How to Fix Bias Drift on Marshall JVM410
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- čas přidán 1. 10. 2021
- JCM2000 And JVM can both suffer from bias drift due to faulty PCBs. Bias drift causes power valves to 'red plate'. The option is to change the whole
board or to carry out this rather tricky modification.
I’m your apprentice now Stuart . Been incredibly sick with flu all week but I’ve learnt a lot . Especially about working on pcb boards. I love how for you it’s relaxed atmosphere even on difficult repairs ❤
Sir , your concubine plays a wonderful koto. Quite soothing.
That's not all she does...
Stuart, I always get excited when you break out your diagrams and explain things. It is extremely helpful and educational. That music sounds somewhat oriental as you put things back together. Thanks again for the tutorial. It was entertaining as always.
Cheers Michael!
Mr. Smith I want to tell you how much I enjoy your videos !! No matter how long they are, I watch every second. I have one question, where might I buy a wire stripper like yours? I've never seen one like that. Please keep making your videos. Have a very MERRY CHRISTMAS.
Excellent job Stuart! Marshall amps could learn a lot by watching your videos.
Thanks Zack. I bet they learned from this epidemic failure! Tens of thousands of amps must have been affected. To be fair to them, it's a very nasty one. They doubtless put out the PCB manufacturing to an external company in good faith. I heard that the PCBs were very slightly conductive (although I've never got the full story on this). They would have stuffed them, fitted them, tested the amp (All okay) then watched in horror as amp after amp came back with bias drift.
Thanks Zack. I bet they learned from this epidemic failure! Tens of thousands of amps must have been affected. To be fair to them, it's a very nasty one. They doubtless put out the PCB manufacturing to an external company in good faith. I heard that the PCBs were very slightly conductive (although I've never got the full story on this). They would have stuffed them, fitted them, tested the amp (All okay) then watched in horror as amp after amp came back with bias drift.
Thank you for another awesome video! I enjoy your channel and repairs! 🍻
Thanks, glad you are enjoying!
Brightly colored magic markers are great for marking the plug-in connectors so that you can plug them all back in correctly in the proper locations. The ink can be removed afterwards with alcohol and Q-tip. Generally faster and easier than referring to a hand-drawn chart, although, since Stuart will likely be doing this again to another amp in the future, it makes sense for him to make up a chart.
Or just leave the keying marks there.
@@marka1986 I wasnt talking about accidentally reversing the polarity, most of those are probably keyed to prevent it. I've worked on lots of 80's stereo equipment with dozens of little molex plugs everywhere, many with the same number of puns, and so color coding them was a good preventive measure against blowing something up by plugging it all back together incorrectly.
@@goodun2974 Zenith TV was like that, most connectors marked well but always 1 or 2 weren't labeled well. Some other stuff was like you mentioned. Whenever I needed to use a marker I just left it on to make it easier for the next person or me. A nice multi color pack of sharpies always kept handy.
i use these little circle multi colored stickers from news stand store and leave them on for next dude
@@patreidcocolditzcastle632 , those stickers fall off when the adhesive gets soft and gummy, or dries up from heat. That has been my experience anyway...
There was this run of marshall tsl's were they accidently loaded temperature sensitive resistors in the bias circuit!! Imagine that misery. Great video
Wow I hadn't heard that one!
Great fix Stuart...Lots of tech tips and I love your running commentary. That was a bit of a beast. You would wonder why Marshall hasn't remedied that issue over the years..? Thanks for sharing..Ed..uk..😁
Thanks Ed. They did eventually sort it in later JCM boards but it seemed to appear also in the later JVM boards. I don't know the full history on this.
@@stuartukguitarampguy5830 Cool, thanks for the info..Stay safe..Ed
I've never seen a JVM board leak yet and I've literally repaired hundreds.
The only time I've seen this. It was due to poor valves (usually JJs) that can't cope with 100W applications and get grid emission because they don't seem to be pumped out well enough.
