Fender Bronco 40 Power Supply Troubleshooting

[Gahoo]

Hi all, got one of these new-ish Fender Bronco 40 combo modelling bass amps on the bench. Customer said the unit won't power on and I've verified that. Took it apart and assumed it'd just be the fuse but the fuse is fine. Measured the voltages coming out of the power supply (PS is on it's own dedicated PCB) and am not getting the correct readings. Called Fender and got the schematic here.



Can anyone help me make sense of this? Looks like that PWM controller creates the AC for the primary side of T301? Not getting any AC on the secondaries...

I'm getting 163VDC at pins 1 and 2 of T301 and -163VDC at pins 4 and 3 of T301. Got those same voltages at the rectified end of D301 and D302 which makes me think something in that PWM mess is duffed.

Thanks!

[52 Bill]

I've not seen one of these before, but generally speaking I find more problems on the secondary side of most of these power supplies rather than the primary side. Especially when the fuse is not blown. I'd test all of the diodes in the low voltage circuits first.

Oh and be very careful working on these, as the primary side will store enough energy to really hurt you.

[Enzo]

These can be lethal, they directly rectify the mains. I always use an isolation transformer to service them.

IMPORTANT: note there are several ground symbols. The power cord has on for the chassis, the secondary have the empty triangle one for the power supplies, and the primary circuits have yet another.

So those "grounds" in the primary circuit are NOT chassis ground. Any voltage readings taken to chassis will not be trustworthy. You need to use the ground symbol there as a reference. Your -163 is probably pretty close, because that "ground" sits at about -170v.

The power supply operated at a very high frequency, I don't know offhand, but I'd guess 50kHz to 100kHz. I doubt yuor meter will respond to that. But that is what will come out of pin 5 of the IC.

You might check all the diodes on the secondary side for shorts.

[Gahoo]

Thanks for pointing me in the right direction. When using the appropriate ground for the section I'm measuring, the values make a lot more sense.

I still wasn't getting the right voltages out of the power supply at J302 so I removed the board, drained the filter caps, cleaned the PCB thoroughly, and retouched some iffy solder points. Plugged the board back in and all was well. Got 30V at VCC, -28V or so at VEE and 9V at the 6V line. (The power supply wasn't plugged in to the rest of the amp so these are unloaded voltages).

Plugged the PS back into the rest of the amp and it still wouldn't power up. Pulled the PS to check voltages again and I notice I'm getting 20V at VCC, -12V at VEE, and 5V at the 6V line. All these voltages are slowly dropping as well.

If I drain the caps and reapply power to the board I can get the correct voltages at first but they slowly start to drop again.

Is something leeching the output here? Should I suspect C318 and C319 on the secondary? Something else I'm missing?

Thanks in advance

[52 Bill]

Do the voltages stay normal when the supply is not connected to the rest of the amp or do they start to drop on their own?

Remember that these types of supplies sense current draw and will start to shut down when there is a problem detected.

[Gahoo]

They start dropping on their own, even when not connected to the rest of the amp

[Enzo]

Scope the output voltages of the supply. Are all CLEAN? If the filtration has failed, the spiky result might trigger the thing to protect itself.

When it starts to fade, what are the primary side thingies doing? PIn 2 of the IC is controlled by feedback through the opto. R315 senses current through the switching FET. That sense is input to the temperature circuit just below, whose output ALSO controls pins 2 of the IC.