Marshall G15R CD Guitar Amp Hum Fix

Investigating and fixing what's causing the 100Hz hum in my Marshall practice guitar amplifier. // Kindly sponsored by my favorite PCB manufacturer PCBWay! www.pcbway.com/
Marshall G15R CD Schematics:
elektrotanya.com/marshall_g15...
Peak Atlas ESR70 ESR/capacitance meter*:
US: amzn.to/3DJIWXY
UK: amzn.to/3EaugT7
DE: amzn.to/3tdrsOS
Teslanol T6 Oszillin contact cleaner*:
DE: amzn.to/3TkKT2G
UK: amzn.to/3Ulr10X
CAPACITOR LIST:
2x 2200uF 35V
2x 100uF 63V
1x 47uF 25V
3x 10uF 63V
2x 22uF 100V
TIME STAMPS:
0:00 Introduction
1:49 Sponsor Message
2:12 Taking the amp apart
3:27 A look at the electronics
11:09 Mains hum
12:20 Replacing the capacitors
21:37 New thermal compound
23:22 Test run
25:16 Cleaning the potentiometers
25:40 Reassembly & Testing
27:19 Bonus silly ground fault!
29:59 Final testing
30:24 Thank you & good night
---------------------------------------­-----
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Thanks for watching!
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#JanBeta #Marshall #Amplifier #Guitar #PCBWay

[@HavingFunRepairs]

Hey Jan, it was nice seeing you go through the schematic and and working through the repair. Enjoyed taking a break from my own filming to see what you are up to. Really enjoyed this video.

[@panopolis8051]

Love your computer videos but also greatly appreciate ones like this where you tinker with other electronics

[@robbyxp1]

Great video. Always enjoyable to see you fiddling with Electronics, no matter what it is.

[@JanBeta]

Thanks! I enjoy the occasional audio repair. The next couple of videos are going to be computery again, though. ;)

[@TorgmanRDP]

I'm a computer nerd, but also interested in music so these guitar/amp videos are really interesting to me. thank you so much for them.

[@scossa_vr]

Very interesting Jan, as usual

[@Lacobrigense79]

Great repair!! I Have a Marshall G30R CD which i have changed the gain potentiometer (which was a pain to get a replacement for). Other than that it has been a real work horse, great little amp perfect for practice @ home!! Greetings from Portugal!

[@CYON4D]

Great repair and video :)

[@8antipode9]

Here's a tip about caps that I learned from Mr. Carlson's Lab (great YT channel for fixing amps btw). Many cap checkers use the amount of time it takes to charge as a way to measure the cap. If a cap is showing a greater capacity than rated, it is not because it's "better than normal", it's because it's taking longer to charge. Why? Because it is LEAKY. ESR is one thing, but leakage is Equivalent PARALLEL resistance. Leaky caps will often show a higher capacitance on most test equipment.

[@cleangoblin2021]

From what i learned about CRT tv nerds, just fookin replace them all caps for old stuff haha. Theyre crazy like that

[@RekEighty]

i appreciate this video! I have one of these amps and though it's working perfectly right now not a scratchy pot or any hum issues.. I like this exists I can reference if I need refresher on taking it apart and working on it. I won't forget the ground lol Good work friend bringing an old amp back to working!

[@richardrichard5409]

These are stupendous little amps, with a proper old school spring reverb so very 70s tone.

[@lestat666666666]

Thanks Man really useful tutorial 👍🤘🎸🎸🎸😊

[@rager1969]

I saw the ground was disconnected but figured you had done it as prep, especially since you were pointing out the spring reverb it was right there.

[@Sloxx701]

Great video ! If you have a chip lifter, the kind used for prying up ICs out of sockets, they also work amazing for gently prying knobs off of potentiometer shafts

[@MegaWimberly]

I love the guitar stuff!

[@meh3247]

Lovely job old chap!

