Fender BJr repair

STRefugee said:
I turned the amp on.
Placed the red probe at the with the stripe, and the black probe at the other end of the diode.
Multimeter set on DC voltage 200.

Reading D7: 14.5
Reading D8: 14
Reading D5: 26.5

Pin 9 reading: 337V and 337
Click to expand...
OK that's right.
The schematic was too blurred to read it.
D7 and D8 should be 15 volts.
Sorry the printing wasn't clear.

High voltage on pin 9 is OK

So now, the amp should be working if the tubes still work.
Go ahead and try it.
Crank it up and wake the neighbors, they will love it.

Note don't hook the speaker with a guitar lead.
Use a speaker cable.
 
Hello Folks!
So, I finally got my replacement Power Transformer delivered, from Canada.
Screenshot_20250508_075712.jpg
The one on the right, Balck and shiny. Sild as a "drop in" replacement for the Blues Junior.
Now you'll notice only 4 wires on the primary side and snother 4 wires on the secondary side.
There are quite a few more on the primary side of the old transfo and a total of six wires on its secondary side.
The colour coding doesn't help much either.
So how will I go about fitting this new transformer in order to achieve this wiring?IMG_20250508_080538.jpgAny step by step guidance you can offer will be greatly appreciated.
Thank you
 
STRefugee said:
Hello Folks!
So, I finally got my replacement Power Transformer delivered, from Canada.
View attachment 106254
The one on the right, Balck and shiny. Sild as a "drop in" replacement for the Blues Junior.
Now you'll notice only 4 wires on the primary side and snother 4 wires on the secondary side.
There are quite a few more on the primary side of the old transfo and a total of six wires on its secondary side.
The colour coding doesn't help much either.
So how will I go about fitting this new transformer in order to achieve this wiring?View attachment 106255Any step by step guidance you can offer will be greatly appreciated.
Thank you
Click to expand...
What's the hammond part number for that transformer?
We have to get the spec sheet from hammond which shoes the winding configuration and color code.
Give me a minute to look at it.

I have to have surgery tomorrow morning.
I'm going to be out of it for a few days.

Don't worry it's going to work OK, just need to get the spec sheet.
 
Last edited:
Amp Mad Scientist said:
Black to P4
White to power switch (mains blue wire side of switch)
Red to P11
Red to P12
Brown P13
Brown P14
Green P15
Green P16
Click to expand...

Hi @Amp Mad Scientist
Hope you're recovering well from that surgery.

I haven't yet got around looking at PCB again but I think one of the Power transformer's wires was connected to lug P2 on the PCB. Is that not the power supply to the rest of the amp's circuit?

Just trying to understand why we can leave it unconnected.

Thank you
 
STRefugee said:
Hi @Amp Mad Scientist
Hope you're recovering well from that surgery.

I haven't yet got around looking at PCB again but I think one of the Power transformer's wires was connected to lug P2 on the PCB. Is that not the power supply to the rest of the amp's circuit?

Just trying to understand why we can leave it unconnected.

Thank you
Click to expand...
It's 5 more hours before they filet my fish.

P2 should be connected to the primary center tap red wire of the output transformer.

We should review all the transformer connections to verify it's correct before you power it up.
Look on the schematic and double check all the transformer connections.
 
Amp Mad Scientist said:
Black to P4
White to power switch (mains blue wire side of switch)
Red to P11
Red to P12
Brown P13
Brown P14
Green P15
Green P16
Click to expand...

Hi @Amp Mad Scientist,
How are you doing? Hope you're in no pain.

Here is a pic of the wiring.
Screenshot_20250510_155708.jpg
I label the side of the transformer where the WHT wire comes out from as (W side). I have:
Red (W side) to P11
BRN (W side) to P13
GRN (W side) to P15
WHT (W side) to switch

Other side
BLK to P4
GRN to P16
BRN to P14
RED to P12

Waiting for your instructions.
Wish you a quick recovery.
 
STRefugee said:
Hi @Amp Mad Scientist,
How are you doing? Hope you're in no pain.

Here is a pic of the wiring.
View attachment 106276
I label the side of the transformer where the WHT wire comes out from as (W side). I have:
Red (W side) to P11
BRN (W side) to P13
GRN (W side) to P15
WHT (W side) to switch

Other side
BLK to P4
GRN to P16
BRN to P14
RED to P12

Waiting for your instructions.
Wish you a quick recovery.
Click to expand...
That looks fine.
What of the O.T.? did you get that hooked correctly?
Double check the O.T wires.

Now power it up without the tubes and make sure the bias is set.
12.3 volts DC is the cold setting.
11.5 would be hotter.
You can set it hotter and it will sound better but don't set it too hot.
I think the original 10.6 volts was too hot.
Some people recommend 12.3 volts just to be on the cold side.

The thing about bias is: hotter sounds better, but the tubes won't last as long.
 
Hi @Amp Mad Scientist,

Glad to hear back from you!
Yes, the Output Transformer wires are correctly connected to the board, thank you.

Is there a primary and a secondary side on this 290XEX power transfo?
Or are they all mixed-up?

Thank you
 
STRefugee said:
Hi @Amp Mad Scientist,

Glad to hear back from you!
Yes, the Output Transformer wires are correctly connected to the board, thank you.

Is there a primary and a secondary side on this 290XEX power transfo?
Or are they all mixed-up?

Thank you
Click to expand...
It has primary and secondary.
The wires come out of the case in different locations that's OK.Screenshot 2025-05-10 at 10-56-27 Fender BJr repair.png
 
Last edited:
Amp Mad Scientist said:
It has primary and secondary.
The wires come out of the case in different locations that's OK.
Click to expand...

Please can I just check if I am getting this right in my head.
I 've done
RED (W side) to P11
RED to P12
It be also fine then to swap them around because the pairs of reds, brown, green are secondary and share the same voltage and Amp.
The black and the white are the primary.
Is that right?
Thank you
 
STRefugee said:
Please can I just check if I am getting this right in my head.
I 've done
RED (W side) to P11
RED to P12
It be also fine then to swap them around because the pairs of reds, brown, green are secondary and share the same voltage and Amp.
The black and the white are the primary.
Is that right?
Thank you
Click to expand...
Yes that's correct.
 
Finally got to the end of it.
It sounds pretty awesome. Massive improvement on how I remember this thing last time I heard it, which was rather stale.
Did a few little mods I picked up along the way and it paid-off!

@Amp Mad Scientist
Thank you Soooo Very Much!! For the support, giving me the confidence to venture on the journey and staying in touch through the whole of it.
Big success story here!


Screenshot_20250511_110425.jpgScreenshot_20250511_110444.jpg
 
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