High voltage problem

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8 months 1 week ago #11082 by RNi
High voltage problem was created by RNi
Hello to everyone.
Just got the 4-digit Wemos kit.
I found some time to put all together last weekend.
Everything went smoothly until I bought multimeter and checked high voltage on test points. It stays 7.7 - 7.8 V all the time. Does not matter how much I turned regulating rezistor: clockwise or conterclockwise. The values of this resistor change when I'm turning it - checked with multimeter.
Wemos works just fine: it starts working, LEDs are lightning, etc.


I've read other topics about smililar issue but I did not find anythich that could help in my case.

Some photos to show how it looks.

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8 months 1 week ago - 8 months 1 week ago #11083 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
Looking at your pictures, the construction looks good. I can't see anything obviously wrong, though a picture of the other side of the board may be useful. So to get started figuring this out...

1) What is your power supply voltage (Vin) ? It would be useful to know this so I can figure out if the boost converter is doing anything at all or if the 7.8V you see is just your Vin leaking through inductor (minus a diode drop or two).

2) What voltage do you read on Pin 5 of the MC34063? If you read about 1.25V then you should examine the voltage divider (R4, R5 and the pot RV1). But if you read a much lower voltage you will need to look elsewhere in the HV circuit, e.g. check polarity of D2 & D3, make sure C11 & C12 are the correct values, and generally make sure all solder joints are good.

Look into it later when the dust is clearing off the crater.
Last edit: 8 months 1 week ago by Ty_Eeberfest.

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8 months 1 week ago #11084 by RNi
Replied by RNi on topic High voltage problem
Thanks for your feedback.

Below you can see picture of solder joints. I tried to double (triple) check them and I believe they are OK.

Input: 9V It's able to provide 3A.

Diodes polarity check and looks OK: side with white stripe goes to the "square" whole.

Capacitors: C12 I put capacitor marked 104 & C11 I put capacitor marked 681

I'll check the voltage on PIN 5 of the MC34063 and let you know asap.
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8 months 1 week ago - 8 months 1 week ago #11085 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
Regarding the Pin 5 voltage what I should have said is that if it is 1.25V or higher then the voltage divider is likely to have a problem.

It's never easy to tell exactly how the solder joints are from a photo, but it seems like VR1, C11, C12 and maybe Q2 solder look a little bit sketchy. It would not be a bad idea to reflow them just in case. Also, you might want to clean off the flux in the same areas. It looks like there is quite a lot of it on the board. Clean with a solvent such as alcohol / methylated spirits.

It also looks like Pin 1 of the DS1307 might not be soldered, which has nothing to do with the HV problem but may cause trouble later.

Look into it later when the dust is clearing off the crater.
Last edit: 8 months 1 week ago by Ty_Eeberfest.

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8 months 1 week ago #11089 by RNi
Replied by RNi on topic High voltage problem
Hi,
Just got back to home and done some work:
1. clean the flux
2. resolder components C11 and C12, RV1, check solder connections on Q2 using magnifying glass
3. voltage on pin 5 of MC34063 is 0.13V (I put multimeter between pin 5 and GND on the board) see photo
4. nothing get hot
5. VIN 9V - see photo
6. HV test points show 7.8V-8.0V - see photo
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8 months 1 week ago #11090 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic High voltage problem
Thanks for the good clear info! It seems to me like you have everything put together correctly. 7.9V on the HV test point means R1, L1, and D3 are okay and connected right. 0.13V on Pin 5 indicates the feedback voltage divider (R4. R5, RV1) is okay and 0.13V makes sense when you do the divider math with 7.9V of HV.

What all that means is the converter isn't running at all. The main suspects then would be the MC34063A chip, Q1, Q2, D2 and R3.

Try taking voltage readings on these MC34063A pins:
Pins 1, 6, 7, 8 should all be all approx +8V
Pin 4 should be at ground
Pins 2 & 3 - let me know what reading you get, both on DC and AC meter scales

Q2 is somewhat static sensitive. Is there and possibility that it got "zapped" during assembly?

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

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