ZX Spectrum issue 6a - yellow border problem

zxuser · May 2015

Hello!

I have problem with my Sinclair ZX Spectrum issue 6A, 48K.

I have changed low memory (all 4116 IC's) and coil, all electrolytics capacitors, TR4 and TR5, ULA - checked, Z80 - checked, ROM - checked, PCF1306P - checked, 4532 - checked, LM1889 - checked, SN74LS04N - checked.

Picture on TV is mainly yellow border with flashing squares (something like this https://www.youtube.com/watch?v=e5_Ytt5QtCw). Sometimes, border is black.

Can you help me with some advice where should I look for the problem?

Thanks in advance!

HZJ · May 2015

Hi,
it looks like a lower RAM fault. Check the voltages +12V, -5V at the pins 8 and 1 resp., of IC13 (4116) .

zxuser · May 2015

HZJ, thanks for replay.

On IC13 socket (all ICs removed) voltages are (pin | value):

1 | -5v 16 | 0v

2 | 2.1v 15 | 3.64v

3 | 4.2v 14 | 2.14v (on IC6 2.63v)

4 | 3.28v 13 | 1.23v

5 | 1.8v 12 | 2.04v

6 | 2.1v 11 | 1.59v

7 | 1.55v 10 | 0.95v

8 | 11.31v 9 | 4.97v

When I put all ICs in sockets, voltages on IC13 are:

1 | -5v 16 | 0v

2* | 3.08v 15 | 3.64v

3 | 4.2v 14* | 3.06

4 | 3.28v 13* | 2.2v

5* | 0.94v 12* | 2.35v

6 | 2.1v 11* | 2.26v

7* | 0.8v 10* | 2.36v

8* | 10.5v 9 | 4.97v

All 4116 ICs are OK - checked on working Spectrum.

zxuser · May 2015

* - changed values

balford · May 2015

Have you tried removing the ROM?

B

1024MAK · May 2015

In that video it looks like the Z80 CPU is not running. What the ULA is displaying on the screen could well be the random data in the RAM after power up.

Add a reset switch (temporary). Now when pressed, does anything on screen change?

What test gear do you have? If you have a logic probe or an oscilloscope test the clock input pin to the Z80 CPU.

Mark

zxuser · May 2015

@balford

Checked with 2 different working ROMs -> same result. Thanks.

@1024MAK

I will add reset and post result. I only have multimeter, but I think that I can find oscilloscope, and I will test clock on pin 6 of Z80. Thanks.

HZJ · May 2015

Hi,
let us interpreete your measurement.

Pin 1: The -5V keeps stable. O.K.

Pin 8: In both cases the voltage is to low, and with ICs plugged in still worse! This could be caused by a dried C44 (100µF) Try changing it or (easier) shunt it by another 100µF! Check the voltage again at Pin8. It should be 12V+-5%!

Pin 6: This is connected to the adress line A6. You have measured 2.1V. This is not TTL! But: Since the signal at this pin is switching between 0V and 5V, your meter has shown some value in between. This learns us both: The value is O.K., AND THE CPU IS RUNNING!

The same for the other pins. You have differences between plugged and unplugged RAMs. This comes from the fact, that the CPU runs in both cases in other parts of the program, without RAMs mostly in NOPs. So, the different voltages are not significant at all.

(Of course, more could be said if you could use an oscilloscope.)

My recommendation: Check C44!!!!! (Also C80 could be critical, but first C44).

zxuser · May 2015

Update:

I replaced C44 and C80, and result is different. 11.6v on pin 8 without ICs, 9.9v with ICs.

Without lower RAM, picture is http://solair.eunet.rs/~mmarinov/01/error01.JPG

and with lower RAM, picture is http://solair.eunet.rs/~mmarinov/01/error02.JPG

It seems to me, that Spectrum doesn't react on reset, when lower RAM is present. When I press reset when lower RAM is not present, border is still yellow, but screen becomes white.

