With out a guitar plugged into it it just hums i chkd the power going to speeker wires and its 40 vdc the leds light up finn turning up or down the vol still no change plug a guitar no change plucking strings no sound just huming the amp is a bassman 60 by fender it belongs to a buddy of mine i pulled the pcb and found that where the 2 big caps are there are 2 750 0hm 2watt resistors under them the board is kinda burnd looking but the reverse side shows no broken joints or traces i chked the resistors and they read arround 750 - 753 ohms caps may be bad but im thinking more along the lines of the two diodes could have something to do with it but im not sure
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Re: amp powers up but just hums
The output transistors are probably shorted. There should not be any DC voltage going to the speakers. DO NOT run it this way for long or you will damage the voice coils of the speakers. This unit needs service. Expect a parts cost in the $25-$40 range.
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Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.
Firstly, do not disconnect the speakers from the amp when it is on. Damage is possible.
You have a bad ground somewhere in the hookup. The first suspect would be your guitar-to-amp cord shielding connections at each plug, then, the wiring in your guitar. Try changing each of these to another as a trial, one at a time.
may be one of the electrolytic capacitors in the power supply has become leaky...this is a common problem among amplifiers...so you let it cool down..then after a period of time the capacitor heats up and then causes the hum...the power supply caps filter out the 60 hertz ac line noise...which is probably the frequency of the sound it is making...this is a good indication of these caps going bad....another thing you can check is to see if the amp is properly grounded(eg: the ground pin on the plug is present) and that you dont have an open cord or input
You are looking at the WRONG end of the guitar cable. You have a VERY unsafe guitar amplifier OR the mixer that is connected to that mic has an UNSAFE grounding condition.
If your guitar amp is an old one that has a two wire cord and a switch you change to reduce hum, take it in to have a three wire power cord installed IMMEDIATELY. Only plug into grounded receptacles. This is needed to reduce electrical noise and hum but MORE importantly is personel safety.
If the problem is with the mixer and the electrical service to the mixer, better get a professional in to analyze the problem before somebody is injured.
AS A START do this: VERIFY that ALL interconnected equipment INCLUDING your guitar amp iis powered from the same source or receptacle. This includes the venue power amps, mixer, any CD players, etc. Professionals run power back from the stage source to the mixer alongside their snake to avoid just the problem you are seeing. When they need more power at a venue, a power distribution box is used that conditions the building power and insures grounding.
Try disconnecting the speaker from the amp with the amp unplugged from your power. Never work on an amp when its plugged in unless you know how dangerous it is. Check for resistance across the speaker terminals with a volt ohm meter to see if you have a blown speaker. Thats the first thing to look for. You should here a hiss or a low hum when your amp is on and you plug a guitar cord into the input without the guitar. Check those two things first.
If you haven't already sprayed plug with penetrating spray or similar (WD-40) then do that first. Next, do yourself a favor, go to a hardware or autoparts store & purchase a 12 VDC test light. (about $5-$10) and it will be one of the best investments you ever made as far as tools go. With it, you can test for blown fuses bad wiring etc. Do you know where I'm going with this as in testing for power in those wires? Start at plug on car, if the power is that far, then it's the trailer side that's causing trouble. If no power at plug on car and your not sure how to proceed, get back to us. You did check for fuses under hood as well right?