buffdriver
Active Member
I'm trying to troubleshoot a problem with an SX-1250, and I'm stuck. This receiver has been completely re-capped using the recommendations found in multiple threads here on AK and had been operating normally for about 3 months.
The problem was noticed as noise in the right speaker, a static rumbling during quiet passages in the music. It was taking the woofer for quite a ride. I pulled the pre-out power-in jumpers and the problem disappeared. I was also able to move the problem to the left speaker by patching the right pre-out to the left power-in.
The noise did not vary with volume setting but did respond to the 30 Hz filter. Bypassing the tone controls had no effect, so I narrowed the problem to the flat amplifier, which is a JFET and BJT circuit.
On my scope, I could see the problem as erratic DC voltage on the right channel output, which is also the Q8 collector. Q8 is a 2SA725 that was already replaced with ksa-992, but I put in a new 992 just in case the first was bad. No luck.
Q6 is a JFET that I understood to rarely go bad, but I decided to swap in a spare from an old board that I had laying around, and the problem appeared fixed. But the problem returned after about 30 minutes. I feel foolish for wasting a JFET without solving the underlying problem, but I'm totally stumped by this circuit.
I would be grateful for any advice on how to proceed at this point.
The problem was noticed as noise in the right speaker, a static rumbling during quiet passages in the music. It was taking the woofer for quite a ride. I pulled the pre-out power-in jumpers and the problem disappeared. I was also able to move the problem to the left speaker by patching the right pre-out to the left power-in.
The noise did not vary with volume setting but did respond to the 30 Hz filter. Bypassing the tone controls had no effect, so I narrowed the problem to the flat amplifier, which is a JFET and BJT circuit.
On my scope, I could see the problem as erratic DC voltage on the right channel output, which is also the Q8 collector. Q8 is a 2SA725 that was already replaced with ksa-992, but I put in a new 992 just in case the first was bad. No luck.
Q6 is a JFET that I understood to rarely go bad, but I decided to swap in a spare from an old board that I had laying around, and the problem appeared fixed. But the problem returned after about 30 minutes. I feel foolish for wasting a JFET without solving the underlying problem, but I'm totally stumped by this circuit.
I would be grateful for any advice on how to proceed at this point.
