High voltage problem

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1 week 23 hours ago #11091 by RNi
Replied by RNi on topic High voltage problem
Hello,

I did what you asked me to do:

pin 1,6,7,8 = 8.22 - 8.23 V
pin 2 = 5.90 VDC & 12.0-12.3 VAC
pin 3 = 0.81-0.83 VDC & 1.1-1.2 VAC

Q1 - do not know how I could check it

D2: it shows some smal value (approx. 158ohm) only when I put "+" probe to te leg without white strip and "-" probe to the other, with white strip. When I change direction it does not show anything (break in wiring)

R3 shows 1015 ohm (1k I believe)

about Q2: I did not noticed anytching strange during the assembly (I'm sure it is not damaged phisicly: hitted, scrathed, etc.) It was protected also with aluminum foil when it comes to me and I keep it like this until it was time to solder this part. Is it possible to check it somehow? (unfortunatley I do have only simple multimeter).

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1 week 22 hours ago - 1 week 22 hours ago #11092 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
Q1 and D2 can't really be tested in-circuit. That D2 reading you got makes me think it's probably okay: you proved it's not shorted but can't really prove it's not open. If you decide to take D2 out then use your diode test range on the meter and look for open circuit with (+) on the stripe end and about .7V with (+) on the other end. But read on...

Your readings strongly suggest that the MC34063A is okay and running. Suggest, not 100% prove, but to prove requires a scope. I am very suspicious of the MOSFET Q2.

Next thing to do, just to be sure there's not a cut trace or something, take readings on all 3 pins of Q2. Pin 1 should read similar to what you read on Pin 5 of the MC34063. Pin 2 will probably be about 8 volts (basically equal to Vin minus a diode drop). Pin 3 should be ground. When testing Pin 3 make sure it's really grounded, i.e. put your (-) probe on Pin 3, put your (+) probe on Vin, and if Pin 3 is truly grounded you'll read Vin volts on your meter.

If these tests don't give expected results then you have a connection problem, either a cut trace or some other sort of bad connection (or maybe bad Q1 or bad D2 though I doubt it).

If they do give expected results the next step is to take Q2 out of the board (sorry!) because it can't be tested in circuit. Then put the meter on diode test range and try every possible combination of pins and probes. Like this:

(+) on 1 (-) on 2 -> read open circuit
(+) on 1 (-) on 3 -> read open circuit
(+) on 2 (-) on 1 -> read open circuit
(+) on 2 (-) on 3 -> read open circuit
(+) on 3 (-) on 1 -> read open circuit
(+) on 3 (-) on 2 -> read about 0.55V!

This is not a perfect test but it's the best you can do with just a meter.

PS - it might be best to cut Q2 off very close to the board instead of trying to desolder it. Clearing the remains of the pins out of the holes one by on is easier than trying to melt all 3 at once. If you cut it close enough I think you will have enough pin length left on the part to put it back in if it tests good. Use your judgement...

Look into it later when the dust is clearing off the crater.
Last edit: 1 week 22 hours ago by Ty_Eeberfest. Reason: Spelling

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1 week 22 hours ago #11093 by RNi
Replied by RNi on topic High voltage problem
Before I start desoldering anything I've just check what was the easiest:

Q2 (looking from the top):
pin1 (left) = 5.6V
pin2 (middle) = 8.26 V
pin3 (right) = when connected this pin and Vin+ it gives 9V, same as Vin

You wrote pin1 should give us something similar like pin5 of MC34063A but it's not even close.....


Should I cut it out and check also combinations you described?

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1 week 21 hours ago #11094 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
Oooops! I'm an idiot... I should have said "same as pin 2" not pin 5!! Sorry - my bad.

It sure looks to me like Q2 is getting the correct input but not giving the correct output, meaning Q2 is the problem. The only way I know of to be more sure (without pulling Q2 off the board) would be to put a scope on pin 1 and see what the driving signal actually looks like (should be a rectangular waveform at approx 50 - 100KHz).

I'm thinking you need to pull Q2 out and test it as I described above. 2 options: cut the leads close to the board so you can clear the holes and put old part or replacement part through the holes, or cut them a bit "tall" and the solder the old part or replacement part to the stubs - no clearing of holes. The second way is easier and less risk of damaging the board but it's not as pretty looking.

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

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1 week 21 hours ago #11095 by RNi
Replied by RNi on topic High voltage problem
OK - if we compare to pin 2 it looks much more optimistic :-)

Just disassembled Q2 and done the test:

(+) on 1 (-) on 2 -> read open circuit - OK
(+) on 1 (-) on 3 -> read open circuit - OK
(+) on 2 (-) on 1 -> read open circuit - OK
(+) on 2 (-) on 3 -> read open circuit - OK
(+) on 3 (-) on 1 -> read open circuit - OK
(+) on 3 (-) on 2 -> read about 0.55V only this configuration gives me read 603 on my multimeter - I believe this is 603 mV

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1 week 21 hours ago #11096 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
I got the 0.55V by testing an IRF730 since I didn't have an IRF640. 603mV (aka 0.6V) is a reasonable reading. What that tells us is that a) the MOSFET isn't shorted source to drain (most common type of failure), b) the body diode is intact, and c) the gate isn't shorted to anything. What it doesn't tell us is whether or not it actually turns on when the gate is brought high. As I said, not a perfect test method.

Do you have a collection of spare parts? Specifically an LED and some miscellaneous resistors (values not real critical)? If you like I can give you a sketch of a circuit you can make on a breadboard in about 5 minutes to do a functional test of that MOSFET.

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

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