Amp Problem

[Scott_F]

Re: Amp Problem

This is one of the best troubleshooting amp sites out there...

http://www.geofex.com/ampdbug/ampdebug.htm




Putt-putting sound or motorboating

In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)

 

[Jeff Seal]

Re: Amp Problem

....as we discussed before, these amps are notorious for having problems....

I'm working on a permanent fix for this design (sometimes re-engineering an amp has to be done....not claiming to be "more intelligent" than the good people at Marshall.....but this one has yet to be resolved..)

....although the "technical" aspects are easy enough to understand..(not exatly "rocket science")...i doubt seriously you're too intersted in the "cause", but rather how to remedy it....

I'll know sometime this coming week the final result......will let you know!

until then... :smoker:

Jeff Seal

 

[Kent S.]

Re: Amp Problem

Every person who's complained about the JTM 60 and JCM 600 has had that same problem. I was told by a tech that those amps have a tendency to overheat the circuit board, causing component failure. It does sound like caps, or possibly a component blew that threw the bias way off. Fix it, then sell it.

Well, as an idea, would it be possible to magnetically shielded small dc pentium style fans inside the unit to check hot spot areas a bit cooler ... I don't know how much it would help, but power amps tend to benefit greatly from forced ventilation.

 

[Kent S.]

Re: Amp Problem

I sure can't explain why, but the motorboating is gone. The amp sounds better than it ever has and I am in tone heaven tonight with my Tele. I'm starting to wonder if there was a powersource issue. Out here on the reservation our power grid fluctuates quite a bit. I hate it when a problem dissapears and I don't know why!

A line regulator/power conditioner is kinda pricey (the line regulator part that is), but is worth it, especially when you can plug everthing into it you got, most will put up with 30A of current draw. Plus the power filtering and RFI/EMI suppression and spike protection is well worth it. Something to think about.

 

[Kent S.]

Re: Amp Problem

....as we discussed before, these amps are notorious for having problems....

I'm working on a permanent fix for this design (sometimes re-engineering an amp has to be done....not claiming to be "more intelligent" than the good people at Marshall.....but this one has yet to be resolved..)

....although the "technical" aspects are easy enough to understand..(not exatly "rocket science")...i doubt seriously you're too intersted in the "cause", but rather how to remedy it....

I'll know sometime this coming week the final result......will let you know!

until then... :smoker:

Jeff Seal

Jeff, off topic, but a lot of amps have a HT (B+) fuse ...well is there a reason that no one incorporates circuit breakers there and at other fuse points?
Other than the urge of a fuse going and prompting you to say, *that shouldn't have happened, let's get it to a shop and find out why it did ...*
(which is why sometimes putting fuses inside the chassis is a good idea, eliminates the want to just thorw in another fuse and roll with it on the spot).
Any reasons for this technically or is it more of a cost issue?

 

[Kent S.]

Re: Amp Problem

This is one of the best troubleshooting amp sites out there...

http://www.geofex.com/ampdbug/ampdebug.htm




Putt-putting sound or motorboating

In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)

Love the site, and Aiken's also ... I have seen power tubes do it also ... Hey anyone have a way to get in touch with R.G. Keen? I though there was something on the site, but I never could find it?
BTW Scott, I gotta couple more suggestions on your amp building, especially the clean/dirty channel affair, might not be anything you haven't seen before ... PM me if your interested.

 

[msplines]

Re: Amp Problem

Acoupla things - If the caps are too heavy (higher value than stock) or defective, then motorboating is more or less guaranteed.
I also live in an area where there is a lot of heavy (old) agricultural machinery and low level popping, noise, whining and shrieks etc are almost an everyday occurrence, so it could just be a simple DIY machine on the circuit because it's intermittant. Filter packs are pricey, but they do offer protection - could be a good bet in the long run.

 

[JumpMarine]

Re: Amp Problem

Thanks again Jeff,
Something changed last night with the amp and it is sounding better than it ever has. Can't explain it! When I asked you about this amp before none of the motorboating had ever occured and within a week it started, lasted for two days and stopped. Is this a normal situation with worn caps? Work fine one day, crappy the next and so on?

....as we discussed before, these amps are notorious for having problems....