[@JanBeta]

Thanks! :)

[@osvaldomoya2655]

Tengo un-Marshall 5213 y tienen mucho ruido ahora se produce, muchas gracias por el video y la explicación saludos desde Chile

[@bandittwothree3765]

fun video. a few tips, "thou shall check voltages" first. First DC voltages on all rails, comparing the positive and negative rails for sag. Then put your DMM on ACV, and see what kind of ripple is present on each rail/cap. Or if you have a diff probe to go with that Keysight, you can visualize capacitor ripple on your scope. You can probe that circuit without a diff probe, but see videos with titles like "how not to blow up your scope" first. Those bulk filtering caps fail 2 ways primarily, they become leaky across, or dry out and their capacitance drops, both of which will manifest with ripple you can measure vs. the time spent plucking and replacing parts. Most modern caps last a long time, even cheapo ones, as long as they aren't abused by heat. Those caps probably suffered from the heat of that undersized bridge rectifier, that thing needs some heatsinking I bet.

[@wimwiddershins]

This was really interesting Jan. My Son is getting into music, so it's good to get a look at what I'm in for.😁

[@JanBeta]

Haha, hope you are not going to have to repair loads of wonky audio equipment then! :D

[@CezarySiw]

That Class-Y capacitor across the mains switch won't filter anything. It is basically shorted when the amp is on. I think the reason that cap is there is to reduce arcing in the switch (which will extend the life of the contacts). It will probably also reduce the loud switching sounds that the speaker / amp may produce.

[@jaycee1980]

Yes, spark suppression is exactly what it's for

[@JanBeta]

Yes, that makes more sense. I didn't realize, maybe I should have taken a bit more time to go through the schematics (which was actually an afterthought after I had already finished working on the amp). :D

[@Four_X]

Spot on, It's technical name is "Switch Bounce"

[@MrRepeters]

LOL, at 11:10 when you open up the amp you can see the green ground wire hanging free. Good instructions with the schematic, I always enjoy your videos. 

[@JanBeta]

Thanks! Yeah, I think I must have unplugged the connector while opening it previously, or it wiggled itself loose while in use. Definitely it was disconnected when I openend it this time. 😅

[@lamezockopa]

Wait, wait,wait! You cleaned the Nuts but not the Knobs? OMG! :D Keep on Rockin'

[@brendonelton]

🤣 Maybe Jan likes to keep the smell of his back stage adventures...

[@Papierzeit]

The Humming bird has come... hi hi hi

[@JanBeta]

More like a humming albatros or something in this case! :D

[@Papierzeit]

@@JanBeta yes :)

[@jaycee1980]

Seen a few amps like this, but wow.. a 1A bridge rectifier.. they really were cutting costs ! Normally I see a 4A "SIP" type thing. LM1875 is very common in such low end guitar amps and was at one point used in slightly-better-than-average PC speakers too. 4558 opamps arent really "hi fi" but good enough for this application :)

[@JanBeta]

I considered replacing the bridge rectifier with something beefier but the ones I have in my parts bin all have a way larger footprint so it would require some hacking to get those in there. :D I think I use the "hifi" term a bit too generously. Just had a quick glance at the data sheet for the 4558s, they indeed don't mention hifi. :D

[@jaycee1980]

@@JanBeta I have a similar problem with an Atari (boo hiss) power supply... they use the same rectifiers and they often fail as they are being pushed too hard. I've managed to find a square 4A part that will fit.. with a little lead bending

[@bandittwothree3765]

yep, I bet that BR is getting mighty hot and caused those caps to fail. The guys who laid this out for Marshall knew it'd fail, and that's why it's in there! 105C caps will help, but getting a heatsink on that BR will help more. A fun test would be to put a temp probe on the BR and see what the temp gets to during a playing session, I bet it sees 60-80C, maybe even 100C.

[@jaycee1980]

@@bandittwothree3765 id probably run wires from the board over to a 15A "KBPC" type thing mounted to the metal chassis

[@jameslewis2635]

I had a Marshall 30w bass amp back in the day which had a similar sound and started smoking. When I took it back to the music store they didn't believe me and plugged it in. It then decided to catch fire at which point they admitted I might have a point. That turned out to be an op-amp issue. And while I know very little about electronics in general one rule I have noted from amp repair videos is 'if in doubt, swap the caps out'.

