Bad 12v line ISSUE 6A with only 5v RAM

bola_dor
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Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

Hi, I ve got this ISSUE 6A TIMEX/PORTUGAL from a CZ2000 (Argentinian rebranded)
20210501_130309.jpg
This one has MCM 4517 low ram IC that only needs +5v being pin compatible with 4116 (this is not common here but not so rare either)
Thing is that 12v line is down to about 1v and -5 to about -2v or less (would be more ... nearer to cero) .. but this RAM doesn't need that.
Thing is the LM1889 is not getting feed..
IMG-20210501-WA0007.jpeg
You can see (c) The sinclair research text there so I know ULA, CPU, ROM and at least Lowwr RAM are working.
14MHZ cristal, ULA pin 39 and 32 and CPU pin6 are OK.
ULA PIN 17gives a nice positive nice syncro signal at 15.6Khz.
BUT LM1889 has pins 14,15,16 near 1v and no signal at al at 17,18..
PAL Crystal has no signal as spected.

I've already replaced tr4 and tr5 using some transistors that worked there on other boards like this. But I still have no luck.. I checked pins of Tr4 and neither has 12v, and checked both pins of C74 and has 5v and a bit less the other. I think I will replace C74 and C46..
I the other hand I had to replace Tr7 and the speaker. It was getting nearly 9v from the tr7
Thing is. I am learning with this process (and this forum). So I may have missed something.. 🙃.. and where do you recommend to check next..
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
bola_dor
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Joined: Wed Oct 02, 2019 5:32 am

Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

Well, I did recap the DC DC circuit,
Found two of them being less than optimal but nothing changed.
This is How I replaced Tr4 and Tr5. This combination is working in another similar board
2021-5-2 20-56-51.jpg
I measured voltages and used my cheap pocket oscilloscope. And they are not pulsing at all...

Tr4 measured from back to front 0v, 5v and 9v (just 0.05v less than the input vcc)
Tr5 measured 4.33, 4.2, 4.5v.

This is my other working board:
20210502_193532.jpg
Looks nice..

The faulty one :
20210502_195159.jpg
At this time I think it may be the coil not charging, but I don't know how to check it.

or that the transistors I used are not working either...
Or any other option you may suggest..

If I give up and decides to inyect 12v directly to the LM1889 (remember this RAM doesn't needs 12v) where would be the best and simplest place to do it?
Thanks!
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by 1024MAK »

bola_dor wrote: Mon May 03, 2021 2:12 am Tr4 measured from back to front 0v, 5v and 9v (just 0.05v less than the input vcc)
Tr5 measured 4.33, 4.2, 4.5v.
These voltages do not make sense.

TR5 emitter is connected to the +5V supply, so should be the same voltage as the +5V regulator output (+4.75V to +5.25V).
If the +12V rail is very low, TR5 base should be at or below 4.3V, but no lower than 4.2V (assuming the +5V is actually at 5V). At the same time, TR5 collector should be about 0.1V to 0.4V less than it’s emitter voltage, as it will be turned hard on.

TR4 emitter is directly connected to 0V/GND.
TR4 base, if it’s not oscillating, should be at 0.6V to 0.7V, but it can’t stay turned on as it will self destruct through overheating.
If TR4 has gone open circuit, or is fully off (less than 0.5V on it’s base), it’s collector will be at the same voltage as the +9V (nominal) supply rail.

From your voltages on TR4, it looks like it’s dead.

Brief circuit description:

TR5 is the error amplifier. It compares the voltage on the +12V rail to a reference voltage (in this case, it’s using the voltage on the +5V rail).

Resistors R58 and R59 act as a potential divider (voltage divider), reducing the voltage from the +12V rail to about +4.3V. This is applied to the base terminal of TR5. If the voltage here goes higher (because the +12V has gone higher), TR5 will start to turn off, reducing the drive to the base terminal of TR4. If the voltage at TR5 base goes lower, TR5 will turn on harder, and increase the current to TR4 base.

When TR4 starts to turns on, current flows from the +9V (nominal) supply rail through the coil. As it does so, it builds up a magnetic field. This causes a current to flow in the other winding of the coil, which turns on TR4 even more. And the current via TR4 collector to emitter increases increasing the current through the coil.

However, there is a limit to how much energy can be stored in the magnetic field of the coil. Once the core of the coil is saturated, the induced current to the base of TR4 stops, and TR4 turns off. This causes the voltage at TR4 collector terminal to rise rapidly, as the coil now becomes a generator.

The resulting current charges up capacitor C80, and in turn flows via diode D15 to feed the +12V rail.

As the -5V rail is not required, C46 can be removed for now.

Things to check/test.

Visual inspection of the coil, does it look like the windings have overheated?
R58, R59, R60, R61
C80, C44.
D15, D17
Polarity of C74
Visual inspection of C43 and C49

For testing, lift one end of R62, that isolates the video circuitry (in case of short circuit),

Use your multimeter to test for shorts on both sides of R62 to 0V/GND.

