How to repair a Marshall JMP 2104 with many problems
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- Опубліковано 24 лип 2024
- Let's repair this Marshall JMP 2104 amp. It has many issues. Some caused by age. Some caused by poor previous work. In this video you will learn how to improve Marshall's grounding, how to replace that awkward capacitor under the PCB, and how to bias an amp using an oscilloscope.
You will also meet my beautiful 1971 black Les Paul Custom called “Doug”, named after my first guitar hero, Doug Ford. Doug Ford and (Beano) Clapton epitomize the perfect Les Paul tone to me. I asked Rod McQueen of Sliders pickups to make me a set of PAF for “Doug”. I love the tone!
The completed JMP 2104 sounded fantastic. I hope it came across on your speakers.
Here are a couple of links referred to in the video:
Sliders Pickups: sliderspickups.com/product-it...
Doug Ford and the Masters Apprentices: • Video
Chapter Index:
00:00 Introduction
01:05 Initial Inspection
03:20 The curse of WD40
04:20 Improving the Marshall ground
06:40 Missing bolts on Output Transformer
10:00 Soldering a chassis ground
15:40 How to replace a Marshall capacitor that is under the PCB
16:50 Schematic overview of the output stage and bias circuit.
18:40 Upgrading the bias capacitors and precaution in not overdoing it.
25:50 How to test for leaky coupling capacitors
31:35 A common cause of signal guitar cutting out
35:40 How to make a tool to clean input jack contacts
37:25 How to bias a Marshall by plate current and oscilloscope methods
38:50 Try different tube positions for a better match of the plate current draw.
40:07 Setting bias with an oscilloscope
49:13 A beautiful pair of original Rola Celestions
50:25 Revision of bias caps and summary
51:50 Homage to Doug Ford and my Les Paul called “Doug”
52:25 Sound Test of the completed Marshall 2104
54:26 End
The more I learn from amp builds, the more important I think the power transformer is. In fact, not just the PT, but the whole power supply. I know, I know: we're talking about guitar amps here, so perfectly clean power, low ripple, beautifully regulated power supplies aren't the objective, etc, etc, etc. That's true. But I'm coming to the view that the power supply is to the signal chain, as the canvas is to the paint. No, the power supply is not strictly 'in' the signal path, but the signal rides on it. So the ropey, primitive power supply is a critical part of the character of a 5E3 Deluxe, just as the stiffer, better-regulated supply of a Twin Reverb is part of that amp's sound. And the PT is the heart of the power supply. So I reckon it's more important than a lot of guitar-playing amp users think.
Phew!
Cool old amp, and a cool video too, mate!
You stated the importance of the PT very well, Bladder! Thank you.
Yeah! The boys are back in town. I had a 1982 black custom just like yours with gold hardware. I had to sell it for another JCM800 to do an outside gig. I thought this was the best choice at the time. Still kicking myself. You know yourself back in the old days you needed all the power you could get. No stadium pa like today. I really love your videos.
I share your pain Monty. For me, my big loss was selling my R8 Les Paul. I think we could stand facing one another and calling out a regretful sale we made. I'm not sure who would collapse first!
@@theguitaramptech Brother you got that right. Im in Texas. which one will cross the big water first.
@@montygore1200 Either way, I'll face up to you with a regret-to-regret showdown. Last man standing buy the beers!
Good job. Good guitar playing. And cool guitar.
Thank you, Zack. I'm glad you like Doug". He's my favouite. I have decided to try to play a sample of songs that typify the amp I'm working on. Sometimes I can do it. Sometimes I sound like sh*t! But I try.
Thank you for another quality video.
I really appreciate that. Thank you.
Very Cool, Thankyou. Informative, Interesting and Entertaining as always. Sounds Great. Cheers
Thank you so much GRB.
This video is a master class..like all your videos. Thanks a lot sir. Greetings.
You are most welcome, Miguel. It's your comments that keep me going with these videos.
I just picked up a 1975 Traynor YGM-4 STUDIO MATE. I went through and replaced some electrolytics, and I noticed that the bias caps were 68uf and 8uf. First time I've seen one that high. I'm glad I saw this
Good one, Soap. We don’t see many traynors in Australia. I’m glad my video was of some help to you.
I'm curious what other amps you.might have seen this implemented?
