Tube Amps / Music Electronics
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|1/9/2006 9:40 PM|
||Intermittant Ampeg B2R|
A hard-working band brought an Ampeg B2R bass amp in to me today... they play 4-hour sets, 5 nights a week.
The complaint was that "it just started cutting out" on them. I put it on the bench, and it makes full power (350W) with a sine wave input.
How do yall handle an intermittant equipment like this?
I'll be poking around with a chopstick, and will clean the effects loop jack contacts. What else would you recommend?
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|1/10/2006 12:55 AM|
First you ask them questions they might not be able to answer.
Do the power lights stay on or flicker off?
Did it cut out like a switch was thrown, or did it fade out or did it gravel up for a second befoe going away?
Was there any change in the sound before the cutout?
Are you using a real speaker cord and not a guitar cord going to the speaker?
Are you using any effects or other units in the loop?
Does the unit get overly hot? Hotter than it did before?
How long has it done this?
How many times did it do it and how long between times if more than once?
Did it do it anywhere else or only at one location?
When it cuts out does anything help?
Does whacking the top of the amp do anything?
If you turn things way up and make a loud thump on the guitar, will it pop back into play?
And of course do we have any way of knowing if the trouble is in the axe, the cords, or the cab?
Any chace the outlet strip or wall socket is at fault?
Assuming it is hte amp itself, we hope we can get clues. A bad power switch or bad connection to the power tranny is one thing, and a break in the signal path is yet another.
FIRST - check the effects return jack. With signal running, probe a plug in and out of this jack a couple times to see if you can trigger a symptom. With power off, measure the resistance between the tip contact on the effects return jack and its cutout contact.
ALSO same test with power amp in jack set.
That resistance needs to be close to zero. Under half an ohm. If ti measures 1 or 2 ohms, it will work OK , but that elevated reading is evidence the contact is dirty and will not be reliable.
Run it a bit, it ought to warm up. Does it get hotter than expected? How about at idle, what is the current draw from the mains? A SS amp biased too hot can overheat.
Apply a steady signal and listen to it as you beat the amp with a rubber mallet - or your fist - from all directions. SHould stay steady.
ANy noisy controls? Try them all.
Push everything with a stick, and tug and wiggle every wire that conntect to the board.
Look for loose hardware, especially jacks.
Check solder under all jacks and controls, and under any larger component. Parts larger than 5w cement resistors have enough mass to shake around and break free of their solder.
Tap on the speaker relay to see if it is l;oose inside. Power iff drops the relay out and it grounds the speaker line, so leave power on but remove the speaker load. Now measure resistance through the relay contacts - that is between the speaker jack tip and the output buss at the end of the power amp
|1/10/2006 12:56 AM|
Oh, and check the input and speaker jacks by applying signal and then while listening for cutouts, flex the input plug up down left right looking for intermitts. Same with speaker jacks.
|1/10/2006 1:12 AM|
Chopstick it end to end (paying especially close attention to any ribbon cables or other connectors), and if you don't find the intermittent inside, check the wiring to the speaker.
If it still won't show up, I usually clean and reseat all contacts, then tell them to try the amp at rehearsal, and if it does it again, bring it to me with whatever instrument and cable they were using when it cut out.
If they can demonstrate to me the dropout, it makes it much easier.
One big thing to remember is that pushing 350W into your dummy load on the bench does not generate the thumps that playing that B2 sitting on top of a speaker would.
|1/16/2006 1:22 PM|
Enzo mentioned the effects loop jack. I've repaired many amps (can I really call this a repair?) by cleaning out the effects loop. If it's a dirty series loop, it could work fine until you get a little heat built up. Then, the amp cuts out. I usually take a hair blower to the jack before cleaning it to get it to fail. That way I know for sure that was the culprit. Enzo also mentioned the relay. I've repaired at least one of these with a cold solder point at the relay.
I usually take a sharpie, and really rap on the components while the amp is cooking at full temp/full load while running the signal generator in sweep mode. Usually an intermittent will show up at some point.
Proably a good idea to put some current limiting (lightbulb) on the AC side.
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