The other one I saw was liquid spillage sitting in one of the cups under the valve socket itself.
NOT ALL MARSHALL BOARDS 'LEAK' although if you believe what you read in forms you will think otherwise.
Some of the old DSL/TSL's did, but there are still lots of them out there with no issues on early boards.
I've replaced several JVM boards for bias drift. Also if you check on line others have experienced it too. JCM boards were worse but they fixed it in later board issues. I have yet to see a definitive factual explanation for exactly WHY these boards 'leaked' and also exactly WHERE on the board this was occurring.
@@stuartukguitarampguy5830 wonder if you could use your megger on a junk board to figure out where the leakage is?
@@marka1986 Actually that's not a bad idea. I will try that.
With those diamonds drills when I've had to use them on tiles, it's best to start drilling at a 45° angle 1st brace the drill so it doesn't run away on you. Once you've made a bit of a groove you can gradually increase the angle of the drill.
ah interesting thanks. You could see I was struggling!
In my vid i used a drill press, you might find that helpful as well
Hi Sturat. I had 2 of these heads at one time. Thses can really sound good. The first one I bought when they firsy came out for 1700.00 brand new.
Ouch that's quite a chunk of change!
Hi Stuart,
I just repaired a JVM 205 with bias drift, Drilled pins 5 on output valves. All appeared stable monitoring voltage and idle current with 4 multimeters. Amp came back the next day redplating again! Set up with 4 meters on the bench all appeared back to normal until I applied heat gun near the PT area, then all hell broke loose! cooling down with fan settled everything down again. Removed the PT. Investigation found the ( invisible) fault was the Power Tranny. the 100v bias winding was O.C. both wires had broken at the transformer solder terminals. I managed to dismantle the cover on the bobbin and reconnect them with some fine insulated stranded wire. Badly constructed Transformers at Marshall once more! Amp is now functioning normally. Something I will check as a matter of course in future. Hope this will help you.
Hi Alan. Wow, nice repair well done.
i like your style!! i heat and lift using sloder like a lubricant.. then wick the holes...my desolder station...sometimes lifts pads...hate that. buddy has the 50 watt version...
At 24:00, probably a good idea to put some tape over that nearby switch before drilling or grinding the board, as fiberglass dust would be quite abrasive to the switch contacts. BTW, other techs I've seen grinding the board away from around the tube-socket pin have used a Dremel tool and a small carbide or diamond bit. I like that hollow diamond-tip bit, I've never seen one of those before. Either method would require a steady hand; and if someone isn't experienced with a Dremel tool, or at doing this type of operation for the very 1st time, it might be smart for them to have a spare pair of tube sockets on hand case they accidentally break or grind that pin right off or down to nothing. Myself, I was born with a Dremel tool in my hands, or so it seems, since I was a professional car stereo installation tech for 15 years and was grinding and filing car dashboards to fit new radios on a daily basis.
Yes I do have a dremel but is runs insanely fast and I judged to quickly for this job.
Psionic Audio, and Brad's Guitar Garage, have actually posted videos where they measured the board-leakage voltage with a multimeter! I would recommend looking under the board before and after the grinding operation shown here as it might be possible to accidentally cut a nearby trace on the underside of the board, running next to the socket at pin 5, if the carbide or diamond bit wobbles a little during the drilling process..
Yes I checked all that and there are no traces near.
Stuart, really enjoy your videos, including this one. I thought this only affects the JCM series, but sadly it looks JVMs are not immune as well. What would be your recommendation as the most reliable Marshall amp produced during the last ten-fifteen years?
That's a really tough one as I am not a guitarist and of course I only see the faulty ones!
