MOSFET is not sending voltage to drain when the gate get 12.2V and source gets12.6V

Semiconductor Discrete Semiconductor Products
1

The MOSFET is a TPC8125 and it was just replaced by a new one that was purchased here, but the problem persists. The MOSFET in question is in the circuit diagram labeled as Q101. How do I troubleshoot it?
Link to the circuit diagram

2

Hi @solver ,
TPC8125 is an enhancement type MOSFET thus it should be off when the gate voltage is near to the source voltage, thus your device works as expected. Your problem is elsewhere.

  1. What is your input voltage?
  2. What is the voltage at Q102 Base?

Cheers, heke

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  1. The gate and source get 12.22V and 12.62V respectively.
  2. The drain get 1.4V
  3. The voltage is being sent by Q100 pin2 is 12.62V and pin3 is 12.22V
  4. Q102 is supposed to send voltage via R116 but neither have continuity with either the gate nor the source.
  5. Q102 pin3 and R116 get 12.18V, the other two pins on Q102 get 0 volts
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close up of PCB
wider view of PCB

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Tested all the Q100 type transistors for resistance and found that everyone except one read either .380M to .400M or 78K when the positive and negative were on a pair of pins and reversed - meaning the readings were the same both ways. On one of the transistors, when one pin was connected to the negative probe the reading was always OL, otherwise the readings were consistent with the rest of the transistors and approximately the same when placed one way and reversed.

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Hi @solver ,

I wonder, could you measure the voltage at the following points:

  1. At wire labelled “SOFTSW_JACK” (Connects to CN104)
  2. At wire labelled “ERP_CTRL” (Connects to R140)

Cheers, heke

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  1. 12.58V throughout
  2. 0V from ERP_CTRL
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Hi @solver ,

The “SOFTSW_JACK” voltage should be zero or close to. Please find out why it is not. What is connected to the connector CN104 (I.e… what is that “SoftSW” stuff anyhow)? If you short the CN104 pins, what happens?

Cheers, heke

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9

The “SOFTSW_JACK” gets its voltage from
Q104 from the same line as Q100. Why should it be zero or close to?

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Hi @solver ,

That’s is exactly why your Q101 stays OFF.
Please answer the questions.
Cheers, heke

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11

After connecting the two dots for CN104 the voltage was taken out of the line, the drain got its voltage, leading to voltage regulator getting its proper voltage and output. But now I altered the design of the circuit and if I left it this way it could take away from some other intended functionality. What needs to be done next?
How did you know that it was that line that was keeping Q102 from sending voltage?

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So now the device stays on and cannot be turned off other than by pulling out the cord. It appears that the line leading to CN104 is activated by default but it was not designed that way. The Q transistors passed the diode test and when checked for resistance every pair of pins had sufficient and equal resistance with the positive-negative probes set one way and reversed.

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Hi @solver ,
Good news. The circuit works. It is expected that you cannot turn the unit off, while the CN104 is permanently shorted.
Now you’d need to check out what kind of thing is normally connected to the CN104 connector. There is probably a tactile push-button switch on the front panel (Power button). I’d guess the button is broken, i.e. cannot create a contact when pressed. Check with continuity tester if the button works or not.
Cheers, heke

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14

The on/off button is just a push button like the TS02-66-43-BK-100-LCR-D and there is continuity between its solder point the rest of the board.
I traced 12.57V from pin 1 to one side of a diode DA2J10100L. The other side measured in mA.
I assumed that the voltage was reaching the cathode, but when testing in diode mode I only got a reading when I put the positive lead on the opposite end, the one that is not getting the voltage. The reading validated it as good 0.588V, but based on the voltage trace it seems to have been installed in a reverse direction.

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The circuit diagram shows that the diode is supposed to face in the opposite direction of the flow of voltage of 12.58V.

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Hi @solver ,
If your amp got alive when sorting the CN104, then there is no need to ponder about diodes.
The schematics says that the CN104 is “UnPop”. It may suggest that the connector is not populated to the board (i.e. is unused - probably an option for remote ON/OFF control). Then the Power ON event must be generated by the “MAIN BOARD” via connector CN7. Please provide the schematics of the main board.
Cheers, heke

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18

Thank you for all your help. I think the problem is solved.

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