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Vectrex blows internal fuse after time

Have a look at your schematic you posted earlier by the speaker. There are quite a lot of 'negative' pins though (including the heatsink).

Oh I see, I can remove the +9v and -9v from T401 Inputs, rather than removing from ic401 itself. Should be easy enough. I'll get on it tomorrow
That sounds like a better bet.

Can you just confirm you have either tested (or replaced) the following capacitors:

109, 117, 118, 119, 404, 411, 511, 512 and 513.

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I changed the caps. All of the ones you list except 491, which I guess is a typo?
I didn't notice last night that you mentioned c120 too, but yes I replaced that also.

With the -9 and +9 removed from ic401 the system no longer blows the fuse.

But the question is now - does the fuse not blow because the current is below the threshold that would cause this.

I think we have to assume the problem is in this circuit initially. Let's see what could cause a problem hete (other than IC401 if course).

I mentioned previously that i found some insulation tubing but wasn't sure which capacitor it should be fitted to. I ended up fitting insulation to BOTH of the capacitors I thought it could be but this didn't fix the problem.

I also briefly removed c404 and c411 from circuit to see if it made any difference but it didn't.

I'm not really sure where else to go with this....

The ic401 thermal pad looked a little dodgy when I last saw it, so perhaps there is intermittent electrical conductivity with the heatsink? I'll order a replacement pad and see what happens.

Also, I'm tempted to buy a new lm379 and replace the one that's there in case the IC is at fault.
The insulation would only come into play to prevent any long(ish) leads from actually shorting out against something else. If nothing was touching the bare wires on the capacitors, then the insulation serves no useful purpose (apart from future preventative issues).

I was looking for potential low impedance paths to ground on the output of IC401. Apart from a faulty IC401 - these components are the only ones that could pass a higher-than-normal current sufficient to blow the fuse.

I will suggest checking the following components:

Y deflection drive:

R412 (1.5 Ohm resistor in series with C408 to GND).
R413 (0.22 Ohm resistor to GND).
C408 (0.1 uF capacitor in series with R412 to GND).

X deflection drive:

R405 (1.5 Ohm resistor in series with C407 to GND).
R406 (0.22 Ohm resistor to GND).
C407 (0.1 uF capacitor in series with R405 to GND).

You might also try disconnecting the X and Y deflection yokes.

Look for overheating of the resistors and potential low impedance path through the capacitors. However, the problem with the capacitor (if there is a problem) may only appear when the machine is switched on.

My component references are from my schematic diagram. We already know that this diagram is not 100% correct with your particular machine. Just check the component values out and check the PCB tracking to IC401 to make sure you have identified the correct components.

After that, yep IC401...

The datasheet for IC401 states that the maximum current is 600 mA (for maximum power output).

If you have disconnected the +9V and -9V rails from IC401, you could put two (2) multimeters in series with the supply rails (+9V and -9V) and see what the low voltage current is at switch on and whether it increases as the machine warms up or not; and whether the increase in current is on both supply rails or just one (i.e. the current flows from one supply rail to GND).

This might give us a bit more information on the manifestation of the fault.

Very good information, thank you. I'll check this out and report back.

I'll need to acquire a second multimeter to run the test in your second post so might take a few days, but hopefully the checks in the first post will be done sooner.
You could always do two tests. First, put the multimeter in one supply with a wire link in the other. Then, swap the link and multimeter around.

You may just need a good supply of fuses...

That's a good idea, I'll do it that way. I've just received 20 more fuses in the post so should be OK for a while. I haven't counted up how many I've blown up to this point! 🤣

Luckily they're cheap and readily available, so no biggie. I'll definitely run through your suggested tests before ordering the new LM379 though, that's less cheap.
>>> I'll definitely run through your suggested tests before ordering the new LM379 though, that's less cheap.

That would be the route that I would take also...

So I checked the components you suggested. I measured the resistors and they're good. The caps I can't measure, but they look fine, no signs of damage or heating etc.

Then I disconnected T401 again and found the fuse was blowing even with it disconnected, so I'm not sure how it's happened but I must have had an erroneous result last time I disconnected it.

In any case, I apologise for leading us both astray with this.
>>> I apologise for leading us both astray with this.

No need...

Ok, so that leads us down a different avenue...

Just to confirm, we have the high voltage AND deflection circuits isolated from both the plus and minus 9 Volt power supply now?

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At the moment I just have the yokes and deflection circuits disconnected (at t401)
I had already reconnected the HV (via t503) but can disconnect again if needed.