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Post subject: Fender Bassman 300 Pro - No output
Posted: Sat Jan 07, 2012 9:59 am
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The other night I turned on my amp and the Power LED did not come on. The power tube LEDs are all Red in Standby and all of them do turn Green when brought out of Standby. Everything on the front of the amp appears to be working; all of the LEDs light up and I see input signal via the Input Level LED.

Today, as I was doing some rudimentary troubleshooting, I noticed that the tubes are not glowing. When I was reseating tubes I also noticed that when just a pair of tubes are plugged in, and the amp is brought out of Standby, all of the power tube LEDs turn green. I am not sure if that is normal, but it seemed a bit odd.

I have checked the four fuses that I know about and they all appear to be in good shape.

I have a show in around a week, so this could not have come at a worse time. If anyone has any ideas as to what might be the cause of this issue I would appreciate the help.


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Sun Jan 08, 2012 6:08 am
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glummoon wrote:
The other night I turned on my amp and the Power LED did not come on. The power tube LEDs are all Red in Standby and all of them do turn Green when brought out of Standby. Everything on the front of the amp appears to be working; all of the LEDs light up and I see input signal via the Input Level LED.

Today, as I was doing some rudimentary troubleshooting, I noticed that the tubes are not glowing. When I was reseating tubes I also noticed that when just a pair of tubes are plugged in, and the amp is brought out of Standby, all of the power tube LEDs turn green. I am not sure if that is normal, but it seemed a bit odd.

I have checked the four fuses that I know about and they all appear to be in good shape.

I have a show in around a week, so this could not have come at a worse time. If anyone has any ideas as to what might be the cause of this issue I would appreciate the help.



In your quote above, I hi-lited one statement. APPEAR to be in good shape may still be an issue. SOMETIMES - especially in a high wattage bass amp or combo - the cabinet and amp is really vibrated by the speaker system. If ANY fuse has enough shake over time, the solder holding the fuse element in place inside the glass envelope can separate causing the fuse to APPEAR to be OK, while in fact, one end or the other is OPEN. This OPEN condition is hidden from view by the metal caps on both ends of the fuse and you can not see the element separation. The ONLY way to troubleshoot fuses is to use a continuity (OHM) meter or simply replace the fuses one at a time with the EXACT same type. The tube heater circuit in that style of amp has it's own fuse, if I remember correctly, and your symptom can be caused by an OPEN filament fuse... not blown out turning the glass black, but simply having the internal element shake loose from it's securing solder by vibration. Before you do ANYTHING on that amp assembly, unplug it from the power outlet and WAIT 30 minutes for power supply capacitors to discharge.


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Sun Jan 08, 2012 10:21 am
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I have pulled the amp from the chassis, and I am letting it rest for a while before I attempt to pull the fuses on the board. Thanks for the assist!


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Sun Jan 08, 2012 2:33 pm
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I have checked the fuses with a multimeter and they are all passing. Hoping our local tech will be able to get it fixed prior to Saturday, but I am not holding my breath.


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Thu Jan 12, 2012 3:53 pm
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Turned out to be a short in the transformer. They were able to save the transformer and it is up and running again.


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Sat Jan 14, 2012 4:36 am
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glummoon wrote:
Turned out to be a short in the transformer. They were able to save the transformer and it is up and running again.


Glad to hear that it wasn't too much of an issue. Weird that a short in any transformer, either primary or secondary, wouldn't take out a line or C+ fuse...


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Post subject: Re: Fender Bassman 300 Pro - No output
Posted: Sat Jan 14, 2012 8:17 am
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Yeah... I don't pretend to know what they did to fix it, but I do know it sounds better than it has in a long time. They were really shocked (Pun intended) that it hadn't shocked me at some point. They also mentioned that there is a cap/resistor mod suggested by Fender to help the head with heat issues. I am going to be taking it back in after my show tonight to get that knocked out.


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