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Old 1st January 2013, 08:10 PM   #21
JMFahey is online now JMFahey  Argentina
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Start by building a light bulb current limiter, to stop blowing fuses and being able to measure *something*:
Light Bulb Limiter
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Old 2nd January 2013, 03:53 AM   #22
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Quote:
Originally Posted by badraven View Post
Plugging in a shorted cable in the Preamp in didn't produce anything I can report other than silence.BR.
I didn't say to short the preamp output. That could be really bad for the preamp. I said to plug the preamp output into the guitar input of your other amp, just be careful to keep the volumes reasonable. You should be able to play guitar thru that setup, just preamping twice in series. You should eventually hear the cracking noise from the other amp, because now the other amp is playting the 'noisy' preamp. When the noise moved with the preamp outout that proved the noise starts in the preamp.

As far as the power problem, that's a different subject, and needs some measurement.
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Old 2nd January 2013, 07:57 AM   #23
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Originally Posted by cyclecamper View Post
I didn't say to short the preamp output. That could be really bad for the preamp.
EXTREMELY unlikely, and a quick glance at the circuit shows 100% definitely not - but even without the 1K resistor it would be unlikely to cause any damage anyway. The 1K would also prevent shorting the preamp out socket from helping at all.

However, I suggested sticking a plug in the PA IN socket, which makes a lot more sense as it disconnects the preamp from the power amp and isolates the problem.
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Old 2nd January 2013, 08:29 AM   #24
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Quote:
Originally Posted by badraven View Post
The joints I thought I had shorted are a ground leg for one of the pots and the ground leg of the adjacent "More Drive" switch.

If there is something shorted from my endeavors I cannot locate it.
I got more 4 amp fuses and removed the PCB from the chassis and hit the on switch and it just hums and then pops the fuse after a few seconds. I'm pretty sure what's happening now is what was going to happen eventually anyway. It used to blink out and hum intermittently, now its doing it non-stop.
I would suggest almost certainly not - it's likely it's something you did, and have blown the power amp.

A DC coupled transistor amp is like a chain of dominoes, any slight short you caused makes them all fall down.

However, the described location probably shouldn't affect the power amp?.

Quote:

Anyone have enough understanding of how to troubleshoot this? I have a Good quality DVOM.
For a start check to see if the output transistors read S/C or low resistance.
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Old 2nd January 2013, 04:59 PM   #25
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Default Reply to Nigel

Nigel

I don't think I did anything detrimental to my amp. Of course I could be wrong on that but I have been looking over everything that I touched and I haven't shorted anything that I can see. The joints that I reflowed are all in the area where the power supply connects. Everything in that area is spaced apart. Not is no chance anything in that area is shorted from what I did. Now there is the chance that I cause the existing/pending problem (intermittent loud hum associated with the amp "shorting out" and becoming non-operational) The only difference now is the non-operation and hum is constant and its blowing the fuse. At this point it doesn't really matter what caused it, I either figure out how to troubleshoot this or it becomes a parts donor. I have to figure something out because I can't afford to buy a new amp at the moment and the one I have left sounds like crap.

I am heading out to my shot to build the light tester thing mentioned yesterday. Hopefully that will allow the amp to stay on long enough to troubleshoot the issue.

BR
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Old 2nd January 2013, 10:10 PM   #26
Enzo is offline Enzo  United States
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It wasn't blowing fuses before, and now it is. That is almost certainly due to something you did. It happens to us all. I*t would be a supreme coincidence if the amp randomly developed a new problem at the exact time you took it apart and put it back together.

COmmon problems would be something like the heat sink shorting out a power supply rail due to a torn/damaged/missing insulator under a power transistor. Also, push-on connectors from the mains wiring or transformer wiring on the wrong pegs. Loose hardware touching the underside of the board. Loose solder beads, etc.
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Old 3rd January 2013, 01:36 AM   #27
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Default FM212R

JMFahey I built the current limiter. Picture is attached.
I understand what's its supposed to do but I am not sure how to check the amp (if it will fire up without blowing the fuse)

Nigel I'm not sure what s/c stands for. Its probably obvious but I have a cold at the moment and my head is pretty cloudy.

Enzo I agree, its probably something that I caused. I said I didn't think it is because its doing what it would do every so often and of course now its blowing the fuse on top of that. My thinking is that I have disturbed it several times by taking it apart. I may well have just disturbed it enough to do what it was eventually going to do anyway. It has been getting gradually worse. It my have been right on the verge of blowing the fuse before I began messing with it. But in all fairness evidence suggests that I have done something and I'm just not recognizing it yet.

I remembered something today. 5 or so years ago when I first began having issues with this amp, the first issue it presented was when I was at church. I would play my Carvin through input 1 and my mic through input 2. I used the clean channel at church and every so often it would switch to overdrive for a moment and then switch back to clean. I never had the foot switch hooked up when that would happen. Fact is I rarely used the foot switch back then. I mainly used the clean channel and only used overdrive when I felt like making some noise and then I normally just used the button on the panel to switch it. Now I use some level of overdrive about 85 percent of the time. Everyone says its ok to have a high ohm load at the speakers than is factory set. But I have to take notice here that I have been running the speakers in series instead of parallel for the couple of days before this amp quit all together.

I am currently reading a book called How to hotrod your Fender Amp. It revolves around Tube amps but I read today that its bad to turn the amp on without the speakers attached. I did do that right after I reflowed some of the joints. It was only on for a few seconds but I did do it. Not sure if its bad to do that on a SS amp though.

I sure do hope we can figure this out. I know its just an SS amp and everyone talks bad about them but I gotta tell you, I have always loved this amp, It doesn't have as fat a sound as a tube but I can make some pretty kick *** tones with this one. But if it doesn't work out I guess it will be that much sooner before I get a Deluxe or Deville or Belair. .
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File Type: jpg Current Limiter Tool.jpg (961.3 KB, 56 views)
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Old 3rd January 2013, 03:40 AM   #28
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Nigel You said
For a start check to see if the output transistors read S/C or low resistance.
The power limiter works, I can turn the amp on and it doesn't blow the fuse.

So how do I check the output transistors?

Is there a possibility I damaged the diodes next to the power supply connections? D53.54.55.and 56 are all right next to the two large Capacitors next to the power supply connectors. I don't know how sensitive these items are. They are all beefy. I can't imagine them being damaged by simply reflowing the joints. I use a 30 watt pencil iron. It doesn't take but a second or so to flow the solder. I know things can be damaged with heat but I have been soldering much smaller items for several years and never burned anything up by over heating them with the iron. Of course these is always the first time... Just wondering.

So should I insert a plug into the preamp in or out and separate the pre and power and and see if it blows the fuse? Not sure where to go from here.

BR
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Old 3rd January 2013, 03:52 AM   #29
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Hey Badraven,
see your having a bit of a problem with your amp.

I fixed one with exactly the same problem. It's 4.45 here and I can't sleep. I'll have a look at my notes when I get to work in the morning.
As far as I remember, it is to do with something noisy on the power supply rail for the pre section, that is why you can hear it with a guitar plugged in the power amp, but at a much reduced volume- it is not interferring with the audio, but is modulating the power supply rail (I think the phase splitter has + and - 15 rail in common.
And whoever suggested plugging in to the power amp to isolate- bravo- that is exactly what I did to get to it.
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Old 3rd January 2013, 04:02 AM   #30
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Bernie

Thanks for the offer. I plug a short patch chord into the preamp in and turned the amp on any it still immediately blew the fuse.

Thanks!

BR
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