Once again bueatiful work by Stuart . I really find the drilling the board trick so innovative. I’ve a JCM 2000 and it’s been so problematic I’ve learnt a great deal ha ha 😂. I’m replacing the main board in it soon as there’s a point where you go wow what next . These amps are a great high gain concept and classic marshy tones but why such poor quality. Shame a great uk company can let it get this cheap . I’d love a hand wired version if one the newer amps . I wonder if there’s hand wired boards for these style of amps
Thanks Pat. A new boardd will sort it out assuming you can get one. I don't think there is a hand wired version of this, Too complicated.
thank you stu.i thought that might be the case re to complex.i,ll get onto marshall aussie on monday see if i can get a board.its been a terrible amp for reliability.i dont like high gain complex amps i use pro reverb 71 ,a tweed with a plexi combination.the tsl 2000 was for my studio and clients.its going but has huge hum atm.ive fixed to date about 5 times.shocking design hey.loving the channel.ive subscribed.your way of explaining issues is A1 cheers mate@@stuartukguitarampguy5830
Great video and very informative. I wonder about some 'mass production' amp manufacturers and why there always seem to be 'issues' with their upgrade or new designs. You'd think by now with the back catalogues, this would have been sorted ! Or am I being naive ?
It's reslly hard to get a complete picture of what went on with these amps. Marshall know of course but they're keeping it quiet for obvious reasons.
An arduous repair, Stuart. I hope you were compensated adequately for this issue which is clearly Marshall's shortsightedness. Save a penny in order to charge a pound. Since you have performed several procedures of this type clearly bolsters my point. I believe that qualifies as a design defect. Marshall should provide corrected PCBs and pay all service technicians to perform the replacements. It victimizes any unsuspecting buyer who saves to buy a Marshall based on reputation. It is criminal in scope and practice. But I digress. Enjoy the weekend and remain safe during this modern-day plague. Cheers!
They are very aware of the problem and understandably leery about the cost implications. It's interesting that they still supply replacement boards for JCM at a silly price - about £35. Shows they are feeling a little guilty! Of course the affected amps are now 10+ years old and so the problem is moving into the past.
@@stuartukguitarampguy5830
The main board is £130 + VAT... 🤔
@@DrWatts-bi1jv Okay thanks, so a fair chunk of money. I have an accont with Marshall so it may be cheaper than that for me. Not sure.
Great fix Stuart. I'm curious if the coupling caps were hard wired to the tubes, pin 5, if that may be an easier repair. I'm still not sure where the issue was. In the end, it sure would be a big problem for tube replacement. That may get the amp owner cranky. Great video Stuart.
Hi Ken. We're not soldering to the actual tube pin of course, just the pin on the valve base (scoket).
@@stuartukguitarampguy5830 OH, I see. I misunderstood the actual repair. I did think you were soldering to the tube pin. However, I think I lost the reason why the bias fluctuating. I'll have to go look at the video again. Crazy though, with all the new technologies in amp building, you'd think the manufacturers would get it right. Seems they all have trouble keeping the amps going. Good for the repair man, but not for the consumer. Cheers 😀
@@kengevers8738 something in those board becomes conductive. that creates a mess with the bias, so isolating the bias connection solves the problem
@@3MonkeysGarage Thanks. I build my amps from glass epoxy boards, and I haven't had any issues. I wish these amp companies would listen to repair people, and not be so concerned about the almighty dollar. I think Leo Fender was pretty cheap, but not so much that his amps wouldn't hold up for 50-75 years. At any rate I missed the issues with lost bias. Now I know. Thanks 😊
Interesting video, as usual :) But my God what a terrible display they have put in that new model tube/bias tester.
Yes I much prefer the old one.
Just a precautionary remark, I had an extreme bias drift on my JVM410H. It would climb up to 130mV on one side. I was ready to do the mod but thought to try new power tubes first. The issue disappeared immediately. Might be a good idea to check your tubes if you haven't.
Ok useful info thanks John.