[@terryraymond7984]

This dates back to the 1950's with Marty Robbins, his TUBE amp had a bug and wasnt that the FUZZ sound that had a really neat sound

[@johnhoogstra8310]

Hey! That is what my commodore speakers do, even when not really turned on! 🙂

[@KeriRautenkranz]

My Harmon Kardon Soundsticks speakers from around the same era were sounding pretty weak and sad... same issue with 3rd-tier capacitors. It has many more small capacitors, a few of which were open and one thought it was a diode. Back to sounding great, as good or better than ever.

[@alisharifian535]

When amplifiers fall into humming suddenly,there is a common cause for it & that is the ground connection. It is not earth connection of mains,but possibly some signal input somewere has shorted to the circuit ground or there is a loose ground in one of ICs,so they are floating and getting EM interference from the mains voltage. ICs themselves sometimes get short circuits internally because of overheating or over voltage.Hums which are caused by bad capacitors usually develope gradually but not overnight(cold solder joints are obviously the exception in this case).

[@mustangj0hn]

I had a similar fault with a Peavey TKO-110 bass amp, but it turned out the earth was missing from the plug socket. Some "joker" had installed the socket before I lived there and didn't connect the earth wire in the plug socket!

[@diymaster1121]

From my experience a hum that sounds like when you touch the plug is usually some sort of ground issue, capacitor issue usually sound like deeper hum like for example when you put your ear up on to power transformer.

[@slevengrungus]

Woah i have this amp. was my first amp

[@davidb.6681]

If the tab the ground connection hooks onto has the option to screw a connection onto it, then replace the spade connector with the screw and nut option to make sure it doesn't come back off. I have a very basic practice amp that's similar and the ground connection was a spade connector just like that and it wouldn't stay on and that's with me barely ever moving the amp. Seems like vibration from the speaker alone is enough to loosen it over time.

[@JanBeta]

Yes, I think it just managed to wiggle itself loose over the years just because of the vibration from the speaker. The amp has not been hurled around much since I got it. I might go back in and crimp a proper screw connector on and drill a hole into the chassis to permanently attach that wire.

[@davidallen346]

My Fender 1959 Bassman amp reissue have the same problem tho the amp techs I took it to for repair couldn't tell the difference but clearly I hear a loud humming sound using it in my bedroom

[@philxknife]

Please do more audio stuff!

[@leeselectronicwidgets]

I'm sure you should feed the preamp circuit through the audio in of a SID and build a small microcontroller to use the SID filter! :-)

[@user-zp6be8gc4e]

What diode goes in space 32 on

[@twocvbloke]

Looking back through the taking apart bit frame-by-frame, I think you are right, that green wire wasn't connected when you removed the PCB tray, some alleged "audio experts" do this to reduce mains noise on their (probably faulty, hence noisy) equipment, and this is a very, very dangerous thing to do, if the audio side is left ungrounded, and mains somehow got through to the audio side due to a fault or bad wiring, the next path to earth is the human holding an instrument or a microphone, which has killed performers live on stage as a result, never, EVER disconnect any ground wires from amps, it's there to save lives...

[@JanBeta]

Yup, it seems to have wiggled itself loose over the years. I might go back in and solder it on or put a proper screw on that connection. The whole chassis is connected with a proper grounding screw but the connection to the circuit board is only through that ground wire and the screws on the heatsink.

[@MrFlint51]

@@JanBeta The green wire might have been removed to "fix" a ground loop. if the amp was connected to another piece of mains equipment.

[@keiiko]

You really should make some outro song by yourself to all of your videos ;)

[@JanBeta]

I tried playing the intro tune on guitar but didn't manage to play a satisfactory rendition, actually. :D I'm working on some tunes in my spare time though so maybe someday I'll have my own self-made outro music. ;)

[@JendaLinda]

Nice repair! Btw the insulating pad under the main amplifier chip is actually made of mica.

[@starsundsternchen802]

No. It should indeed be Mylar (Polyesterfilm by Dupont).

