Hello, my friends.

I recently bought an old 2mhz function generator, however, it doesn't work very well.

When I opened it for the first time, I noticed that some components were burnt (resistors and transistors), I immediately replaced them, it "works" but shows a lot of heat in this region (figure 01).

Apparently it was a "white label" project in which companies registered with their brand, but it shares the same project with other names: VC 2002, MFG 4202, GV 2002.

I'm a beginner and would like some help on possible problems and, mainly, techniques for identifying a problem in general.

I have part of the electronic diagram of the device (I couldn't find the complete diagram), however, I have exactly the part that "roasted" and that after replacing the skins, it became very hot (figure 02).

Figure 03 is a complement to the circuit and is one of the only two things I found on the internet.

Thanks!




# Figure 01:



# Figure 02:


# Figure 03:

 

Which part is getting warm, the large resistor R161?

 

Hello! The components that are overheating are the: R169, Q111, Q112, Q113 and Q114. (Sometimes: Q107 e Q108)

 

Sounds like the base driver bias is wrong, I think preset W111 sets the current, is there anything in the manual for setting it up?

 

I only have this fragment of the electrical diagram. I took the continuity of Q111 and Q112 and they are positioned correctly. When I add resistor R169, transistors Q111 and Q112 get very hot.

 

Is the power supply the correct voltage?

 

Thank you very much for your effort in helping me!

I took some measurements and I believe you have a clue about the problem, I found two divergent measurements (precisely in the components that came toasted and I replaced when I bought the device) [as shown in the figures].

I took two measurements, one without and without R164. I removed it to do the tests, considering that when it is on the board, transistor Q112 heats up A LOT.

Which components would be responsible for these "wrong" measured voltages?

Thanks again!

 

Update: I replaced transistor Q109 (which was shorted between the base and collector) with another one (it didn't have the SMD). The specifications are below:



I ask that you disregard the inverted measurements in the previous answer, they were "correct" in terms of the sign (positive and negative). After replacing Q109, Q112 stopped overheating, but now it is Q109 that is overheating.

I placed the tension measures on the board:

 

What does W111 preset do when altered?

 

It's alive!!!

Now the function generator is showing signs of life, responding to amplitude commands, changing the shape of the waveform, etc. However, when I leave it on, transistor Q113 is heating up A LOT (I can't leave it on for more than 5 minutes).

I measured the voltage and the values of +25V and -25V are altered, both are hitting +27V and -27V, I believe that this is what is causing the components to heat up.