| Author |
Topic: Question for the power gurus |
Rick Tyson
From:
Ohio
|
Posted 23 Mar 2000 7:33 pm |
|
I use a Randal rack mount power amp (300WT) & dearly love its sound.
Lately though it seems the volume drops after an hour or so of use & I must increase volume gradually.
Ihave used contact cleaner on all connections & plug-ins with no results.
My question is,,do these type of power amps loose volume due to age?? or do they just go bad all at once?? Its a fairly old amp ,maybe 10 or 15 years old. Someone told me to check for (cold solder connections??)
If I didnt love the sound of this amp, I would just replace it.
Thanks in advance
------------------
Steel Guitar Forum Scrapbook Pictures
|
|
|
|
Jack Stoner
From:
Kansas City, MO
|
Posted 24 Mar 2000 3:05 am
|
|
I suspect with the age of the amp, the power supply filter capacitors are weak. The symptom is typical of devices that lose voltage as the unit warms up. There could be other problems but I'd attack the power supply first.
|
|
|
|
Bill Llewellyn
From:
San Jose, CA
|
Posted 24 Mar 2000 8:04 am
|
|
Yeeesh.... this one is odd. It may be the power supply, but amplifier gain generally should not be a function of supply voltage -- though a sagging power supply would limit the maximum available output power of the amp (i.e., it would distort more easily). If the amp has a stage whose gain is a function of the power supply voltage, that would do it, I suppose (but such a stage wouldn't have very good PSRR). Anyway, checking the filter caps certainly is a worth doing.
Rick, is the problem that the amplifier distorts more ealisy, or does the sound actually get softer? Is there a volume control you can spray clean? Are there internal connectors between subsections of the amp which can be pulled apart, cleaned, and re-connected? Is the problem possibly other external gear, either before or after the amplifier? When the sound softens, does is loose more bass or treble than the rest of the spectrum? Any crackling? Any increased humming? Heat?
(Does the amplifier get heavier? Does the PSG begin to sound like a b@njo? Why do I ask such silly questions?)
Ya' know, one problem I found plagues aging audio gear, even the pro stuff, is solder joint fatigue. It can do exactly what you describe. This is particularly true of gear which is moved a lot or which as a fan which continuously vibrates it. I made many repairs on an old MCI 24-track 2" tape recorder in a studio where I used to work wherein solder joints would form hairline cracks and become intermittant. Big headache, though easily fixed by de-soldering and resoldering (if you could find the culprit joints!).
------------------
Bill * MSA Classic U12 * email * homepage |
|
|
|
Rick Tyson
From:
Ohio
|
Posted 24 Mar 2000 9:52 am
|
|
Thanks fellas for the Info.
The amp does not distort or pop, just fades in volume?? It is convection cooled, no fan.
One thing I forgot to mention was there is a switch on the back that says, either constant voltage or constant current. Since i purchased this amp used ,I dont have an owners manual & dont know which position to switch to????
I will check those solder connections & re-solder.
Thanks again
Rick |
|
|
|
Rich Paton
From:
Santa Maria, CA.,
|
Posted 24 Mar 2000 6:58 pm
|
|
It could be the "bias" is out of adjustment.
This sets the amp's "idling current" minimizes any "crossover distortion", and of course, affects overall power and sound quality.
There's normally one per each channel.
It's usually a simple adjustment, but the amp should be well warmed up first.
I would also check the output for excessive dc level, usually another (often touchy) critical adjustment.
Try to get the service manual.
Good Luck, "dOc" [This message was edited by Rich Paton on 24 March 2000 at 07:00 PM.] |
|
|
|
Mark Amundson
From:
Cambridge, MN USA
|
Posted 24 Mar 2000 7:27 pm
|
|
I can't say I've ever seen it on solid state amps. Usually, my first line of troubleshooting is aimed at battery powered stuff. They have the nasty habit of fade out as the battery gets to end of life.
Mark,
|
|
|
|