[@jaycee1980]

@@starsundsternchen802 Nope, they are traditionally mica. Nowadays you may see "sil pad" and some of those are based on Kapton

[@JanBeta]

Oh, I confused the two. I even have a stash of replacement mica insulators in my parts bin (but didn't remember the name correctly)!

[@fu1r4]

Try turning the power connector 180 degrees. It can sometimes get rid of hum.

[@anticuchodechina]

Hello, great video. How to solve the crack sound from the potentiometers

[@JanBeta]

I usually use a good non-corrosive contact cleaner like Teslanol T6 (German product, not sure if it’s readily available elsewhere). Deoxit is the US alternative. Spray some into the little cutouts in the potentiometers, move pots for a bit, spray again, let the contact cleaner evaporate. Usually that fixes noisy pots. If you still have problems, the coating in the pots might be damaged and they need to be replaced altogether (but that rarely ever happens in my experience). Hope that helps!

[@terryraymond7984]

Jan have you ever worked on Midi keyboards. I have a Concert Mate Midi keyboard that I messed up the TOP plastic cover mainly that messed up the speaker, I dont know if Concert Mate was sold in Europe though. This seems not very popular and older I wish I could fix the speaker mainly with the plastic, but it needs a complete TOP piece to fix this. Or if anyone may have a Concertmate you would give me good deal on please let me know.

[@terryraymond7984]

I still like the Marty Robbins Fuzz amp that was of course the old Tube type

[@tuonofly]

Hi Jan, gutes Video...ich habe gerade einen Valvestate 8080 Combo gekauft und er macht 100% genau Hummmm wie deinen. Egal, ob einen Gitarrenkabel verbunden ist oder nicht...Meist du, wenn ich ein extra Erdung Kabel von Platine bis dem Gehäuse verbinde, kann ich das reduzieren? Vielen Dank, Danilo

[@milk-it]

The ground wire was not connected at the start of the video ;-). Great effort recapping it, though. Giving that Marshall amp some TLC will pay off in the long term!

[@SalsaLover08]

funny, I noticed the green ground cable disconnected at 3:27 when you just opened the encasing....

[@Stefan_Payne]

SamXon are seen as pretty good caps. Though they go lower end than Nippon CHemicon/Nichicon and co. and don't forget that Japanese Companies have Duds too -> NCC KZG/KZJ, Panasonic FJ, Nichicon HM, which should be replaced on sight by now. Sadly you have to go to 16 or 25V Caps (and 12,5mm diameter) for 10mm/6,3V ones...

[@jaycee1980]

They are OK in undemanding applications, but in anything involving switched mode power supplies or high ripple currents, they are total crap

[@JanBeta]

I've seen a lot of Samxon caps fail in early 2000s DVD players and similar devices. They seem to have been used a lot in those days.

[@Stefan_Payne]

@@JanBeta Yeah, that's the bad Caps era, when a lot of low ESR Caps were abused, misused and of not great quality... It was common back then to put entry level caps right next to the hot MOSFETs and let them run at 70°C or more. And as all new things, as low ESR Caps were at the time, they failed a bit more than conventional Caps.

[@terryraymond7984]

yeah I have a little cheap amp for playing my Midi keyboard but yeah it works

[@davidellenberger3852]

I'm replacing the caps on my amp at the moment and I can't confirm capacitors values. I guess there's different versions of it. My one has a 2x 16V 1000uF and 2x 50V 2.2uf. Weird, if it was just a bit off I'd understand but they are so different. Strange world. Anyway, can you make suggestions when replacing caps. You're saying audio quality isn't needed, in this case we go for lowest impedance I'd say or what? Thanks

[@MacMelmac]

Why are the guitar amps always open at the back and not a close cabinet as normal speakers?

[@arcifric969]

One wire problem LOL :-) thanks for tihis video . . .

