# Borrow a ProJr III in Kitchener-Waterloo area?



## LexxM3 (Oct 12, 2009)

Perhaps a strange favour request: if anyone in the KW area has a Fender Pro Jr. III, could I borrow it for a couple of weeks?

I am doing some trouble shooting on mine and it would be very very convenient to have a 1:1 comparable unit for measurements at every point. Just walking through the schematic test points wasn’t quite enough so far.

Thanks in advance!


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## Budda (May 29, 2007)

L&M?


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## Guest (Feb 16, 2019)

Does L & M rent them?


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## BSTheTech (Sep 30, 2015)

If it came with Groove Tubes I’d start there.


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## Budda (May 29, 2007)

Player99 said:


> Does L & M rent them?


They should. Cheaper than an HRD.


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## LexxM3 (Oct 12, 2009)

I was thinking of L&M rental as well, so I definitely will investigate, thanks for the suggestions.

It’s been pointed out that maybe I should outline the core issue and turn this into a proper repair thread; thank you as always, @jb welder . Here is an initial overview:

- purchased the Pro Jr III second hand a little while back; sounded awesome for a while and quite loud; then ...
- in a whiff of smoke and cutouts, all sound died
- opened it up, found the 1/4W R26 fully smoked, nothing else obviously damaged; tubes seemed fine
- replaced R26 by a 1/2W same value; added a biasing pot (R29); while examining the rest of the amp, noticed that it looks like the power transformer was replaced from original (Pro Jrs are known to have bad factory power transformers, so it seems this one has experienced a previous failure; the replacement job looks professional, so was/am not concerned, yet)
- it works now, but is pretty quiet, so that’s what I am trying to figure out ...


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## copperhead (May 24, 2006)

Red Platting can smoke that resistor . keep us informed .


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## Guitar101 (Jan 19, 2011)

LexxM3 said:


> I was thinking of L&M rental as well, so I definitely will investigate, thanks for the suggestions.
> 
> It’s been pointed out that maybe I should outline the core issue and turn this into a proper repair thread; thank you as always, @jb welder . Here is an initial overview:
> 
> ...


Just curious why you replaced the 1/4W R26 with a 1/2W same value and added a biasing pot (R29).


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## greco (Jul 15, 2007)

Following with ("academic") interest.


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## LexxM3 (Oct 12, 2009)

Guitar101 said:


> Just curious why you replaced the 1/4W R26 with a 1/2W same value and added a biasing pot (R29).


Ok, easy one first: R29 biasing pot added so I could ... (drum roll please) ... bias . Pro Jrs are known to be factory set up near tube destruction bias of most tubes (mine was), so seems like a good idea to be ready to adjust as I likely need to play with tubes if something is smoking in the amp. No regrets so far, bias pot works as expected.

Regarding R26, is the question why I replaced at all (because original was smoke), why same value (because I figured someone at Fender didn’t quite randomly chose resistor values and it matched the schematic as a bonus), or why 1/2W (because I am a fan of margins that are easy to achieve even if unnecessary).

Apologies, but not clear on the context of these two questions yet.


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## Guitar101 (Jan 19, 2011)

LexxM3 said:


> Ok, easy one first: R29 biasing pot added so I could ... (drum roll please) ... bias . Pro Jrs are known to be factory set up near tube destruction bias of most tubes (mine was), so seems like a good idea to be ready to adjust as I likely need to play with tubes if something is smoking in the amp. No regrets so far, bias pot works as expected.
> 
> Regarding R26, is the question why I replaced at all (because original was smoke), why same value (because I figured someone at Fender didn’t quite randomly chose resistor values and it matched the schematic as a bonus), or why 1/2W (because I am a fan of margins that are easy to achieve even if unnecessary).
> 
> Apologies, but not clear on the context of these two questions yet.


I know nothing about amps and I'm impressed by those of you that do. I was just wondering why you would throw a few more variables into the mix. Thanks for explaining and I hope you eventually get the results you are looking for. If you need to rent from L&M, they have a few half price rental days each year if you can wait for one.


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## LexxM3 (Oct 12, 2009)

Guitar101 said:


> I know nothing about amps and I'm impressed by those of you that do.


I have a significant background in electronics and know enough about high voltage to have survived this long. But amplifiers and in particular tube amplifiers, and in double-particular guitar tone-critical guitar tube amplifiers are interesting and specialized beasts and I am definitely very much still learning about those .


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## greco (Jul 15, 2007)

LexxM3 said:


> ...double-particular...


Look at what I learned today!


