This one turned into a marathon! Nasty faults which took a while to fix but I got there eventually. Folow me step by step as I diagnose and fix a succession of faults on this amp caused by an arcing EL84 output vakve.
I have to totally agree with Thomas. Your explanations throughout the video are what other repair videos are lacking. Great job Stuart ! I subscribed. Keep the amp repair vids coming
One last thing and I'll stop! Actually my favorite el84 tube for the blues junior or most other el84 amplifiers is the electro-harmonix el84. It got really great reviews and it's just a really warm super reliable tube that breaks up beautifully and gives you that creamy beautiful sustain and in an AC15/30 just gives you the chime that we all love! Okay, I know everybody's tired of my comments! So that's it for me on this one! Cheers everybody, Jim from Seattle!
Hi Stuart,When you want to read voltage on brown and blue output transformer leads, Youll have to unplug the speaker (load) on the amp output trransformer). Then youll be able to read volage with your multimeter. (2017 blues junior circuit pcb). Thanks for sharing all those videos.
Overall a good video with reasonable explanations. What I do not like is your fear to take out the main board and fix everything the best way possible .... without doing this you get from one additional issue to the next.
Great stuff. I purchased new, a USA manufactured BJ in1999 (Rev 2 1995 green board) and have been unable to find any information regarding a parallel resistor except mentioned here. As I understand R37.33k and R31.22k (resistors per the schematic) make the voltage divider in this board model so the 33k ohm R37 would receive the parallel resistor. What would be a reasonable resistor value to parallel across R37 to reduce the bias? Thank you...
Had the same diode problem recently on a Fender 65 Reverb reissue. I pulled the board and the trace was untraceable (totally vaporized). I temporarily removed both diodes, while waiting on delivery, putting the outputs back in balance and ran it a short time. this stopped blowing the fuse. It worked to some extent, once you hit 4 on the volume it went into oscillation, motorboating then squealing as you went up. When the diodes arrived, I set the missing trace end flush to the board. I insulated the diode wire and ran it under then back up the old hole, made a nice little hook for the little wire to the Plate. Then I took then attached the other end normally, but leaving the diode in an upright position. This seemed like the best route on this board to maintain its original configuration, though just running the plate to the diode would have been a bit simpler. The Princeton board is a bit easier to remove, you pull a few screws, pop off the wire connectors from the front panels, then remove the ground lead and unmount the bias pot. Luckily the output transformer survived in this case and the screen resistor did as well. Thanks for the video, and hope this helps someone along the way. Sorry for the length.
Any suggestions what might cause a blues junior that has a decaying swelling sound when picking one string that decays and is somewhat fuzzy/raspy but not heavily distorted. Tubes are newish. Thank you.
Interesting video :) That was a serious punch. I do think the reason this Blues Jr might be a little different on the inside than other Jr's is because it is an LTD model in lacquered tweed with a Jensen speaker, not a regular Blues Jr IV tweed (originally a Celestion speaker). And I am wondering over the initial problem...a red plating (due to hot bias) or a shortcut transformer perhaps.
So I know you can't diagnose an amp without even seeing it but I have a Blues Junior three possibly a four. It doesn't give a number on the back plate though. Anyway I was playing it and it really had quite a bit of a hum that was really annoying but I kept playing anyway. Shortly after I got home it blew the main Slow blow fuse. I went and got another fuse, put it in and it blew immediately. The problem was this one wasn't a Slow blow fuse. You know if that would make a difference? Also I took it apart and looked at the PCB board and did not see any cooked resistors or transistors or any sign of smoke or heat. I turned it over and looked at the traces and I didn't see any signs of arcing or smoke. Does any of this ring a bell that you might be able to steer me in the right direction? Like could it be an output Transformer or something in the power supply? Again, this stuff is way above my paygrade so I'll have to take it somewhere but I'd like to have a rough idea of what I may or may not be looking at. Anyway I'm a super fan of your channel, I've liked and subscribed and shared to several of my musician buddies! Keep up the good work man, you do such a fantastic job of taking your time and explaining things to us in layman's terms in a very articulate way so we can wrap our heads around some of these problems regardless of our skill set. So anyway well done mate! Your friend Jim from Seattle Washington, Home of everybody's friend Jimi Hendrix!!