[@JanBeta]

Yeah, it was a real face palm moment when I realized that the ground connection was unplugged. Didn't hurt to replace the caps I guess, but that was definitely causing the majority of the hum... :D

[@alanmann1731]

It’s possible the green earth connection came off as you pulled the chassis out as it’s disconnected in a very early part of the video

[@dougjohnson4266]

Crank it to 11! I am having the same issue with mine and have been too lazy to work on it.

[@JanBeta]

Maybe you just have a disconnected ground wire, too! Worth a quick look, I guess. ;)

[@reddragon27284]

Did anyone else spot the detached ground wire?

[@benbaselet2026]

Nice. I picked up an even older and cheaper Marshall amp from the trash to take apart. I was not impressed by the build quality in any way :) Same basic board as yours pretty much as far as I remember. The real bothers were cleaning the pots and then figuring out how the hell I'm going to test the thing since I don't have a guitar :D I would have doubled the value of those main filter caps, just because you can be sure they used the cheapest parts they could get away with.

[@ApocalypseRider]

Hi and thanks for the very informative video. I'm just about to start to repair this same amp. But is there fault in the parts list? I can't find any 22uF 100V caps from my amp nor there seems not be those in schematics either... But from amp I found two of 2.2uF 100V caps And the schematics has a fault when it says there is 47uF 10V cap in reverb circuit when it is in reality 47uF 25V like in your list.

[@JanBeta]

Hi! The parts list may differ from the amp revision you have, they seem to have gone through quite some changes over the years with component values. I’d advise to always replace the capacitance with the same you actually find in your circuit. The voltage rating is not important as long as it’s the same or higher rating as the original caps! Hope you manage to fix the amp! 🤞

[@zbaby82]

Do you like F&T capacitors? Anyway more guitar amp repair videos would be nice. 🎸

[@andrewclegg9501]

Yeah, the ground connection wasn't connected when you took the amp out of the cab. At least its got good caps in it now.

[@JanBeta]

Yup, I just realized when I edited the footage. It might have wiggled itself loose over the years just from the speaker vibrating. I had the amp open a while back though to have a quick look, maybe I accidentally disconnected the spade plug then. In any case, I think it was a good idea to replace the caps anyway. :D

[@gitplayer55]

I have the G100R CD, which is the 100w head version of this amp. I bought it in the early 2000s when I thought I needed a half stack but didn't use it much. I don't think it sounds that good tbh. It is a cheap transistor amp and doesn't compare to a real tube amp. These days you are better off with a Spark, Katana or THR10 or something. And those mini head versions of bigger tube amps, like 6505, Rectifier or even Marshall themselves offer enough volume for live applications also. So, I wouldn't bother fixing die G100 if it were broken some day.

[@fu1r4]

23:46 Using a brush to make it cleaner will not work. You are only moving it around with that brush. You need to use a washcloth or paper to wipe it off.

[@davidellenberger3852]

Did anyone tried to modify and power it with a 7x lithium pack? 22-29V? Would be nice to turn it into a portable version

[@kriswillems5661]

It's possible it's not about the caps at all. Maybe the ground connection just got better by screwing in the board again....

[@JanBeta]

Yeah, the missing ground connection definitely played a major part in producing the hum. It was a lot better with the new caps than when I previously tried it though, the guitar signal was barely audible over the hum before I started the repair. So most likely a mix of bad capacitors and the ground fault in the end.

[@DiverCTH]

26:33 That reminds me, Marty. You better not hook up to the amplifier. There's a slight possibility of overload.

[@bradnelson3595]

Is that an eight bit guitar?

[@fuzzlabrador]

if it's humming check the grounding - just ask Uncle Doug

[@boscogonsalves2757]

That was a fantastic job going thro' d schematic diagram. I'm trying to repair my friend's G15RCDR. similar to your model. The potentiometers are beyond repair n needs to be replaced. I 'm not able to find it's valves in any circuit diagram. I would be obliged if you cud let me know valves of each potentiometer. This wud enable me purchase n replace my potentiometers with correct values. Bosco Gonsalves, Mumbai, India

[@JanBeta]

Unfortunately I don’t have the amp anymore, passed it on to a friend and upgraded to a slightly more powerful Fender in the meantime. I am sure there are some forum posts about replacing the potentiometers somewhere though. Most of the times the values should be engraved or printed on the pots themselves, too. Should be visible once you remove them from the circuit board. Hope you manage to get the correct values!

