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Thread: Fender Blues Jr 3

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    Supporting Member bsco's Avatar
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    Fender Blues Jr 3

    Good day....I have this one here and it is a weird one for me.....It has no volume......and it is distorted when turned up.....when it is at low volume it does sound good but the volume is not as loud as it should be.....I installed a complete set of new tubes just to rule weak tubes out....can't find any bad resistors, filter caps, etc....sockets are good and solder is good as well.....

    B+ voltage is 321V; Z = 294V; Y=258V and X =236V....these measurements are taken from the hot side of the filter caps...

    V1 Voltages at PIN1=164V, PIN2=0V; PIN3=1.078V; Pins 4&5 are for the heater; PIN6=162V; PIN7=0V; PIN8=1.09V; PIN9=0V..........

    V2 voltages are PIN1,2 AND 3 re not connected......PIN4and 5 are for heaters.....PIN6=158V; PIN7= 0V; PIN8=1.17V; PIN9=0V.......

    V3 voltages are PIN1= 190V; PIN2= 33.4V; PIN3=52.4V; PIN4 and 5 are heaters;PIN6= 187V; PIN7=33V; PIN8= 52.5V; Pin9 0V....

    V4 voltages are PIN1=0V; PIN2= -10.35V; PIN3=0V; PINs4 and 5 are heater; PIN6= voltage is drifting above and below 1V; PIN7=320V; PIN8=0V; PIN9=294V...

    V5 voltages are....PIN1=N/C; PIN2=-10.34V; PIN3=3.4mV; PINS 4 and 5 are heater; PIN6= drifts around the 2V mark; PIN7=318V; PIN8=drifting around 1.5V; PIN9= 294V.....

    anybody want to share any thoughts or want more info....I will see if I can post the schematic for you......Click image for larger version. 

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    Old Timer Leo_Gnardo's Avatar
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    FWIW all those voltages look normal. The plate v's on the output tubes being 2V different suggest one tube's drawing a good deal more current than the other. You could do a bias current test the approved MEF way - measure resistance of each OT primary winding, then measure voltages across those windings, current = v/r.

    Have a dental mirror? Scan the back of the main PC board, especially where the leads to the output tubes are. Make sure you don't see any signs of fire, arcing, like that. What's left? Speaker - unlikely but possible. Fender to their credit puts a rather heavy duty speaker in these. Drive a known working speaker from the amp & you'll know fast. Output transformer? Personally I haven't had one go bad, but others have. Lucky Fender has QC connectors on OT leads so you can disconnect the one in the amp & hook another known working OT with clip leads.

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    Enjoy. Every. Sandwich.

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    Or use the 'neon trick' to check to OT.

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    Supporting Member bsco's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    FWIW all those voltages look normal. The plate v's on the output tubes being 2V different suggest one tube's drawing a good deal more current than the other. You could do a bias current test the approved MEF way - measure resistance of each OT primary winding, then measure voltages across those windings, current = v/r.

    Have a dental mirror? Scan the back of the main PC board, especially where the leads to the output tubes are. Make sure you don't see any signs of fire, arcing, like that. What's left? Speaker - unlikely but possible. Fender to their credit puts a rather heavy duty speaker in these. Drive a known working speaker from the amp & you'll know fast. Output transformer? Personally I haven't had one go bad, but others have. Lucky Fender has QC connectors on OT leads so you can disconnect the one in the amp & hook another known working OT with clip leads.
    I tied a different speaker...no difference.....I'll check those output plate voltages again just to be sure....I don't have another O/P transformer from one of these amps.....or anything even remotely close to it....Thanks for the reply....I'll check out the current draw....

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    Supporting Member bsco's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    Or use the 'neon trick' to check to OT.
    Right....I used to have one of those bulbs before...I will have to take a look for it or get a couple of more........Thanks....

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