TR4 cannot be tested in circuit, because the coil is a low resistance to DC. So to test it, it has to be removed from the circuit.

With TR4 removed from the circuit, TR5 can be tested in circuit. You can also test between the position for TR4 base and TR4 collector to check that there is no short circuit between the two windings of the coil.

Only fit a new transistor in TR4 position after doing the above checks/tests.

If you want to feed in +12V from a separate test supply (I recommend a PSU that has a sensible current limit in case of a short circuit) connect the 0V/GND to the ZX Spectrums 0V/GND. Connect the +12V to the cathode of D15. R62 does need to be reinstated.

Issue 6 TR4, TR5
Issue 6 TR4, TR5

Reference information:
Issue 6 DC-DC converter schematic
Issue 6 DC-DC converter schematic
Data sheets
ZTX213
ZTX313
ZTX450
ZTX650 and ZTX651

BC549
BC557

Image
NOTE all lead identification, as viewed from below the transistor.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Tiger
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by Tiger »

If you really think it's only a 12V-fault (and not the 1889 itself) I would swap the 7805 with an OKI-78SR-5. In this case you can take a 12V power supply instead of 9V and grab the 12V directly from the "power in" to pin 14 (best use a diode to prevent a shorting if the LM is broken) of the LM1889.

By the way - in the first picture there is a 10µF/25V cap for composite vid build in the modulator - I think it's better to replace it with an 100µF type...
bola_dor
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

1024MAK wrote: Mon May 03, 2021 12:51 pm
bola_dor wrote: Mon May 03, 2021 2:12 am Tr4 measured from back to front 0v, 5v and 9v (just 0.05v less than the input vcc)
Tr5 measured 4.33, 4.2, 4.5v.
These voltages do not make sense.
I see :?

TR5 emitter is connected to the +5V supply, so should be the same voltage as the +5V regulator output (+4.75V to +5.25V).
Checked
If the +12V rail is very low, TR5 base should be at or below 4.3V, but no lower than 4.2V (assuming the +5V is actually at 5V). At the same time, TR5 collector should be about 0.1V to 0.4V less than it’s emitter voltage, as it will be turned hard on.

TR4 emitter is directly connected to 0V/GND.
Checked

TR4 base, if it’s not oscillating, should be at 0.6V to 0.7V, but it can’t stay turned on as it will self destruct through overheating.
If TR4 has gone open circuit, or is fully off (less than 0.5V on it’s base), it’s collector will be at the same voltage as the +9V (nominal) supply rail.

From your voltages on TR4, it looks like it’s dead.
Will change it again..

Brief circuit description:

TR5 is the error amplifier. It compares the voltage on the +12V rail to a reference voltage (in this case, it’s using the voltage on the +5V rail).

Resistors R58 and R59 act as a potential divider (voltage divider), reducing the voltage from the +12V rail to about +4.3V.
Resistors are OK
This is applied to the base terminal of TR5. If the voltage here goes higher (because the +12V has gone higher),
It's lower than 2v
TR5 will start to turn off, reducing the drive to the base terminal of TR4. If the voltage at TR5 base goes lower, TR5 will turn on harder, and increase the current to TR4 base.
I think that is shown by the oscilloscope in picture of my working board

When TR4 starts to turns on, current flows from the +9V (nominal) supply rail through the coil. As it does so, it builds up a magnetic field. This causes a current to flow in the other winding of the coil, which turns on TR4 even more. And the current via TR4 collector to emitter increases increasing the current through the coil.

However, there is a limit to how much energy can be stored in the magnetic field of the coil. Once the core of the coil is saturated, the induced current to the base of TR4 stops, and TR4 turns off. This causes the voltage at TR4 collector terminal to rise rapidly, as the coil now becomes a generator.

The resulting current charges up capacitor C80, and in turn flows via diode D15 to feed the +12V rail.

As the -5V rail is not required, C46 can be removed for now.
Done
Things to check/test.

Visual inspection of the coil, does it look like the windings have overheated?
Looks good
R58, R59, R60, R61
C80, C44.
D15, D17
Polarity of C74
It's not marked in the diagram.. it is like in the serigraph, anode leads to r59 and cathode to +5v input
Visual inspection of C43 and C49
For testing, lift one end of R62, that isolates the video circuitry (in case of short circuit),

Use your multimeter to test for shorts on both sides of R62 to 0V/GND.
TR4 cannot be tested in circuit, because the coil is a low resistance to DC. So to test it, it has to be removed from the circuit.
Removed and is open in any direction with the diode tester :twisted: thing is this was new and the failure was the same before .. previous was not original either..
With TR4 removed from the circuit, TR5 can be tested in circuit.
B-E is open 0.6/0.7v both ways.. 😡
You can also test between the position for TR4 base and TR4 collector to check that there is no short circuit between the two windings of the coil.
✔ open circuit..

