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Problem with PAS

68.88.238.129
Posted on October 22, 2009 at 22:55:21
tubtub
Audiophile

Posts: 123
Location: US
Joined: June 5, 2007
 One of my PAS has recently developed a weird problem.

It would play fine for 10 minutes then a surging/hissing/popcorn sound came one for a minute then go away on left channel; a few minutes later, that unwanted noise came on again.

Switching tube on the line stage board or phono board did not make any different, that left channel still has the problem.

All resistors and capacitors had been replaced with new parts recently (except for the can cap). What could be the problem.

Thank you gentlemen for any pointer.

The most LIKELY suspect is..., posted on October 23, 2009 at 16:05:46
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 a cold solder joint or a bad resistor. It could also be caused by a solder "bridge" that only makes intermittent contact when the unit warms up.

This gives me an opportunity to (once again) ask that people who have problems with their equipment PLEASE describe the conditions more completely. In this particular case, you didn't mention whether this problem occurs when listening to a phono cartridge, or using a line input. Does the noise increase or decrease when you change the setting of the Volume Control? Unless you describe FULLY the operating conditions where the problem shows up, any "suggestions" we might make will only be general in nature.

With that little reminder out of the way, I can only *assume* that you are talking about a noise that is NOT affected by the Volume Control setting. That isolates the problem to a point AFTER the Volume Control. Since you mentioned that all resistors and capacitors have been recently changed, you can *try* to find the offending part by "wiggling" each part a bit to see if any one part affects the noise. Since there are high Voltages involved, use the eraser end of a pencil to push on each part. If THAT doesn't locate the area of the problem, it's time to break out the soldering iron and "touch up" each solder point that is associated with the left channel on the Line Stage Board. If THAT doesn't solve the problem, you will have to break out a magnifying glass or small "hobby-type" microscope and go over EVERY circuit trace to find a possible break. Wiggle components while following each trace.

There are no "short-cuts" here. EVERY soldered connection MUST have good integrity and since the PC boards in a PAS preamp are VERY fragile, it can be quite a challenge to successfully replace all of the components without causing more problems than you might solve.

Honestly, it really sounds like a cold solder joint, or a bad resistor somewhere in the Left Channel of the Line Stage board. Also, check the Blend Switch to make SURE the contacts are clean as well.

As mentioned in the other reply, the can capacitor is probably NOT the culprit (although you should plan NOW for its ultimate failure.)

Hope this helps.

Bill

RE: The most LIKELY suspect is..., posted on October 24, 2009 at 22:53:09
tubtub
Audiophile

Posts: 123
Location: US
Joined: June 5, 2007
 Thank you Bill and everyone for the suggestion. I touched up all the joints on the linestage and the problem went away. Again, I greatly appreciated the help very much.

tubtub

Lucky Guess on my part. GREAT work on YOUR part!, posted on October 25, 2009 at 17:15:59
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 Bravo!

Sincerely,

Bill Thomas

RE: The most LIKELY suspect is..., posted on October 23, 2009 at 17:06:40
llwhtt
Audiophile

Posts: 26
Location: SOCAL
Joined: October 28, 2008
 One more thing to add that I think Bill missed in all of his good suggestions. Did it do it BEFORE you replaced anything? If it didn't look at your work with a magnifying glass, inspect EVERY solder joint that you touched or didn't touch. If it's wired with the original solid core wire solder joints are even more suspect. That's probably what it is, no biggy.

Craig

RE: Problem with PAS, posted on October 23, 2009 at 08:54:35
paddeo
 Replace the can cap.

Ciao

Probably NOT the quad cap., posted on October 23, 2009 at 15:46:07
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 Since the problem affects only ONE channel, the quad cap isn't a likely suspect. Sorry.

Bill

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