Fender BJr repair

STRefugee said:
Yes, there's power.
measured between those 2 top terminals with the 2 black wires attached.

View attachment 106112
Click to expand...
OK now looking at the back of the switch: switch on
Keep one probe on the top left terminal,
Use the other probe to test both sides of the fuse.
Is there voltage on both sides of that fuse?
Fender BJ fuse.png
JohnnyGoo said:
Were following along.
Click to expand...
You can talk about the smoke and flames, for years to come.
:rolf::run::rolf:
 
STRefugee said:
Top end of fuse
View attachment 106115
and bottom end of fuse. View attachment 106117
Power on.
Click to expand...
hold on I'm wrong it's wired 240V
Don't power it up yet.
240V
P4 violet
P5 white no stripe
P6 white black stripe
P7 Black red stripe
P8 no conn
P9 Black green stripe
P10 Black yellow stripe
Black no stripe to Switch top left terminal S2B

Can you verify that's all plugged in correctly?


Look at the transformer 240V diagram.

You see how each wire is a blade / socket connection?
Make sure each blade is centered into each socket. Sometimes the blade can miss the socket when it's plugged in.
It looks like it's connected but it's not, it' pushed in off center, sometimes. Verify each one.

Wait don't power it up yet.
 
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Amp Mad Scientist said:
Looks like the power transformer wires are mixed up.
Don't power it up yet.
230V wiring
P4 Violet
P5 white no stripe
P6 Black no stripe
P7 Black red stripe
P8 No connection
P9 Black green stripe
P10 Black yellow stripe
White black stripe to switch

You see how each wire is a blade / socket connection?
Make sure each blade is centered into each socket. Sometimes the blade can miss the socket when it's plugged in.
Click to expand...
That was part of the problem. They sell those BJr here set for the EU with the 230V setting. But in the UK it needs to be the 240V set-up. That is swapping the WHT/BLK stripe from the switch with the BLK no stripe to P6. As per the attached screenshot.
Screenshot_20250421_110432.jpg

I tried both 230V & 240V set-ups and get to the same dead end.

But what I have not tried yet is to plug the BLK/RED to P4 as it were before I made any changes, with the Violet left dangling, and both P8 and P7 unconnected.

And that is the mistery. Why on earth was it set-up and running that way?..

From the schematic, can we understand the intended wiring and identify the function of each wire?

Is it possible to perform some testings on the tranformer to confirm it is delivering as expected?
 
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STRefugee said:
That was part of the problem. They sell those BJr here set for the EU with the 230V setting. But in the UK it needs to be the 240V set-up. That is swapping the WHT/BLK stripe from the switch with the BLK no stripe to P6. As per the attached screenshot.
View attachment 106118

I tried both 230V & 240V set-ups and get to the same dead end.

But what I have not tried yet is to plug the BLK/RED to P4 as it were before I made any changes, with the Violet left dangling, and both P8 and P7 unconnected.

And that is the mistery. Why on earth was it set-up and running that way?..

From the schematic, can we understand the intended wiring and identify the function of each wire?

Is it possible to perform some testings on the tranformer to confirm it is delivering as expected?
Click to expand...
Let me think about that for a minute...
Interesting
We measured voltage and it seemed roughly correct.
Go ahead and wire it back to what it was when you started.
 
Amp Mad Scientist said:
Let me think about that for a minute...
Interesting
We measured voltage and it seemed roughly correct.
Go ahead and wire it back to what it was when you started.
Click to expand...
I will test each lug in turn for AC voltage starting with P4. I would expect it to show around 230v to 240v as well since looking at the tracing on the pcb it seems to be the first port of call after the fuse. But here I'm guessiing of course.

And I don't know what, if anything at all happens with the Violet wire feed to the transfo, or what comes out of it and where it goes. Same with the other feeds.

Perhaps that under a previous owner some mess-up damaged the Transformer resulting in the Violet wire feed to somehow die in the Transfo. Whereas it still produces an output from the BLK/RED wire. But although the resulting output may allow the amp to run, it may not be the same as what would have been sourced from the Violet wire.

I got that BJnr 2nd hand for dirt cheap well over 10 years ago. Never did anything to it, apart from replacing the cooked tubes a couple of times. Probably because I only used it at low volume as a practice amp, that it delivered for as long as it did. And my use of it was not constant as it's always been a rather dull experience.

So will test those lugs. Re-wire as it were in the past and see if the amp comes to life that way.

