the diag is successful for all components, when starting, vertical bars are displayed. I have some doubts about the Floating bus test, as it never displays ok, and as screen is garbled (what is floating bus ??).

Would be great if you could help me Mark ;)
Thanks

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Fri Jun 29, 2018 12:27 pm

Floating bus refers to the data bus floating to a high level (FF) when there is no device driving it (due to the pull-up resistors). In the Spectrum community, this is normally when the CPU attempts to read from the I/O address of a non-existent I/O device. [The Z80 CPU having a separate address range for I/O devices so that they don't have to be mapped in the normal memory map].

However, because the ULA uses the 'lower' RAM (the 16k bytes provided by the 4116 chips) to get screen data, and the only separation of this part of the data bus and the CPU data bus is eight resistors, the CPU can instead of reading FF, read whatever byte of data that the ULA fetched.

Some games use this quirk to synchronise when they update the screen data.

In a ZX Spectrum 16k or 48k machine with no expansions plugged in, the only I/O device is the ULA. It responds to any I/O address where A0 is low (any even number). So if the CPU reads an I/O address where A0 is high, it will get the floating bus value. Normally I/O address FF is used to get the floating bus value, as this is least likely to activate any external expansions that may be plugged in.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Fri Jun 29, 2018 5:56 pm

I have fixed sound thanks to the scope.
On a working speccy, I could see a waveform on each side of D9. On my non working speccy, I could see a waveform on one side only... I had found the suspect !
After replacement, sound is now working !

On this board, only one problem remaining : picture is sometimes in b&w, sometimes in colors but colors are wrong... I suspect the lm1889.

On the other board, everything is ok excepted that it does not boot normally. I can see black stripes. I suspect the rom, but under diag, checksum seems.ok. so what is it ?

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Fri Jun 29, 2018 6:45 pm

LM1889 chips are normally fairly reliable. Check that it is getting a 11.5V to 12V supply. Loss or low voltage supply results in loss of colour. Also that the PAL frequency crystal oscillator is working.

Mark

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