Marshall G15R CD Guitar Amp Hum Fix

Marshall G15R CD Schematics:

Peak Atlas ESR70 ESR/capacitance meter*:

Teslanol T6 Oszillin contact cleaner*:

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

[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.

[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. 

[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.

[antipode]

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.

[Lacobrigense]

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!

[robbyxp]

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

[bandittwothree]

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.

[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.

[panopolis]

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

[rager-]

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.

[richardrichard]

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

[jaycee]

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 :)

[osvaldomoya]

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

[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!

[jameslewis]

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'.

[Sloxx]

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

[lamezockopa]

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

[alisharifian]

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).

[MarzLast]

i would allways inspect the joints around the pots and the jack sockets with the board out, i have also replaced a lot of pots on these and similar amps so worth checking all of them for noise before taking the board out to do caps , and also bigger capacitors may need draining before putting your fingers inside !.

[terryraymond]

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