# Marshall JCM600 noise



## mkaye (Jan 25, 2009)

i have been troubleshooting this for a bit now
i have to leave it on for a few hours before the noise starts & stays
it is a kind of random crunching sound
it seems to be on the overdrive side only
i have tracked it down to V103 (replaced it with no change)
if V103 is removed the noise is gone
V101 & V102 are removed
i can see the noise at BK & AA with a scope
tapping components doesn't change the noise at all
unplug CN109 & CN110 - no change
could it be a bad resistor??

mark


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## Wild Bill (May 3, 2006)

mkaye said:


> i have been troubleshooting this for a bit now
> i have to leave it on for a few hours before the noise starts & stays
> it is a kind of random crunching sound
> it seems to be on the overdrive side only
> ...


I assume you've already changed the tube, V103?:smile:

Yeah, resistors are the most likely culprits. Not so often now, since the industry got away from those horrible carbon comp noise generator resistors but it still happens once in a while.

Plate resistors first, then cathode resistors, especially in a cathode follower.

These are the most PITA type of problems. A psychic might be of some help!:smile:

:food-smiley-004:


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## greco (Jul 15, 2007)

Wild Bill said:


> ...... A psychic might be of some help!:smile:
> 
> :food-smiley-004:


Good one !!


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## mkaye (Jan 25, 2009)

i consulted the I Ching & my Tarot cards, but neither shed any light on the problem yet (moon probably in the wrong phase!)

mark


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## mkaye (Jan 25, 2009)

went to change the resistors & found what may have been a bad solder joint on the cathode resistor of the A section of V103
redid all the solder joints and am letting it heat up
unfortunately, if the problem doesn't show up it doesn't necessarily mean it's fixed

mark


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## mkaye (Jan 25, 2009)

changed the resistors on the A side of V103 - no change
did some more analysis & noticed that the output of V103 feeds both the clean & overdrive tone circuits and the clean side is noise free
so, assumed it must be something in the tone control circuit
changed C12 470pF cap on the treble control & the noise appears to be gone

mark


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## mkaye (Jan 25, 2009)

the cap change fixed the major noise problem
still have a similar rumble/sqeal/pop/bang that comes & goes
this is on both channels and is generated by V103 (V101 & V102 removed)
tried 4 different tubes and problem persists
changed the 3 resistors around V103 - no change
definitely need some divine intervention...

mark


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## mkaye (Jan 25, 2009)

replaced the 3 dropping resistors in the P/S chain & this got rid of a lot of noise
changed the V103 tube & this helped too
i still have some noise for the 1st 5 minutes as things warm up
finally got some freeze mist & the noise returns in spades if i freeze the cathode resistor on the 1st stage of v103 (the only resistor i didn't change, because it was difficult to get at!!)
also 1 of the resistors i changed introduces some noise when frozen

is there any way to weed out bad resistors or is it just trial & error

mark

as you can i am gradually working my way thru all the culprits, unfortunately they all seem to introduce the same sort of noise and as i eliminate 1, the next 1 comes into play
hopefully getting closer to a quiet amp


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## Wild Bill (May 3, 2006)

mkaye said:


> replaced the 3 dropping resistors in the P/S chain & this got rid of a lot of noise
> changed the V103 tube & this helped too
> i still have some noise for the 1st 5 minutes as things warm up
> finally got some freeze mist & the noise returns in spades if i freeze the cathode resistor on the 1st stage of v103 (the only resistor i didn't change, because it was difficult to get at!!)
> ...


There's no simple way to weed out noisy resistors. The good news is that with modern resistors getting a bad one is extremely rare, like getting a bad nail out of a box of thousands.

Metal film resistors are the quietest but are not made in high wattages. 1/2 watt versions for cathode resistors in preamp stages are available. Carbon film are the cheapest but are normally more than quiet enough for 'rock and roll'.

The absolute worst for noise are the old-fashioned carbon comp resistors, especially when they get old. Many's the old Fender brought to me that hisses like frying eggs! Replacing the old carbon comp plate resistors with modern ones usually makes the amp quiet as a mouse.

The other nasty thing about carbon comps is that if they get overloaded and burn out they can burst into flame!