HZJ · May 2015

Hi,
the voltage at pin8 (+12V) is still to low. But, since the CPU does not react on reset, might be that Mark is right: He assumed that the CPU is not running. Check this! If you don't have an oscilloscope, try measuring the voltage at pin6 of the CPU (Z80). The meter should show around 2.5V.

zxuser · May 2015

Hello!

Voltage on pin 6 is 2.22V. I noticed that after ~5 min., TR7 is very hot.

Update: replaced TR7 and D9. TR7 is still very hot.

blueowl0708 · May 2015

Have the symptoms changed at all while you were replacing components?

i.e. when you started, was the border yellow, etc or did this start after working on a particular component?

1024MAK · May 2015

Hello!
Voltage on pin 6 is 2.22V. I noticed that after ~5 min., TR7 is very hot.
Update: replaced TR7 and D9. TR7 is still very hot.

With the power off, unplug all the 4116 chips. Then measure using the resistance range on your multimeter between each pin of one of the sockets (for the 4116 RAM chips) and 0V/GND (pin 16). Post the results here please.

Mark

HZJ · May 2015

Did you already check the voltage at pin6 of the CPU Z80?

zxuser · May 2015

@blueowl0708

No. Symptoms changed when I replaced ULA. Border became white, but screen is still the same. Both ULAs are tested on working Spectrum and there is no any sign of some type of error.

@1024MAK

Resistances are (measured on IC13) (20k range) :

1-16: over the range (on 2Mohm)

2-16: 6.22k

3-16: -

4-16: -

5-16: 0.99k

6-16: 1k

7-16: 0.97k

8-16: 2.8k

9-16: 0.4k

10-16: 0.98k

11-16: 1k

12-16: 0.99k

13-16: 0.99k

14-16: 6.22k

15-16: -

16-16: 0

@HZJ

Yes, it's 2.22V.

HZJ · May 2015

9-16 seems not o.k., it should be about 1k. Try measuring 9-16 again with swapped probes.

zxuser · May 2015

Checked again - same result with swapped probes - 0.4k.

HZJ · May 2015

If you have another working Issue6a, try measuring 9-16 there (with RAMs, if you cannot remove them.). Might be that IC27 is faulty.

zxuser · May 2015

On working 6A, it's 0.7k.

IC27 (PCF1306p) is ok, tested on working 6A Spectrum.

HZJ · May 2015

zxuser said:
Hello!
Voltage on pin 6 is 2.22V. I noticed that after ~5 min., TR7 is very hot.
Update: replaced TR7 and D9. TR7 is still very hot.

If TR7 gets hot, it could indicate that the Spectrum is permanently in a state were Pin28 of the ULA is high. Check the voltage at Pin28. I suppose almost 5V? But the main problem remains. Could there be a hidden short at pin9 of one of the sockets of the lower RAM (IC16-IC22)? Check the sockets at Pin9 with a magnifier!

Do you have an oscilloscope?

zxuser · May 2015

Hello!

ULA voltage on pin 28 is 4.67V.

I checked all tracks around pin 9 with a buzzer and with a magnifier (lower and upper memory) and couldn't find any short.

I don't have an oscilloscope, but I will try to find. What should I need to check (beside clock on Z80 as Mark suggested)?

HZJ · May 2015

Hello,
to rest TR7 you could open the connection TR7 - D9 by dissolving one leg of the diode.
With the oscilloscope you should check the quality of the signals:
Check all adresslines A0 - A15 directly at the CPU Z80: You will see a running picture on the screen, but the slopes should be steep and the voltage should clean switching between 0 and 5V. No values in between!
The same for the data lines D0 - D7. The signals there are not as good as on the adress lines, the slopes are not as steep, but no plateau of voltage in between 0 and 3-5V should appear.
Then check A0 - A6 on IC13 and on IC16 or 22.
Post here, if there are peculiarities.

