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Post subject: fender blues junior troubleshooting
Posted: Sat Feb 07, 2015 10:01 am
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i am trying to fix a Fender Blues Junior. it had no sound whatsoever. The R47 resistor was fried. i replaced it and 5 new tubes (probably didn’t need to replace tubes) after replacing it with a 2 watt 2k2 resistor I now
get a very muted low distorted sound when i strike a string on the guitar. improvement but..... what do I do next? also in replacing the power transformer pins does it matter which red green or brown goes on what pin? lastly the black wire off the transformer has 5 pins to plug it into which is correct? the other 4 are empty thank you


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Post subject: Re: fender blues junior troubleshooting
Posted: Sat Feb 07, 2015 8:37 pm
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Hi Matt Gourlay,

It absolutely matters to which connectors that the transformer wires connect! You can cause severe damage if not connected properly. Check the schematic for proper connection first, then see if the sound is good.

Do you have the newer version III (black face plate) or an older one (chrome face plate)? The connections are labeled differently, and there are also circuit differences between versions. R47 is in the power supply on the version III, 2.2K ohm 2 watt, which is the screen grid supply for the output tubes. For the previous versions, R47 is in the reverb return circuit, 470K ohm. So it seems like you have the version III. Take a look at the screen grid resistors, R35 and R36. Check the resistors with a meter, they are 100 ohm. The screen grid resistors often fail when an output tube fails, and it sounds like a bad output tube also took out the supply resistor R47. Hopefully that is all.

BJr III:
http://support.fender.com/schematics/guitar_amplifiers/BluesJuniorIII_schematic_Rev-D.pdf

BJr older version:
http://support.fender.com/schematics/guitar_amplifiers/Blues_Junior_schematic.pdf

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Post subject: Re: fender blues junior troubleshooting
Posted: Sun Feb 08, 2015 1:48 pm
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thank you kindly for your reply it is a crome III and i have the correct schematic it is the R47 that is in the power supply 2.2K ohm 2 watt, which is the screen grid supply for the output tubes. it is replaced. I also bot all new tubes. I checked the resistors 35 and 36 they look ok and read 100 ohms. I presume i can check them terminal to terminal without power on? i checked the B+ DC voltage and it starts out 330volts and when heaters warm up it drops to 295 volts. And getting back to the transformer pins i did go 2 red 2 green and 2 brown but they are twisted and for red for example it is P11 and P12 but there is no designation on the red wires as to which red pin to place on which pin.


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Post subject: Re: fender blues junior troubleshooting
Posted: Sun Feb 08, 2015 1:56 pm
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All these wires are not polarized any red can go to P11 or P12 , same for brown and green

Any Browns ; P13, P14

Any Green ; P15,P16

Next time use tag number :lol:


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Post subject: Re: fender blues junior troubleshooting
Posted: Sun Feb 08, 2015 2:32 pm
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+1 stratele52, I didn't realize he was referring to polarity.

Hmmm, the B+ should be 330 volts with the tubes heated, so I'm not sure about your voltage dropping like that. Maybe you should also check your diode bridge in the power supply, D1, D2, D3, D4. If a diode is open, you could have lower voltage with a load. And/or a poor filter cap(s), C25, C26, C27, C28.

Check all voltages compared to those listed on the schematic, with tubes heated and ready to play, check Plate voltages, screen voltages and grid voltages, for all tubes, starting at the supply and check at the tubes pins. Even if the Plate resistors and screen grid resistors check OK, confirm by checking the voltages at the tubes' pins compared to those listed on the schematics.

You should take notes, such as; V3 pin 7, 330 VDC... V3 pin 9, 307 VDC... etc.

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Post subject: Re: fender blues junior troubleshooting
Posted: Sun Feb 08, 2015 4:13 pm
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thank you again i have it playing and it sounds good not sure what was wrong i believe i had the rj jacks on the reverb plugged in reverse the inlets are both white and plugs are red and black i was checking that too. problem now is the v5 EL 84 is lit up like a red Christmas bulb so something is wrong i shut it down and need to figure out what is up i want to get a safe long tester before i do all the voltage testing with amp on.


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Post subject: Re: fender blues junior troubleshooting
Posted: Sun Feb 08, 2015 4:26 pm
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Matt Gourlay wrote:
thank you again i have it playing and it sounds good not sure what was wrong i believe i had the rj jacks on the reverb plugged in reverse the inlets are both white and plugs are red and black i was checking that too.

This can't damage a amp

problem now is the v5 EL 84 is lit up like a red Christmas bulb so something is wrong
Tube is short or bias is wrong , toooo hot

i shut it down and need to figure out what is up i want to get a safe long tester before i do all the voltage testing with amp on.


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