ZX Spectrum +2 booting problem

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CapSmasher
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ZX Spectrum +2 booting problem

Post by CapSmasher » Mon Oct 22, 2018 9:11 am

Hello people,

I've recently aquired a ZX Spectrum +2 issue 1 but when I powered it on I have this (fuzzy) image:
Can anyone tell me what could possibly be wrong? Is the CPU, RAM, etc?


Thank you

Image
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1024MAK
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Mon Oct 22, 2018 1:43 pm

Simple answer: not without more information.

You will get a better picture if you connect via the RGB video socket.

Most failures of ZX Spectrum computers are caused by either one (or both of) the transistors that form a DC/DC converter/inverter internal power supply having failed, one (or more) faulty DRAM chips, a faulty Z80 CPU or a faulty ULA.

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Re: ZX Spectrum +2 booting problem

Post by CapSmasher » Sat Oct 27, 2018 9:08 pm

Hey thank you for the reply!

Seemed that the voltage regulator went crazy... I had more than 5v in the output. I tried with another one, voltages are correct now, but... :D

I also observed that I have only 4.72V on the ram chips from the right side of PCB and 4.69v in the far left side. The TEA2000 has 1.37v on the pin 11
No image on screen anymore - only white noise...

If I measure the resistance of the circuit probing the power socket connections, I have around 60k Ohms ... is it normal? It seems that the voltage regulator have the same resistance.

I detached TR8 (ZTX213) and TR9(ZTX650) and the TR9 was bad, TR8 gave good readings. While soldered on board, TR9 had .712 and 1.131 while TR8 .755 and .758.
I changed TR8 (with a BC577A with emitter to 5V) and TR9 (with ZTX653 with emitter to 9v). Now I'm not sure if I put it properly because the previous TR9 had the collector to 9V. Anyway, I think is some short in the way, because TR9 burned in a couple of seconds after powering on... I don't think it has to do with it being (or not) reversed. I think there is something wrong between TR8 way to TR9... Can you please give me some advice what should I check to see where is the problem - I'm not that advanced in this kind of troubleshooting?

Another thing is that the inductor coil has some resistance between its + and -, but after some readings I learned that this should not exist (is around 700 Ohms)?

Thank you

*forgot to mention that I'm using a 48k ZX Spectrum power source - that has around 13v with no load and 1.4 amps - but I'm testing the board without the cassette player so I think it should be enough?
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Sun Oct 28, 2018 12:27 pm

A schematic of the ZX Spectrum +2 (grey) power supply section is here

TR9 HAS to fitted the correct way round. Collector to +9V via the primary winding of the coil. Emitter to 0V/GND. Base to the secondary winding of the coil. The coil being a high frequency transformer.

I don’t know the resistances of the coil off the top of my head. But the resistance of the primary (+9V to TR9 collector) should not be as high as 700 ohms. But if it was this high, then the transistor fitted in position TR9 would not be able to overheat and die.

TR8, TR9 and the “coil” form the heart of the DC/DC converter/inverter. The transistor fitted in position TR9 can overheat and die if the coil has been damaged by overheating (the insulated coating on the copper wire melts, resulting in internal short circuits) or if there is a short circuit on one of the outputs of the DC/DC converter/inverter circuit, such as the +12V rail or the -5V rail. TR9 can also die if there is a problem in it’s base circuit that keeps it switched on.

In the ZX Spectrum +2, the -5V and the 12V AC rails are not used internally. The -12V is only used by the serial port and aux port buffer chip IC3 (1488). So if there is a short, it is most likely to be on the +12V rail. This also feeds IC3, and it feeds the +12V ANALOG supply, which in turn supplies the colour encoder chip IC2 (TEA2000) along with the +12V to the RGB/video DIN socket.

For short duration testing only, yes, a Sinclair 9V 1.4A PSU is okay.

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Re: ZX Spectrum +2 booting problem

Post by CapSmasher » Sun Oct 28, 2018 3:16 pm

Thank you for your reply, Mark!

Is there any way to test the coil? Visually it look fine ... I was wondering, is possible that a resistor to allow this failure of the transistor? I checked on circuit the R79 transistor and I have no reading on it - I also checked R54, R55, R30,R78 and these have some values but a much less than they hould have. Strangely enough seems that resistors that were closer to the coil have the highest error and while goes far, resistors are ok (is an issue1 board and most of resistors are close to the coil).

There is a R62 transistor with 2k2 in the schematic (on -5v) but I couldn't spot it on this board I think the schematic is wrong, because I see the same R62 resistor in the main circuit schematic connected to TEA2000 on X1/X2 and it has 1k5 value. I think the correct resistor number should be R52 -but still couldn't find it on my board :)

I will try to test them outside circuit but at least I want to know if that can be the cause of the power failure, so I will know that I'm not butchering the board for nothing :D

Thank you!
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher » Sun Oct 28, 2018 4:05 pm

Think that I found R52, it has continuity with D19 and I think is the one.. The thing is that I'm reading seomething like 10k value on it (on circuit), and the colors doesn't seem to match the color charts - it has a gold band but it should be brown so to be 2.2k ? And the Tolerance band is black, but this color is not having any value. I also attached some photos with the coil and the R52 resistor

Image

Image

Image
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Sun Oct 28, 2018 4:30 pm

Hello

I’ve not had the chance to go through everything that you have said. But wanted to tell you immediately that in-circuit testing on resistance and capacitance can give very misleading results. This is because the test current from the multimeter will flow through other components, this then affects the displayed figure.

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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Sun Oct 28, 2018 5:33 pm

On a 48k ZX Spectrum (I don’t have a ZX Spectrum +2 open at the moment, as far as I know, the same type/design of coil is used), the coil in that tests as follows:-
Primary winding (connected to the transistor collector) 0.21 ohms.
Secondary winding (connected to the transistor base) 0.05 ohms.
Primary winding to secondary winding: greater than 3.7M ohms.

Please note that the test lead resistance and how accurate your multimeter is will affect the results.

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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Sun Oct 28, 2018 6:40 pm

So going through your post in a bit more detail...
R79 - the in-circuit resistance test results will be affected by the coil being in parallel with it. So it is not possible to test it in-circuit. It is not likely to fail without damage being visible though.
R54, R55 - both these will be affected by whatever is connected to the +12V Rail.
R30 - will be affected by R76, as it is effectively in parallel with it.
R78 - can’t see that in the power supply section. Do you mean R76? If yes, see R30 above.
R62 - yes, there is an error in this schematic. The numbering must be wrong, as there must be a resistor in this part of the circuit.

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Re: ZX Spectrum +2 booting problem

Post by 1024MAK » Sun Oct 28, 2018 6:52 pm

CapSmasher wrote:
Sun Oct 28, 2018 4:05 pm
Think that I found R52, it has continuity with D19 and I think is the one.. The thing is that I'm reading seomething like 10k value on it (on circuit), and the colors doesn't seem to match the color charts - it has a gold band but it should be brown so to be 2.2k ? And the Tolerance band is black, but this color is not having any value. I also attached some photos with the coil and the R52 resistor

Image

Image

Image
I’m not sure what is going on here. That resistor that you think is R52 looks like a 22 ohm resistor going by the colour code!

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