I try to list failures which I saw with the AG9015 and explain my way of resolving these.

In general it is a good idea to replace electrolytes. Also, the carbon (brown small) resistors are a source of issues (always replace Replace R26, R30, R126, R130). Avoid heat stress on the small caps as these tend to get noisy with the repair. Swap things in pair and measure if both matching in value! Philips Mustard caps do not like heat, else they work forever, they are teh best.

Symptom: One channel has much stronger sound

Solution: Seeing the stronger 1kHz signal on scope on the 800Ohm output, became clear, that the stronger channel is wobling. As signal was fed in on C21, the end amp was clearly the suspect for issue. Checking resistances (especial the brown carbon one’s) became clear that R30 gone into the MOhm range and is not fulfilling any negative feedback(normal value is 120kOhm). Carbon resistors have perfect sound, but can easier fail. Please replace both channel with carbon resistor at the same time. This is the most common part which fails.

Symptom: hissing noise in one of the channel.

Solution: Remove one leg of C21 to separate circuit for end and pre amplifier. In my case issue remained (so end amplifier.) Such noise can be generated by a cap, so hard to think it is caused by an resistor. Through a high voltage, smaller cap try to short audio signal of the end amp. In my case in area of B3, B3′ was canceling the noise. After unsoldering C23, noise disappeared. As previously the area had several resistor exchanges the cap became damaged with heat of my solder iron.

Symptom: No sound, R38 burned down.

Replace resistor(both channel, take care on power rating), check tubes, avoid short circuit at the output

Symptom: No sound, R1, R2, R3 burned down

Replace resistors (take care on power rating), check tubes, avoid short circuit at the output . Please do not use bigger fuses then 125mA for the anodes.

Symptom: hissing noise in one of the channel

Similar hiss noise in the upper region, but noise is more white and is hard to hear (I realized it only with the 800Ohm speakers)

Solution: The 820Ohm carbon resistor became noisy. Replace it. (R26, R126)

Ticking noise (see video:)

https://www.facebook.com/groups/elektroncsoveserositok/permalink/670649066708000

Replace R26, R30, R126, R130, the NFB is cuasing as it becames positive.