Fender Pro Jr Schematic Question

[AmpGal]

I have two Pro Jrs, one virgin that works great (well, if really REALLY loud is great) & one modded that has problems. I'm using the schematic to learn how all the pieces go together.

Question 1 - Why does my schematic have 2 sets of 12AX7s on it, when there's only one physical set? Does this have something to do with a solid state part that's supposed to be in there somewhere?

Question 2 - Which set on the schematic (V1A/B or V2A/B) goes with my physical tubes?

So, I'm looking at the schematic and I can see where the tube points trace out to the resistors, capacitors & I see all the TPs, which I'm assuming means "Test Point" & what readings I should get there, but I'm afraid to test them.

[Phostenix]

Can you post the schematic?

[AmpGal]

Can you post the schematic?

http://www.schematicheaven.com/fenderamps/pro_jr.pdf

[Phostenix]

You said there's only 1 physical set in your amp. Does your amp have 1 12AX7 tube or 2?

[AmpGal]

You said there's only 1 physical set in your amp. Does your amp have 1 12AX7 tube or 2?

2 tubes... 1 set (well, the way I see it).

[AmpGal]

I guess I shouldn't say I'm "afraid to" take these readings, since none of these are any higher than readings I'm taking on the tubes. What I'm trying to say is I want to know if I'm right that TP points on a schematic are what they look like... places to check voltages throughout the board; and if something doesn't fall within the acceptable variance from the voltage listed at that TP, then you know something's wrong at that point in the circuitry.

I have a lot of experience with working on electronics, but it's always been where I either knew exactly what was wrong & how to fix it or it's been in a "this doesn't work, let me dick around with it & see if I can fix it" mode. This is the first time I've ever tried delving deeper into the "exact science" of it.

I'm excited about learning to look at a schematic & "see" the finished product's flow, since that is how my brain tends to work... I can "see" the house when I look at the floorplans, etc.

[Phostenix]

Each 12AX7 tube has 2 triodes inside it, so the "4" tubes you see on the schematic are actually just 2 physical tubes. They normally will be labelled something like V1A & V1B, V2A & V2B, etc.

TP is test point & you are correct that they are places to test to narrow down the problem.

What exactly is the problem(s) with the modded amp?

[AmpGal]

Each 12AX7 tube has 2 triodes inside it, so the "4" tubes you see on the schematic are actually just 2 physical tubes. They normally will be labelled something like V1B & V1B, V2A & V2B, etc.

TP is test point & you are correct that they are places to test to narrow down the problem.

What exactly is the problem(s) with the modded amp?

Ah, that makes sense. and proves I do need to study up on tubes. I had the thought, since each showed different pins, but didn't trust my gut feeling. I searched for over an hour on the internet trying to figure it out for myself, but couldn't find what I needed. My husband kept saying "ask the forum," so I finally did.


The modded pro jr doesn't have the volume it should. I did the "complete mod kit" from Fromel Electronics (http://www.fromelelectronics.com/mod...-Fender-Pro-Jr). While the mod is supposed to make the amp quieter, we're talking noise, hum, etc. not making it sound like grandma would like it. I'm trying to troubleshoot in a more logical manner and learn more about everything along the way.

[hasserl]

Gina, did you do the mods yourself, or were they done by someone else?

Did you have the amp before it was modified? If so, was it louder before it was modified?

[AmpGal]

Gina, did you do the mods yourself, or were they done by someone else?

Did you have the amp before it was modified? If so, was it louder before it was modified?

I did the mods. It was louder before, yes. There isn't all that much to the mod, mostly just updating resistors. The point was to take the harshness out of the sound. Instead of being cleaner, more refined like it was supposed to be, it's farty & doesn't get sound until 5 (if the guitar vol is max) or 8 (if the guitar vol is at half).

I'm messing with tubes right now. My husband has an entire box full of them. Lord only knows what's good & what's bad. A few minutes ago, I realized he had a 12AT7 in there with a 12AX7. Replaced it with another 12AX7 and it sounds a little different but not much. I'm going to take the two 12AX7s out of the working Jr & see what they do in this one. It just sounds like the one from last week that ended up having a bad preamp tube, so I thought I'd start there.

[hasserl]

That's a good place to start, use the tubes from the good amp in the bad amp and see if that corrects the problem. You might also try plugging the bad amp into the good amp's speaker, and vice versa.

If you've still got problems posting here what you did for mods would help identify possible problem areas causing the troubles.

[52 Bill]

Checking and swapping tubes is always a good starting point.

I'd suggest double checking all of the mods and swapped parts, as they really are the cause of the problem. Maybe a wrong value resistor, or a broken trace on the board, or a cold or bridged solder joint.

[AmpGal]

Checking and swapping tubes is always a good starting point.

I'd suggest double checking all of the mods and swapped parts, as they really are the cause of the problem. Maybe a wrong value resistor, or a broken trace on the board, or a cold or bridged solder joint.

I checked all the solder points & resistor values. I didn't have any trace problems, but I'm going to take it apart & go over everything again anyway. Maybe I missed something. It really is too bad the problem child is this one... she's a pain to dismantle.

gotta get the kids settled for the evening. Will post the mods in the am. Thanks!

[AmpGal]

Okay, life is settled a bit. I'm back. Here are the mods done to the Pro Jr.

Upgraded the following resistors: R1, R3, R5, R6 (chg from 56k to 22k) R9, R10, R17, R18, R21, R22, R26, R27 (chg from 100k to 82k), R29 (chg from 15k to 20k), R31.

And the following caps:
C1 (.01) to an orange drop .022
C2 (22P) to a silver mica 100pf
C4 (.01) to a orange drop .0047

Added a 47uf filter cap in parallel at C15.

Switched out the input jack.

I've tried a dozen tube configurations, to no avail. I'm ready to throw this one out the window. I'm tired of looking at her. I read VDC values at all the TPs on the schematic & they're all within 15% of spec, most less than 15%.

Any suggestions would be greatly appreciated.
Gina

[Enzo]

OK, you have two, one that works and one that doesn;t. If I read you, you didn;t really change the circuit much, just the parts in it. In that case, apply the EXACT same input signal to each amp and trace it down the signal path. Set all knobs the same on both amps. All you need is to take the back off for this, no need to pull the boards. See where the signal loses altitude versus the good amp.

I will believe your DC voltages are OK, but the signal path has a problem. DOn;t fall into the trap of assuming all problems are parts. Two resistors need to be OK, but so does the trace connecting them together. Likewise, two things next to each other that are not supposed to be connected better not have a little solder bridge connecting them anyway.

ANd double check that nowhere has some error like 100 ohm instead of 100k or vice versa.

Your test signal should be steady preferably. If you have no signal generator, some sort of keyboard will work. Hold down a key with a weight, or jam a matchbook between keys to hold one. And if nothing else, tune your FM tuner on your stereo to a non station, turn off the stereo mute so it makes white noise, and run a line out from that into the amp.

Best to follow with a scope, but if you lack a scope, use your meter set on AC volts and measure the signal level at each point.