I have a pcb that takes 5v from a main board on pins 3&6 and takes this current through the T1 Transformer (obsolete, no datasheet S1673305) and should output some voltage (service manual does not say what current).

After the transformer I have two voltage regulators (IC4 & 5) that should output 3v and 1.5v. Both receiving 0v and outputting 0v.

My issue is: I am getting 0v on the transformer output. I checked the coils for continuity and they seem fine.

Not sure where to go from here and what to do to trouble shoot this circuit or if my problem is definitely my transformer.


  • \$\begingroup\$ Do you have any drive signal on pin 16 input..? \$\endgroup\$ – Thedoc8 Jan 3 at 2:10
  • \$\begingroup\$ yes, 0v when DMM is enabled and 5v when it is disabled. \$\endgroup\$ – GuiSoySauce Jan 3 at 3:57

If all of the coils have continuity, then I would suspect the transistors.

First thing is that you seem to expect DC out of your transformer secondary when you put DC to the primary.

Transformers don't work that way.

Only a changing voltage on the primary will cause a voltage to appear on the secondary.

You connected the transformer to a DC powersupply, and got 0VDC on the secondary. This is correct. But, that was a bad idea. Connected that way, a lot of current will flow through the coil, possibly overheating - that can either melt the insulation and cause short circuits in the coil, or cause the wire itself to burn through.

Q9 is supposed to be driven by a pulsed signal. I expect DMMP should be a series of pulses rather than a simple DC signal.

The circuitry around D7 and ZD1 (together with the lower coil and Q10) might form some kind of oscillator (in which case DMMP would be a simple on/off control.)

In any case, Q10 would be my first suspect. Your diagram says you measured 5V on the collector of Q10. That shouldn't be. The voltage there should be fluctuating - that's the changing voltage that drives the transformer. It should measure less than 5VDC if the circuit is operating (your voltmeter can't measure the fast changes, so you will get a sort of average of the high and low values of the pulses.)

Check Q10. Use a diode test from base to collector and base to emitter. Put the red lead on the base. Black lead to collector and emitter. Both should read about 0.7V. If not, then Q10 is toast.

Q9 might be bad, check it the same way as Q10.

But, I suspect Q10. It is driving a coil, and has no protection against the voltage spikes that can occur when switching current through a coil. That transistor can switch around 1A. That amount of current through that coil can cause a large voltage spike if switched off suddenly.

  • 1
    \$\begingroup\$ You were spot on. Q10 was toast! Thanks so much for the explanation. Makes much more sense now. \$\endgroup\$ – GuiSoySauce Jan 7 at 23:14

Could be the transformer. The circuit works this way: when DMMP is high, Q10-C is pulled low, disabling the power supply. When DMMP is low, Q10-C should be high and is no longer part of the circuit. To troubleshoot, remove Q10 and the oscillator network containing Q9 should run, giving the desired outputs. R41 pulls up the base and turns Q9 (NPN, not a PNP) which should then provide a negative voltage pulse on T1-1, turning off Q9 and starting the oscillator. From your measurements, Q9 may be blown, but that could be a secondary failure to the transformer if it is bad.

  • \$\begingroup\$ Thanks for your answer! Was extremely useful. I did a couple tests — Removed Q9 and tested as a diode. BE and BC getting 0.6v. Nothing on reverse. I imagine it is ok then — Removed the Transformer and hooked pins 1&5 and 2&4to my power supply. Getting Zero volts on the other side — The transformer was pulling a lot of current from my power supply… 1A @ 5v not sure if its right. \$\endgroup\$ – GuiSoySauce Jan 4 at 8:45
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    \$\begingroup\$ Not sure you understood - the unit should run without Q10 but must have a transistor in Q9 to oscillate. Connecting DC directly to a transformer will not work - they need an alternating current. The coil on 2 and 4 must be turned on and off using the feedback from the coil on 1 and 5. When the circuit is working properly, expect a periodic waveform on 2 and 4. \$\endgroup\$ – John Birckhead Jan 4 at 13:21

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