Fender Passport 500 Pro SMPS question

[JP-Stereo]

Hi, i have a Passport 500 Pro that began making some strange noise and worsen to flashing Line Level leds red and green than no sound.
I seem like a few members are knowing this amp better than me...

After studying a few threads about this amp i made the following verifications.
- All the voltages on JA1 connector are OK.
- D615 is sending 15vdc to U605, should it be 18vdc ?.
- But i do not have +/- 50vdc at JA2 only +12/-13vdc.
- Q600 and Q601 are OK.

The amp board and the mixer are disconnected from the SMPS

Should i trick a fake a protection or another signal line to enable the high voltage rails ?

Thanks

[x-pro]

U605 receives 18 V via D615 from transformer winding 16-15.

What kind of weed do the circuit designers at Fender smoke that they come up with such! circuits (just kidding) and substitute the usual designation of TIP31C/TIP32C transistors with a chip designation.

First, check C653, be sure to unsolder it to check. Next, unsolder C655, put a 100-200 ohm resistor in parallel with D627 and observe pulses on pin 6 of U605 with an oscilloscope.

[JP-Stereo]

Hi, thanks a lot for the hints.
I have about 15.5 vdc after D612.
I have removed and check C653, it was reading good but with a small instability so i change it for a new one.
C655 was 102pf, i let it beside and put a 120ohm 1/2 watt at it's place.

Now proceeding to read U605 pin 6...

[x-pro]

Next, unsolder C655, put a 100-200 ohm resistor in parallel with D627 and observe pulses on pin 6 of U605 with an oscilloscope.

the point is to temporarily disconnect the transformer winding from pin 6 of U605.
120 ohms not instead of the capacitor, but in parallel with the D627

[JP-Stereo]

Hi, i did power it on and nothing serious on U605 pin 6, AC or DC.
I do detect positive and negative assimetric spikes of about 250mv, a little stronger on the negative side.



It's begining to look like a dead UC3844 ?

[x-pro]

do you have a 15-20V 0.5A lab power supply ?

[x-pro]

"a dead UC3844 ?"
maybe.
To be sure, you need an external 18V-20V power supply.

[x-pro]

you can try connecting 2 9V batteries in series
------------
You don't have to do the tambourine dance. Install a new chip at once, if you have one on hand.

[JP-Stereo]

OKI i got it.
The idea is to put a load on pin 6 to ground in order to see the output.
So C655 removed and D627 bypassed by a 120 ohm 1/4w this time.
I still have the same reading, a little more small spike in DC than AC.



Yes i do have a variable power supply, butr is it safer with 2 batteries ?
And yes i have an IC on hand.

[x-pro]

Connect a different power source to the U605, without mains power. When were 9V batteries dangerous?

[JP-Stereo]

Hi, about the batteries, i mean probably using batteries IS safer than an AC connected power supply because of grounding issues.
The 15v dc line following D615 is powering a lot of components around U605, but is it enough to wake it up ?
But if the main power if off, what kind of signal can we expect from this IC ?

[x-pro]

Connect an external power supply to test the operation of the U605 without the mains power supply.
I have indicated this in post #10.
15V supply only powers the chip. Other elements connected to it are various protection circuits.

Neither D608 nor D615 even need to be unsoldered. Connect 15V between pins 7 and 5 of U605.
If the chip is good and the surrounding elements are also good, there should be rectangular pulses on pin 6

[JP-Stereo]

Good day !
I change U605 and feed it 15vdc directly to pin 7 and ground to pin 5.
Now absolutly no output even the previous spikes have disapeared.
I have verified the ground and VCC at the IC pins.
And D627 is not short because i read only the 110ohm of the bypass resistor from pin 6 to ground and reverse.

I will begin to look for an obvious short, it must be one of those diodes or transistors around RT/CT, VFB or COMP.

Do i miss something ?

[x-pro]

Okay. Then unsolder Q605

[x-pro]

U605 needs 18V