Solved robotron screen sprites/colour

[bones]

Used to get problems with pots particularly in power supplies. Where they sit in the same position the wiper contact could get 'dirty' and lose the connection to the carbon track. Normally we'd just tweak them either way a few times and return them to the original position.
Sorry I thought you'd lost the 2nd colour by the loom wiggle,my bad. It could well be the colour drive transistors on the neck board then. I suppose you could pop a meter on to diode test and compare the red and blue transistors against the green transistors. They probably won't read exactly the same because of the pots setting and they're still in circuit but they should be similar so an open circuit on the red or blue where the green gives a reading might indicate a blown transistor. Obviously don't have it powered up when you do it and be aware that on diode test the probes are polarity sensitive so the same probe has to go on exactly the same leg as the one you're comparing it with. I'd still try the wire swapping first though 😁

 

[Lastspaceship]

Now trying to locate pots on this board/monitor!! has them for freq etc on the back not sure where/if for rgb..

 

[pooman2084]

Are they not on the neck PCB on this model?

 

[Lastspaceship]

ah yes. looks like it on the top.

 

[Lastspaceship]

Ok Update, Things done/tested with a bit of help:

- We confirmed good signals off the JROC board by swapping RGB pins as suggested (didn't cut them put unplugged them), and also they looked good on an oscilloscope.

-Monitor discharged with the HV (thanks for the tips in the other thread), neckboard off then we swept RGB gain and cutoff pots on the neck board but it didn’t fix it.

-Next was checking the transistors on the neck board as suggested: By each RGB gain pot there are 3 transistors, all marked C548B so we think these are BC548B NPN transistors. For the G channel all transistors showed sensible PN junction voltages using the multimeter in diode mode, I.e. about 0.68V. For the R channel one of the transistors showed a base-emitter voltage of about 1.47V. On the B channel one transistor had a base-emitter voltage of 1.9V. Highly suspicious of these transistors but aware the readings were taken in-circuit.

Thinking the next step would be to desolder the suspect transistors and check them out of circuit, or simply replace them? Wondering if there was anything else on the board worth checking that could be causing these transistors to fail?

tips welcome!! Also wondering if should just wack it and it'll work again

 

[bones]

Yeah the transistors will be BC548B's, probably worth checking out of circuit.
When you done the continuity testing to the chassis did you put the probe on the solder pads of the chassis rather than the top of the crimp to eliminate it being a bad crimp in the plug, just a thought. Sometimes the crimp suffers a bit of metal fatigue and 'opens up' thus not making contact. It'll probably be worth checking the schematics you have (they'll be similar to a 9000a) to see what components the colours go through to get to the neck board, I'd imagine there'd be 3 identical circuits along the way and if you unplug the connector going to the neck board then won't have the whole circuit affecting any comparison readings you take,though it'll still be in circuit. Hopefully one of the monitor people might offer some advice, I've not touched these in 30+years. Not sure if you can swap the soldered green wire on the neck board with the red and blue soldered wires to prove that the red and blue 'guns' are working. I'd get a 2nd opinion on this before trying as I don't know if it's a no no or not.

 

[Lastspaceship]

didn't do it to the solder pads, might have to get the chassis out to do that, can try though. I think the 9000 to 9000a is a bit different, or at least the 9000 schematic is a bit different to what's there. just trying to run a 9000a to ground.

 

[Lastspaceship]

Update....

Tonight had a go at this, with my super assistant !! Basically the suspect was two of the transistors, having checked PN junctions in-circuit for transistors TR1 to TR9 using a multimeter in diode mode. TR2 (red) gave a suspicious base-emitter voltage of 1.47 and TR9 gave dodgy voltage of 1.92V for base-collector and 1.67V for base-emitter.



So got the chassis out (after discharging) which was relatively straightforward apart from one painful connector that didn't want to unplug.
The back of the chassis looked like it had had a repair or two, but left that bit alone...



Desoldered TR2 and found the base-emitter junction was completely open circuit. We also desoldered TR9 and found both junctions open circuit. We fitted 2 new BC548 transistors that we believed to be BC548B (because they both had a gain of about 270).



We also desoldered and checked the 47uF capacitor connected to the base of some these transistors but it seemed ok (based on measured capacitance)

put it all back. Turned it on.....



working! Thanks for all the tips and assistance. Was a proper buzz when it fired up. outrun next..

 

[bones]

Nice, well done, glad you're up and running.

 

[Lastspaceship]

cheers and thanks for tips. Gratuitous pic!!:

 

[Lastspaceship]

quietly unsolving this thread.. was running fine for a few days, then did notice a "red" background on startup that then sorted itself a few mins in (back to black background, then monitor stopped showing and some electrical burning smell. turned it off, investigating tonight, maybe red and blue channels running too high after transistor change, tips welcome.

 

[bones]

Hmm, may need some expert advice from gunblade or tb2000. Hopefully the transistors are ok and one of the resistors has given up. I was told once that putting a new component can cause a domino effect on the other components along the line. I wonder if there might be a dodgy electrolytic cap or something that caused the transistors to blow in the first place. Will be following this thread, good luck.

 

[Lastspaceship]

Found a blown resistor, neckboard seemed fine as were the transistors that were replaced, just trying to see if anything else effected.

 

[bones]

Fingers crossed for you, bear in mind that those electrolytic capacitors life span has probably been exceeded by many years if you've not done a cap kit replacement.

 

[Lastspaceship]

Yeah it might be worth doing that too, though they looked ok on this one.I have an outrun Caberet also with an issue thinking of recapping that chassis too. Are there kits for these or is it a case of working it out for each chassis ?

Ordered another resistor to replace this, but wondering if there was too much current coming through after replacing the transistors for red and blue, maybe the cutoffs need to be adjusted, also saw there is a brightness pot near where this resistor blew. It did run with a red background for a bit on startup before this happened, though sorted itself after a few mins.

 

[Lastspaceship]

Replaced the resistor, but it basically blew or got a small smoke from it straight away. I think this is around the brightness circuit. Not given up yet, but might be starting to think of a replacement chassis..

 

[pooman2084]

Send it off for repair.

 

[gunblade]

correct about that resistor being in the sub brightness circuit
what flyback is installed, a hr7508?

 

[Lastspaceship]

correct about that resistor being in the sub brightness circuit
what flyback is installed, a hr7508?

Not 100 percent sure as away from the machine at the mo, I think so, just got pic to hand:



Did wonder if it was running v bright when it did work, but wasn't sure if that meant it could blow it. This time though this resistor went immediately.

 

[Lastspaceship]

correct about that resistor being in the sub brightness circuit
what flyback is installed, a hr7508?

Sure now, checked