Also , would non polarized caps be ok to use ?
@@soapboxearth2 non-polarized capacitors can replace polarised electrolytics. Non-polarised applications are often used for coupling, tone caps, feedback, and oscillation. In amps, we mainly see polarised capacitors for filtering ripple or cathode bypass applications. Electrolytic capacitors usually have larger capacitance values for a given voltage. It is not so easy to make a large-capacity non-polarized capacitor, because the size and cost make them unpopular choices.
@theguitaramptech so the bias circuit caps are filter caps? I guess I should use polarized?
Nice job, on a nice old Marshall.👍
Thank you, John. I appreciate that, coming from a Marshall guru.
Excellent work there on a real basket case. I really enjoy your channel as well as Brad’s Guitar Garage. You two always seem to fix the impossible.
Thanks David. I have a lot of respect for @bradsguitargarage…both the man and his channel.
Great video as always! 👌All the best from Austria! Keep up that great stuff!
You are awesome! Like your voice so much!
Thank you so much, Philip!!
Thanks for another great video, Chris . I appreciate you going through the schematic and didn't realize the high input was feeding back through the low input jack to the 2nd gain stage. Nice work brother man!!.
Thank you for your "thank you"! I appreciate it. Yes those contacts work great when the amp is new, just as the Send/Return problems. But in time they clog up and it "welcome to the world of intermittent problems".
Thanks Chris. I really like the way you set bias with the oscilloscope. It seems to be way more accurate that setting grid voltage. I looked up Doug Ford. What a sound!
Thanks Les. Setting bias by grid voltage will get you in the ball park. Setting by plate current is much more accurate, but setting by "Loud" playing levels using a signal generator and an oscilloscope shows you exactly what the tubes are doing. I use 440Hz at 250mV
If you have Nickelodeon on Spotify, listen to Doug's tone on that live album!!
Hey Chris, Absolutely fab video..Just so much info and explained methodically for a newbie like myself..Still learning, but it's all beginning to make sense..Love the play off at the end, sounds great..Top job..Stay safe .. You're lucky I'm not your neighbour, you would never get rid of me...Ed..UK..🤣
I'd love to have you as a neighbour, Ed. I am very hppy that you're getting value from the videos.
Generally incoming (mains) earth connection should be always made on a separate point from internal grounds (safety feature).
Also if I remember correctly should be made with a screw and nut because this sort of mechanical connection is considered more safe than just a solder joint to chassis.
Thank you for taking the time to reply, Jernej. You may be right, but I have not heard of either. I have seen too many poor nut and bolt connections, but if you come across a reference to educate me, I would very much appreciate it. Either reply here or through my www.guitaramptech.com.au website. Thanks Jernej.
Got one of these with the Rola Celstion speakers, awesome amp, hardly gets played because it's so loud but it has the best tone of all the amps I own., hands down.
Perhaps I should get an attenuator and play it more.
Great video and sound.
You bet. It's a great sounding Marshall. I hope you get to play it more.
Wonderful stuff Chris! To be a good tech, you need to know why, and you do a great job of explaining that! For this business, "just doing it" can get you in trouble. If you know why, you are able to adapt to situations and give you peace of mind that all is done right. Cheers from America!
Thank you Bruce. You are right. I get very annoyed/disappointed when some Techs are too lazy/uneducated/unethical and just replace everything in sight.
I love the USA. Between doing shows there, or NAMMs, or holidays, I have been there 34 times. Which part are you from?
@theguitaramptech Ohio, all my life. Retired EE, worked at Wright Patterson AFB, where we stored the little green men 😀. Would love to make it down your way someday!
@@brucebuckeye I reckon you would love Aus as much as I love the USA, Bruce. Sadly I haven't been to Ohio. Other than one show in Chicago, all of my work was in a smiley shape across the map From WA to CA, TX, FL, GA, VA, MD, NY, MA
thanks
You're welcome, Stephen. I am glad you liked the video.
Thanks.
Thank you for taking the time to say "thank you"....I really appreciate it.
Hi Chris,
I own a Marshall JCM800 4010 (2204 in a Box :-) ) from the same era, but in much better condition. In my opinion, however, my Marshall sounds much better than this one. The distortion sounds a bit muddy, but that may also be due to the miking or the high output of the pickups.
