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Post subject: USA Blues Deluxe Blowing fuses
Posted: Mon Apr 01, 2013 11:48 pm
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Hope fully someone can shed some light on whats going on with my blues deluxe.
First It all started with the volume nob crackling and getting loud and quite when turning the nob. Then a blown fuse. Replaced fuse it turned on but still had the volume nob problem. i played a chord and turned the volume nob then pop there goes the fuse again this time around i could not get the amp out of standby with out a 5 sec hum and pop! Blown fuse!. So i started diagnostics. here is what I've done.

1. Pulled all tubes out and put in standby. Flick on switch hummmmmm pop there goes fuse.
2. decide to just buy new tubes anyway. JJs
3. Got my hands on another power transformer and still same problem.
4. Figured it could be CAPS. Purchased frommel kit and changed the caps and some resistors still same problem
5. have some carbon on the pcb under r86 , r85 dont know if that will cause fuses to blow.

Don't know what else to do. PLEASE HELP!!!!


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 3:12 am
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Hi acarabes,

Have you checked for a shorted diode(s), CR14, CR15, CR16, CR17?

R85 and R86 both get very hot and discolor the board, but I doubt that it's carbon or would cause the fuse to blow.

http://support.fender.com/schematics/guitar_amplifiers/Blues_Deluxe_schematic.pdf

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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 9:08 am
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Ok so did what you said. to test them i pulled the cathode end off of board set DMM to 20k i get a reading cr14 .614 cr15 .568 cr16 .577 cr17 .598 Not sure if thats how you wanted me to test them


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 9:38 am
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Well, they should read very low resistance, like the reading that you have, in one direction, and read very high resistance in the other direction (swap meter leads). If a diode reads low in both directions, it's shorted.

If the fuse doesn't blow until the standby is switched on, with all tubes removed, that pretty much leaves the high voltage supplies, and the OT. Four diodes, six caps, six resistors, a choke and the OT, are about all that you have to check. I doubt that the ceramic cap, C51, is the problem, but it's possible. Unplug the OT primary (three wires, red, blue, brown) from the board to further isolate the supplies.

It is also possible that you have replaced a faulty component(s), but then wired something up wrong while doing it. Double-check your caps' polarities, and that the new transformer is hooked up to the correct terminals as shown on the schematic, there are other terminals close to the correct ones. If all else checks out, still with the OT primary unplugged, you can pull one leg of C51 from the circuit and see if the fuse holds.

BTW, what value fuse did you use as a replacement? It should be a 3 amp fast-blo if you have a 120 volt input. :idea:

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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 12:04 pm
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Dude shimmielou you are Genius! So i went ahead and checked all my components to make sure they were all good solders (no cold ones) i did what you said and left the output transformer cable (Red) that connects to cp3 off. The amp stays on. If I connect cp3 with red wire i get the hummm. I didnt even bother to remove the brown and blue from the tube board, but it just leads me to believe it is the OT. Any thoughts. It kinda makes sense this is probably why my volume output changed drastically with an ever so slightly touch of the volume knob.

p. s. The diodes cr14-cr17 read only in one direction with a zero flipping the leads. not sure if i did it right.


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 6:06 pm
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Could be the OT, but two more things to check to verify. Diodes CR24 and CR25, which are connected to the OT brown and blue wires. If these two diodes check out, then that leaves the OT. These two diodes are connected across the output tubes, in the reverse polarity to the tubes, to conduct the collapsing field of the OT, protecting the tubes.

Oh yeah, I had to reread your explanation of the diodes that you checked, and yes close to zero ohms in one direction, and a higher ohms reading in the other direction means that the diodes are good. Check the remaining two diodes and that should tell you what to do next.

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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 6:23 pm
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will do thanks for the help!


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 7:09 pm
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ok tested diodes with diode test on DMM. cr 24 and cr 25. CR 25 read over at 10.11 with no reading when leads where switched cr 24 read the same on both ends which leads me to believe its shorted correct me if im wrong


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 7:34 pm
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Sounds right to me. Lift one end of CR24 from the board, plug the OT and everything else back in and try it. It won't hurt to leave the output tubes removed, and power up with one end of CR24 lifted. I have used the amp with those two diodes lifted with no ill affects and no affect on sound, but it's probably better to have both diodes in the circuit while using the amp, to protect the output tubes.

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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 7:57 pm
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pulled cr24 from back connected all cables voila it stayed on. a .25 cent diode caused me a headache. I put the diode back it just to be 100000000% sure. and it replicated the problem.
Whats your advice. Should i test with tubes in


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 8:50 pm
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Good job! You might be just fine with the diode(s) removed just for a brief test, but someone, likely smarter than me, put those diodes in for a seemingly good reason. I would also have a hard time waiting for new diodes to arrive and would probably at least test the amp with the output tubes installed.

shimmilou wrote:
...I have used the amp with those two diodes lifted with no ill affects and no affect on sound, but it's probably better to have both diodes in the circuit while using the amp, to protect the output tubes.

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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 8:58 pm
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Yeah. Ill try it out in the morning. One more thing Its about biasing this amp i have some jj's i just purchased would like to throw in there will i have biasing problems should i put in a biasing pot and Where? exactly would it go.


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Post subject: Re: USA Blues Deluxe Blowing fuses
Posted: Tue Apr 02, 2013 9:31 pm
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They might bias fine, or maybe need adjustment, depends on the tube rating. You could try a different value resistor for R87 to change the bias, maybe down to a 22K ohm. Or replace R87 with a 25K ohm multi turn pot in series with a 10K ohm or 15K ohm resistor. You really have to check the tube current to know where the bias is anyway.

This is the pot that I've used for just such applications. Connect the wiper of the pot to one end of the pot, making a variable resistor, then series with the 10 K or 15K resistor to replace R87. Mount the pot and resistor directly on the board with some silicone on the pot. For a good mounting job, drill a small hole between the two holes of R87 for the middle pot terminal, being sure not to drill into another trace underneath, and put the middle and one end of the pot terminals through the holes, leaving the one end terminal on top to connect to the resistor, bending the two pot terminals together under the board.

http://www.mouser.com/ProductDetail/Bourns/RJ24FX253/?qs=sGAEpiMZZMvygUB3GLcD7oYQkxX9lDAfsMaFgDyYQN4%3d

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