ZX Spectrum +2 booting problem

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CapSmasher
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ZX Spectrum +2 booting problem

Post by CapSmasher »

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 »

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 »

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 »

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 »

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 »

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

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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

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 »

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 »

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 »

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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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Hey Mark, thank you for your replies

I checked the R79 and R78 outside circuit and indeed they are good, thank you for the tip!
R78 is correct at least on my board - and is indeed connected with C27

TR9 can be burned if is reversed (sorry my noobines)? I initially intalled it with the emitter on the 9v... Saw that on a video on youtube that the emitter had to be connected to 9v - I'm confused now... I guess if the diodes and resistors are OK the coil may be damaged? Is there the possibility that the caps to be to blame?
I will try to investigate that R52 because it seems is tied with D19 like in the power schematic so it must be that.

You said that TR9 is mainly feeding the TEA2000 - that means if the TR8 is working and in charge with the +5V, I still should need to have +5V on the chips, no? But I have only 4.7V ... Let's assume that all circuit is OK except the coil and TR9, I still should have sound when pressing the keys throught the headphone jack, if the rest of the board is OK?

Do you think I can replace the coil with one from a 48k ZX Spectrum? Does it have a specific name/specifications, or some commercialy alternatives?

Alternatively (and maybe more extreme), can I bypass all this power circuit and feed the chips directly to see at least that the rest of the components are good?

Thank you
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

The 7805 voltage regulator supplies the +5V Rail. If the voltage on the input to the 7805 voltage regulator is at least 7.5V, then the output should be within +5V +/-5% at worst. So 4.75V to 5.25V. Most 7805 voltage regulators manage better than +/-5% though.

If the regulator and it’s heatsink are hot, and the output voltage is lower than expected, measure the current on the +9V input.
It may be that if the +5V line went above 6V during the time that the old regulator was faulty, it may have damaged some of the other chips. So see if any are hot (apart from the ULA which runs hot anyway). The Z80 may be warm, but should not be hot. The other chips should be cool or slightly warm.

Whatever the actual voltage from the 7805 voltage regulator, there will be a gradual loss across the board. So a little bit of voltage drop on the +5V rail is to be expected.

I’m confused by your reference to D19. The 2.2k ohm resistor for the -5V line should be between diode D37 (1N4148) / capacitor C22 (47uF) and diode D18 / capacitor C24 (10uF) according to the schematic I’m looking at. Or is this another error on the schematic?

Normally, the transistor (TR9 in this case) dies before the coil becomes damaged. But occasionally the coil may overheat. There are no know suppliers of new coils. As I said earlier, it is believed that it is the same as those used on 16K / 48K boards. A very limited number of second hand coils may be available from a repairer. But the normal fix for a damaged coil, is to remove it from the board. Then very carefully make notes and take photos as you unwind the old enamelled wire. You have to record where the wire starts, which direction it is wound in, and the number of turns, and where it finishes. For both coils of wire. Then obtain some new enamelled wire of the exact same diameter. Then wind the new wire on the former following the notes that were made earlier to the letter. Before soldering the enamelled wire, scrape the enamel off with a sharp knife. Then tin it.

For test purposes, you can remove / leave out TR9. Then using a current limited, but regulated +12V power supply, you can feed +12V to the cathode of diode D20. Remember to also connect the 0V line to the Spectrums 0V/GND line.

It is also possible to use an external current limited, but regulated +5V power supply. I recommend that you remove the 7805 voltage regulator and then feed in via the appropriate solder pads. The 7805 voltage regulators don’t like voltage on their outputs if there is no voltage (or a lower voltage) on their inputs.

If you use the RGB video outputs and the display you are using can display the RGB video inputs without needing active function switching inputs (RGB monitor or TV with a SCART socket that can be switched to RGB inputs manually), then the missing +12V is not a problem.

I can’t remember if the keys make a keyboard click when the ZX Spectrum +2 powers up, because it goes to the menu. But certainly once in BASIC the keys should produce the keyboard click. How are you listening to the sound? Only if the video output is not functioning due to the +12V line not being operational, there will be no sound on the UHF TV output, and no sound out on the composite video output (on the RGB/video DIN socket).

