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Post by 1150lefty on Apr 21, 2015 22:04:12 GMT -5
The problem was intermittent in the past, and I think it was the reason a Desert Storm Vet sold it so cheap, but now the amp won't get loud anymore. It seemed that EQ pots and reverb wires were the culprits. The reverb wires would sound scratchy when I touched them and the volume kinda comes back momentarily. The mid and the push/pull pot for the highs and distortion/clean channel switch have both been culprits for this problem also. Any suggestions before I start poking around?   |
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Post by sumgai on Apr 22, 2015 16:48:54 GMT -5
.... Any suggestions before I start poking around? Yeah. Start poking with a warm/hot soldering iron. Touch absolutely every solder joint you can find, even if it looks good. I mean every one of 'em. For that cable connector on the board, the one that has wires running over to the sockets, pull out and re-seat every wire in that connector. This may be best described as a shotgun approach, but then again, it takes only a few minutes, and when yer done, it'll eliminate the largest part of a headache in finding intermittent problems. Preventing future ones, too.  If the problem persists after that, then we'll look at doing some resistance checks, but let's try the easy way first. sumgai |
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Post by 1150lefty on Apr 22, 2015 21:11:14 GMT -5
Thank you, sir! That was pretty much my first approach. I used what little time I had today to release some of the tension on the reverb cable connectors - it seemed before that if I turned the reverb dial the volume problem would go away. Plugged it in, turned the power on, and ran my hand over the tip of a guitar cable. Had to be quiet, so that might have fixed it. I'm pretty sure that was one of the bigger problems; I'll sand and re-seat the contacts later and do the other connectors. Next, I'll get some pot cleaner and work over the pots. Then comes the solder iron.
The QA sheet (build sheet?) has 1993 all over it. I didn't think it was that old. I think I bought it around 2000. Past time for an overhaul regardless.
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Post by 1150lefty on Apr 29, 2015 17:51:57 GMT -5
Volume problem seems to be gone(and ouch, this baby is loud!). Still has a "scratchy" sound - reverb off/any setting, volume up/down, clean/gain channel, switched speakers, cables, guitars, sounds kinda cool with my gold foil neck pickup. Is something blown or dying? Where should I test first? I haven't found any wiring diagrams with values yet, just user manuals of this and similar models. Could running at 4ohms(amp's minimum)for so many years cause this?
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Post by sumgai on Apr 29, 2015 23:14:33 GMT -5
Ok...
I generally dislike troublshooting something without a schematic, but then again, I have been known to walk on the wild side, on occasion.
Does this scratchiness occur only whilst playing, or does it occur whenever you rotate any of the controls? If it's only while playing, then I'd suspect the filter capacitors. Put a multimeter on the primary side of the output transformer, and watch carefully* for excursions of the high-voltage into the stratosphere. It really should drop a bit when you hit a note, and have the volume turned up - that's the current flowing like crazy. But if the voltage goes up in response, then I'd say your filter caps are about to buy a ticket in the Southward direction. Moreover, you say this thing is 15 years old? Replace the caps out of hand, even if that doesn't solve the issue, just to be safe.
sumgai
* Nicer multimeters have the ability to "capture and hold" a maximum reading, which make chores like this easy-peasy.
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Post by 1150lefty on Apr 30, 2015 10:01:47 GMT -5
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!It happens when I play. I have a basic "good" DMM, but I'm sure I can find somebody who has a decent Fluke if I feel the need. Looks like I have a new project...on top of all the rest.  Thank you so much sumgai! I seem to have made new buddies down at the electronics supply house with all my repairs and work-related "I can't find this thing anywhere else" cries. There is one guy down there who use to do tube amp and microwave repairs and has all kinds of old stuff in his personal stash. I don't think I have ever seen capacitors that big!!! Needless to say, he'd like to take a look at my amp.
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Post by sumgai on Apr 30, 2015 23:05:26 GMT -5
'lefty, So let him look at it - what can go wrong?  Seriously, if he's got lots of parts and gear from the old days (when tubes were still in fashion), and has repair experience, then he's probably well enough educated to figure out your amp's problem. And the price might be just about right, too - who knows?  I've had two amps come across my bench, one a Fender and the other a Crate, they both had filter caps that were marginal at best. When I used my "normal" troubleshooting methods, the problem didn't show up - only when playing, just like you. I finally just replaced the caps in the Crate, and that did the trick. (That was the first of the two.) For the Fender, I actually put an oscilloscope on the output, and saw the distortion (massive noise) as the signal level dropped. That was good enough for me, I replaced the caps, and once again, all was well in customer-land. Can't say this'll be the cure for your particular situation, but like I said earlier, if they're 15 years old, then it's not a dumb idea to just replace 'em it anyways. HTH sumgai |
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Post by 1150lefty on May 3, 2015 1:33:50 GMT -5
I got sidetracked. The "crackle" was gone and I got to messin' around...I didn't get to checking the transformer. I did put a switched output jack on a little Danelectro Honeytone amp and plugged it into my 2x12 cab - surprisingly loud, but the sound isn't really that great. Maybe I could do some St. Vincent covers or something.  |
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