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FDP Forum / Amp Mods, Repairs, and Projects / What's wrong with my Fender Pro Jr III ?

Eddie5323

Nashville, TN USA

Nov 18th, 2017 12:51 PM   Edit   Profile   Print Topic   Search Topic

I picked up a used Pro Jr recently. It worked fine for a while. Played a couple rehearsals, 1 gig, and practicing at house. Then I noticed the sound getting buzzy. I looked at the power tubes and one was red plating. I shut it off and let it cool. I had a used set of EL84's laying around so I swapped them out and powered it up. All seemed fine for a week or two.

In the mean time I ordered a new matched set of JJ EL84's. I anxiously put in the new JJ's and upon firing up the sound was good for a moment, but as I tried adjusting volume on the amp I saw a quick increase in tube glow but it dimmed right down. I shut down the amp again and pulled the JJ's. Then I reinstalled the old set previously installed. Upon power up the amp seemed fine for a short bit but as I adjusted the volume up it started crackling and significant volume changes started occurring. I shut down the amp again and let set over night.

Upon trying the amp the next day, it sounded fine for a few minutes until it warmed up again and then the same problem reoccurred. I figure there was a loose solder connection that became a problem when it heated up.

First, I am not an amp expert but have changed tubes, set bias, re-soldered connection in the past many time successfully. I took apart the Pro Jr and did not observe and loose connections or burnt parts or smell of burnt parts. I did try to heat up the solder connections on the tubes to make sure I had a good connection. I even re-tensioned the tube sockets. No luck... problem still exists. Unfortunately when I first disassembled the amp I suspect the amp had been previously worked on by the looks at the back of the circuit boards. There definitely was some sloppy soldering work.

So here I am with an amp I've only had a short while and it's not working like it should. Any suggestion what might be the problem? I am hesitant to take it to a real amp repair person as the repair costs will quickly exceed what it's worth. I have had amps repaired a few times over the years by the experts in "Nashville" and they do a great job, but you never get out of there cheap. If I can't fix this one, it will become a parts amp.

Can I buy replacement only boards from Fender? Since I've had the amp apart I know I can easily do it. If not, what component might have failed? Are those parts available? I've seen the Fromel kits available. Would one of those kits allow me to replace a bad part not knowing what the bad part actually is? I can buy a new Pro Jr so I will not be without one for long. I am hesitant to buy used as I have been repeatedly burnt by people not fully disclosing problems that may have had with their amp. Are there any suggestions for alternative amps with higher reliability?

Sorry for being long winded. I have been searching internet and forums for suggestions to fix my problem, but have not found anything addressing this one. Thanks in advance.

Peegoo
Contributing Member
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Nov 18th, 2017 03:49 PM   Edit   Profile   Print Topic   Search Topic

a red-plating power tube means it has lost its bias voltage. The causes are poor tube connections (socket & tube pins) or bad solder connections on the tube sockets.

But you went over these items and all is good. Next thing I'd do with that amp is pull out the schematic and use a DMM to check for proper voltages.

Something's outta whack, and it sounds like it's heat related.

Eddie5323

Nashville, TN USA

Nov 18th, 2017 08:21 PM   Edit   Profile   Print Topic   Search Topic

Thanks for the response. I've never done trouble shooting like you suggested. I do have the schematic I downloaded and a DMM. How do you go about it? Do you start at the AC power or guitar input point to measure voltages? How do you go about applying a 1kHz signal to the input jack to read the AC voltages? I am aware of the lethal voltages and capacitors holding a charge so I won't just poke around in there without a better understanding. How do you know if you have a bad resistor or other component? Thanks again for any help or suggestions.

Peegoo
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Nov 18th, 2017 09:24 PM   Edit   Profile   Print Topic   Search Topic

No need to run a test signal to the input to check DC voltage levels. You need a signal generator (or an O-scope that has a signal function) to test AC voltage levels.

Look at the schematic (link below) and blow it up to enlarge the image. You'll see several ovals and boxes in the drawing labeled TP# xxx VAC or TP# xxx VDC.

TP = test point
# = the test point ID
xxx = proper voltage
VAC/VDC = AC or DC voltage

Testing an energized amp chassis is dangerous.

Clip the black test lead to ground (metal chassis). Always keep one hand in your pocket as you set the meter to read AC or DC, and carefully probe with the red lead.

Test the following TPs: 13, 14, 16, 17, 18, and 19. These are the voltages for the power tubes--that's apparently where you're having the problem.

Keep notes as you work.

An out-of spec voltage reading at a test point will indicate which component or connection may be faulty.

Click

(This message was last edited by Peegoo at 11:35 PM, Nov 18th, 2017)

Eddie5323

Nashville, TN USA

Nov 19th, 2017 06:36 AM   Edit   Profile   Print Topic   Search Topic

Thanks again Peegoo. You are a wealth of information. I just need to figure out where to get a signal generator (rent or borrow). Again, that part is new to me. If that's the only way I can trouble shoot it, then I might have to take it to someone.

Peegoo
Contributing Member
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Enjoying

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Nov 19th, 2017 07:05 AM   Edit   Profile   Print Topic   Search Topic

Where are willie, Steve Dallman, pdf64, Hammond101, ejm, JJuran, and a host of others when we need them? They know way more about this stuff than I do.

(This message was last edited by Peegoo at 09:05 AM, Nov 19th, 2017)

FDP Forum / Amp Mods, Repairs, and Projects / What's wrong with my Fender Pro Jr III ?




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