Fender Passport 500 Pro SMPS question

[x-pro]

The operating logic is as follows.
Q605 must be "closed" (off) for U605 to start.
For this to happen, Q607 must be "closed". It will be closed if there is no positive voltage greater than 0.6V on its base.
And the positive voltage comes from two lines: through open Q604 and D631. Q604 is open with voltage b-e 0.6V. Earlier you stated its base voltage as 17.338V(post 34). It can be so if Z608 is "leaking". Voltage between Q604 base and D629 cathode = 2.4V.

[x-pro]

The description on R632 turned out to be a bit wrong.
To find out if it affects the operation of the circuit, you need to measure the voltage at the anode of D631.

[JP-Stereo]

Hi, I have removed R631 and nothing change.

With R631 removed, i am measuring this:
Vcc = 15.35 vdc
Q604 base = 14.4 vdc, (So SCR601 is not loading the line.)
D631 anode = 0 vdc
Q607 base = 0 vdc

How much current is needed to kick in Q604 ?
Because with R631 removed Q604 base is at 42 kohm from the Vcc through R629 and R630.

Even if Q605 was off, U605 still do not have the required voltage.


I am meditating on that...

[x-pro]

The voltage b is 14.4 V of Q604 and 15.35 V at the emitter.
Thus, the voltage at the b-e junction is 0.95 V. That's a lot of voltage.
Replace Q604 with a new one.

[x-pro]

Even if Q605 was off, U605 still do not have the required voltage.

I remove Q605 and it pulse again, ...

How?
You'd better make up your mind.

[JP-Stereo]

I will replace Q604 they are easily available.

The board own VCC is still under the U605 required voltage threshold,
But it's working is confirmed when powered with an external PS of more than 16.36 vdc.

"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."
So it was with the external PS.

The VCC from the boards PS is at 15.35 vdc, that why i said that something is loading the VCC line.
The voltage went to 18.585 vdc when i unload the line.

"i have lifted the D615 cathode and read the voltage ---> +18.585 vdc, more than enough to start U605, so something else is loading this VCC line to 15.345 vdc."
Yes there is a problem in the VREF / COMP circuitry, but the VCC line is still too low from this board.
Maybe it's the same defective component that cause both...?

[x-pro]

Try replacing diode D615. It happens that the diode is normal during testing, but under pulse voltage it gets sick.

[JP-Stereo]

Hi, i will order new Q604 and D615 today.
Do you think it's safe to swap this U4003 for a 1N4003, specs look similar and i have a few on hand...?

I've reinstall everything an take some measurments again:
VCC at D615 cathode after 2 minutes = +15.358 vdc.
Q605 = 0.0 vdc
Q607 = 0.0 vdc
Q604 = +14.4 vdc
SCR601 gate = 0.0 vdc
U605 pin 6 = no pulse

I should have look at the REF (Q605 pin8) while powering it with an external PS to confirm if the low voltage cut off also kill the REF, because i don't have any voltage at pin 8 on the board PS.
This is driving the SCR601 and maybe it's why Q604 stay on ?
It's also feeding Q605 base.

[Jazz P Bass]

I cannot find any info on a "U4003" diode.
UF, yes.
U, no.
If it is a UF4003 that is an ultrafast recovery diode.
In that case a plain Jane 1N4003 is not a suitable replacement.

[JP-Stereo]

Your right it's a UF4003, another typo.
I've found a source for "through hole" version but not for the surface mount yet.
Looking at the datasheet, i don't see any particular parameter linked to speed...
But it's called fast !

[x-pro]

es1d-e3/61t, hs1d r3g, uf4001 - uf4004, us1a, us1b, us1d, us1g-e3/61t, es1j, her101 - her105, us1b, us1b m2g, us1d, us1g

[x-pro]

VCC at D615 cathode after 2 minutes = +15.358 vdc.
Q605 = 0.0 vdc
Q607 = 0.0 vdc
Q604 = +14.4 vdc
SCR601 gate = 0.0 vdc
U605 pin 6 = no pulse

You don't need to measure like that. Where are these voltages, collector, base, emitter?

[JP-Stereo]

Hi, All (base) or (gate) voltages...
I found a source for the diode, preparing an order,

[x-pro]

Q605 = 0.0 vdc
Q607 = 0.0 vdc
Q604 = +14.4 vdc

If Q605 base is 0V, then Q607 base cannot be 0V! Especially when Q604 is open.

[JP-Stereo]

Hi, sorry for the lag, i was working on something else while waiting for a D615 replacement.

I have written down the measurment at the time so i will double check or redo them.
But remember that on the low board VCC the REF output is not providing any voltage.
Q605 seam to be triggered by this REF output (U605 pin 8) so it may be normal in the context...