Hello. I have to change the valves on my jvm 410h. How do you change pairs? A pair V1 and V2, on the one hand. And another pair, v3 and v4? Or on the contrary, a pair at the extremities, and another pair in the middle.? thank you
Hi I'm not totally sure what you are asking. It's a 4-power valve amp. Are you thinking about only changing one pair and leaving the other pair in place? If so, why? Thanks Stuart
just wondering if anyone can help, I recently bought a 2016-17 model jvm410c tested at the sellers house and everything sounded as is should be. Got it home and had about 3 sessions getting to know the amp played through all the channels and modes sounded great and was loving it, but on the 4th session, the next day, there developed a terrible hiss and crackle from all the channels and even slightly on the clean channel green mode but especially from the red od1 and od2 channels red and orange mode ..I understand that high gain amps make noise but this is not right it's way too much basically making the amp unplayable anyone got any ideas would be greatly appreciated.. thanks!
What did you decide to do with the EAR mixer? Hope you do it at as it would be a fun watch to at least get the PSU up and running and see what happens as you never know..
It's the next project up on the bench!
@@stuartukguitarampguy5830 Very much looking forward to it!
Couldn't it all be done from the top without taking the board out? Can't you check for a DC leak/creep at the output of the coupling cap or on the next component after them?
That's actually the first time I have done that mod, I usually replace the whole board. If I have to do another one I'll see if it can be done 'top side'.
Interesting. I'd thought they had licked the bias drift issue but I guessed wrong. Hope you cooked it good and proper before you sent it home.
Yes it's been back a few months now and seems ok. Just had another one in...
How do you remove those connectors on the board?
You just pull them and they come apart. Is that what you meant?
thanks for the great I got TSL 100 jcm 2000 and gain is so weak, gain is almost 9 on the crucnh channel but there is only bearly gives distortion sound, preamp tubes also new.. but result is same i wonder that have you any video that kind of problem? By the way i faced same issue with all channels , clean-crunch and lead😢 no any issue with the volume knob and level its quite powerfull
Thanks
Thanks Orcun. Unfortinately your problem could be a number of things. I'm afraid you'll need to find an amp tech to sort it. All the best Stuart
@@stuartukguitarampguy5830 Thanks flr the reply! i hope that you ll fix one oe them on here as video🤘😊🙏
Have you got a feedback from the customer if this did the trick? I wonder how a 2 year old modern PCB can get conductive. Maybe it has also something to do with the choke 'mod' ?
Yes it seemed to do the trick.
I am experiencing the red platting on a new late 2020 Marshall JVM 410h 😭 Where can I get a replacement board to be prepared if something goes horribly wrong?
Hi I'd be surprised if this late version board suffered from the bias drift problem. Have you checked the bias cct to make sure it's giving out the right negative voltage? Marshall sell boards but they've recently stopped selling them to the general public and forbiddem people like me from acting as a middle man. So that's not very helpful!!!
@@stuartukguitarampguy5830 I will check the Bias cct voltage as you suggested, I am not sure of what was the reading. 😅
Hi Stuart, I was reading about a Mod that some people do while adding the choke. They replace there 220uf electrolytic caps on C50 and C81 with 100uf caps. Have you ever heard of this? In your opinion, would that be a good idea? I've been looking for more info on it and haven't seen whole lot. Just thought I would try and get your thoughts on it. Thank you!
Hi Mike. That's quite a big increase. The only reason for increasing the cap value is if you have residual hum. This should improve it. It also makes for less 'sag' when you play a loud chord - but many guitarists like this sag and consider it a positive feature of valve amps.
I feel like this is a stupid question and no puns intended, I’m having a hard time understanding what some of these description words mean. Where can I learn about what they mean, sag, stiffness, etc.? there are more but I can’t think of them right now. I’ve been searching but haven’t been able to clarify.
@@rustyzipperband Hi Mike. Not a stupid question at all! Part of the journey about learning anything is mastering the jargon. There is no one place to learn this. You pick it up as you go along. If you want to send me a list of words, I'm happy to explain them!
Sag and stiffness are a good start. I’m really trying to comprehend what that means. Lol I will have to reread the JVM forum to get the other words. Thank you very much!