Hi Jim Remove all the valves, put a new fuse in and see if it blows (ideally don;t use a slow blow one). If it blows straight away it's something to do either with the mains transformer or the poower supply (e.g. it could be a shorted HT cap). The next thing I would do is disconnect all the secondaries from the transformer and try again, but this may be a bit above what you want to do.
I've had thermal runaway with two el84 amplifiers. One of them being a doctor Z Maz 18 Junior and the other one being a Vox ac15 hwx. Both amplifiers had JJ tubes in them which I will never ever purchase again! I like The tung-sol Replacements and I like The mullard Replacements as well as Mesa Boogie. They seem sturdier and they sound better. I've forgotten the exact number of the EL84 type tubes that Dr. Z recommends for the Maz 18 it's something like a PN14 something-or-other. But it's a very heavy duty tube to able to take High plate loads and he claims that you should get 5,000 hours out of a pair. So what are your thoughts about thermal runaway and what can cause it? And, can it cause fuses to fail? Thanks again so much for taking the time to do all this hard work and to answer all of our questions! Your friend Jim in America, Seattle Washington Home of Our Friend Jimi Hendrix!!
Hi Jim. I've used JJ for years and they're pretty good. Thermal runaway (red plating) is fairly rare so it's unusual to get it in two amps. It's often caused by a problem in the bias circuit, not the valve itself. Also 9/10 it will take out the HT fuse.
I enjoy your channel, Stuart. Nice, practical repair work. I've had three GT EL84 tubes fail spectacularly in two of my three FBJIII amps (Red, Black, Tweed). One took R34 with it. Totally gone; nothing but leads. None of these amps were abused or even had heavy use. Needless to say, I'm eyeing my Tweed BJIII with great suspicion. Changing all my tubes to JJ, installing an adjustable bias, and changing filter caps to F&T. Is this a common experience with Groove Tubes (in FBJ amps or otherwise)? Or are these failures because they bias them so unnecessarily hot? Poor matching?
Hi. No I haven;t heard anything particularly bad about Groove Tubes, they seem ok. Hmm, I wonder what has gone on with your amps? If it was just one amp I'd suspect a fault, but 3 amps kind of rules that out. As you say, get new tubes and bias them on the cool side and see if all is well.
How on earth, blown power tube , which is typical maintenance issue , kind of normal and expected behaviour in tube amps, can cause damage on output transformer?,What kind of fuses protection is used in? or do I miss something in here?
I';m not sure without rewatching the whole video. Quite often if a tube blows it will take out other components. Just had one in with screen resistors popped due to bad tube. Arcing tubes cause all sorts of nasty very high voiltages to float around and this can, rarely, take out a transformer.
I did some work on my own Pro Jr. I think I had the main board out only twice before those useless ribbon cables started breaking. Fender should have went with something like a molex connector setup or similar.
Agreed. There is a trick where you can run some hot melt glue along where they join the noard. Also I'm VERY careful and slow bending the board back and that seems to help. Touch wood, I haven;t lost one yet. Not great design though. But of course they don;t design things for maintenance, just cost.
I stumbled on your channel while looking for fixes for blown fuses in the Blues junior! My amp seems to have pretty much the same problems as the amp that you're repairing here. It's beyond the scope of my abilities to repair this amp so I will be taking it to an authorized repair station here in town. But I wanted to thank you for taking the time and explaining it to those of us that don't have the level of ability such as somebody like yourself. I'm a new fan and a new subscriber! Well done.