[@rebeccaschade3987]

I thought ground faults were basically the default for cheap guitar amps... And some not so cheap ones as well. I can't remember ever hearing a guitar amp with no hum what so ever, but I could be wrong.

[@jaycee1980]

Guitar amps normally have fairly high gain on the pickup, which means hum gets picked up quite easily... Also of course the pickups themselves pick up mains hum which is why you have "Humbucker" pickups :)

[@rebeccaschade3987]

@@jaycee1980 I did mean for it to be slightly tongue in cheek. It's surprising sometimes how often electronics are designed with ground faults in them though. It's almost certainly by accident, exacerbated by cost saving, but you hear it so often that I feel it probably should have a meme ^^

[@pikadroo]

Why?

[@rawberg_se]

Doesn't seem like the ground was connected in the first place? Unless you unplugged it for some reason, maybe it got loose in the while in casing?

[@Liofa73]

Spoiler...

[@thomasandrews9355]

@@Liofa73 never read comments before watching

[@JanBeta]

Yup, looking back I think that was what happened. I briefly had the amp open a while ago after it started humming to have a look but didn't notice anything unconnected. It might have wiggled itself loose or I accidentally disconnected it while I opened the amp previously. Just glad that I found it in the end! :D

[@rastislavzima]

@@JanBeta Yeah he disconnected it deliberately so he had reason to make a video, didn't you Jan? 😁

[@katho8472]

74656! LLAP!

[@Liofa73]

3:40 -- No spoilers...

[@JanBeta]

:D

[@sonic2000gr]

Interestingly, it seems the new models of their practice amps are even worse now. We bought one recently and the sound quality was completely underwhelming. They carry the name but not the quality you'd expect from Marshall.

[@fichambawelby2632]

The isolation piece is made from mica, not mylar.

[@JanBeta]

Oh, I confused those. Thanks!

[@fichambawelby2632]

@@JanBetayou’re welcome, Jan!

[@LeftoverBeefcake]

Well of course it hums, it doesn't know the words! ... I'll see myself out.

[@M0UAW_IO83]

If my experience of repairing equipment for bands and DJs is anything to go by, that hum is "authentic valve sound" and some guitarists would not thank you for "fixing" it 😉

[@meh3247]

Which perfectly illustrates the lack of knowledge of "some" guitarists!

[@JanBeta]

But valve hum is good hum while transistor hum is bad! :D

[@M0UAW_IO83]

@@meh3247 I'm only partly joking, plenty of guitarists play the amp as well as the guitar, even using the microphony of the valves, the springline reverb etc. (where fitted) and the hum of the amp is a part of their sound.

[@hooman1439]

The good video! I don't scrodriver :(

[@nobaiat5621]

Probably poor grounding?

[@JanBeta]

Yup.

[@UpLateGeek]

You've fixed your amp, now you just need to work on your guitar face! 🤣

[@AG-cg7lk]

These cheaply made solid state Marshall amps are junk as are their modern pcb-based tube amps. I repaired one of their modern tube amps that was humming loudly. It was the bridge rectifier, which was a cheap component that was getting so hot as to gradually desolder itself from the pcb because they chose a component that was borderline underrated for the circuit.

[@terryraymond7984]

My little AMP is a Walmart special

[@SimonZerafa]

So you didn't modify it so it goes to 11?! 🤨🤷‍♂️ You also need to post some actual music! Let's hear a track to two and listen to the soul ripping sounds of The Jan Beta in action 🙂👊

[@TheGreatAtario]

Protip: "potentiometer" is pronounced " puh ten shee *om* i ter "

[@JanBeta]

Isn't that what I said? Sorry if I mispronounced it! :D

[@TheGreatAtario]

@@JanBeta Heh, sorry, it's hard to see the boldface on here. Emphasis should be on the "om" syllable. Cheers!