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?
Return of the (slightly less) faulty ZX Spectrum
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- Drutt
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- 1024MAK
- Bugaboo
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Re: Return of the (slightly less) faulty ZX Spectrum
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
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
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
Replace TR4 and TR5.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.
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
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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- Drutt
- Posts: 6
- Joined: Sat Nov 17, 2018 10:41 am
Re: Return of the (slightly less) faulty ZX Spectrum
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.
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.
- 1024MAK
- Bugaboo
- Posts: 3104
- Joined: Wed Nov 15, 2017 2:52 pm
- Location: Sunny Somerset in the U.K. in Europe
Re: Return of the (slightly less) faulty ZX Spectrum
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
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.