Finding a replacement for an undocumented transformer

[bsfeechannel]

Will that may have caused one specific leg of Q10 to fail?

And this may well be the cause of the failure of Q10. C24 is part of the AGC (Automatic Gain Control) loop of the oscillator. Installed the wrong way, it may have forced Q10 to work outside its ideal operating point and caused premature failure. Just a guess.

The one that failed was B-C which flows from R41 which flows from DMMV.

Q10 base to emitter was fine.

Would it be more likely a surge DMMV considering the supply block of the MB was affected by an electrolytic leak?

Q10 should take 50v/150ma. Q9 50v/5a.

If not from the capacitor you mention. From where else and what could have caused the failure of Q10?

I said Q10, but I had Q9 in mind. So thank you for correcting me.

[gkmaia]

That is encouraging! update and a bit of a background!

When I bought this scope all caps on the motherboard power supply block were leaking and some damage was made. For the DMM that was a battery leak that damaged most of the power supply block.

So the mother board is now fixed, traces rebuilt and new inductors are quiet! Could manager to get both scope channels working and the logic probe as well! Without the DMM board installed.

The signal generator should arrive this week and I will be able to test the transformer.

In the mean time I will start replacing the caps, diodes and Q10 on DMM board.

[gkmaia]

The signal generator finally arrived and I finally tested the transformer

To start with a Sine, 15khz, 5v straight to my scope reads as a 0.5v as well as on my multimeter. I am not sure why, but seems the signal generator says it generates at 5v but what comes out is 0.5v.

So all tests have been made with this 5v / 0.5v reading

@15khz input on pins 1-5 outputs 0.82v on pins 6-8
@15khz, 5v input on pins 2-4 outputs 0.82v on pins 6-8

If I raise the frequency to peak output

@190khz, 5v input on pins 1-5 outputs 1.93v on pins 6-8
@190khz, 5v input on pins 2-4 outputs 0.9v on pins 6-8

I cannot really scope the DMM board as it is part of the Scope.

Would it be a good idea to attach a 5v power supply to DMMV and see if it oscillates and if it does capture the frequency?

I imagine the DMMP does not have an influence in the oscillation as it seems to be there just to get Q10 active and divert the current away from Q9.

[james_s]

It sounds like the transformer is probably ok. I'm not surprised the DMM doesn't properly read a 15kHz signal but it should display properly on a scope. Are you using a 10x probe with the scope set up for a 1x probe?

[Armadillo]

You are risking your signal generator.

The transformer resistance is so low and the impedance could be low enough to fried the output of the signal generator if you are in the 50ohms output mode.

Apparently, its a impedance mismatch. It's not a way to use your signal generator without an appropriate buffer for the type of load.

Why don't you ring test the transformer if you are so concern about.?

The signal generator finally arrived and I finally tested the transformer

[gkmaia]

It sounds like the transformer is probably ok. I'm not surprised the DMM doesn't properly read a 15kHz signal but it should display properly on a scope. Are you using a 10x probe with the scope set up for a 1x probe?

Yes, you are right. It attenuates by 10x.

So I am getting then 8v on the secondary. That feeds two regulators. Both with max input of 15v and output of 1.5v and 3v.  So I am in range here.

Considering the input/output differential for these regulators is 0.5v to 1.7v. That gives me a dropout range of 5.0v and 4.0v respectively. Around 50% of the voltage there is lost.

Am I correct?

[bsfeechannel]

This is not a linear transformer. The voltage on the secondary will be a function of the turns ratio AND the duty cycle. For a flyback transformer, V_S = V_P · N_S · D /( ( 1 - D ) · N_P ), where:

V_S is the voltage on the secondary;
V_P is the voltage on the primary;
N_S is the number of turns of the secondary;
D is the duty cycle;
and N_P: is number of turns of the primary.

Duty cycles tend to be around 40%, which means that the voltage on the secondary will be around 5V.

I presume this flyback converter is there just because the circuitry around IC1 seems to be galvanically isolated from the rest.

[gkmaia]

I am planning to test if the circuit oscillates before I assemble the whole thing.

For that I am planning to plug a 5v DC source to pins GND and DMM. Also remove R48 & 49 so I stop the current from flowing after it passes the transformer.

By leaving DMMV low on Q9 base I imagine it will force the current through the oscillation block.

I will use my bench top DC digital power supply.

Should I do not do that? Anything I should think ahead so I do not blow anything? Any advise?

[gkmaia]

I got great news!!! So decided to plug it in and it WORKS!

Was indeed a faulty Q10.

Thanks to all of you guys for helping me learn more about electronics and fix my gear. Also thanks for the patience with my beginners silly questions!