Welcome, Guest. Please login or register.

December 02, 2020, 08:09:05 PM

Login with username, password and session length

Recent Posts

 

Author Topic: Marshall MG15CDR OD channel hum and feedback, HELP!  (Read 5797 times)

DIYmastermind

  • Regular
  • **
  • Posts: 23
  • Chip Points: 0
    • View Profile
Marshall MG15CDR OD channel hum and feedback, HELP!
« on: September 11, 2013, 08:22:51 PM »
Hey people.

So a couple years ago i was at a pawn shop, and was looking for a cheap marshall amp (i was in a "marshalls are the best, period!" phase, which i very quickly grew out of). i looked around and spotted the MG15CDR. i plugged up, played a couple of riffs, and dropped sixty bones for it (they wanted 70 but i pointed out the loose vol. knob and a small tear in the tolex) i took it home, tightened the vol. knob, and played the amp for about a week until i got sick of the way it sounded.

Fast forward to now. well, two weeks ago actually. i had chopped a squier sidekick (now named the SP-10, i believe) up and turned it into an 8 ohm head out of boredom. realising most of the not so musical sounds that were coming from that amp were the fault of the dinky, no-name 6" speaker, i decided to also convert my Marshall practice amp combo to an 8 ohm head. so i did, and the process went just as smoothly as the sidekick did. about one week ago i plugged into the marshall which was plugged into my 12" extension cab, and switched to the OD channel (something i had not done since the combo to head conversion. i had only played it clean). i first turned the volume on the OD channel up, and noticed a slight hum. on a side note, i was using a guitar with humbuckers, and the volume on the guitar was all the way down. also, the guitar i was playing with has never had any humming problems with any of my other amps. anywho.. i turned the gain up to about half way, and the hum got pretty intense. i tried cranking the gain all the way up to see how bad it would get and it got to the point of being unbearable. mind you, the volume for this channel was only on 2 of 10. if i turned it up anymore then that it started to feedback terribly. i was standing a few feet from my ext. cab so i thought that might be it. i backed away to the other side of my practice area. the feedback stopped, but the loud hum persisted. also, i use high quality cables running from my guitar to amp, and from amp to cab, so i've ruled that out. playing this amp on the clean channel sounds good (IMO) as it always has. it's just the OD channel that hums and gives me feedback

I've spent a fair bit of time with this amp, not to mention the time and effort of converting it from combo to head. so please don't advise me to get another amp. i have superior sounding amps. but this amp also has sentimental value for the time i've spent on modding it. i mainly use it for the CD/AUX input to play music on my phone, but i also run my guitar through it on occasion.

Does anyone have a diagnosis, or any tips to help me figure out what the problem is? i have decent electronics knowledge, and i'm okay with a soldering iron, so please don't advise me to take it to a tech. please, someone help me out. i can't stand to see one of my amps in such pain... haha. seriously though, any help would be greatly appreciated because, i'm stumped  :-\
« Last Edit: September 11, 2013, 08:25:09 PM by DIYmastermind »

Enzo

  • Legendary
  • ******
  • Posts: 1984
  • Chip Points: 216
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #1 on: September 11, 2013, 11:47:11 PM »
Turn the reverb down, any difference?

DIYmastermind

  • Regular
  • **
  • Posts: 23
  • Chip Points: 0
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #2 on: September 11, 2013, 11:58:01 PM »
Update: so I just now turned the amp on and began poking around with a chopstick to try and find the culprit. the only thing I am getting (I have poked every cap and resistor, twice) is when I poke the resistor labeled D2, I also get a tapping sound out of my speaker cabinet. Does this mean the resistor needs replacing? I am now going to try to re-solder it to the board, in case it somehow came loose.

Well, that did nothing. Now I'm going to remove the board from the chassis and try touching up all of the solder joints to see if anything might have come loose. Will update again later, wish me luck.

DIYmastermind

  • Regular
  • **
  • Posts: 23
  • Chip Points: 0
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #3 on: September 12, 2013, 12:55:50 AM »
Hey enzo. There is no difference with the reverb. I've turned it all the way up and down, and it has no effect on the hum  :-\

another update: I have re-soldered all of the joints on the bottom of the PCB, no difference what-so-ever. No better, no worse :(

another note, I have an out of commission crate gx-60 amp that I could probably source for spare parts, if need be. I am currently trying to fix both amps, but the Marshall means more to me than the crate.

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #4 on: September 12, 2013, 04:08:57 AM »
Okay, we're not going to tell you to get a new amp, or to take it to a tech.

Quote from: DIYmastermind
resistor labeled D2

quĂ©?  Sounds like it's having a personal identity crisis.  I'd expect a diode D2 or a resistor R2, but it seems that you have something microphonic in that area.  Normally I'd suspect a disk ceramic, but whatever, this microphony is unlikely to be related to hum.

Combo cut down to a head - what's it sitting on?  I have an otherwise fine Yamaha 4-track that is quite intolerant of operating on top of anything with a mains transformer in it.

The ground for the first stage is drawn differently to other grounds, implying that it has its own path back to supply -ve.  In your mods you haven't accidentally created another ground path, and thus an earth loop?

The main supply voltages aren't given, but check that you have +/-15V +/-5% across ZD1 and ZD2, and that the main supplies across C33 and C34 are significantly higher.

If you have a CRO you could check the +/-15V preamp rails for excessive hum (there should be practically none).  If you don't have a CRO, your could try tacking a couple of known good electros across C27 and C28, 47-220uF, 16V or more, and see if that make any difference.

Are the two clipping LED's, LED1 and LED2, exposed to artificial light?  LED's are normally photo-sensitive and this is a possible path for 100/120Hz hum from a light globe to get in.

