Transistor tester

[Cactus]

Well i built one of those transistor testers bought from china, its the M12684 graphics type with the red circuit board. The kit seems to work ok but with one problem, when i conect it up to a power supply, be it a bench supply or a battery, it reads the voltage as 20v and i dont know why. 

I have checked the board and components but i can't seem to find anything I have done wrong and I am not sure where to go next. Im hoping i have just made a small error somewhere and cant see it.

I would appreciate any helpful thoughts other forum members may have as im still very much a beginner.

Thanks

[JoeO]

How about posting clear pictures of both sides of your board.

[ledtester]

If the following schematic of the M12864 matches what you have, the battery voltage is read on pin 28 via the resistor divider network formed by R11 (10K)  and R12 (3.3K), so I would check those resistor values.

I would also check the voltage on pin 28 when the device is powered up. If the tester is reporting a battery voltage of 20 V, that means it's seeing 20*(3.3/13.3) = 4.96 V on pin 28. A 9V battery should present 9*(3.3/13.3) = 2.23 V.

[Cactus]

Wow folks thanks for such quick replies. I will check this out when i get home and report back.

[Cactus]

 Ledtester you were bang on target. Im afraid i made an error and had a 33k resistor where i should have had a 3.3k resistor. with your help  i have learned from it and fixed te problem. My tester is now showing 9.1v, happy days!

I hope one day i may know enough to be able to help someone on here! Thank you so much

[JoeO]

So where was the 33K ohm resistor supposed to be soldered?

[Cactus]

Looking at the schematic above - R10 i believe

[Cactus]

Looking at the schematic above - R10 i believe