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## LexxM3 (Oct 12, 2009)

greco said:


> Look at what I learned today!


1984. Double plus good.


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## jb welder (Sep 14, 2010)

LexxM3 said:


> It’s been pointed out that maybe I should outline the core issue and turn this into a proper repair thread; thank you as always, @jb welder .


lol, I hope you didn't think that's what I meant. No pressure. 

R26 in series with C11 across the OT primary form a conjunctive filter. It (filter) rolls off ultra-highs and helps kill high freq. oscillations.
If the resistor burnt, there was likely HF oscillation above audible freq. that cooked it. The HF oscillation may still be occurring and robbing you of power in the audible range.
Once of the first suspects when encountering such HF or feedback is whether there have been any non-stock changes regarding the OT or it's wiring. You mentioned the PT replacement, so maybe the OT was also disconnected for some reason during the repair. 
So there's a chance that the OT phasing has been reversed, and the NFB (negative feedback) loop has been made positive, creating feedback (oscillation).
Do the colors of the OT wires match up with what is shown at CP1 and CP3 ? And the green and black to the speaker jack?

One other thing that could cook R26 would be a shorted C11. So recheck resistance on R26 to make sure it's still ok, and check that C11 is not shorted (you will need to disconnect either CP1 or CP3 while checking resistance of R26 and C11).


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## greco (Jul 15, 2007)

This Pro Jr. schematic is old (1994) ...but I'm trying to follow what
@jb welder is describing. If it is inaccurate, please tell me and I will delete this post so that there is no confusion in future.










WARNING: The schem becomes massive when you click to enlarge.
However, it is kind for old eyes.


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## LexxM3 (Oct 12, 2009)

@jb welder, that was no pressure at all, your suggestion was right on and it will be interesting to learn. An HF oscillation failure, if that's what it turns out to be, should not be a common problem on a production amp (although I understand that it's the primary cause of destroyed prototypes . While the tone is awesome, considering all the reliability issues the community has found with Pro Jr (at least pre-IV, jury is out on IV), it may very well be just a slightly more mature design than a prototype . What we tolerate for tone ...

I have a maybe more up-to-date schematic (created 1993, plotted 2005), but at least the section you've zoomed in, @greco, is identical circuit, values, and labeling, so it's good for now. Thanks.

To the details. @jb welder, awesome explanation -- I was not completely clear what the R26/C11 circuit was supposed to be doing, this is very helpful. To address your OT phase reversal question, it looks correct. CP1, CP2, and CP3 are lugs and all wire colours match schematics. My assertion that the PT may have been replaced is based solely to the observation that the two green secondary PT wires (tube heaters) going to the tubes PCB were clearly re-soldered, yet the indicator light wires right beside those were not; so it's also possible that someone needed to get at the tubes PCB and had to desolder these two wires to do so, in which case the PT may very well be stock; in fact, the tube PCB mounting screws seems to still have clean undisturbed locktite/lacquer on them, so perhaps even this last assertion is not correct and it's just that the factory soldering job is a little amateurish on those two wires.

Regarding smoked 270R R26, even though it was smoked, it still measured right around 270R after I took it out. I did not measure C11, but it's easy to lift one side of my new R26 so I'll try to measure that shortly and report.


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## LexxM3 (Oct 12, 2009)

Ok, C11 does not appear to be short. It does, however reliably settles at almost exactly 4K resistance across it after charging (from fully discharged). This is with one side of R26 lifted, so there should be no other current path around C11.

So that does not seem right. A replacement cap is $0.60 at DigiKey, but $8 shipping. Anyone in KW need anything from DigiKey?


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## jb welder (Sep 14, 2010)

I forgot to mention to also check R24 & R25 screen resistors, especially if you need to order parts. 
They will usually open if a bad power tube and would create a noticeable volume drop if one opened. I had figured there were probably test points there and you had said they all checked out, but I see there are no test points for pin9's of 6BQ5's.


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## LexxM3 (Oct 12, 2009)

R24 and R25 look good and both measure very close to spec 100R.

So a question is how would C11 get damaged. Is one way that someone ran the amp without a speaker load and the HF oscillation damaged the cap, but not as a complete short?

Here is an interesting description that might fit: What is a typical failure mode for ceramic capacitors? | Engineering Center ; it's interesting that moisture over time is asserted as the ultimate cause of final failure.


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## greco (Jul 15, 2007)

LexxM3 said:


> So a question is how would C11 get damaged


Exciting stuff!

(I don't get out much any more.)


