Fender Passport 500 Pro SMPS question

[x-pro]

UNDERVOLTAGE LOCKOUT SECTION Startup Threshold UCX844 - 16 V (14.5-17.5V)
https://www.onsemi.com/pdf/datasheet/uc3844-d.pdf

If there are no pulses on pin 6, connect temporarily 10kOhm between pin 7 (+Vcc) of U605 and the top pin of the TP600 trim resistor

[x-pro]

In the general case of troubleshooting in SMPS in the absence of visible defects of elements, we start with checking all active elements with a multimeter (in diode mode) directly on the board, without unsoldering anything. Then we switch the multimeter to the small resistance measurement mode and check small resistors and capacitors.
In most cases this speeds up the work.
And then we turn on our brains.

[JP-Stereo]

I've not followed this exact path but i was trying to prioritize the elimination of the mostly obvious suspects...
Now i understand a little more these SMPS, this one have 2 or 3 interlaced switcher from wath i understand...

After removing Q605 the circuit is coming alive !!!
But i have to crank the VCC to at least 16.36vdc and then i get a strong 15vdc pulse train.



I hope the old IC was draining the line, because with it i was reading only about 15.3vdc on the VCC so it's not enough to start it.
I've tried VCC up to 18vdc before the removal of Q605 and it was not working.

At this point i figure i have to replace Q605 and hope... ?

[x-pro]

Q605 can also be replaced.
But it may not be the culprit.
There are many other elements in front of it that can be faulty. And they affect Q605 by their action.

[x-pro]

Inspection (may require unsoldering of the part)
In order of priority:
1. Q605
2. С670
3. Q607
4. SCR601 check for open circuit
5. Z607
6. Q606
7. Q604

[JP-Stereo]

You are right on !

I am making progress:
Q605: It pass the diode test for a PNP device, i didn't look at the HFE.
C670: Testing half the specified value at 500nf and the reading is not stable = replaced.
Q607: Pass the diode test as a NPN desoldered.
SCR601: Always open, diode reading of 0.688v Gate to Anode, i feed it 5v directly and cannot get more than 0.100v at the cathode when triggered. (Prime suspect)
Z607: Pass the diode test (0.727v) on the board.
Q606: Pass the diode test as a NPN desoldered.
Q604: Pass the diode test as a PNP

I replace SCR601 or is there anything else to verify ?

[x-pro]

SCR601 ! Are you checking it properly? It's a thyristor. It will have a voltage drop of 0.6-0.7V at the G-C junction.
We should try to replace SCR601. Check TH600 for open circuit.
The other parts from the list above should be soldered in place.
Leave the parts on pin 6 of U605 for now. We need to get the chip to work from an external lab power supply.

[JP-Stereo]

"SCR601 ! Are you checking it properly? It's a thyristor.
We should try to replace SCR601."

First i test it with a multimeter in diode mode.
No continuity anode to cathode and reverse, but the gate act like a diode toward the anode.

I supplied 5vdc to the anode and trigger the gate with 5v via a 1kohm resistor.
It should latch closed but it's not, instead i have only about 0.100 vdc when triggered.
I will compare these readings with a new one when i find a supply.
It look like these MCR100-6 are no longer manufactured and somewhat rare.
I read about substituing a 2SF656, it seem easier to source.


About T600, i think i will have to desolder it because everything after pin 10 is testing short/connected exept 15 and 16.

I've tested all the transistor and SCR unsoldered, i will reinstal them.
Pin 6 will stay in "test mode"...

That's a lot of debugging help : )

[x-pro]

"but the gate act like a diode toward the anode."
That's not right. The control is relative to the cathode. This is the cathode, since it has one pn junction. Look at the structure of the thyristor.

"I supplied 5vdc to the anode and trigger the gate with 5v via a 1kohm resistor.
It should latch closed but it's not, instead i have only about 0.100 vdc when triggered."
Also a misconception.
Some load with some current is connected to the anode. Obligatory. And the cathode is to the minus of the power supply.
If the current flowing through the thyristor is low, it will not latch.

"About T600, i think i will have to desolder it because everything after pin 10 is testing short/connected exept 15 and 16."
Message 24 reads incorrectly. It refers to the TH600, not the T600.
T600. The windings are not shorted, just have low impedance resistance.

Here's what we're going to do next.
Reinstall all items except SCR601.
C670 is new.
We check it in the test mode.
If there are no pulses, measure the voltages on the elements.

[JP-Stereo]

Hi, i've reinstall everything except SCR601 but U605 still dead.
I remove Q605 and it pulse again, so it's something else or something additionnal to SCR601 that is blocking U605.

What should i probe at this point ? and with or without Q605 ?

Thank you very much again.

[x-pro]

what is the voltage on b Q605 and b Q607 ? or unsolder D631 and check the pulses

Also, check RJ617 (0 ohm), R651(10 ohm) for compliance with the rating.

[JP-Stereo]

Hi again !

Q605 = (b) 0.015vdc
Q606 = (b) 0.700vdc

D631 unsoldered = No Pulse

RJ617 = 0.08 ohm
R651 = 10.01 ohm

D630 pass the diode test

[x-pro]

If there are no pulses, unsolder R636.

[JP-Stereo]

Hi, we are near something...
When i removed R636 the pulse came back strong.
So i reinstalled D631 and loose the pulse again.
This mean that there is something before both that affect U605

SCR601, D631 and R636 removed.
Pin 6 in test mode.
We have a pulse.

[JP-Stereo]

I reinstall SCR601 and still have a pulse.

D629, Z608 and Z607 all pass the diode test on board.