ZX Spectrum +2 booting problem

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

This method gets increasingly impractical as the current increases in a design. As more power is lost in the resistor, and you also have the problem of the voltage ‘lost’ across the resistor means that the circuit that is being supplied no longer sees the correct supply voltage.
I just read this again and makes sense, but I had high voltage drop also without using the resistor...

In the meantime I found that the Composite Sync is using 1.2v and that is just the current value on TEA2000's pin11. I guess it could be a sign that TEA2000 may be functional? If is true, is curious that it receives 1.2v without any loss , compared to the ram chips. The only chip that was getting warm was the CPU.

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Ok maybe the plan with using an external source was too ambitious with my current tech and experience...

What if I'll do like this:

1. connect the Spectrum as usual from a 48k power supply - will be safe to leave TR9 out just for testing 5v functionality ?
2. improvise a wiring from the RGB port to some SCART jack (don't have a cable for this) - do you think bare wires will work for this or I have to put some resistors on R,G,B?
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3. observe if something is going on screen

I'm still a bit surprised that I had +12v dead at some point but still had some fuzzy image with analog output (when voltage regulator gone berserk). I remember that I still had something like 1. something volts on TEA2000 so in theory analog video should be dead?

LATER EDIT:
I just read on some comments on other sources that the Serial Transceiver Chip (I think is about that MC1488P ?) is common to kill the +12v line. It is safe to remove this chip (or just some pins) just to test if is indeed the short's cause ?

EVEN LATER EDIT:
I got my hands on some TIP31C (to replace ZTX653 / TR9), do you think this could handle better the short and survive the tests?
CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Say no more, it works!

I just put a TIP31C in place of the ZTX653 (TR9) - ok I may overdo it with that radiator improvisation, but it was for testing :D
RAM seems to have those 4.7v - 4.72v but it seems it works like that. TEA2000 has around 10.5v or something

After 15 minutes these are the approximate temps (tested with IR temp with case open, ambient temperature of ~22):
CPU - 45 degrees
voltage regulator - 40
ULA ~38-40
TEA2000 -41
MC1488 - 42 (this one is hot compared with the other MC1489 which was pretty cold) - it may be this the problem that triggered the burn ZTX653 couldn't handle? Maybe if I use a device to test the connection? What device could I use ?
TIP31C - 25
RAM chips were pretty cold

Thank you for all the help and tips!

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CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

I found out from the datasheet that the installed RAM (M3764) actually permits a minimum recommended voltage of 4.5 to 5.5 - and supports spikes of 7v (but for short time) maybe that explains why is still working. Maybe will help others this info if they will encounter the same problem.
CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Sorry to keep reviving this thread, but I have some new info that could help some inexperienced enthusiasts.
I found why it was only 4.7v on the RAM chips... Because I needed to verify first the voltage regulator, in all these tryouts I used some breadboard cables jumpers that seemed to drastically increase the resistance. After I soldered the original wires on the voltage regulator, 5v came back and all seems to function with normal parameters - the red led is not going on though - maybe it was burned or a loose wire (cassette player seems to work).

Can anyone give me some information about this 1488 chip? It seems that is heating as much as the CPU - pretty fast at around 40-45 degrees. The 1489 chip stays cold. It could be shorted? I have a lightgun, can this be used to test its functionality?
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1024MAK
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

CapSmasher wrote: Mon Nov 12, 2018 8:23 am Sorry to keep reviving this thread, but I have some new info that could help some inexperienced enthusiasts.
I found why it was only 4.7v on the RAM chips... Because I needed to verify first the voltage regulator, in all these tryouts I used some breadboard cables jumpers that seemed to drastically increase the resistance. After I soldered the original wires on the voltage regulator, 5v came back and all seems to function with normal parameters - the red led is not going on though - maybe it was burned or a loose wire (cassette player seems to work).
Voltage drop via thin wires is not unusual. The circuit for the power on LED is very simple, just a resistor, the red LED and the wires/connections.
CapSmasher wrote: Mon Nov 12, 2018 8:23 amCan anyone give me some information about this 1488 chip? It seems that is heating as much as the CPU - pretty fast at around 40-45 degrees. The 1489 chip stays cold. It could be shorted? I have a lightgun, can this be used to test its functionality?
Keep in mind that one is a receiver only and the other is a transmitter/level shifter. So of course the temperatures they run at will be very different. For the 1488, can you test the input pins and the output pins with a multimeter on the voltage range. A short circuit on an output will cause the chip to get warmer than normal.

Mark
Last edited by 1024MAK on Mon Nov 12, 2018 2:02 pm, edited 1 time in total.
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1024MAK
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

Links to three datasheets for 1488 chips:- One thing I forgot to say in my earlier post. 1488 chips are based on old bipolar technology. This type runs warm even when doing nothing. If you look at the datasheets, the typical power consumption/ dissipation is around 576mW. That’s over half a watt of power in a normal DIL case.

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