For generations they were the only kind of cheap resistor we knew how to make. When carbon and metal film resistors were invented and came on stream in the mid 60's industry couldn't get away from carbon comps fast enough! The only hold outs were the military. Since they are incapable of changing their paperwork for mil specs in anything less than 100 years they still insisted on buying the old fashioned resistors until the late 90's. I worked for a distributor that still sold them, made by Allen Bradley. It had gotten so ridiculous that we were getting over $1 each for them, when modern resistors cost a penny. Even with such price gouging, the volume the military could offer to buy was not great enough to make it worth while to keep making them so Allen Bradley finally packed it in. The military was forced to modernize it's specs!

About that time the audiophile market decided there was some holy mojo about tone and carbon comp resistors. A few off shore specialty houses started offering them, at ferocious prices! The quality tends to suck. I've seen some where the wire leads actually fell off in the package! Noisy ones are very common, even when new.

I had one audiophile customer who still insisted I put carbon comps in his amp. I told him I would but that I would NOT guarantee anything about hiss and noise! He had to bring it back 3 times in one year for me to replace resistors but he still wanted me to use the carbon comps. I didn't care, as long as I got paid!

:food-smiley-004:


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## nonreverb (Sep 19, 2006)

I've had the "pleasure" of working on one of these in the past...not one of Marshall's better products...


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## mkaye (Jan 25, 2009)

definitely a frustrating amp to work on
i had fixed all the noise with V103 and it was nice & quiet on the bench
put it all back together, plugged the guitar in & lots of noise again, this time before the clean/overdrive volume controls
i changed almost every plate & cathode resistor on V101/2, some of them 3 or 4 times (even had to dig out some high quality metal films, instead of the regulars) to find a quiet one
is this amp more susceptible to noisy resistors than most?
hopefully when i turn it on again tomorrow, it will be quiet

mark


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## mkaye (Jan 25, 2009)

still frustrating...
i thought i had it quiet again, but powered it up today & had to change 2 of the resistors that i had already changed
i might have been shooting myself in the foot!
once i had it quiet, i put the original preamp tubes back in to see if they were quiet too & they were, but what if 1 of those tubes is bad in such a way that as i power down it stresses the plate/cathode resistors
i really don't have any other explanation as to why it seems quiet on the bench, but the next day i power it on & i have noise, either on the clean, overdrive or common 3rd stage and i change the resistor again & it is quiet

mark


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## Wild Bill (May 3, 2006)

mkaye said:


> still frustrating...
> i thought i had it quiet again, but powered it up today & had to change 2 of the resistors that i had already changed
> i might have been shooting myself in the foot!
> once i had it quiet, i put the original preamp tubes back in to see if they were quiet too & they were, but what if 1 of those tubes is bad in such a way that as i power down it stresses the plate/cathode resistors
> ...


Mark, two more long shots came to mind. First off, dirty effects loop jacks? They after all normally have shorting contacts to let the signal go straight through if there's nothing plugged in externally. A bit of "whoosh" as you insert a plug a few times will clean it. A jumper cable between the jacks will also tell you of dirt problems, if it kills the noise.

The other long shot is a circuit board that is just too cheaply made! You may have bad solder connections all over the place. A "shotgun" approach by touching up as many as you can see with your soldering iron might hit the bad one(s) and cure the problem. Also, there may be flux and crud between the traces. This can become conductive from picking up grease and dust particles over the years. It acts as a high value, extremely noisy resistor between the traces it covers. An alcohol wash or scrub with an old toothbrush followed by a few minutes under a hair dryer might fix that up.

One problem with old-fashioned eyelet boards is the actual board material picking up tiny amounts of moisture over the years and allowing high voltage to leak through the board itself, again like a high value, crappy resistor. I had one old SilverFace Bassman with this problem. You could stick your multimeter probe anywhere on the bare eyelet board and read anywhere up to 60-70 mv of voltage! This of course wound up in signal paths to give a very loud 'frying eggs" hiss! Most times when you have this problem you can cure it by letting the amp run all night so that the heat will drive out the moisture but in this amp things were so far gone that it just wouldn't work. I ended up removing the board, drilling and mounting a few tie strips and wiring the amp true point-to-point.

Failing that, you just might have a tiny break or kink in a trace, not enough to kill the flow but enough to provide a crappy connection that changes under heat or cold. There's a product techs have used for years called "Cold Spray" or some variation of that name. You let the circuit run and when it acts up you give suspicious parts a zot or two of the stuff. It instantly cools them down. If the part was acting up when warm it should instantly behave, giving itself away.

Engineers often may have had a lot of schooling in printed circuit boards but if it was all with mainstream electronics they never would have seen such problems. The transistor world is all low voltage, which makes such problems much less likely. Having to run tube voltages of several hundred volts makes these problems much more possible.