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## jb welder (Sep 14, 2010)

It's measuring 4K now, but may be changing, especially under power. So it may be less when running, or at least when it was burning that resistor.
1KV rating sounds like a lot, but it's not really for the spot it's in, the 270ohm is comparatively pretty small resistance there. The peak voltages across the OT primary are big. So you might even want to put a 2KV there, or two 4700pF 1KV's in series. Cap values in series divide, opposite of resistors, so two 4700pF 1KV in series would create a 2350pF 2KV equivalent.
It could have been subjected to some 'lightning' type spikes if the amp was run unloaded or oscillation as you suggested.
Or it may have just failed. Some percentage will, and there are a lot of those amps out there. A quick search turned up the same part failing on the Fender forum, no obvious cause.


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## greco (Jul 15, 2007)

LexxM3 said:


> A replacement cap is $0.60 at DigiKey, but $8 shipping. Anyone in KW need anything from DigiKey?


You might want to call Orion One at Victoria St. and Lancaster St.
Ask for Michael to see if they have any in stock.
https://www.orionone.ca/contact
They apparently still have a lot of passive components not on display but "out in the back".


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## davetcan (Feb 27, 2006)

What a great community!


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## LexxM3 (Oct 12, 2009)

Update: IT’S BACK, BABY!

Everything is back to normal on the ProJr with the C11 replacement. I went with a 3KV-rated cap — margins are good.

There was another available diagnostic step that confirmed C11 as the issue before replacement: run the amp with the R26 lifted; that eliminates C11 from circuit and it was at full volume then, confirming that C11 was semi-shorted and shunt attenuating the output. Knowing that C11/R26 was a HF oscillation filter (I should have done the RC time constant/frequency calculation myself to have identified that) and not fundamentally critical to the tone, allows this leap and test for a short period.

@jb welder is a rock star. This isn’t even slightly the first time that his knowledge, reasoning, attention to detail, sharp eye, and approach has helped me and others. Thank you!


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## greco (Jul 15, 2007)

LexxM3 said:


> (I should have done the RC time constant/frequency calculation myself to have identified that)


I'm so very, very disappointed that you didn't do this calculation. I hold you in such high esteem and this is just so out of character for you. I know that it takes courage for you to admit not doing the calculation, and that is certainly impressive and honourable, but it is going to take me some time to adjust nonetheless. Try not to do this again...OK? (j/k)



LexxM3 said:


> @jb welder is a rock star. This isn’t even slightly the first time that his knowledge, reasoning, attention to detail, sharp eye, and approach has helped me and others.


@jb welder has always been extremely helpful and thorough in answering my basic electronics theory questions. Once again, I (also) thank him very much.


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## LexxM3 (Oct 12, 2009)

I know you’re joking, @greco, but if you calculate the f0 frequency of C11=2200pF and series R26=270R, you get just under 1.7MHz, which is so above audible frequency range that there was no chance it was there for any audio purposes.

But interestingly, I am reading that lots of other tube amp designs do use this RC structure across primary side of the OT for high audio frequency roll off with much larger RC values (larger RC is lower frequency attenuation, reaching the audible range). It also seems to be used to compensate (even out) the reactive load over frequency as seen by the power amp stage.


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## davetcan (Feb 27, 2006)

oops


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## davetcan (Feb 27, 2006)




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## greco (Jul 15, 2007)

Very nice!


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## Guitar101 (Jan 19, 2011)

LexxM3 said:


> I know you’re joking, @greco, but if you calculate the f0 frequency of C11=2200pF and series R26=270R, you get just under 1.7MHz, which is so above audible frequency range that there was no chance it was there for any audio purposes.


I'm not sure Greco is joking Lexx. He went after curlers in another thread. If he was joking, wouldn't he add a smiley face.


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## greco (Jul 15, 2007)




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## greco (Jul 15, 2007)

error


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## Guitar101 (Jan 19, 2011)

greco said:


> error


I see your getting a few double reply posts lately Greco. I've noticed for quite awhile now that when I hit reply to a post, sometimes it doesn't post immediately. I'm really tempted to hit reply again but now I don't and when I go back out of the forum and then back in, the post is there. I'm guessing it's a flaw in the forum and I just live with it. Not sure if this is happening to you but it works for me.


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## jb welder (Sep 14, 2010)

Glad to hear it's up and running and hope you're enjoying it. And thanks for the kind words guys.
Just wanted to mention there is a vast resource for all kinds of info on repair and mods of these amps at billmaudio.com (R.I.P. Bill).


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