sunn concert lead strange noise

[themusic]

Recently repaired a Concert Lead.  New caps ,power transistors. Worked great for about 2 weeks
then started making noise when playing at higher levels , especially on lower notes. sounds like broken speaker, bad solder,
distortion like, Check speaker, ok. Ran another preamp thru power amp in and no problem. Ran preamp out to another amp
noise. Pull preamp boards, checked all solder joints, pots, inputs, pulled and check caps. What's weird is both inputs
create the same noise. I'm out off idea's ...thanks

[loudthud]

I assume you have the older dual channel version. There isn't much in common between both channels up to the preamp output. Just Q105, the components around it and the power supplys.

A simple technique that can help narrow down the source of the trouble is signal sniffer. Just connect a small cap like .022uF between points in the circuit and any kind of little practice amp.

Was the power amp connected when you tested the preamp out? A cable to the power amp input would disconnect it.

[george]

Bypass and coupling capacitors? Dying JFET?

[loudthud]

No, what you do is monitor the signal along the signal path to see where it gets corrupted with the noise. It might be comming from some defect in the power supply, but it's also possible that a circuit problem injects the noise into the power supply. The 0.022uF cap simply blocks DC from the input of the practice amp.

Check that all the electrolytics are installed with the correct polarity. Simply measure the DC voltage across the caps with a DVM. It's possible that a 50V capacitor might not fail for two weeks if it only has 5V reverse voltage across it.

[robbymiseries]

Recently repaired a Concert Lead.  New caps ,power transistors. Worked great for about 2 weeks
then started making noise when playing at higher levels , especially on lower notes. sounds like broken speaker, bad solder,
distortion like, Check speaker, ok. Ran another preamp thru power amp in and no problem. Ran preamp out to another amp
noise. Pull preamp boards, checked all solder joints, pots, inputs, pulled and check caps. What's weird is both inputs
create the same noise. I'm out off idea's ...thanks

Currently running into the same/similar problem. Were you able to figure it out?