Return of the (slightly less) faulty ZX Spectrum

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Macross_VF1
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Return of the (slightly less) faulty ZX Spectrum

Post by Macross_VF1 » Sun Dec 02, 2018 8:17 pm

A few weeks back I asked for help with a problematic ZX Spectrum (viewtopic.php?f=22&t=1107) and it was pretty much decided that the voltage regulator was faulty and that it might have damaged other components as well.

I've changed the regulator and the unit is now actually responding though not without glitches as can be seen in the picture I've attached. The glitches/artifacts appear after some button presses, though admittedly I can't test it properly as the keyboard ribbons are pretty crap and needs replacing. It sometimes also reset but I haven't been able to get a picture of that.

I've done the same tests as before and here's the results

The edge connector
Pin 3B/+5V: +4.94V
Pin 4B/+9V: +10.76V
Pin 20A/-5V: -4.9V
Pin 22A/+12V: +10.7V

4116 DRam
Pin 1-16: 0.004V
Pin 8-16: +9.95V
Pin 9-16: +4.94V

7805 Voltage regulator
Pin 1: +10.77V
Pin 3: +4.95V

Is it safe to assume that the previously faulty voltage regulator might have damaged the DRam chips?


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1024MAK
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Re: Return of the (slightly less) faulty ZX Spectrum

Post by 1024MAK » Mon Dec 03, 2018 4:40 am

4116 DRAM
Pin 1-16: 0.004V 👎 :( This should be between -4.3V and -5.5V
Pin 8-16: +9.95V 👎 :( This should be between +11.5V and +12.5V
Pin 9-16: +4.94V This is okay 👌 :)
I previously wrote:...the on board DC/DC converter/inverter formed around TR4, TR5 and ‘the coil’ will not work correctly, and it is very likely that TR4 and TR5 have been damaged.
Replace TR4 and TR5.

Use one of these in place of the ZTX650 in position TR4:

ZTX651 Order code 9525572
ZTX653 Order code 9525580

https://uk.farnell.com/search?st=ZTX65

Use a ZTX550 in place of the ZTX213 in position TR5:
Order code 9525548

https://uk.farnell.com/diodes-inc/ztx55 ... 548?st=ZTX

You should also be able to get these transistors from other suppliers.

Mark
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Macross_VF1
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Re: Return of the (slightly less) faulty ZX Spectrum

Post by Macross_VF1 » Fri Dec 07, 2018 8:09 pm

As usual the help I'm getting is highly appreciated and helpful. Anyway here's an update. I might have actually done it! I've replaced the TR4 and TR5 as suggested and I've also changed all the caps.

Now I get the proper voltages on the DRam and I seem to get a stable picture, though since I haven't received the replacement keyboard membrane yet I can't fully test it.

There's just one strange thing left however. Whenever I power on the unit I get a low humming/buzzing noise, and it's not the speaker. I've tried recording it but no real luck on that front. However I'm fairly certain the noise comes from the inductor coil or at least very from a component close to it.

*Edit* I've heard from other sources that this humming is in fact normal for a 48k.
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1024MAK
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Re: Return of the (slightly less) faulty ZX Spectrum

Post by 1024MAK » Fri Dec 07, 2018 10:39 pm

Yes, the coil is actually vibrating at the frequency generated by transistors TR4 and TR5. This is normal for coils in issue 1 and issue 2 machines. Sinclair used better made coils in later machines that make less noise and also revised the circuit design a number of times. But they still often make some noise.

Mark
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