If it does, we can then test various part of the board to check if we find something deadly or else I guess
 
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STRefugee said:
Top end of fuse
View attachment 106115
and bottom end of fuse. View attachment 106117
Power on.
Click to expand...
I think what happened is that some primary tap in the transformer are actually bad.
Somebody rewired the primary to bypass the bad tap.
We checked the voltage and it was working before you started.
The amp will work, but you can get a new power transformer and swap it out in the future..
I think it will still go even though the transformer is apparently not 100% good.

I don't know how much the transformer is in England, but here it's not expensive. It's $51 here.
 
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Amp Mad Scientist said:
I think what happened is that some primary tap in the transformer are actually bad.
Somebody rewired the primary to bypass the bad tap.
We checked the voltage and it was working before you started.
The amp will work, but you can get a new power transformer and swap it out in the future..
I think it will still go even though the transformer is apparently not 100% good.

I don't know how much the transformer is in England, but here it's not expensive. It's $51 here.
Click to expand...
Yes, I think we nailed it. We now know what the cause for that unconventional wiring is.
The Violet wire must be the primary thermal circuit breaker. ( For the audience : this is a circuit board protection. When an over current occurs, heat is generated as the current flows through the reed causing the reed to deflect and snap open.)
And here, the design does not allow for a reset contrary to your typical domestic circuit breaker.
So that is why that wire was left dangling to bypass the thermal circuit breaker.
Had I started from there, would I still have cracked over £100 in caps, resistors, etc...?

When back home, I might tinker some more and update.
Cheers!
 
STRefugee said:
Yes, I think we nailed it. We now know what the cause for that unconventional wiring is.
The Violet wire must be the primary thermal circuit breaker. ( For the audience : this is a circuit board protection. When an over current occurs, heat is generated as the current flows through the reed causing the reed to deflect and snap open.)
And here, the design does not allow for a reset contrary to your typical domestic circuit breaker.
So that is why that wire was left dangling to bypass the thermal circuit breaker.
Had I started from there, would I still have cracked over £100 in caps, resistors, etc...?

When back home, I might tinker some more and update.
Cheers!
Click to expand...
I don't see any reason why the amp will still not work.
It should work, we saw the voltages working before you started replacing parts.

I did not know there was a thermal breaker in your amp.
I don't think I ever saw a BJ with a thermal breaker.
But thermal breaker resets itself when the temperature cools down.

When I modify amplifiers:
I do install a thermal breaker, which is a safety regulation, which I follow.
But that is a requirement for custom modified appliances, not all production amplifiers.
Some amplifiers like Peavey: you will see thermal breakers installed by the factory.
 
Amp Mad Scientist said:
I don't see any reason why the amp will still not work.
It should work, we saw the voltages working before you started replacing parts.

I did not know there was a thermal breaker in your amp.
I don't think I ever saw a BJ with a thermal breaker.
But thermal breaker resets itself when the temperature cools down.

When I modify amplifiers:
I do install a thermal breaker, which is a safety regulation, which I follow.
But that is a requirement for custom modified appliances, not all production amplifiers.
Some amplifiers like Peavey: you will see thermal breakers installed by the factory.
Click to expand...

I 'm likely wrong then. Saw a replacement product by a (Tube Amp doctors) including in the description:
Connection with Board labeled 2001, 2002, 2003, 2010 (Blues Junior III)
Primary:
Purple wire (Thermo) to P4 (formerly Vio) etc..

But that may be an upgrade on the original then, based on what you are saying. The price tag is to cry too at around £140.
 
STRefugee said:
I 'm likely wrong then. Saw a replacement product by a (Tube Amp doctors) including in the description:
Connection with Board labeled 2001, 2002, 2003, 2010 (Blues Junior III)
Primary:
Purple wire (Thermo) to P4 (formerly Vio) etc..

But that may be an upgrade on the original then, based on what you are saying. The price tag is to cry too at around £140.
Click to expand...
There may be a thermal breaker required in some countries.
Hook the old transformer back up.
Then let's check the voltages.
But don't put the output tubes in yet until we verify the voltage.
The bias voltage has to work.

When you do put the output tubes in, the speaker has to be connected.
The bias has to be set.

There are 2 jacks.
A footswitch jack and a speaker jack. Make sure the speaker jack is labeled so it won't be mixed up.
 
Hi @Amp Mad Scientist
See the pics. Rewired as it was before the changes. That is the Violet wire unplugged and BLK/RED wire on P4.
Screenshot_20250423_174932.jpg
And as expected it comes back to life!
Screenshot_20250423_174747.jpg
And plenty of volts to the caps
Screenshot_20250423_174819.jpg
 
For biasing the power tubes I went for one of those little trimpods as you can see on the pic here.Screenshot_20250423_180000.jpg
But I'll have to learn what measurements are needed and how to go about it.
 
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