Only fit a new transistor in TR4 position after doing the above checks/tests.
I Guess I have to replace Tr4 and Tr5 again.. and perhaps think about other replacements.. the original ones are impossible to get here and most of the replacements offered on internet are difficult to get too.. what should be the key specifications of these to work? I suppose Tr4 replacement I am using may be short in some specs..

Mark
I have no words to thank you 🙏 enough for this teaching..
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
bola_dor
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

Tiger wrote: Mon May 03, 2021 1:13 pm If you really think it's only a 12V-fault (and not the 1889 itself) I would swap the 7805 with an OKI-78SR-5. In this case you can take a 12V power supply instead of 9V and grab the 12V directly from the "power in" to pin 14 (best use a diode to prevent a shorting if the LM is broken) of the LM1889.

By the way - in the first picture there is a 10µF/25V cap for composite vid build in the modulator - I think it's better to replace it with an 100µF type...
Yes you are right about the cap.. I have changed it already...
And will take note about the regulator
Thanks 😊
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
bola_dor
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

Well I've made some progress...
I replaced again both tr4 and tr5 and got 12v again..
With R62 lifted in the right lead I feed it wit 12v using a LM317 to limit current at about 40mA and a 12v zener to keep voltage..
It worked good taking about 25mA and with good signal at the PAL Crystal..
So I resoldered r62 in place and test the final results...
....
20210505_231401.jpg
Ok.. took a deep breath.. tried a couple of things.. visually checked the board.. and went to sleep.. sad.


...
At least video looks good😬🤕

Image is not static.. kind of a flashing pattern.. but freezes when reset and start flashing again.. good sign..
Z80 CLK and refresh ar OK reseting works OK too, so my 1st suspect was the ROM.
I checked for continuity all addresses and Data lines and RD line between Z80 and ROM, all OK . Vcc and GND of course checked.. and no shorts between neighbors pins either..
Checked with the oscilloscope and all lines seems to pulse..
Z80 control lines are pulsing to but pin20. So I started to follow it and from ROM to whatever and found ULA p33 with a noisy High signal while A0 keeps pulsing..
20210506_202151(1).jpg
R27 is Ok and continuity from Tr6 to 5v, ULA 33 and A0 is good.. So, I don't know how ULA pin 33 should look but I guess Tr6 should be doing something..
All IC are soldered to the board but the ULA. I don't really know How the IORQL signal works but ULA and Z80 seems to be otherwise OK...
By this time I don't know if should I replace Tr6 or or light five candles in a circle of salt with five black cat hairs in the middle of the night...
Any ideas before turning to the dark arts?
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by 1024MAK »

Z80 pin 20 is /IORQ and is normally high unless an input/output (I/O) device is being accessed. In an unexpanded ZX Spectrum, there is only one I/O device - the ULA (the register/outputs for border colour/tape mic/speaker and the input port for tape ear and keyboard).

If the Z80 is not running the code from the ROM as intended, it’s perfectly possible that pin 20 will be high most or all of the time.

The job of TR6 is to overcome a bug in the ULA that has always been present since the issue one board (this is the spider transistor, just given a proper position on the board). It prevents the ULA being enabled for I/O unless A0 is low, by pulling the ULA pin 33 high when A0 is high.

Obviously if /IORQ is high, ULA pin 33 will also be high, TR6 will have no effect regardless of the state of A0.

The screenshot you showed is typical of a RAM fault.

Does the border always change to white after a power up? If yes, then it’s likely the ROM and the Z80 are fine. But neither can be fully eliminated at this stage. I don’t suspect the ULA.

The fastest and easiest way to diagnose the problem now, is to use a ROM based diagnostic program.

Do you have any interfaces/expansions that take ROMs or act like ROMs and which can override the BASIC ROM?

I presume that the BASIC ROM is soldered in.

What do you get on screen if you connect pin 25 on the underside of the edge-connector (/ROMCS) to +5V then power on. This connection needs to be continuous during testing. It will disable the BASIC ROM. The Z80 should then see 0xFF whenever it reads an instruction, this is a RST &38 instruction. It will push the current PC to the stack, then jump to memory location 0x0038. Then the cycle repeats. Because it keeps pushing the same value of PC to the stack, as the stack pointer changes each time, this causes all RAM to be filled with the same pattern. Which should show up on screen when it fills the screen memory area.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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bola_dor
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by bola_dor »

Border blinks from Grey (white?) to green continuously.. it just pauses at reset..
I tried an Interfase 1 and nothing changed..I guess that rules out the ROM.. will do the pin 25 test..
I don't have a rom based memory tester..
I have a DIVMMC can that be used somehow ?
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Re: Bad 12v line ISSUE 6A with only 5v RAM

Post by 1024MAK »

The ZX Spectrum does not do grey, that will be non-bright white (the border can’t do any of the bright colours).

Strange that it’s blinking to green.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: 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.
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