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Blues Jr mod disaster - help needed!
Blues Jr mod disaster - help needed!
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Old 21st November 2019, 01:09 AM   #1
cobraclutch is offline cobraclutch
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Join Date: Nov 2019
Default Blues Jr mod disaster - help needed!

Hey guys,
I know I'm just a new member, but would appreciate any help for a damn old fool!

I recently purchased a Fromel Complete mod kit for my BJ.
I have a Blue Junior III 2010 (REVISION D I assume) and decided to install a Fromel kit, but have stuffed something!

It powers on fine, light comes on, but I can't get anything from the guitar.
There is a slight stable hum that is a bit louder than before, which ramps up with the volume (gain), not the master.
And also changes pitch with the treble, which it did do at high volumes before also, if you paid attention while not playing.
There is also a pitch change when you touch the guitar input, or put the jack in, which used to be silent.

In fact the whole amp was very quiet until I threw a spanner in the works.

The tubes fire up and everything, but just no signal.
No smoke from anything.

The original large capacitor always read 6 volts after it self-discharged, and the new one read exactly the same.
It's the only thing I put in that is polarized (apart from the jack).

I did accidentally lift the pad on the PCB when installing the new WIMA cap on the bass pot (C6), but I scratched back the lead and it seemed to solder on okay, that's only thing I can think of that didn't go as planned (or so I thought).
To be fair, I've never seen a pad lift so fast.

I'm no tech, but I live out in the sticks in Australia, so there was no real option but for me to do it myself.
I did start an electronics degree when I was young and both my brother and father are electronic engineers, so I thought I would still retain some of the skills I had, but maybe not!
First time I'd even used a multimeter for a long while.

How can I problem solve this thing to track down the problem?
If you could give me some easy rookie tips, that would be great.

The only vague instructions were with the input jack, so maybe I screwed that, but not sure how.


I did the Fromel Complete for MIM (made in Mexico) set, which is:
Replaced C25 main filter electrolytic cap with 100uF/450v.
Replaced C5 with a 250pf Silver Mica
Replaced C6 + C7 with .1uF + .015uF WIMA caps respectively.
Put a jumper on the mid pot.
Replaced bias resistor with a 33k at R52
Installed new input jack, not mounted to the board.
I disconnected the trans leads and dressed them better and replaced them.
This is the kit:
Fender Blues Junior Modifications – Fromel Electronics


Click the image to open in full size.

Note: The white resistor is just to discharge caps.
And I didn't put the extra little cap in (disable sparkle mod)

Here's the schematic:
https://www.thetubestore.com/lib/the...atic-Rev-D.pdf



Anyway, thanks for your time...
:?

Last edited by cobraclutch; 21st November 2019 at 02:34 AM.
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Old 21st November 2019, 03:24 AM   #2
PRR is offline PRR  United States
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> a slight stable hum that is a bit louder than before, which ramps up with the volume (gain), not the master.

This suggests the problem is *before* that Volume control.

So the first amplifier stage or the input jack.

You didn't touch the first stage.

You did touch the input jack.

I know what I'd look at first, but I can't see your work from here.
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Old 21st November 2019, 03:28 AM   #3
cobraclutch is offline cobraclutch
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I thought from the start it was the jack.
I didn't even want to put it in, but it came with the kit, so i thought, why not? lol

Those plastic washers were just in the bag, so i only put in one, on the outside under the the input nut.
Should the other have gone under it between the chassis to stop it being grounded or something??
Though I think it gets ground to the chassis at some point anyway, doesn't it?

I dunno...

I'll take a photo when I get home.
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Old 21st November 2019, 10:33 PM   #4
cobraclutch is offline cobraclutch
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Got it!
Thanks for the help, so were 100% right.

For anyone googling, the washer was the major problem.
They are not mentioned in the instructions.
But from their website: Included is a shoulder washer and isolation washer so the input jack does not ground directly to the chassis and prevents ground loops.


Might seem obvious, but the last input was grounded to chassis, so I didn't even think about it.
Once I put them in properly, I measured 0 to ground straight away and realised it wasn't grounded properly on the PCB either.