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Re: ZX Spectrum +2 booting problem

Post by Ast A. Moore »

1024MAK wrote: Sun Oct 28, 2018 10:39 pm I can’t remember if the keys make a keyboard click when the ZX Spectrum +2 powers up, because it goes to the menu.
Yup, they do.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

1024MAK wrote: Sun Oct 28, 2018 10:39 pm The 7805 voltage regulator supplies the +5V Rail. If the voltage on the input to the 7805 voltage regulator is at least 7.5V, then the output should be within +5V +/-5% at worst. So 4.75V to 5.25V. Most 7805 voltage regulators manage better than +/-5% though.

If the regulator and it’s heatsink are hot, and the output voltage is lower than expected, measure the current on the +9V input.
It may be that if the +5V line went above 6V during the time that the old regulator was faulty, it may have damaged some of the other chips. So see if any are hot (apart from the ULA which runs hot anyway). The Z80 may be warm, but should not be hot. The other chips should be cool or slightly warm.

Whatever the actual voltage from the 7805 voltage regulator, there will be a gradual loss across the board. So a little bit of voltage drop on the +5V rail is to be expected.

I’m confused by your reference to D19. The 2.2k ohm resistor for the -5V line should be between diode D37 (1N4148) / capacitor C22 (47uF) and diode D18 / capacitor C24 (10uF) according to the schematic I’m looking at. Or is this another error on the schematic?

Normally, the transistor (TR9 in this case) dies before the coil becomes damaged. But occasionally the coil may overheat. There are no know suppliers of new coils. As I said earlier, it is believed that it is the same as those used on 16K / 48K boards. A very limited number of second hand coils may be available from a repairer. But the normal fix for a damaged coil, is to remove it from the board. Then very carefully make notes and take photos as you unwind the old enamelled wire. You have to record where the wire starts, which direction it is wound in, and the number of turns, and where it finishes. For both coils of wire. Then obtain some new enamelled wire of the exact same diameter. Then wind the new wire on the former following the notes that were made earlier to the letter. Before soldering the enamelled wire, scrape the enamel off with a sharp knife. Then tin it.

For test purposes, you can remove / leave out TR9. Then using a current limited, but regulated +12V power supply, you can feed +12V to the cathode of diode D20. Remember to also connect the 0V line to the Spectrums 0V/GND line.

It is also possible to use an external current limited, but regulated +5V power supply. I recommend that you remove the 7805 voltage regulator and then feed in via the appropriate solder pads. The 7805 voltage regulators don’t like voltage on their outputs if there is no voltage (or a lower voltage) on their inputs.

If you use the RGB video outputs and the display you are using can display the RGB video inputs without needing active function switching inputs (RGB monitor or TV with a SCART socket that can be switched to RGB inputs manually), then the missing +12V is not a problem.

I can’t remember if the keys make a keyboard click when the ZX Spectrum +2 powers up, because it goes to the menu. But certainly once in BASIC the keys should produce the keyboard click. How are you listening to the sound? Only if the video output is not functioning due to the +12V line not being operational, there will be no sound on the UHF TV output, and no sound out on the composite video output (on the RGB/video DIN socket).

Mark
Hi Mark,

Thank you for the info!

The voltage regulator last time I checked had correct values of 9v input and 5v output. The problem is that even with the 5v output I had only 4.7v on the RAM chips.

My board is issue1 and the schematics are for Issue3, maybe some resistors/capacitors changed a bit. Didn't find any schematic for Issue1..

OK so in theory I should be able to power the ZX spectrum +2 like this - I've re-written the info from you from what I understood so to be sure I will not mess up:

1 - desolder the TR9 (just to prevent frying it because is my last piece)

2 - connect +12v to the negative (cathode) pole of the diode D20 and the GND from this power source to the Spectrums' GND.

Here I will need just a bit of clarification... Do I need to desolder the D20 cathode then wiring it with 12v, or I'll wire it directly on the solder pad of that cathode? The GND from the +12v power supply will connect directly to the ground of the power supply jack solder pad? Will I need this +12v if I will use only the analog TV cable?

3 - remove the 7805 voltage regulator

4 - connect +5v and GND (from external power supply) to the +5v and GND pin where the 7805 was attached

5 - check ram voltages and see what chips get warm

6 - try to search for some signal on TV

7 - if no image on screen, I will connect some headphones and check if there is some sound while pressing keys (but only if I will have the +12v line functional?)

