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Old 30th December 2012, 08:18 PM   #11
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I agree with the rest - the flickering LED can only be caused by something in the very simple PSU, and MOST likely the primary - you're not going to lose one side of the secondary without MAJOR problems (speaker blown, amp blown etc.) It's VERY possible that the fault is external, and is a faulty mains socket or lead.

As for the noises, when it's happening plug a 1/4 inch jack plug in the PA-IN socket, just a bare plug is all you need - does that istantly stop it or not?.
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Old 31st December 2012, 04:47 AM   #12
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Thanks for all the advice. I am still trying to recreate the symptoms described earlier. The scratchy paper sound started earlier and I went to plug in the plug to the preamp in and it went away before I could to it. I have tried several chords and can't seem to recreate these things on demand. I think it only happens when a guitar is plugged in. But before anyone makes the assumption that its the guitar or the chords etc. These issues do not happen when I am plugged in to my other amp. I am thinking of opening the amp anyway. Which I guess I will go ahead and do that since I can still run it out of the enclosure. Then I will check all the solder joints and plug ins. When I plug a shorted plug into the power amp in the amp is dead silent.

BR
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Old 31st December 2012, 07:23 PM   #13
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Enzo is absolutely correct. I saw the center tap, but you're right, it's a full-wave bridge and the center-tap mid reference is re-established later. Could be a center-tap or ground problem but more generally it can't lose half the voltage due to a transformer open or thermal-cutout. The bridge means it would be full voltage between the rails or nothing. So when it goes out, and the power light goes off, where is the power for the hum coming from? Of course the power transformer could still have an intermittent short.

If you could just reproduce it at-will, and watch with an infra-red camera...see what gets colder or gets really hot. I'm still focused on the light going out.

Whacking it is of course a perfectly valid and valuable test for solid state equipemnt. I remember when a large mainframe channel-attached storage director kept having intermittent problems at Morton Thiokol, nearly 30 years ago. They cleaned and reseated cards, then sprayed freon coolant on it, hoping to get some reaction. As service level agreements got violated IBM automatically dispatched more guys in suits. After a while, the machine room was full of guys in nicer and nicer dark suits mumbling "think we need the big guns...yeah call for an xc-5444885850-C...will it fit in the elevator..." and a few minutes later a guy appears with a redwood 4X4 about 6 feet long with a brass band at one ends and a plate with an IBM part number, and a procedure describing how several guys swing it like a battering ram and exactly where to impact the frame. Definitely poke around with insulated chopsticks.

Loose screws used to be more common than you'd imagine on the old power transistor cases. Now not so much.

Ribbon cables?

The first diagnostic step has to be finding out whether the current is not being delivered or is being burned up somewhere. If some load is drawing the power down so much the light goes off (even an LED) then something is getting hot. If it's an open circuit making voltage and curernt low, something is getting colder than usual. Still strange it's not blowing a fuse, no smoke, no hot capacitors, no burnign speaker voice coils indicating RF. To drop the rail enough to turn out the light would require a serious short or load, more than the normal output of the amp. You'd notice a 100-watt "heater". Makes an intermittent connection or rectifier in the power supply very suspect; some open-circuit condition. The fact is still hums makes me suspect a partial transformer winding short or part of the bridge. If it drops to exactly half, that might be a clue half the bridge isn't working.

Intermittents are a b*tch. I would hope your amp breaks for Christmas, then it can probably be fixed much easier; but then you hope the failure doesn't take out additional expensive parts.

If it was mine, I might solder a small fast-blow fuse into each rail right after the bridge rectifier and a 1/4 ohm resistor in series with the fuse. I have a bunch of really cheap $8 digital VOMs I bought in bulk and gave away last Christmas, so I would wire in two of them watching the two rail voltages, and two more VOMs as current meters measuring the voltage across the resistors.
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Old 31st December 2012, 07:54 PM   #14
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Quote:
Originally Posted by badraven View Post
When I plug a shorted plug into the power amp in the amp is dead silent.
BR
With it off, spray inside all the input jacks and the preamp/poweramp jacks while inserting and removing a plug or pressing on the internal switches with some wooden shish-kabob skewer or shopstick.

Plug the Fender's "preamp out" into your other amp's input, and put a shorted guitar plug into the Fender's poweramp input and another shorted plug into the Fender's guitar input.

Leave it like that until you hear the crackling noise from your other amp. That proves the problem is in your Fender's preamp.

Fiddle with knobs & buttons on the Fender to see whether it makes any difference.