Are we still having fun?:smile:

:food-smiley-004:


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## mkaye (Jan 25, 2009)

thanks for the tips

i'll clean the effects loop contacts

i touched up all the solder connections a while ago
also, cleaned all the connections with isopropyl alcohol to remove any flux
the circuit boards are very poor, the traces lift with just a gentle touch of the temperature controlled iron (of course, changing the same resistor 4 times doesn't help either)

i've been using the 'cold spray' for the past 2 weeks to weed out all the bad resistors (which were almost all the plate/cathode/PS resistors)
but, i solder in a new resistor, it's quiet, small shot of spray, still quiet
let everything cool down for a day & the same resistors are noisy again
almost like something is stressing the resistors every time i power it down

definitely a learning experience!

mark


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## sfx70 (Sep 16, 2009)

isn't the jcm600 the model with overheating issues? I would install a a fan as it appears to solve the issue.


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## Wild Bill (May 3, 2006)

mkaye said:


> thanks for the tips
> 
> i'll clean the effects loop contacts
> 
> ...


Mark, the more you describe things the more it sounds like the board itself!

I know it's a helluva lot of work but you've already done a helluva lot of work with no success!

At this point, if it was me I would be strongly tempted to rip out the board, take some tie strips and hand-wire the amp!

:food-smiley-004:


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## nonreverb (Sep 19, 2006)

Wild Bill said:


> Mark, the more you describe things the more it sounds like the board itself!
> 
> I know it's a helluva lot of work but you've already done a helluva lot of work with no success!
> 
> ...


This is where techs in today's world are at a disadvantge. We take in repairs like this only to find the product is poorly manufactured and are stuck trying to find solutions to problems that are, in a lot of cases, manufacturing or design shortcomings. (See my recent repair odyssey with my AC30). Lately, I've been seeing some tube amps with 'cringe' SMT boards in them. Unfortunately, I simply cannot repair this type of technology even if I could identify the problem as I don't own the equipment to repair it....besides, it seems that most of the time the boards are just swapped out....so much for repair techs. Thank heavens that the old Fenders and Traynors etc. will probably in service for many years to come:smile:


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## mkaye (Jan 25, 2009)

i think the problem i have been fighting is poor resistors
i had some high quality metal films and every one of these remains quiet
all the others are carbon films and are noisy even after replacing 3 or 4 times
i have 1 remaining 2.2K that i need to find a MF to replace it & hopefully that will be it

mark


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## Wild Bill (May 3, 2006)

mkaye said:


> i think the problem i have been fighting is poor resistors
> i had some high quality metal films and every one of these remains quiet
> all the others are carbon films and are noisy even after replacing 3 or 4 times
> i have 1 remaining 2.2K that i need to find a MF to replace it & hopefully that will be it
> ...


Mark, I hope you're right but I've never found carbon films to be noisy! At least, not like the problems you've been describing.

In fact, you seem to have had more bad resistors than I've SEEN in over 50 years! You have far exceeded the Laws of Chance in what you've replaced. I just cannot believe that you have had that many bad resistors. It would be like filling an inside straight in poker, 5 hands in a row!

Something else is going on. Something perhaps that you're repeatedly doing that we can't see 'cuz we're not there, like way too much heat damaging parts when you solder. Maybe not, since the parts are apparently noisy before you change them. Still, SOMETHING more than bad resistors must be going on!

Forgive me if you've already told us but if you did I've forgotten. What kind of noise are you hearing? Loud, 'bacon frying'? Snake hiss? Intermittent snaps or crackles?

Is the noise really loud, or just noticeable if you're not actually playing? Have you ever heard the same noise in another amp?

We need to widen our perspective here.

:food-smiley-004:


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## mkaye (Jan 25, 2009)

scratching, hissing & popping that lasts for about 5 minutes as it warms up, after that it is virtually quiet
at the moment it is being generated by V103
once it is quiet, if i spray the offending 2.2K resistor (cathode resistor of stage 1 of V103) with the cold mist, the noise will start again & settle down after a minute or 2
the plate resistor was acting the same, but after replacing it with a MF i seem to be OK
however i have been what i though was OK before too & after letting it sit overnight, i have noise again the next day
the noise is very loud, but gradually settles over the 3-5 minutes

mark


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## Rugburn (Jan 14, 2009)

This sounds a lot like a problem an amp tech friend described to me. He'd not been able to track down the same type of noises you describe. After a lot of failed efforts, he decided to replace the filter caps that tested right and looked fine visually. This did the trick. He could *hear* the problem, but not "see" it. Apparantly he's experienced this a few times since. You've done just about everything else....who knows? 

