No high voltage, no help from the seller...

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8 months 1 week ago #11081 by Ty_Eeberfest
Before you just start throwing parts at it again, check the voltage on Pin 5 of the chip. If it is 1.25V (or very close to that) then the circuit is running but there's something wrong with the divider network R22, R23, pot.

But if the Pin 5 voltage is something less than 1.25V it's time to look somewhere other than the divider because that means it's out of regulation, i.e. not running.

You've swapped so many parts so many times that I'd be very unsure about the condition of pads and traces. You should inspect them thoroughly under as much magnification as possible and if in doubt use continuity test on your meter to do point by point testing of traces.

Look into it later when the dust is clearing off the crater.

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8 months 1 week ago #11086 by jonboy
This is weird, there is definitely something wrong or loose.

Sometimes when I connect to power, I can put my multimeter across the 50v test point and see the voltage slowly climbing up.
Sometimes it stops at 20v and at other times continues up to 40+ volts. My irf640 gets like warm, but nowhere near warm or hot.

The times it goes up to 40+, I am happy. But if I wait a few minutes and check again, it dropped down to 20 again.

Could this be because there is no load on it at the moment, and so it’s dropping the voltage as it not needed?

If I think I have a faulty pot or pad for the pot, can I remove it and add one in ‘fly by wire’ with pin 1 connected straight to R23 and pins 2-3 connected straight to a ground point.

I don’t think my pot pads will take another removal! But all the other pads and traces look and test out okay.

I was really happy when I first got over the 40v mark, but don’t understand the fluctuations.

I will continue testing and might reflow my joints again. My mc34063 is still running okay, still get the 5v and 1.25 volt on the correct legs.

I have a feeling it’s my pot, so can I wire it directly as above?

Thanks all.

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8 months 1 week ago #11087 by Ty_Eeberfest
I don't think the varying output is caused by not having a load but will admit I am not an expert on this boost converter's behaviors. Some boost designs will regulate just fine with no load and some will not. I don't have one of these clocks to test with here. I'd gladly run over to Ian's house and get one but unfortunately he lives over 8,000 miles away from me and I've got to be at work in a couple hours.

If you want to see how your boost converter does with a load that's easily done. Just put a resistor temporarily across the +HV test point and ground. I'd use a 10K (or 5K or 15K or something like that, exact value not important) and see what the voltage looks like.

I see no problem with direct wiring your pot as described if the pads are looking bad. The technical term for this solution is "adding a bodge wire". Really.

Look into it later when the dust is clearing off the crater.

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8 months 1 week ago #11088 by Ty_Eeberfest

Torsten Lang wrote: Hello Ty,
I just received an answer from threeneurons: He really did place C1 on purpose as it is to delay the turnoff. The design was changed over time.

I still think there's a certain risk. The threeneurons PSU set the peak current much higher anyway, so the capacitor has somewhat less influence as in Ians design.

Regards,
Torsten


Thanks for following up Torsten. Seems like a strange design decision to me. I would really have expected the 100uF to be on Vin serving as a local "reservoir" with maybe a 100nF in parallel for noise...

Look into it later when the dust is clearing off the crater.

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