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Because some component in the amp is failing/failed and drawing too much current. The amp needs a repair shop. Poking around without knowing what you're doing can expose you to leather voltage. Don't do it.
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:
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.
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.
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.
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.
A voltage "regulator" would only limit the voltage from going above a set amount - it wouldn't do anything for a power dip. To protect against that you really need an Uninterruptable Power Supply (UPS) which would switch to battery power in case of a power dip. Do you really think you're home electrical system is that unstable? Are your problems happening in different locations?
Your AC adapter actually should have a fuse inside it to meet EU safety
and CE safety requirments. Getting that apart is the challenge.
Test it with a voltmeter first.
If there is voltage, check to make sure the coaxial connector center post is the right size for the plug. If it is, you will need to open the cabinet. There might be a fuse present because of the battery option, in which case that fuse may be failed, a resistor in close vicinity may have failed as well. A broken wire or solder track would be more likely.
If no voltage output from supply, 2 ways to gain entry into a glued wall wart are not the prettiest, but it gets you into them. It saves buying a new one. One is to use a knife carefully and slice through the plastic at the seam where the 2 shells come together, it takes a while. The other method is brutish and quick- a side cutter/diagonal cutter on the corners is usually enough to break them on the seam.
Then check the fuse itself for continuity. If the fuse checks okay, carefully set it so you can apply power to it and check the rectifiers to see if any burned out- you want to essentially check the unit to see if the rectifiers simply failed.If it is a transformer supply, measure at the point where the transformer leads join the bridge rectifier quad. If there is AC present, the rectifiers failed. If there is no voltage/currnet- just check for voltage- the transformer burned out. If the unit is a switch mode type supply, check the rectifiers with an ohm meter with no power applied from the mains. You may or may not see any visigly failing capacitors- if you do those can be replaced, or you can simply replace the supply. This latter type might be held together with screws- it will certainly have less heft to it.
Transformers USUALLY don't get burned as they are usually protected by a fuse. This one supposedly uses a 3 amp fuse which should have protected it from getting burned. If you can find a schematic or if you can find the specs on the output amp power supplies we can probably calculate the voltage of secondary. We know only that it is supposedly 120 Watts and uses an 8 ohm speaker. The RMS voltage should be about square-root 120*8 = sqrt 960 = 31V*1.4 = approx 44 volts peak... so you will need to get about +/-45 volts out of the supply for headroom... which will need approximately 34 volts AC given diode drop and droop of filter caps. These are really rough educated calculations from the tiny bit of info we have.
Finding a transformer to replace this matching these requirements will be near impossible without getting device from the original manufacturer OR having your rewound.
I would first verify the transformer is bad (only plausible cause is somebody over-fused it or a winding shorted)... Remove all the secondary loads. Put a 3 amp fuse in AND put a 60 Watt light bulb in series with teh power cord to act as a limiting device to test the transformer... Note that transformers DO have a bit of odor as they are usually impregnated with a varnish...
The YPT220 is similar to many of the PSR keyboards and will use the same adapter. MOST, like your YPT220, use an adapter with the center pin positive AND are protected by a series diode in the keyboard such that damage will NOT occur if wrong polarity is used. You should use an adapter that provides 10 to 12 volts with positive on the pin and negative on the shell capable of 1 amp (1000ma).
http://www.fender.com/support/amp_schematics/pdfs/Bassman_400_Schematic.pdf Also note there is a thermal protection circuit and a current overload. If the amp turns on and there is no sound start with the speaker to make sure it isn't blown and work backwards through the wiring and connections up to the crossover circuit (all in one cabinet version). On the head separate versions check for a broken output jack problem. The fuses are 10 amp 250 volts for the 100-120 volt models and 5 amps 250 volts for the 230-240 volt models. NOTE: If you are at all unsure about opening your amp up to work on it, take it in to be serviced. It doesn't sound like a fuse problem to me. Necer work on an amp while the power cord is plugged in. Always replace parts with the proper replacement especially fuses. They are there to protect the circuits. If they keep blowing it means there is an internal problem. Remember that capacitors store a lethal charge and it is easy to touch the wrong thing when you are looking inside an amp even if it has been disconnected from power for a while.
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