1 minute fix and sounds great.
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Old 22nd November 2019, 06:32 AM   #5
PRR is offline PRR  United States
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Mis-wired In and Out jacks are very "popular" on guitar amp and pedal forums.

There's 2 to 6 pins, different on different brands, easy to get wrong.

Guitar pedals have two jacks. *On the stage*, convention is signal flows right to left (works well with a single pedal and right-handed guitarist). On the bench, upside down or backward, it is easy to go in-the-out and out-the-in, which is not what was intended.

Glad you sorted it.

Last edited by PRR; 22nd November 2019 at 06:47 AM.
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Old 22nd November 2019, 10:02 PM   #6
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by PRR View Post
*On the stage*, convention is signal flows right to left
This convention has annoyed me for years, because I learned a different convention as a child, from mostly British and European electronics books, and I've have been using that convention ever since: on every schematic, signal flows from left to right, and DC voltages increase from bottom to top.

Interestingly, most guitar amps I've encountered have the input jack on the left (when you're facing the amp, as you are when you plug in the guitar.) But most guitar FX pedals have the input jack on the right. And when you find schematics for amp or pedal, the input jack is always on the left, and the signal flows left to right through the schematic...

I suppose it's no worse than my old Hyundai, which had the headlight switch where my current Dodge has its windshield wiper switch. After the switch, there were probably a dozen times when I turned on the wipers when I actually intended to turn on my headlights.


-Gnobuddy
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Old 22nd November 2019, 10:16 PM   #7
cobraclutch is offline cobraclutch
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Join Date: Nov 2019
Quote:
Originally Posted by PRR View Post
Mis-wired In and Out jacks are very "popular" on guitar amp and pedal forums.

There's 2 to 6 pins, different on different brands, easy to get wrong.

Guitar pedals have two jacks. *On the stage*, convention is signal flows right to left (works well with a single pedal and right-handed guitarist). On the bench, upside down or backward, it is easy to go in-the-out and out-the-in, which is not what was intended.

Glad you sorted it.
Exactly
I went from a 5 pin to 2. Thatís alotta spare holes!
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Old 22nd November 2019, 10:21 PM   #8
cobraclutch is offline cobraclutch
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Quote:
Originally Posted by Gnobuddy View Post
I suppose it's no worse than my old Hyundai, which had the headlight switch where my current Dodge has its windshield wiper switch. After the switch, there were probably a dozen times when I turned on the wipers when I actually intended to turn on my headlights.
-Gnobuddy
happens every day in Australia!
All of our cars are a mix of aussie/jap (right side indicator) and us/eu (left side)
I own a jeep and the work car is a toyota, so happens every time...
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Old 23rd November 2019, 01:04 AM   #9
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by cobraclutch View Post
Exactly
I went from a 5 pin to 2. Thatís alotta spare holes!
Some 1/4" jacks have really unintuitive pinouts (Switchcraft, for one!)

To avoid mis-wiring them, I stick the appropriate mono or stereo 1/4" plug into each one, then use my DMM to find out which pins on the plug connect to which pins on the jack. This means I'll at least get my ground and "hot" pins correct, or ground, left and right channels if it's a stereo plug / jack.

If one or more switching contacts are involved, still more fun with the ohmmeter is required. Pull out the plug, use the ohmmeter to find out which jack contacts connect to which other jack contact. Put the plug back in, repeat.

Combine that with physical examination (look at the guts of the thing), and you'll usually have all the jack pins sussed out.

All this still won't prevent those braid-to-core shorts if you overcook your solder joint to shielded wire (coaxial cable), but at least it's a start.


-Gnobuddy
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Old 23rd November 2019, 01:11 AM   #10
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by cobraclutch View Post
happens every day in Australia!
As if having kangaroos that live in trees, mammals that lay eggs, and Christmas in summer wasn't confusing enough!

(The Bennet's tree kangaroo in the pic is found in Queensland, Australia. It's on the verge of extinction, like all species of tree kangaroos. Not to mention the Great Barrier Reef. )


-Gnobuddy
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