Hello and How do I check the voltage of this SMPS?

[Switchy Cat]

I am a novice teaching myself basic electronic repair. I want to fix this SMPS which seems to have a problem with voltage regulation and I am just starting to troubleshoot it.

***The power LED is blinking every few seconds (protection I assume)

The first question, is how do I check the voltage output? I assumed I could check VDD and VSS with a DMM, is that wrong? Can I check it without a load?

I attached a detail of the secondary SMPS section below.

 

[ramussons]

Sorry, but if you are a novice, you have a long way ahead before trying to repair SMPS units. Please be aware that everything left of that transformer has Lethal High Voltage enough to be fatal.

Having given that caution, most SMPS need to have a minimum load for proper operation. I don't see the specifications of this PS unit, and I cannot give more suggestions.

 

[(*steve*)]

I would 1) check the output caps for high esr, 2) check for shorted rectifiers, 3) remove it from its load, possibly applying a smaller known load to it and then see how it goes.

 

[Switchy Cat]

I would 1) check the output caps for high esr, 2) check for shorted rectifiers, 3) remove it from its load, possibly applying a smaller known load to it and then see how it goes.

I only had a chance to check the voltage without a load. I got as high as 80v DC and it should be closer to 28v.
I'm believing the problem is in the feedback and reference section? Possibly even the optocoupler.

 

[(*steve*)]

You need a load. I'm not sure what the rates load is for this supply, but if it exceeds an amp it so then a 100W light bulb may be an acceptable load (don't expect it to light brightly (it may just glow a little red).

Measure the voltage on both the AC and DC ranged of your multimeter, and tell us exactly what multimeter you have.

 

[73's de Edd]

Sir Switchy Cat . . . . .

Is the LED you mention, being the D29 RED main power indicator LED ?
If so, its power is being derived from the +15 DC regulated buss being provided by U200.
See it in the bottom left corners GREY inset reference snippet, with those 2 parts being RED flagged / outlined.

Monitor back at U200 pin 3 with DVM in DC mode and with ground at the shared junction of diodes D207 and 208.
Is it THIS supply that is dropping its power to the LED at the instant of blink ?

Now . . . .IF it is being, the mentioned SMPS section, the regulatory portion is located within the YELLOW lined in area.
Three branches from power supply secondary voltage samplings and ground come down into the circuitry and pair up in the GREEN groupings of resistors that make up a voltage divider to feed to the control input pin #1 of the precision adjustable zener . . .U303.
That U303 unit is in a series loop from the Vdd supply to the D320 zener that needs the VCC-VSS voltages to exceed 30 V for conduction. Then the LED within VIOLET flagged U302 optical isolator will proportionattively conduct and illuminate its other halfs phototransistor over at the other RED star location into the controller pin 2.
So monitor across the pins 3-4 of the U302 to see if any voltage variance there, is mating up timing with your RED LED drop out .

Not really expecting it . . . but the overtemp circuit has its RED star connection going to that same other RED star junction of pin 2 (FB) of U301 main controller, and shunts that junction to ground thru a 100 ohm resistance when that OT circuitry activates.
Temporary shorting transistor Q304's C to E could defeat its effect on pin 2 for an instant of testing of its possible engagement.

Thaaaaaaasssssssit . . . . . .

The WHOLE Enchilada . . . . .

https://i.ibb.co/6ZYK7tZ/FENDER-Sw-Mode-Power-Supply-Snippet.png


73's de Edd . . . . . .

If the police arrest a mute, do they even need to tell him that he has the right to remain silent ?


.

 

[Switchy Cat]

Sir Switchy Cat . . . . .

Is the LED you mention, being the D29 RED main power indicator LED ?
If so, its power is being derived from the +15 DC regulated buss being provided by U200.
See it in the bottom left corners GREY inset reference snippet, with those 2 parts being RED flagged / outlined.

Monitor back at U200 pin 3 with DVM in DC mode and with ground at the shared junction of diodes D207 and 208.
Is it THIS supply that is dropping its power to the LED at the instant of blink ?

Now . . . .IF it is being, the mentioned SMPS section, the regulatory portion is located within the YELLOW lined in area.
Three branches from power supply secondary voltage samplings and ground come down into the circuitry and pair up in the GREEN groupings of resistors that make up a voltage divider to feed to the control input pin #1 of the precision adjustable zener . . .U303.
That U303 unit is in a series loop from the Vdd supply to the D320 zener that needs the VCC-VSS voltages to exceed 30 V for conduction. Then the LED within VIOLET flagged U302 optical isolator will proportionattively conduct and illuminate its other halfs phototransistor over at the other RED star location into the controller pin 2.
So monitor across the pins 3-4 of the U302 to see if any voltage variance there, is mating up timing with your RED LED drop out .

Not really expecting it . . . but the overtemp circuit has its RED star connection going to that same other RED star junction of pin 2 (FB) of U301 main controller, and shunts that junction to ground thru a 100 ohm resistance when that OT circuitry activates.
Temporary shorting transistor Q304's C to E could defeat its effect on pin 2 for an instant of testing of its possible engagement.

Thaaaaaaasssssssit . . . . . .

The WHOLE Enchilada . . . . .

https://i.ibb.co/6ZYK7tZ/FENDER-Sw-Mode-Power-Supply-Snippet.png


73's de Edd . . . . . .

If the police arrest a mute, do they even need to tell him that he has the right to remain silent ?


.

Wow! This is most helpful and insightful! I appreciate your guidance and time! I've been asking for help in other forums, and most people tell me to junk the amp. But it's not about the amp value, I want to learn what I can from this.

And yes, it is D29 when I say the power led is blinking.

*I do have reservations of running the whole amp assembled because the POWER AMP board blew two capacitors when I left the amp on for too long.

I'll take my time and follow your instructions. Thank you very much!