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System 9 - one controlled lamp row seemingly low on power.

Ashbo

Ashbo

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So, I have an intermittent issue that's worse since I have come back on holiday.

Every ounce of my being tells me it's a capacitor issue, though why it would be is a mystery.

On connector 6 row 3 every column is flickery on led bulb and cannot power an incandescent bulb.

This is on the board, I tested by creating a simple test setup.

IMG_20230804_151248503.jpg

Pinwiki suggests checking the transistors, however I have confirmed the led bulbs do turn on and off as they should, just not very powerfully.

I don't have a logic probe, but as the right bulbs are firing when they should, I feel this isn't a IC issue, it's like the bulbs are firing but without enough overall voltage.

I gather the voltage is 6v dc average and the real voltage varies between 0 and 18v, so I suppose I need to test this somehow (logic probe?)

If the voltage on one column is low, which component should I be checking / replacing?

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I think the voltage on controlled lamps should be DC and not AC, GI would be AC.

Possibly check the resistors that feed the base on the transistor thats connected to that light column, make sure they are still in spec, could be they are out of spec and not allowing enough current to flow through the transistor.

If an LED is lighting a bit and an incandescent is not then I would say its a current issue as LEDs require less current to light up.

I had an issue recently on a Williams System 7 where a column of lights were out and testing with a multimeter showed voltage was there but it was twice what normally there and found it was the resistor connected to the base on the transistor that feeds the lamp column was completely open circuit. Im not massively familiar about how transistors work, but that would be what I would check first.

Actually just read that wrong I think, you saying a row is having issues and not a column, which is the different way round than what I had it, but I think the same may apply if the return on the row goes through a transistor, it could be that row transistor is not allowing enough current to flow through it. Again I would check the resistors feeding that transistor, and the transistor that controls that transistor if its anything like a System7 where the PIA controls a transistor that then controls the main power transistor.
 
Fantazia2 said:
I think the voltage on controlled lamps should be DC and not AC, GI would be AC.

Possibly check the resistors that feed the base on the transistor thats connected to that light column, make sure they are still in spec, could be they are out of spec and not allowing enough current to flow through the transistor.

If an LED is lighting a bit and an incandescent is not then I would say its a current issue as LEDs require less current to light up.

I had an issue recently on a Williams System 7 where a column of lights were out and testing with a multimeter showed voltage was there but it was twice what normally there and found it was the resistor connected to the base on the transistor that feeds the lamp column was completely open circuit. Im not massively familiar about how transistors work, but that would be what I would check first.

Actually just read that wrong I think, you saying a row is having issues and not a column, which is the different way round than what I had it, but I think the same may apply if the return on the row goes through a transistor, it could be that row transistor is not allowing enough current to flow through it. Again I would check the resistors feeding that transistor, and the transistor that controls that transistor if its anything like a System7 where the PIA controls a transistor that then controls the main power transistor.
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Thanks @Fantazia2 , that's my next check then :)

On replacing the board following removal for inspection, I now have lamps affecting sound. An earth issue somewhere perhaps :(
 

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So my investigation will be around these elements....

Screenshot_20230805-083734-772.png

1J6-3 is the connection where all the issues manifest

The suspect circuit has.

U59 - which is an OC shared with other rows. Unlikely but possible.

That connects to the base of the TIP122 through a diode.

I don't know what I'm talking about, I'm a narrow things down and change one thing kind of guy, but tip122 is an obvious thing to change first so I'll order a few.

I'll also pick up the resistors, diodes and the 7406 whilst I'm at it.

If System 9 boards weren't so precious, i would swap components from another row to help identify the issue, but I don't fancy adding the "wear and tear" onto the board needlessly.
 
AlanJ said:
also consider the scr 2n5060, plus d11 , d13 are cheap n easy to replace.

a logic probe would help greatly here.
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Thanks @AlanJ - any recommendations on a good logic probe, many cheap ones on Amazon but it's not an area I'm familiar with?
 
On a pedantic note, 2N 5060/1 isn't a transistor as such; it's an 'SCR', which could be described as a 'controlled diode'. It only conducts if the voltage on the additional 'gate' terminal allows it to. Williams used it in this situation for short-circuit protection.
 
Having spent the day feeling really good about myself, I had a good play tonight and it has started to flicker the lights on that row again. I'll test the row again tomorrow with an incandescent, following that, off to Amazon for a logic probe.
 
It's possible that a bulb is causing this by dawning too much current (perhaps about to blow but hasn't yet). Try replacing all the bulbs in the row and then replacing the scr (as that's probably been damaged which is why the fault has returned). Good luck as intermittent faults like this are awkward to resolve.
 
Moonraker said:
It's possible that a bulb is causing this by dawning too much current (perhaps about to blow but hasn't yet). Try replacing all the bulbs in the row and then replacing the scr (as that's probably been damaged which is why the fault has returned). Good luck as intermittent faults like this are awkward to resolve.
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I like your thinking thanks @Moonraker
 
Moonraker said:
It's possible that a bulb is causing this by dawning too much current (perhaps about to blow but hasn't yet). Try replacing all the bulbs in the row and then replacing the scr (as that's probably been damaged which is why the fault has returned). Good luck as intermittent faults like this are awkward to resolve.
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You may have a point, testing the board again this morning, it's all good on incandescent, plenty of current driving the lamp on row 3.

Next replace all bulbs.....
 

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