Thank you
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

In the meantime I tested the coil resistance:

Primary winding (connected to the transistor collector) 0.05 ohms.
Secondary winding (connected to the transistor base) 0.05 ohms.
Primary winding to secondary winding: greater than 20M ohms (it kept growing, 20M is maximum on my multimeter)

Do you think are OK these values?
I also checked the resistors, seems all OK - didn't find the R52 but don't think is that important since is on -5V

Thank you

LATER EDIT:
So while I was looking at the wonderful speccy board that I have (since couldn't power it on) I just found this capacitor (C41) which paradoxical it has some connection with +12.. It looks a bit bulky and like there is some green leaking on its wire? Could this been causing the TR9 (ZTX653) sudden death?


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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

Electrolytic capacitors can cause all sorts of problems. If you are good at soldering and desoldering, you should indeed replace any that look like they are leaking, or may leak.

The resistance tests on the coil look good. Resistance tests alone can’t prove that it is good, but it is unlikely to be faulty.

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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

1024MAK wrote: Wed Oct 31, 2018 3:55 pm Electrolytic capacitors can cause all sorts of problems. If you are good at soldering and desoldering, you should indeed replace any that look like they are leaking, or may leak.

Mark
Hey Mark,

Thank you for the reply, can you please tell me if the steps from this previous post:
viewtopic.php?f=22&t=1062&start=10#p15314

..are correct? I may try first to power the Spectrum from an external source, just to check the status of the chips

Thank you!
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

CapSmasher wrote: Mon Oct 29, 2018 1:22 pm 1 - desolder the TR9 (just to prevent frying it because is my last piece)

2 - connect +12v to the negative (cathode) pole of the diode D20 and the GND from this power source to the Spectrums' GND.

Here I will need just a bit of clarification... Do I need to desolder the D20 cathode then wiring it with 12v, or I'll wire it directly on the solder pad of that cathode? The GND from the +12v power supply will connect directly to the ground of the power supply jack solder pad? Will I need this +12v if I will use only the analog TV cable?

3 - remove the 7805 voltage regulator

4 - connect +5v and GND (from external power supply) to the +5v and GND pin where the 7805 was attached

5 - check ram voltages and see what chips get warm

6 - try to search for some signal on TV

7 - if no image on screen, I will connect some headphones and check if there is some sound while pressing keys (but only if I will have the +12v line functional?)
Yes, but note the following:-
Solder the +12V to the pad of the cathode of D20, or any other convenient point on that track. So you could also connect to the positive (+) pad of capacitor C41 (100uF).

If the +12V AND the +5V are being supplied by the same PSU and already share a common 0V/GND, you don’t need two different 0V/GND connections. If however the +12V and +5V do not share a common 0V/GND, OR you are using different PSUs, then two 0V/GND leads (wires) are needed.

You must use PSUs where they limit the current in the event of a fault. Otherwise you risk damaging the PCB tracks if there is a short circuit somewhere on the board. If possible, I recommend no more than 1.2A on the +5V line. And no more than 100mA on the +12V line.

If different PSUs are used, power up the +5V supply first, then the +12V supply.

The audio output from the ZX Spectrum +2 will need to be amplified if you are going to use headphones. This does not use the +12V rail. Only the composite video (on the RGB/monitor DIN socket) and the TV UHF outputs need the +12V for the TV audio to work (it is combined with the video signal hence the need for the +12V to be working).

Good luck.

Mark
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

Oh, by the way, to reduce stress on the PCB, when removing a component where you are not intending on reusing it (such as an electrolytic capacitor), it’s better to note down the holes it uses and the polarity (or take photos), then cut it’s legs, then desolder the legs, then fit the new one.

Mark
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

1024MAK wrote: Wed Oct 31, 2018 4:21 pm Oh, by the way, to reduce stress on the PCB, when removing a component where you are not intending on reusing it (such as an electrolytic capacitor), it’s better to note down the holes it uses and the polarity (or take photos), then cut it’s legs, then desolder the legs, then fit the new one.

Mark
Hello,

I managed to desolder and check the C41 capacitor and it was in good parameters despite its looks (I will change it anyway just to be sure). Unfortunately I don't have a power supply that I can limit to a certain current value, I will receive one in a month or so.