Shawn :wave:


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## mkaye (Jan 25, 2009)

i can definitely see it on the scope, just not sure why every resistor i put in turns noisy very quickly
i soldered a 20uF cap in parallel with the existing P/S caps and it didn't make any difference, but didn't do every cap down the P/S chain and it is really hard to to remove the existing cap on the circuit board
but i can try a 20uF cap across each cap & see

mark



Rugburn said:


> This sounds a lot like a problem an amp tech friend described to me. He'd not been able to track down the same type of noises you describe. After a lot of failed efforts, he decided to replace the filter caps that tested right and looked fine visually. This did the trick. He could *hear* the problem, but not "see" it. Apparantly he's experienced this a few times since. You've done just about everything else....who knows?
> 
> Shawn :wave:


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## Rugburn (Jan 14, 2009)

It was just a thought Mark. Wild Bill would know infinitely more about troubleshooting a Marshall than I ever will. I really know very little about the amps that came after the JTM 45(Bassman). However, my friend has encountered this issue enough to suspect the filter caps regardless of how the test or appear, after exhausting all other avenues. I hope this bugger comes out well for you soon!

Akso maybe have a look here:
http://blog.tubedepot.com/?tag=tube-amplifier

Shawn


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## Wild Bill (May 3, 2006)

mkaye said:


> i can definitely see it on the scope, just not sure why every resistor i put in turns noisy very quickly
> i soldered a 20uF cap in parallel with the existing P/S caps and it didn't make any difference, but didn't do every cap down the P/S chain and it is really hard to to remove the existing cap on the circuit board
> but i can try a 20uF cap across each cap & see
> 
> mark


Mark, if it's a filter unfortunately paralleling a good cap across it may not fix the problem. If we were just looking for a bad cap and dealing with filter hum it's a good trick. If the problem is a filter that is leaking or has a tiny bit of arcing inside it will do it even if there's a cap in parallel.

I know it's gonna be a PITA but that's been the story all along! You should be numb to more pain by now!

:food-smiley-004:


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## nonreverb (Sep 19, 2006)

Have you considered or eliminated transformer arching? I recall this particular design had transformer issues.


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## mkaye (Jan 25, 2009)

no, i haven't
i definitely see crap on the P/S with the scope, but i don't know what came 1st as it could be reacting to the power draw of the preamp tube(s) that are generating the noise
is there an easy way to check the xformer?
it has a solid state rectifier & any of these could be the culprit too (very difficult to get at)



nonreverb said:


> Have you considered or eliminated transformer arching? I recall this particular design had transformer issues.


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## nonreverb (Sep 19, 2006)

mkaye said:


> no, i haven't
> i definitely see crap on the P/S with the scope, but i don't know what came 1st as it could be reacting to the power draw of the preamp tube(s) that are generating the noise
> is there an easy way to check the xformer?
> it has a solid state rectifier & any of these could be the culprit too (very difficult to get at)


Have you tried scoping the feed to the PI with the power tubes in and out?


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## mkaye (Jan 25, 2009)

i pulled the power tubes & i still see crap on the B+ line at the 3rd stage that is giving me problems
i see some 120Hz noise spikes, but what concerns me most are the occasional large spikes or dips
i'm using 10mV/div & 20uS trace, so the variations aren't large in respect to the 450V, but it's hard to tell just how large as they show on the scope for a very short time
as difficult as it may be i guess i have to remove the existing caps & try some temporary replacements to at least rule them out
still leaves the xformer & bridge diodes

is there an easier way to troubleshoot?
i am almost thinking of wiring the P/S from another amp in there to bypass the xformer & didoes

mark


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## mkaye (Jan 25, 2009)

replaced the 1st two 47uF caps and i still have the spikes & dips
it could be 1 of the smaller caps further down the chain, so i'll try them next

mark


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## mkaye (Jan 25, 2009)

i have the output tubes out & removed the 2nd dropping resistor so that i just have the xformer, 270 ohm/5W dropping resistor & 2 large caps (which i replaced with no change, but have the originals back in)
i still have the A/C ripple on the P/S dancing all over the place
any troubleshooting help?
i guess i could replace the 270 ohm with something close & not worry that i don't have a 5W as i am not drawing very much current at the moment

mark


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