Hi, I have a Leader 300 oscilloscope and the DMM is not working. I could manage to get the service manual from Leader and pin point a few issues with a multimeter.

The Model 300 has 4 boards. DMM, Scope, Main board and LCD. DMM and scope grounds are isolated.

My DMM board seems to receive power from main board just fine on pins 3 and 6. When DMM function is enabled pin 16 reads 0v and when it is disabled it reads 5v. It then outputs the data through photo couplers direct to the LCD, which shows blank.

My power stops at T1 (transformer S1673305). The transformer's passed a continuity test. But if you look at the diagram attached you will see no current flows to it's output block. Leader also does not inform the output voltage of the transformer, neither I could find a datasheet for it.

Something else noticed was. Pin 16 connects to Q10 base. When the DMM is enabled the base reads 0v and when it is disabled the base reads 5v. Should it be the other way around? Then it is DMM is enabled base gets 5v signal and then enables C to E current? I checked it several times and that is what it is.

Can anyone throw some light here?!

 

Hi, regret cannot help with your problem, but would be interested to know where and how you got the service manual as I have the same instrument.

 

Test both transistors on the primary side of the transformer.
Incidentally they are both NPN and not as marked on the diagram.