spectrum 48 iss. 2 screen image not sharp/focused

[jojo]

the ill speccie ;

one of the other speccies ;

image done via the same adapter with the same settings darker is iss 2

hope this will help

[jojo]

@ tiger ,

all components are as they should be .

next is freq counter to verify .

[Tiger]

The dark screen of the ill speccy looks like a poor signal output. Have you swapped the ULA's ? Any other result with the ULA from the good speccy? I suspect one of the components in the IC 14 area if the ULA swap has no effect.

[1024MAK]

The dark screen of the ill speccy looks like a poor signal output. Have you swapped the ULA's ? Any other result with the ULA from the good speccy? I suspect one of the components in the IC 14 area if the ULA swap has no effect.

This looks like an analogue problem to me. Either, the video output driver stage of the ULA pin 17 (least likely I feel), or in the analogue video circuitry from ULA pin 17 to the video output (via R47/R48/R49/R50/TR1/R51/R52/TR2/R53.

If jojo has done as I asked and left C65 lifted and left out IC14 (LM1889), then these and their circuitry including TC2, VR1 and VR2 cannot be the cause.

jojo, do you have an oscilloscope that you can use?

If not, but you have another working ZX Spectrum, you may want to try this experiment. Read through the following at least twice before trying this.

Stage One - good machine feeding poorly machine

1. On the poorly issue two board, remove IC1 (ULA), if the Z80A is in a socket, remove this as well.
2. On the poorly issue two board, temporary attach a test lead to any convenient 0V/GND point. Connect the other end to a convenient 0V/GND point on the good ZX Spectrum. Keep the lead as short as possible.
3. On the poorly issue two board, temporary attach a test lead to the left hand lead of R48 (as viewed with the sockets/edge-connector furthest away from you). Connect the other end to the left hand lead of R48 on the good ZX Spectrum. Keep the lead as short as possible.
4. Connect your video display to the poorly issue two board.
5. Power up both ZX Spectrums simultaneously (use a power strip).
6. Check the video output, is it worse/about the same/better?
7. Switch off both ZX Spectrums simultaneously.
8. Connect your video display to the good ZX Spectrum.
9. Power up both ZX Spectrums simultaneously (use a power strip).
10. Check the video output, is it about the same as you would expect from this machine (there may be less contrast and it may be less bright, that’s to be expected),
11. Switch off both ZX Spectrums simultaneously.

Stage Two - poorly machine feeding good machine

1. On the poorly issue two board, refit IC1 (ULA), if the Z80A is in a socket, refit this as well.
2. On the good ZX Spectrum, remove IC1 (ULA), if the Z80A is in a socket, remove this as well.
3. On the poorly issue two board, temporary attach a test lead to any convenient 0V/GND point. Connect the other end to a convenient 0V/GND point on the good ZX Spectrum. Keep the lead as short as possible.
4. On the poorly issue two board, temporary attach a test lead to the left hand lead of R48 (as viewed with the sockets/edge-connector furthest away from you). Connect the other end to the left hand lead of R48 on the good ZX Spectrum. Keep the lead as short as possible.
5. Connect your video display to the good ZX Spectrum.
6. Power up both ZX Spectrums simultaneously (use a power strip).
7. Check the video output, is it worse or about the same as you would expect from this machine (there may be less contrast and it may be less bright, that’s to be expected), did the blurry symptom show up?
8. Switch off both ZX Spectrums simultaneously.
9. Disconnect both test leads and refit the ULA (and Z80A if removed).

Mark

[1024MAK]

Here are the in-circuit test results on one of my issue two ZX Spectrum boards:

Code: Select all

C42              R53 (390) =390
R51 (2.2k)=2.2k  C65
TR1              TR2
R49 (8.2k)=2k
R52 (2.2k)=2.2k
R50 (4.7k)=1.35k
C36
R40
R41
R72
R42
R48 (2.2k)=1.31k
R47 (220)=210
C40

Note:
On some later issue boards, the following resistor values were changed as follows:
R47 now 1k
R49 now 10k
Over the years, some different values were tried for various other resistors in this circuitry.

Mark

[jojo]

C42 50nF R53 (390) =390 360
R51 (2.2k)=2.2k 2.2k C65 24uF
TR1 bc549c TR2 bc549c
R49 (8.2k)=2k 3k
R52 (2.2k)=2.2k 2.2k
R50 (4.7k)=1.35k 1.4k
C36 43nf
R40 .7k
R41 .9k
R72 9.8k
R42 .9k
R48 (2.2k)=1.31k 1.3k
R47 (220)=210 212
C40 46nF

[1024MAK]

So some slight variation for some resistors, but nothing that is far enough out to be concerned about. Note that where I listed only the component references, but no value, that was purely to make it easier when taking readings from a board, as the order I put them in is the same as the board layout. So where you have listed values for these, where I did not, I have not compared these results to check them.

Mark

[jojo]

@ mark ,

Stage one ,

good driving ill;

image good when driving ill ;

Stage two ,

ill driving good;

so the problem must lie in the last components before the modulator as i understand ?

[jojo]

So , how to spent an afternoon , well.......

i have replaced about every component between R48 en the "video-mod" including the caps and tr1/2 once more .

and tadaah , the result ;

not too bad for an iss. 2 .

slight blue/yellow glare but in focus and nice bright colors as with the other one .

THANK YOU ALL !! for all your assistance and wise words.


now for the non-working 128

[1024MAK]

Mark