Then I'd use the other amp as an AC probe at various test points; just convert a guitar cable, with a wire to an alligator clip on the ground and a pointy solid wire on the hot. If you want to be careful you should put a big (10m?) resistor and nonpolar cap in series with the hot, but I've probed around with just the stock 10K guitar input network before without damaging either amp. Don't know whether I was just lucky or desperate, but it was a useful technique when my old 'scope had drifted completely off-screen. Maybe Enzo can tell whether that's dangerous at the test points labelled on the schematic. Anyway, the point is that you should be able to use the second amp to listen and find the first test point where the cracking noise appears. If it's everywhere but TP7 then suspect the zeners and caps that give you the =/- 16V.
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Old 31st December 2012, 08:08 PM   #15
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The "Bump Test" didn't produce anything other than the sound of the spring in the reverb tank rattling. Plugging in a shorted cable in the Preamp in didn't produce anything I can report other than silence. The amp has a very low hum like all electronics seem to have but its is very low and I have to listen for it. Plugging in the shorted cable didn't change it. The only sound I got was when the metal of the plug came into contact with the socket. Of course my fingers were touching the metal of the plug and we all know that creates hums and scratches when we do that. But all went silent when the plug slid into home base.

I removed the amp from the cabinet last night and went over the top of the PC board looking for anything obvious. Nothing is burnt, scorched or broken. There were a couple small caps lightly touching a resister body but no metal to metal connections. I will be removing the the PC board shortly and checking all the solder joints. I plan to re-flow all the joints unless someone things that is a bad idea.

I would like to note though, last night after I removed the amp and was looking over the board I laid my hand on top of the transformer and it was pretty warm. Not hot but it seemed hotter than I expected. I don't know how hot these things get while the amp is being played but I played it on low volume - about 1 to 1.5 on the drive channel for about half an hour.

I rechecked all the connections from the power supply to the PC board and all connections are correct. I labeled each before I removed them last time. So if they were correct from the factory, they're still correct.

Cyclecamper. Tell me where to solder in these connections on the schematic so I can get these points installed and ready to check things if it starts shorting again.

BR.
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Old 31st December 2012, 08:13 PM   #16
Enzo is offline Enzo  United States
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Power transformers by their nature do tend to get pretty warm.
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Old 1st January 2013, 01:34 AM   #17
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Default FM212R in pieces w/pics

Its all apart and I have been over it visually and I don't see anything out of order, no strange smells, burn marks etc...

Attached are some pics. The Power Supply has varnish that ran out of the coil(s) which I assume happened when it was originally dipped.

The two big Caps are not discolored, misshaped etc.

There is the residue all over the solder joints that I am under the understanding is supposed to be cleaned off after soldering because that substance is supposedly corrosive. But I am not positive about that.

Not sure if I should reflow all the joints or not. I don't want to cause myself any more problems.

I think I will re-assemble the amp and leave it out of the cabinet, hook up the speakers and power and try wriggling wires etc.

BR
Attached Images
File Type: jpg Fender FM212R PCB1.jpg (840.7 KB, 89 views)
File Type: jpg Fender FM212R PCB2.jpg (923.5 KB, 82 views)
File Type: jpg Fender FM212R PCB3.jpg (608.8 KB, 79 views)
File Type: jpg Fender FM212R PCB4.jpg (686.5 KB, 80 views)
File Type: jpg Fender FM212R PS1.jpg (512.8 KB, 77 views)
File Type: jpg Fender FM212R PS2.jpg (473.8 KB, 26 views)
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Old 1st January 2013, 05:02 AM   #18
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I reflowed the solder on the main connections. i.e. Power supply terminals, the large Caps, all 5 of the jacks I recently installed and a few of the smaller ones. No I didn't over heat anything that I know off. I then mounted the PC board back in the chassis. I made a quick check of all the connections and discovered that the crimp connection on P7 moved within the connector. It wasn't flopping about, it was just loose enough to move it if I tugged and pushed back. I got really happy assuming that I had found the underlying problem. I recrimped all the push-on connectors and then I plugged the amp in without the speaker hooked up. I flipped the switch and it took about 2 seconds and it blew the fuse on the PCB.

I pulled the PCB again and rechecked all my solder joints and found what looked like a hair sized piece of solder between two of the small joints I had resoldered. I desoldered it and then soldered again. I checked all the joints again and found nothing so I reinstalled the PCB and installed a new fuse and hooked up the speakers this time and when I turned it on the speakers hummed for about 2 seconds and the fuss blew again.

So... Either I am missing something or I caused another problem OR I inadvertently caused the humming issue to finish "going out". So... I'm not sure where to look now but either way I don't have any more fuses. Tomorrow will be another day I guess...
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Old 1st January 2013, 12:17 PM   #19
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Exactly where were the joints you shorted out?.
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Old 1st January 2013, 08:51 PM   #20
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Default Still blowing fuses

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.

Anyone have enough understanding of how to troubleshoot this? I have a Good quality DVOM.

BR
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