Time/Heat induced crackle? Help (Lace Acoustic TA35cr)
I have a Trace Audio TA35cr Solid State acoustic guitar amplifier. The schematic can be viewed here:
It turns on and functions correctly when cold (all knobs, reverb, chorus, active/passive preamp, etc). After a while, like 10-15 minutes it begins to randomly crackle and distort. Like an egg frying with pops. This is intermittent and seems to increase with time (which I suspect is relative to heat). This happens with no instrument plugged in and not touching any of the controls. Completely unprovoked. If an instrument is plugged in to either pre amp section the amp functions fine and sounds good for a while, then the random crackle will still come and go seemingly unprovoked by the guitar and gets worse with time. The guitar sound being amplified starts out fine but becomes more distorted over time consistent with the random crackle noises.
I am familiar with this type crackle and distortion from bad oil filled power caps in tube amps and sometimes even the tubes themselves but the crackle usually comes on immediately and does not change over time. I have less experience with solid state amps so I am not sure if I should be looking for shifting voltages over time to isolate a bad cap in the power section or if from my description YOU might suggest a place to focus. I would be forever indebted to any advice / guidance you could provide. Thank you in advance for your time. I look forward to solving this problem.
If there's a way to disconnect the preamp from the amp, you at least ought to be able to nail down which section of the unit the problem is located in.
Notice how the knobs on the front are exactly the same as the Carlsbro Cobra amplifiers of the late 80's - I used to have to deal with hundreds of these.
the designer from Carlsbro was the friend of a colleague of mine and moved to work for Trace. The same problem with his Cobra design manifested itself into the Trace Products.
If it's the one with Send/Return sockets or Pre-out/PO in at the rear - when unused the contacts go filthy causing fried egg sound.
If you see in the image - where the contacts fall a tiny needle file and some contact cleaner will do.
I would start by looking for a bad solder joint.....
If not visible then power it up from cold (beware of the MAINS) and then carefully tap on each components with an insulated stick (ex. the handle of a small screwdriver).
Or if you have cooling spray, let it warm up so it cracles and apply the spray to small areas at a time.
Good luck :)
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