spectrum 48 iss. 2 screen image not sharp/focused

[jojo]

hi forum ,

a very long story to repair a defective spectrum48 ,

finally got it working again ( after a bit too much money spent on it ) ,
but now a problem remains which i can not get sorted;

so got a blurry screen image not focussed and ghostly .

any ideas what i can do to get it better .

=> what has been done ;
tr1 and tr2 replaced with bc549c
lm1889 socketed and replaced with new one
simple video mod with capacitor

i configured my screen with a known good speccie , before installing this annoying thing

jojo

[Tiger]

Only a thought 'cause I had seen some faulty recapped issue 2's: is C65 (the one directly under the modulator) placed in the right direction? Plus must be to pin13 IC14 and negative to TR2. If you look on your pcb and the modulator is on top, plus is to the right and minus to the left. On the other hand I think the issue 1 and 2 aren't the issues with the best picture ...

[jojo]

thanks for answering ,

but cap has the correct position .

i have another iss. 2 which has a good image ......

and according to the owner before the fire it had a razorsharp image ( which i doubt , iss. 2 have a bit of a woolish image to them )

puzzled here


jojo

[Tiger]

I think it would be good to compare the signal values of the two Iss 2 around the LM1889 to find the cause...

[sP1d3r]

You might need to adjust the variable resistors and/or trimmer capacitors.
The variable resistors are mostly colour balance, IIRC.

See;

https://www.dataserve-retro.co.uk/conte ... k/d29.html

for an explanation of them.

[1024MAK]

We need to do some elimination.

I suggest that you desolder and lift one leg of C65 (22µF) and assuming IC14 is now in a socket, remove the LM1889, so that the colour encoding circuitry can be disabled.

If the displayed image (now monochrome) is still blurry, then at least we know the cause is not the LM1889, or the rest of the colour encoding circuitry.

Are the BC549C transistors that you used from a reputable supplier? And I assume you have double checked that they are fitted the opposite way round to the original ZTX313 transistors?

Is the voltage on both sides of resistor R64 (15Ω) okay (>4.75V)?

Are C25 (22µF), C42 and C40 (both 47nF and both axial types the size of resistors on most boards) okay?
Are the PCB tracks in the area of C25, TR1, TR2 okay?

Mark

[jojo]

I suggest that you desolder and lift one leg of C65 (22µF) and assuming IC14 is now in a socket, remove the LM1889, so that the colour encoding circuitry can be disabled.

If the displayed image (now monochrome) is still blurry, then at least we know the cause is not the LM1889, or the rest of the colour encoding circuitry.

Are the BC549C transistors that you used from a reputable supplier? And I assume you have double checked that they are fitted the opposite way round to the original ZTX313 transistors?

=> the same image with the ztx313 transistors

Is the voltage on both sides of resistor R64 (15Ω) okay (>4.75V)?

=> 4.920 and 5.020 V

Are C25 (22µF), C42 and C40 (both 47nF and both axial types the size of resistors on most boards) okay?

=> c25 fresh new one , c42-c40 42-44nF .

Are the PCB tracks in the area of C25, TR1, TR2 okay?

tracks are clean an connected

[1024MAK]

Okay, a close look at the latest picture that you have posted reveals that the problem is signal reflections / signal echos. If you look carefully, each character is being shown multiple times. Each ‘copy’ being fainter each time.

But you have indicated that your other ZX Spectrum works okay. Are they both using the same video display, same cable, and same composite modification?

Leave C65 and the LM1889 disconnected/removed for now. As it makes fault finding easier.

Mark

[1024MAK]

Try this simple program to see how bad the problem is.

It draws a single vertical line in the centre of the screen.

Code: Select all

10 CLS
20 PLOT 128,0
30 DRAW 0,175
40 PAUSE 0

Here’s what the output looks like on an emulator:

Single vertical line test screen

Mark

[Tiger]

IC 3 an IC 4 are 74LS157 (and not 74HC157) types? Is it possible, that X2 is out of range (moving TC2 could sharpen the pic)? But if you touch TC2 it is better to follow the Service-Manual chapter 3.1.4. Look also for the different values for R47, R49 if you use a 5C112 or a 6C001 ULA.