Power supply circuit diagram

Blowing fuses, was caused by many thing.
Short of rectifier,resistor,elco,tube heater,tube pin, and also trafo itself (primary or secondary winding). It must check step by step.
CAUTION :
THIS IS HIGH VOLTAGE AREA.
RISK OF ELECTRIC SHOCK THAT CAN KILL.
DON'T DO THIS IF YOU ARE NOT TECHNICIAN OR NOT FAMILIAR WITH VACUM TUBE CIRCUIT. THE CIRCUIT MAY STILL CONTAINS ELECTRIC SHOCK EVEN IF IF THE AMP IS NOT POWER PLUGGED.

The simple steps (see picture) are:

1. Disconnect all connection from secondary of power trafo (PLV,PDCH,PACH and PHV).
   Power ON the amp. If fuse is not blowing,trafo is good.
2. Power OFF the amp. Unplug the cord.
   Connect PLV1-PLV2-PLV3 (this are Bias voltage,+/-15VDC and Backlight PCB voltage) to power trafo.Power ON the amp. If fuse is not blowing,PLV circuit is good.If fuse blowing,there are parts is damage (dioda,elco,7815/7915,resistor) in this PLV circuit.
3. If step 2 is good,go to next step.
   Power OFF the amp. Unplug the cord.
   With PLV circuit stay connected,connect PDCH 1-2 and PACH 1-2 to power trafo. Power ON the amp.
   If fuse is not blowing,PDCH and PACH circuit is good and all tubes heater/filaments are lights after a few seconds.
   If fuse blowing,there are parts is damage (dioda,elco,tube heater pins) in this PDCH/PACH circuit.
4. If step 3 is good,go to next step.
   Power OFF the amp. Unplug the cord.
   With PLV,PDCH,PACH circuit stay connected,connect PHV 1-2 to power trafo.
   Power ON the amp. CAUTION: HIGH VOLTAGE AREA is ON.
   If fuse is not blowing,PHV circuit is good.
   If fuse blowing,there are parts is damage (dioda,elco,tube pins) in this PHV circuit.

.
Notes:

• Steps had to be done sequentially from 1 to 4, not upside down.
• If tube/heater pin is short,it can't be repair. The tube must be replacing.
• PDCH circuit is voltage for V1-2-3-4 heater/filaments.
• PACH circuit is voltage for V5-6-7 heater/filaments.
• Always replace fuse with same type.

John
With AC power off.
Use a meter (on Ohms scale) to check internal fuse for blown (open) fuse.
If you feel qualified & safe with AC Power On.
Check input to internal power supply on AC Scale (Caution when AC is on) for volts in.
Use meter on internal Power Supply for "probably" DC volts leaving internal Power Supply electronics to power the rest of the electronic circuits.
Switch mode power supplies can break down. They stop doing their high frequency switching so they don't convert AC to DC.
If this P/Sup is a separate box or circuit board, inside the unit, then it can have a part number & be replaced.
Good Luck

Blowing a fuse means that it has a dead short. The fuse does its job by protecting the other components while the circuit fails. Something is definitely burned out inside.

Transformers RARELY fail. It is likely that either one side of the amplifier has shorted or there is a failure in the rectifier/filter circuit of the power supply. Take it for repair unless you are adept at electronic repair. Do NOT continue to replace the fuse as you MAY cause futher damage. Use a 25 or 40 watt lamp in series with your power cord to limit current while troubleshooting.

Either a short in the power supply such as diode(s), capacitor, or transistor(s) or in the power amp. It will require service. Replacing the fuse before repair will likely damage more components... don't do it!

The relay shorts out two power resistors that limit the input power surge. The circuit is complex a bit. An off-line small switching power supply fires up and "heats up" the rest of the circuit for the main power switcher. I have repaired about four of these. Usually a failure due to arcing in the power amp section fries three transistors there and often a Zener diode. This then wipes out the main +/- 70 volt supply. The two IGBT's are usually blown including the four driver traansistors and the switching regulator chip and another small surface mounted transistor and one or both speedup surface mounted 1N4148's. IF THE fuse had been replaced to try again, often the two power resistors (sand coated ceramic) that ar 10 Watt either 10 or 20 ohms will be blown. Some of the hardest thing is to get the heatsinks loose with the IGBT's. When restarting the supply AFTER repair, one puts about 100 Watt lamp bulb in series with the 120 power input to act as a limiting fuse. You will NOT get far with this without a schematic. You can contact me by direct email fredy2 @ aol.com

Why did you replace the fuse? If it was blown then you need to take the amp in for servicing. Iffthe fuse is glass and it was silvery on the inside, that is a hard short and is likely a power supply problem. Probably a rectifier diode or filter cap. Pray that it isn't the power transformer. Diodes are cheap... few pennies... caps a few bucks at most. Power transformer... you DO NOT want to know.

The fuse blows because of a short in the Power Supply. The power transformers primary windings are probably shorted. next in the chain would be a bridge rectifier or a quad of diodes forming a bridge rectifier with a couple surge diodes off of the filter caps. so if you can disconnect the power from the amp, on the power tranny disconnect the primary wires ( the input AC power wires) usually white and black and take an OHM reading with a mulimeter accross the white and black wires. you should get a reading of 15 ohms or higher up to 30 or so. if so the primary windings are good. so then check the secondary (output windings) red and red yellow, etc. ohm reading and you should get a relatively low ohm reading 2-6 approx. if so its a good tranny. on the primary side if you get a real low reading for ohms , like 1 or something, its toast. once you rule out the tranny, switch your digital mulitmeter to read diodes (in the ohms area looks like a diode symbol (a line and a solid triangle with the line being at the tip end of the triangle). desolder one leg of the diode and put one probe on one side and the other probe on the other side. it should only ring out in one direction. you will be testing approx 6 diodes in this power grid on the pcb. you can also desolder the filter caps after you safely discharge the voltage with a 1 ohm 1 watt resistor to ground and use an analog volage meter to see the needle rise in one direction and not in the other (switching the test probes from one side to the other on the cap). usually what i do is just spend 20 bucks on all those parts from mouser.com and instead of desoldering to test i desolder to replace with new. diodes usually part number 1N4003 or 1N4004 somewhere in that ballbark. hope that helps. i think its your power tranny primary winding that is shorted, be aware that this might domino effect into the rectifier and caps, so expect to replace them. its super easy!! you can email me from my website yostamplifier.com

This is a serious problem. If you spilled liquid into the unit, expect serious internal damage.

DO NOT keep replacing fuses as you may be creating further damage.

Best scenario is if the input to the power supply has shorted diodes or capacitors. Worst is that part of the circuit board has been burned.

Taking it to a shop or having electronic skills and opening the unit up to troubleshoot are the options.

As you probably know the regulator also supplies power to the other ICs on the board. If the fuse keeps blowing that means that one of the parts drawing power is shorted. By looking the pin configuration up for each IC you can find which one is the + and which one is the- etc. The faulty IC will be showing voltages on the other pins where they shouldn't be. That is usually the defective part. You will also want to check parts that are related to or in the same circuit. The worse case scenario is that the main IC is shorted. Without the schematic its a long arduous troubleshooting procedure but it can be done and I just thought I would suggest it. I mean know pffence and just wanted to help somewhat.