Hi Mike. These and many similar terms are not strictly defined of course. However, my understanding is that Sag and Stiffness are the opposite of each other. They refer to the power supply, usually the HT (High Voltage). When you hit a loud chord, the power valves pull a lot of current. If the HT supply is not well regulated and smoothed, the output voltage can suddenly sag (drop). say from 360V to 290V or whatever. It recovers after the loud chord. Early valve amps weren't very well smoothed and so they suffered from this 'sag'. BUT ... guitarists seemed to quite like it, or got used to it. When later, better power supplies came out ('stiffer' supples) some guitarists missed the sag effect. I can't really describe this effect as I am not a guitarist. This is why many new valve amps have a valve rectifiers rarther than 4 diodes at a fraction of the cost. That's the extent of my knowledge!
Hello Sir, thank you for the video. I have the same Amp and it's not working. I have done all of the troubleshooting possible. I took the chassis out of the case. I thought that I would have one of the fuses blown but they all look and test fine. I thought I would find something obvious but can't find anything. The tubes all light up. When plugged into a guitar you can hear an ever so slight whisper of the guitar. It's definitely not the guitar or speaker. Like I said I've gone through all of the troubleshooting. Do you have any advice? Thank you I would greatly appreciate any help.
Hi Mike. No sorry I can;t really help as it could be so many things. You'll need to take it to a local tech. Good luck,.
@@stuartukguitarampguy5830 I figured it out. It was R26 and R27 both are blown. They don't look bad but they are. Thanks
@@rustyzipperband Well done Mike. Result!
I live in the middle of nowhere in Wisconsin. I haven't had any luck finding any decent technicians.
@@rustyzipperband Hi Mike. I;ve just noticed that this video is incorrectly labelled as the JCM410. It should be JVM410. I'll need the change that. R26 and R27 are 1 ohm cathode resistors in the cathodes of each valve pair. If they have both gone open circuit that means your power valves pulled way too much current at some point, so you need to find out why that is. So, it's 1 ohm not 180ohm. You're probably reading the bars incorecctly, they can be confusing on these low ohm resistors.
Hi, I would like to know if anybody experienced that problem - and perhaps has an advice for me. My JVM 410 head (6 years old) had already a big noise problem (strange crash noise when you play) two years ago. Local repair music store solved by changing Marshall tubes with JJ. Last week when I was doing a sound check the amp stopped working - no sound at all from the 1*8 OHM out to the Marshall 1960 4x12 cab. Used a JCM 900 for the gig. Then I tried to make the JVM 410 work again : Tried a different cab, tried a different guitar - cables, changed the 5 fuses, changed the ECC83 preamp valves, changed the EL34 power valves. I connected send to return the parralell fx loop, connected send to return the serial fx loop and stll no sound. Reset to factory default, nothing more. But I have a normal sound out of the emulated line out (used for silent recording). Do you have an idea, what could be wrong with my JVM 410? Any idea? Thanks in advance for your feedback. Kind regards Th
Hi Really hard to diagnose over the internet of course. My best guess would be you have no HT (high voltage DC) on the tubes. If you are able to measure that, do so. The most common reason for no HT is HT fuse blown, so double check that though you already said you had (don't check the fuse in circui, pop one end of it). If that's the end of your technical ability I'm afraid you'll need to find an amp tech.
@@stuartukguitarampguy5830 Thanks for your kind feedback, you are right I have 0 on the 4 tubes :( I fear it becomes to difficult for me ;) Strange because all fuses are ok. KR Thierry
@@musicnetwork7840 So just checking... you have no HT right? Pin 3 of any output valve. Are you able to check the AC winding on the transformer which generates the HT?
@@stuartukguitarampguy5830 Actually R26 and R27 are dead, I will try to follow what you have shown in this video and then replace the two 1w 1r0 resistors. Can't we risk to do it without removing the Main Board? Thanks again, I really love your videos! KR Th
@@musicnetwork7840 Yes try and do it topside. If those resistors are open circuit, at some point the power valves pulled a lot of current.
plus...i didn't hear usual swearing as i would be doing ( shhhh..LOL) . i've dome some pin isolating on an engle to get to pin 8....