All in all a great video and first class work.
Thanks for sharing and many greetings from Germany
Markus
PS: still praying for your doughter❤
Vielen danke, Markus. I will let my daughter know that my friend from Koln is still praying for her.
The 2104 sounded amazing in this room. My Sliders PAF pickups are vintage (low Z) windings with minimal wax potting. Any sound defects were solely my inability to record properly.
I bet your 4010 sounds great. I wish Marshall stopped making new amps after the JCM800, and just focussed on reissues. Markus, your amp uses the same method of using an extra gain stage as the 2104, so make sure you keep the shorting contact on the LOW input extra clean.
Just noticed that this Marshall amp doesn't have any grid-stopper resistors between R23, R24 and the grids of the two power tubes, normally you'd be seeing two 5k6 grid-stopper resistors there, or maybe 2k2 grid-stopper resistors.
Yes, I noticed that and I like to have grid stoppers, but from memory the schematic didn't show grid stoppers. When you say "normally", are you referring to a 2104 or Marshalls in general? Thanks for the comment, Simon.
@@theguitaramptech , I also installed the 1KOhm 5 Watt screen grid resistors, and the 5K6 grid stoppers after I sent marshall an email that they were missing in my 1980 JMP 2104. There answer was that in those days the tubes were from a far better quality and so it was not neccesary for 50W amps back then. They advised me to install them if I use current production EL34 tubes for safety. It changes the tone a very little bit.
that amp must have some great gig stories to tell
I reckon so, Tim.
one of my all-time favourite players is the late John McGeogh of Magazine, Siouxsie & the Banshees and PiL. He used one of these and a Jazz Chorus, with an MXR flanger on a mic stand so he could play with the knobs standing up.
I didn't know John McGeoch played a 2104! Thanks for that. I really liked his attitude to playing even more than what he played AND I respect anyone who can hang one of those Yamaha SG's off his shoulder for 3 hours.
Hello thanks for the video, do you have the pdf schematic? Thanks again
Hi Robin. I found the schem online. I can't attach doc on UA-cam
That my cup BTW
Me too Steve…except for the weight. I can’t believe I used to lug a tall 4x12 Marshall cab up flights of stairs. 😂
Intresting video Chris! What is your leakage tester device exactly? A led with a little resitor in series? Could you share the schematic ?
Its basically what you said. From memory, I had a Zener reverse biased to protect the green LED. The trick with the LED was to find one that was sensitive (ie turn on) to low currents. I can't recall the resistor values. I'll open it up one day and draw out the schem. I promised Uncle Doug ages ago. I'm just too busy with repairs at the moment.
No problem Chris , If and when it suits you ;)
21:00 So these PTs don't have a dedicated bias tap, interesting.
Why are you using the jcm800 schematic for this JMP? It might be confusing to some people, especially to those who don’t know that only the very early jcm models are exactly the same circuit as a JMP
Thank you for taking the time to comment, John. I think you have answered your question better than I could. :)
Why change the bias cap value when you can just install 10uf 160v in there?
That's a good question. Without having to endure watching myself again, I hope I answered it in the video. In short, I like to have a stable bias that is well-filtered, so I increase it. But, you can't go too high or the lag in the bias cap charging can lead to other issues. The cap value won't affect the tone. If it did, I would not change it.
It's a blurrrr 22 years ago ..42 years ago ...same thing .
🤣
81-23 = 42 years old.
😂 yep. For a moment there I thought it was 2003… just like an old fella.
@@theguitaramptech I do that at least twice a day! Lol. Love your videos sir. You are an amazing teacher.
if you are doing electrical work you meant to test and tag every unit
Thanks for that headsup, Ian. That's news to me, so I checked with SafeWork NSW to confirm I'm doing the right thing.
@@theguitaramptech I'm licensed to test and tag I can do it for you
Rust stopper and ... "WD40?" But you literally just said don't put WD40 anywhere near an amp! 😮
Hahaha! Got me. You’re right!! there’s no way I would use WD-40 on anything electrical, but I do use it to protect things from rust or remove rust.
@@theguitaramptech just ribbing ya, great vlog, learning heaps! Thanks for your efforts.
MY pleasure. I am so happy that you are getting a benefit from my video efforts.