HZJ · May 2015

Hello,
I have to correct a former posting concerning yor measurements of the resistance of the pins against ground! I mixed up in the scheme the 4116 with the 4532! No value of your measurements shows an anomaly! Also 9-16 is correct, since 9=+5V-power line. So 0.4k is OK.

With the oscilloscope check also the quality of the +12V at pin8 of IC13. There shouldn't be any high voltage spikes!

1024MAK · May 2015

I think HZJ means desolder rather than dissolving :D

Keep in mind that when using an oscilloscope, the logic levels for "TTL" are not actually the same as the supply rails.
A logic low/zero should be between 0V and 0.8V (max). Normally less than 0.5V.
A logic high/one should be between 2V (min) and 5V. Normally greater than 3.5V.

You should use a oscilloscope rated at 20MHz or higher. If the 'scope has one than one channel, try to synchronise to the Z80 /MREQ pin.

Note that if the Z80 does not have a clock signal, it will do nothing and it's output pins may not be set to sensible states. It is also good to check every pin on the Z80 (except the 0V/GND and the +5V power pins).

Mark

1024MAK · May 2015

General Z80 fault finding:-
Check the supply voltages (including checking the ground connections)

Using your 'scope, check that

/RESET is high (pin 26) unless the reset button (where fitted) is pressed
/BUSRQ and /BUSAK are high (pins 25 and 23)
/HALT is high (pin 18)

then check that the following are pulsing

clock (pin 6)
/M1 (pin 27)
/RD (pin 21)
/MREQ (pin 19)
/REFSH (pin 28)

also worth checking

/WR (pin 22)
/IORQ (pin 20)
/WAIT (pin 24) this is normally high.
/INT and /NMI (pins 16 and 17) should normally be high unless the other hardware is requesting an interrupt service. On a Spectrum, the ULA should pulse /INT low every 50mS.

Then test the /CS or /CE pins on the ROM chip.

Mark

zxuser · May 2015

Thanks for your responses, HZJ and Mark!

I will try to find an oscilloscope in next week, and when I do, I will post results here.

zxuser · June 2015

Hello!

Pictures from oscilloscope (Z80):

pin 06:

pin 16,17, 18, 20, 23, 24,25 ,26 (around 5V)

pin 19

pin 21

pin 22

pin 27

pin 28

Oscilloscope - Hameg HM103

Y (5V scale), X (1us scale).

HZJ · June 2015

Hi,
the pictures look good so far. Two recommendations:

1) With the oscilloscope check also the quality of the +12V at pin8 of IC13. There shouldn't be any high voltage spikes!
2) As Mark said, try to synchronise to the Z80 /MREQ pin. Monitor pin19 again and check the data lines D0-D7 (pins 7-10 and 12-15). The pulses should be as good as in your pictures above.

Heinz

HZJ · June 2015

...synchronising to the Z80 /MREQ pin on falling edge.

zxuser · June 2015

Hello!

I checked voltage on pin8 and it is:

It is around 12V on oscilloscope (10.9V when I check with multimeter) with this little peak. X-scale is 0.1ms.

How to synchronise Z80 /MREQ pin (with what I need to synchronise it)?

HZJ · June 2015

Hi,
the picture shows that the 12V line is working well. Next, check the data lines as I wrote above synchronising to the Z80 /MREQ pin on falling edge. This is done as follows:
I don't know which scope you're using, but I'm sure that your device can do it. There should be a switch somewhere labelled "extern synchronization" or "extern trigger" as well as a jack with the same marking. Don't worry that the beam now will disappear. You will need a second cable to connect this jack with the /MREQ-pin at the Z80. Then turn slowly the "level"-controller (somewhere close to the jack) until an x-line will appear again on the screen. (Time base approx. 1µs/cm or shorter) Now you can check the data lines using your measuring cable . Please, post a picture of one of the signals (D0...D7). If you have problems to understand my instruction, then first tell me the name of your scope.

ZX Spectrum issue 6a - yellow border problem

Comments