I'm working on a permanent fix for this design (sometimes re-engineering an amp has to be done....not claiming to be "more intelligent" than the good people at Marshall.....but this one has yet to be resolved..)

....although the "technical" aspects are easy enough to understand..(not exatly "rocket science")...i doubt seriously you're too intersted in the "cause", but rather how to remedy it....

I'll know sometime this coming week the final result......will let you know!

until then... :smoker:

Jeff Seal

 

[JumpMarine]

Re: Amp Problem

And a special thanks to Kent and everyone else who chimmed in. This amp is a work in progress and I don't want to give up on it just yet.

 

[Kent S.]

Re: Amp Problem

And a special thanks to Kent and everyone else who chimmed in. This amp is a work in progress and I don't want to give up on it just yet.

No problem man, to toss in once again, regarding the cap thing ... as far as I know power conditions can aggravate conditions ... also, some *border line* things (read a little to far out of spec) will behave normally, until they heat up ... then toss in power flucuations and spurious line noise ... you can get things that come and go. I think though that once the caps start to go, and start motorboating that they generally continue doing so. I've never really observed it under a continued test ... Maybe Jeff and Scott could give a concise, and concrete yes or no on that. Heck, I still have SS practice amps that pop and buzz sometimes, I clank 'em on top one time with my fist and they stop ... does it ever couple of years, hit it, works fine ... never have figured out what that deal was ... everything checked out inside. ??????
I couldn't find a schematic on the 600 series, I found the manual for it, but not the schematic ... you might want to order one from Korg though, especially if you plan on keeping the amp, which seems to be a toneful winner.
Oh, if you want the manual you can either get it from the Korg or Marshall sites, or PM me with an e-mail address and I'll send the pdf, as it's just big enough not to be able to post to the forum. No super secrets or anything, but it does mention that the XLR line out is pre power amp with speaker emulation or post power amp without speaker emulation, also they have specific connections from certain pins for certain level applications. And the speaker outputs are discuused impedance wise (there something that is a bit misleading on the back of one of the amps or something). Anyway, offers there.

 

[JumpMarine]

Re: Amp Problem

Thank you Kent, I have already downloaded the manual for the amp. I still can't get over how my amp sounds right now, very chimey, full, clear, great solid bass and midrange, almost like there is a small amount of reverb on at all times, but it's turned all the way off. Wish the power company would get it together! BTW, I lost a DVD player to a power fluctuation just a few days ago as well. And it was hooked up to a good surge protector.

 

[Kent S.]

Re: Amp Problem

BTW, I lost a DVD player to a power fluctuation just a few days ago as well. And it was hooked up to a good surge protector.

Move ... :laugh2: Seriously though, a descent surge protector is not the same as those sold as *power strips*, some are good, and some just aren't!
Buy a good one, heck, just get a Furman, and call it a day ... BTW, don't laugh Furman also makes computer, and Home Theatre power conditioner/protectors also ... Big thumbs up!

Also, buy one of those outlet testers that show if your outlets are wired correctly, radio shack carries them along with all electronics suppliers ... they are like $7 or so, very good investment. Great to have in a gig bag.

 

[JumpMarine]

Re: Amp Problem

So here's an update on the amp situation. The amp continues to sound wonderful, BUT now I don't have any control over the eq on the gain channel. Arrrggg! The eq on the clean works just fine, but the gain eq is dead. Anyone know of a good amp tech in Washington State? Or would I be better off just sending it off to Jeff Seal? Although, I don't think Jeff wants to see anymore of these amps for a while. J/K Jeff!

 

[Kent S.]

Re: Amp Problem

So here's an update on the amp situation. The amp continues to sound wonderful, BUT now I don't have any control over the eq on the gain channel. Arrrggg! The eq on the clean works just fine, but the gain eq is dead. Anyone know of a good amp tech in Washington State? Or would I be better off just sending it off to Jeff Seal? Although, I don't think Jeff wants to see anymore of these amps for a while. J/K Jeff!

Sounds like you got a short around the entire tone stack? How does something get rerouted around that? ... Any descent amp tech can find the problem once they look over the unit ... Just call around to the local music shops ... Better yet, find an authorized marshall tech in your area. That one has me scratching my heead, and I couldn't find a schematic on it off the web.