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.
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!
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.
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
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.
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.
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. 
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. 😅
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 :)
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
@@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
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.
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!
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.
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.
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
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'.
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).
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...
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.
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.
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
It's an ESD safe mat, there's many different variants/sizes available from a number of sellers. I have one that can be connected to ground/mains earth which makes it very effective for preventing electrostatic damage to components I'm working with. :)
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.
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. ;)
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.
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...
@@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.
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!
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!
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
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.
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
Might just be a dirty/scratchy potentiometer. Maybe some contact cleaner helps (make sure to use non-corrosive contact cleaner meant to be used on pots).
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!
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
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.
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.
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 :)
@@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 ^^
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.
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.
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! 🤞
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
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!
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.
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
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 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.
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
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.
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.
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 🙂👊