In the meantime...

...is there any other way to detect where is this short happening? Do I really need to desolder any component and check? I read something that the TR9 instant burning can be triggered by a short in at least one lower RAM on the 48k Spectrums. In my case the only chip that uses +12 would be TEA2000? Could I test if this is the fault if I will desolder the PIN11 from the board and if the TR9 transistor will not burn that means this chip was the fault?

Thank you
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

You could do that. Keep in mind however that unlike the 4116 DRAM chips fitted in the 16k and 48k machines which are known for sometimes failing. The TEA2000 chips are normally fairly reliable. That does not mean that it has not failed. The +12V and -12V supply also goes to IC3 (1488) which is used for level shifting to RS232 voltages (for the serial and aux/keypad ports). +12V also goes to various resistors and to TR4.

If you don’t have a current limited supply for the +12V, you could use any regulated PSU, but wire a 100 ohm resistor between the +12V output of the PSU and the +12V rail on the ZX Spectrum +2. Then you can measure the voltage across this 100 ohm resistor. The resistor will also limit the current if there is a short circuit (it will get hot in this case).

Voltage (U) in volts (V) divided by resistance (R) in ohms gives the current (I) in Amps (A)

So if you get 2.5V...

Code: Select all

U       2.5V
— = I = ———— = 0.025A = 25mA
R       100
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.

There are other ways of working out where short circuits are, but in practice, on printed circuit boards, most are impractical. In order to measure current using a multimeter, the meter needs to be connected in series, so the current flows through it. This means breaking one of the conductors of the circuit you want to test.

The only other ways (without disturbing the PCB air the components) are to supply the circuit with power and either use a very sensitive voltmeter to compare the chip pin voltages with a known good unit. Or use an expensive IR camera to detect components that are warmer than they should be.

Mark
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Thank you very much for your answer...

Sorry for asking all these questions that the answer can be pretty obvious for an experimented person. Is clearly making some light in the understanding of how these circuits work and hope that will also help others.

So basically I could first try checking if TR4 (2N3904) is not the short's cause, then the TEA2000? If I exclude these there will be only a matter of a failed resistor/capacitor in the way - or that IC3 (MC1488P)?

Thank you
CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Following the post above,
TR4 seems to work... Maybe I should try pumping 12v + 120 Ohm (0.1A) resistor to the D20 diode and check some voltages? Do you have some advice what voltages should I check first to spot this cursed short?
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1024MAK
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Re: ZX Spectrum +2 booting problem

Post by 1024MAK »

CapSmasher wrote: Tue Nov 06, 2018 9:00 am TR4 seems to work... Maybe I should try pumping 12v + 120 Ohm (0.1A) resistor to the D20 diode and check some voltages? Do you have some advice what voltages should I check first to spot this cursed short?
It’s really a case of trying it, and then seeing what the results are.

This test will may not tell you what is causing the short, but each step narrows down the possibilities.

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CapSmasher
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Re: ZX Spectrum +2 booting problem

Post by CapSmasher »

Ok, so I tried with some primitive PSUs only for the 5v line. I connected the positive terminal from the power source to the positive pin of the voltage regulator output, and the negative to the negative pin nearby.

I observed that I have some massive voltage drop - somewhere like 4.1v are reaching the RAM chips - it would be wise to find a power source of let's say 6v and test to see if ~5v would reach the chips? Or there is something on the board that cause this voltage drop. The power source alone had at least 5.1v.. Another observation was (with another -new- multimeter) that when I first touched the RAM power pin it showed 5. something volts, but then instantly dropped at 4.1v

I also put a resistor of 10 Ohm and 100 Ohm, and indeed they are getting hot pretty fast (the 10 Ohm one got it to the point that it smell, but I saved it in the last moment). In the end I saw that the power source was not going more then 1.2A and I removed the resistor . Interesting is that the TEA2000 have the same voltage as it was powered before (of 1.2v something) that means the +12v line was dead and this 1.2 voltage is in fact an 'echo' from the 5v line !? I think that using the resistors also drastically increase the voltage drop to something like 2.5v

It is strange that this voltage drop is higher than expected, with the voltage reglator when I had 5v output, on RAM chips I had somewhere like 4.7v...
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