Fantastic amps, had nos for a decade and still love it. Last weekend started some loud humming, but would still play and after a few hours the hum stopped eventually. Next day hum was back with a vengeance. Turned off & back on a couple times next day hoping it would be okay lol. Later that evening turned it on and it promptly died. Any clue as to whether this sound like a blown fuse, bad tubes(last changed ca. 2017-18), or even resistor cap? Grounding issue even? Plan to take it to be checked out this weekend, I’m just wanting some reassurance that it’s nothing major. Anybody with any ideas or who’ve experienced similar issues to what I’ve described, a comment or message with some advice/suggestions, or even diagnosis would be greatly appreciated!
Thanks for the tips I’ll have it all checked out asap. Took it apart and found a blown 3A fuse(the long thin one) and locally could only find a 5A, changed it and viola! It started right up and plays great so far
@@stuartukguitarampguy5830 One question, will using a 5A fuse instead of the 3A for maybe 3 days cause any problems? Or otherwise risk making things worse? Again thanks in advance
@@stuartukguitarampguy5830 yeah finally did track down some 2A 250V fuses and swapped it out. Good as new, no waiting on the shop and no repair bill! Really appreciate the advice everybody, saved me lots of time and money and got me back to rockin’!
It’s a real shame Allen amplification aren’t selling the Heyboer TO20 at the moment. It’s an awesome replacement transformer for the BJ. I think Mojotone also do a replacement. Probably also worth speaking to “primary windings” for replacements.
@@ralfhedstrom ps I might be misremembering the number of the one Bill M used to sell but thought it was to20 or to22. Isn't the the to26 the output transformer for the deluxe reverb? I think that one would work but it wasn't the one I was thinking of (I was thinking of the one co designed by Bill M and Allen amps)..
Hi Stuart, I don't know if you've noticed but all fenders come to the UK wired for 230v. I have found that what comes out of our sockets is generally still much closer to 240v. If you look at the schematics for "Export" Fender and EVH amplifiers they have a 240v tap. I switch every Fender and EVH to 240v wiring and find the B+ and Bias voltages to be bang on what the schematics specify. The amps sound much better and Blues Jr's don't eat valves as much.
That's VERY interesting, thank you very much. Yes agreed we are still 240V. I read something about the '230V' being a fudge figure +/-15% to cover everything, but can't recall the details.
Great work Stuart, Fender Blues Junior not a great design for the service tech, I've been fortunate not to have had one for repair! the unfortunate thing is there are many amps out there that are just as much of a pig to work on, it would take very little effort on behalf of the designer/manufacturer to make things much easier but of course it comes down to cost/profits etc and of course some thought (do they really care though?) re possible cost of repair making their wonderful designs only fit for landfill should they go wrong, our eco age eh?????
Just got one from a friend to try out and it fried the R47 diode on start up. One power tube has seriously red plating and sparks!!! It’s a poor design unfortunately.
@@stuartukguitarampguy5830 shortcut repair means, although you wanted to save your customer some cash, you could've taken the board out and repaired the amp properly. I repair amps as well, and I tried that once, and lost the customer and earned a bad reputation along with it. Never did that again!!!
@@stuartukguitarampguy5830 Just so you know, I'm not trying to be mean spirited, but I was taught a valuable lesson. It becomes a matter of pride and workmanship, and word of mouth can cut like a knife. Cheers 🙂
@@kengevers8738 Hi Ken yes I understand. Ideally I'd like to do everything 'perfectly' but my customers would not be happy with the bill! I'm also on the enviable position of not needing to do this, and having more work than I can possibly handle! But the important thing is I never let a repair go out that I'm not 100% happy will last.
bush league repair, pull pots back pull board back without disconnecting ribbons, repair like a pro. ohhh and diodes did their job, protected your most expensive component, PT
Hi Monty. I can;t put my email here. If you go to the end of any one of my vids there is a splash screen. On there is my website address and via the website you can contact me. All the best.
