ZX spectrum 48k repair

tyrell
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ZX spectrum 48k repair

Post by tyrell » Mon Jun 25, 2018 4:12 pm

Hi all !

I'm trying to repair a defective ZX spectrum 48k board.
TR4 was defective (I first knew it when testing the transformer which was in 'short circuit' while it shouldn't). I replaced it as well as some capacitors whose value had drifted too much.

I removed all the lower bank ram chips and plugged the power supply.

Voltages were ok : I got the expected +5v, -5v and +12v.

Ok, next step was to plug the ram chips again. That's where it went bad ;) I couldn't hear the transformer buzzing anymore, and after 30 seconds, I could see some smoke. I quickly unplugged the adapter...

So my question is,how to diagnose the failing ram chip(s) ? I've heard that I could use my metter to check resistance between pin 2 and 16 or between pin 14 and 16 ? What should be the resistance there in a normal ram chip ?
Any other way to put the finger on the failing ram chips ?

Thanks !

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Tue Jun 26, 2018 8:59 pm

The trouble with 4116 DRAM, is it needs three power supply lines plus ground. Most of the chip runs off the +12V supply. The remainder (the input/output buffers) works from the +5V, and the chip needs a -5V bias supply or it may self destruct.

With a 4116 chip out of circuit, you can try this:-

4116 DRAM out of circuit tests

The following are performed using a digital multimeter switched to the 20k ohms range (or equivalent range).

Test between pin 1 (VBB, -5V) and pin 16 (VSS, GND). You should get more than 11k ohms.

Test also between pin 8 (VDD, +12V). and pin 16. You should get more than 20k ohms.

Any that test below these values are probably defective.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Tue Jun 26, 2018 10:53 pm

Thanks. I'll try and i'll keep you updated.

Also I'm looking at replacing failed 4116 with 4164 coming from a c64 ;)
Don't use -5/12v, consume less power... Only a small modification is required. Not sure it will work, but it should.

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Wed Jun 27, 2018 12:17 am

Yes, other DRAM chips with a similar pin-out will work. Just make sure that you don't kill it with the -5V and the +12V supplies...

Another option, is to buy and use a 'lower RAM' replacement module. This is a small circuit board with modern SRAM and some interface circuitry, it plugs in (or solders in), in place of all eight 4116 DRAM chips.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Wed Jun 27, 2018 7:31 am

Ok, I replaced the ram chips with a mix of new ones and modified 4164.
Transformer is 'buzzing', all voltages are ok, I get a picture with vertical multicolored lines, which are kinda "moving".
At least, some progress ;)
When I plug the smartCard V2, I get a "whoah, the spectrum seems to be bad" message. Any idea ?

On the other 48k board (issue 6), after replacement of TR4 and 2 x 22Mfd caps, I get a picture, but something is wrong. I get a mix of black and white vertical stripes + blinking 8x8 areas.
When I plug the smartCard V2, I can enter the menu when pressing the NMI button.
I even launched games, and they are 100% working (head over heels, lancelot).

Could it be the ROM ?

But if I take a look here: http://hardware.speccy.org/reparar-i.html

It exhibits exactly the same symptoms as the ones explained in the column "Spectrum 48K issue 6" (the first one)

Explanation is: "RAM test showed problems in low memory. I tested some chips but there were ok. I tested PCF1306P, ROM y Z-80 chips but there also were ok.Finally, I found that the scope showed some ac signal in the +12v. C44 capacitor was not working well.
After C44 changed, Spectrum started well. I decided to change some electrolytic capacitors. Now the Spectrum has very good image and low noise."

So, maybe some caps ?? but given that the games are working, i'd be tempted to point the rom, no?

Thanks for your help :)

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kokkiklhs
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Re: ZX spectrum 48k repair

Post by kokkiklhs » Wed Jun 27, 2018 1:52 pm

Have you swapped the ULA with a known good, working one?

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Wed Jun 27, 2018 2:23 pm

I swapped the ULA with the other board (issue 3b, where I can play games, so obviously ULA is ok), but still same situation.
(Ok, I know I should pick the ULA from a *fully* working board ...)

What is strange is the message from the smart card ("whoah, the spectrum seems to be bad"), which is displayed whatever the ULA, so I think we can exclude this one. I'm trying to contact Phil Ruston, to know when exactly this message is displayed.

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Wed Jun 27, 2018 9:45 pm

If the games run (and are not using the ROM at all), then yes, it could be the ROM, or the configuration links on the PCB for the ROM.

If the ROM is in a socket, try swapping it, or just power the machine up with no ROM. With no ROM, the data lines should float high, the CPU will then execute a RST command, with will push the return address onto the stack, the stack will eventually pass through screen memory before wrapping back around. This normally causes a distinctive pattern on the display.

If the system can display an error message in normal text, then the ULA, CPU and lower RAM must be working.

Mark

tyrell
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Re: ZX spectrum 48k repair

Post by tyrell » Wed Jun 27, 2018 10:28 pm

Unfortunately the rom is soldered.i have some socket I could use, as soon as I have some free time I'll try swapping. I have some other roms I can use, or I'll flash one but I need some eprom flasher (+ modifications to use it).

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1024MAK
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Re: ZX spectrum 48k repair

Post by 1024MAK » Thu Jun 28, 2018 12:28 am

If you have a spare edge-connector, you can disable the on board ROM by taking /ROMCS high (wire to the +5V supply).
Make sure you get the correct edge-connector contact. See this wiki entry.

Mark

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