{JACK2 must be in error because if the tonestack input was brought back to the ring contact of the input connector a mono guitar plug would short any signals; not to mention the missing values of C2,3 and 7.}

It could also be that given a better speaker you can hear background hum the previous speaker couldn't reproduce.

{keep the Crate for a rainy day project}

HTH

Circuit attached, thanks to www.amparchives.com
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #5 on: September 12, 2013, 09:42:58 AM »
Make the amp feedback, then turn all its volume and tone controls in your guitar to 0.
Any change?

g1

  • Legendary
  • ******
  • Posts: 670
  • Chip Points: 80
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #6 on: September 12, 2013, 11:52:15 AM »
  This may not be your problem but I will mention it anyway.  When you convert a combo to be a head, you often have to add a speaker jack, did you?  It is important that this jack be completely insulated from the chassis, otherwise you can get a ground loop/hum.  If you use a plastic jack, it should be insulated, but if you use a metal jack, you will probably have to use insulating washers so the jack ground does not contact the chassis.
  In your case, it sounds like the hum is only from the OD channel.  But it is possible it is there in both channels and just less noticeable on clean due to the lower gain.

Kaz Kylheku

  • Elite
  • ****
  • Posts: 112
  • Chip Points: 18
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #7 on: September 12, 2013, 08:23:21 PM »
another update: I have re-soldered all of the joints on the bottom of the PCB, no difference what-so-ever. No better, no worse :(

The hum may still be there, but the component that previously made a noise when tapped with a chopstick doesn't do that any more, right? If so, that is progress. And if it weren't for the hum, you might not have been alerted to this problem.
ADA MP-1 Mailing ListMusic DIY Mailing List
http://www.kylheku.com/mp1http://www.kylheku.com/diy

DIYmastermind

  • Regular
  • **
  • Posts: 23
  • Chip Points: 0
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #8 on: September 13, 2013, 03:51:35 AM »
Hey roly. The combo I cut down to a head is just sitting on the base of what was the bottom of the combo. Essentially all I did in the conversion was cut the midsection out of the amp and glue it together with wood glue. I cut the baffle board to fit perfectly and used the old grill cloth to cover it. I cut the back part of the amp (don't know what it's called, but the part that made the combo a closed and not open back) to fit on the back just right, so there is no way the leds could be coming into contact with a light source. The project turned out better (cosmetically) then I had expected. When i added the extension cab output, i just drilled a hole in the back of the chassis to fit the speaker jack in. There are washers on both the inside of the chassis, and on the outside with the screw on nut holding it all in place. I even plugged a speaker cable in the ext. cab output when I had the chassis outside of the head to see if maybe the end of the cable or maybe part of the jack was coming into contact with anything inside the amp. Nope. I hope this helps you guys help me. And if I missed anything you asked or didn't answer fully, let me know.

On a completely unrelated topic.. today i was at a pawn shop, and I spotted a peavey bandit. It looked like an older model and it was loaded with a single 12" Sheffield (idk if I spelled that right) speaker. It was 80 watts and looked to be in good condition (at first glance). It had a tag on it for $250, which seemed decent to me. Upon further inspection I noticed the speaker cone was completely torn. I went and talked to one of the sales guys, and he said he would lower the price to $50, with a guarantee that I could bring it back in two days for a full cash refund. Is it worth it? I play any kind of rock, a fair bit of metal, and some clean stuff I write. Would this amp be worth my time and money to buy a replacement speaker for? Thanks guys

DIYmastermind

  • Regular
  • **
  • Posts: 23
  • Chip Points: 0
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #9 on: September 13, 2013, 04:07:55 AM »
I forgot to mention a couple of things. I have an orange ppc108 cab with an 8" "professional loudspeaker" in it. I hooked the Marshall up to that, and I still get the hum. I even tried swapping the "professional loudspeaker" in the orange cab with the 8" speaker that was originally in the Marshall before I converted it to a head. Still get the super loud hum. I have noticed however, when I play my guitar through the OD channel, the him seems to go away (but that could very well just be the way my ears perceive it). Also with my guitars vol. and tone controls turned to 0, the him is still there. The hum even stays when my guitar is unplugged from the amp. So by process of elimination, it HAS to be something within the amp. Am I wrong? Thanks again guys, I feel like even if this amp can't be fixed, I am learning a lot  :dbtu:

phatt

  • Legendary
  • ******
  • Posts: 2148
  • Chip Points: 262
    • View Profile
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #10 on: September 13, 2013, 10:19:38 AM »
I'd guess Something like *g1* mentioned,, You have inadvertently created an extra ground connection (thus creating excess hum) or removed a ground point without realizing so,,
Either way hum will often be the result. :(
Most hum issues are due to grounding paths which are not always easy to spot and can send you round the twist.  :loco

Maybe retrace each step in the conversion and see if you can find that added or missed ground point.
A photo of the internals of your amp chassis may help us pin down some problems. :tu:

BTW,, Can I suggest you stick to 1 Amplifier subject as it just adds confusion when speaking about all the amps you have and makes it harder for us to follow.
Phil.

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Marshall MG15CDR OD channel hum and feedback, HELP!
« Reply #11 on: September 13, 2013, 11:48:51 AM »
Quote from: DIYmastermind
It had a tag on it for $250, which seemed decent to me. Upon further inspection I noticed the speaker cone was completely torn. I went and talked to one of the sales guys, and he said he would lower the price to $50

I would call this a classic example of how a fault can demolish the price of an amp.  The real question is not "is this cheap" because it is, but "do I really need another amp?".  Me?  I nearly did something similar last week despite the fact that my home is already packed to the rafters with "good junk".  It's not a bad deal but the economics are a question only you can really answer.


Talking of which; you didn't answer JM's question - do the controls change the hum in any way?
If you say theory and practice don't agree you haven't applied enough theory.