help needed with old detector

[dbanner]

I think the 30K resistor should actually be a 10K. I also suspect this transistor is faulty. Lift up(desolder) one leg of this diode(shown in picture by blue arrow) then repeat the test I indicated in thread #51. Remember to lift up leg of diode(remove from circuit). keep black probe as shown on picture and then switch other red probe to other leg of transistor as shown. what are the results.(the first test is like thread #51, the second test is like thread #51 but with red probe to other leg of transistor)

[keith777]

Hi again dbanner.

i desoldered one leg of the diode and if i have done the tests correctly. this is what i got on the meter. see pic bellow.

[dbanner]

This transistor is good! Now i want you to check the 2N4403 on mainboard. you will have to remove the base leg of 2N4403. Give me a few minutes to draw picture to illustrate. I'll be back with you soon.

[dbanner]

O.K. Remove base leg of 2N4403(middle leg as shown in picture)from circuit. This maybe a bit difficult, but I'm sure you can do it. Then do same transistor test as before. The black probe goes on middle leg, then test each remaining legs with red probe. Remember to set up meter for transistor testas before.

[dbanner]

You are working with some assumptions, namely:
1- You have correct batteries installed
How can you be sure its not a pp9 battery(9 volts)? Seems like it would fit.
2- Your speaker is good.
Did you check the speaker?
3- Your search coil is good.
we can check this later
4- Replacement resistor is correct vaIue
I cannot find a manual for questor but the fieldmaster 33, which is the next model up, uses a 9 volt battery.

[dbanner]

When you first opened up the detector housing, did you find any wires loose?

[keith777]

there wasn't any wires loose when i first opened it. then one of the speaker wires came off. the battery terminals are the same type fitting as the 9v ones but they are way to big to say on. plus only one would go on at a time.

here is the test results for the transistor. see pic.

[dbanner]

This transistor is good. So we have so far eliminated these components that we have checked as the possible cause of problem. Now you can check the remaining transistors on mainboard. You do same test as the last one, remove base(centre)leg and place black probe on centre leg, red probe to each of the other two legs, they should all read similar results as last test. I know its a lot of testing, but at least you will know that all your transistors are good or maybe we will find a bad one.

[keith777]

which is the other transistors. is them the black round thinks. if so how do i know which is the centre leg.

[dbanner]

Yes the four black round ones. Here is the datasheet for those 2N3638 transistors. Have a look, the centre leg is the base leg. You may not have to remove the base leg, just do the tests with them soldered in and post results. You should be able to determine which ones the centre leg by looking at the diagram on datasheet. Its the one in the middle of the triangle. But before you get to that, please check the resistance of this potentiometer(refer to picture). you need to remove the wire shown on picture. Set up meter as shown. Place probes as shown on picture. Then post the results before checking other transistors.

[keith777]

Hi again dbanner.


the test result i got for the potentiometer test was. 011

[dbanner]

Was that .011 or 011?

[keith777]

sorry forgot the .

.011

[dbanner]

Switch down to the 200K scale on multimeter and do the same measurement again. Then place one of the probes on centre lug of potentiometer and other probe on the outer lug where wire was removed. Do this while turning knob. you should see the reading on meter changing as you turn knob.

[keith777]

200k test i got 11.2
on turning the potentiometer the reading on a full turn went from 03.9 to 11.2