It's a shame the JVMs are showing the same issue. I've had the bias drift issue on a few of the JCM2000 amps myself. The first couple I followed a couple of the modifications available online and while they worked initially, they again started presenting the same symptoms a while later after only a hundred or so hours of use. I've had more than a few JCM2000s and whenever the issue started happening just replaced the board as it was just a lasting fix (my current TSL100s replacement board is knocking on 10 years old now). I didn't realise the JVMs were starting to present the same issue though. Very tiresome. I'd be interested to see if the mod you did there will last the test of time. Hopefully it will :)
I hope it doesn;t come back on this one. I think those boards are getting harder to get.
@@stuartukguitarampguy5830 out of interest do you know what year that amp was? It looks like the stealth model which I believe was fairly recent.
@@jonsick445 Made in 2015 apparently.
I have a few I've modified just in case boards become unobtainium.
Good plan.
unfortunately i think my jvm is on the same boat. One pair is redplating.
Oh dear, yes it sounds like it.
@@stuartukguitarampguy5830 The weird thing is I swapped the pairs around and the redplating followed the tubes.
@@hippyamp hmm, that IS weird, so maybe it's not the board? Hard to believe though that a PAIR of tubes could be faulty. Is it 4 tubes or 2?
Always a good thing to save a board. But, sadly, I will have to completely rewatch this, the music forced me into a meditative state and I got distracted eating noodles. :p (don't change it)
The only channel where you get great tech tips AND enlightenment. What's not to like???
@@stuartukguitarampguy5830 all good stuff. Maybe you should pre mod a few of your spare boards for the next one.
Alta, I watched this video last night, and today I had a sudden craving for Pho 🍜 at the new Thai restaurant in town, and so that's what I ate for dinner!
@@goodun2974 a little pavlovs treat for us
@@goodun2974 Aha! My subliniminal advertising is working then. I'll call them for a percentage.
Not enough knobs 😄
Ho ho!
The flippin’ JVM does this too??!! Do those boneheads at Marshall know how to manufacture a proper amplifier any more??
My belief (not backed up by anything!) is that one generation of engineers moves on the does NOT pass on knowledge to the next generation. I see the same old mistakes being repeated such as mounting power resistors close to the PCB. Using small pads for large power diodes and suchlike. This very nasty bias drift problem came about because they used a cheap PCB manufacturer and the boards had a minute amount of moisture retention - enough to cause some current leak at the high voltages which are present. Took them a long time to realise what was going on, by which time they'd manufactured hundreds if not thousands of amps. Whilst I'm moaning, I notice that (e.g.) the present version number of a JCM2000 board is something like 25!!! That means they have gone through 25 iterations of the board to fix this or that issue. That seems like very poor initial design to me. You'd expect maybe 5 or 6 over several years, but 25???
Modern marshalls .... not at all what they used to be . Pure way over priced junk now . The tubes are worth more than the amp. Not worth the money they ask . Like disposable bic lighters now . High price for flimsy low quality high priced junk .
Yes it's a real shame how so many brands embark on the same path. Mercedes= top class, can only be owned by a select few. "I know, let's make a CHEAP Mercedes for the masses so they can get behind the badge..." And so on.
@@stuartukguitarampguy5830 You are a very patient and remarkable technician with great experience ! Much appreciate all your highly informative and very valuable videos . Most others dislike to work on these and the infamous JCM 2000 DSL bias drifters .
@@tonedriverss8629 Thanks I am so pleased you enjoy the videos!
YOUR BETTER OFF TO TALK THEN PLAY MUSIC
It kind of breaks up a long video. Also I only do the music when it's obvious what I'm doing. No point in saying "And now I am screwing this screw into this hole here." The alternative is that sort of 'fast forward' effect you get on some videos. I don't like that. My view (I could be wrong) is that there is a market out there for 'slow video' - almost real time. No idea really!