Family_Dog
Administrator
Me, Jan 1, 2018:
So this bad boy started misbehaving again - playing perfectly when set to the low-power position, but cutting out and going into protection when switched to high-power. In order to preserve the meter lights (expensive and snotty to replace), I normally run it in the 30w low-power mode which switches the lights off when running. The other day I wanted to show off the luvverly illuminated VU meters to someone and switched to high-power mode, and the lights came on displaying the true beauty of this magnificent beast. Five seconds later the amp went into protection mode. Back to low-power mode, runs perfectly. High-power mode - protection.
Opened it up and took bias measurements - all was fine. Unsoldered the 10k resistor which I had fitted to the input of the protection IC and the amp worked perfectly, all measurements were still in line. Left it on for four hours or so and took measurements again. Immediately noticeable was the left channel heatsink which was running hot, not too hot to touch, but much warmer than the right channel. Measured the voltage of the Bias settings control pot that controls the quiescent collector current of the output transistors and it was running very high at 65mV (should have been 15mV), resulting in the over-heating. So it had drifted. Resetting this to 12mV seems to have cured the problem. Not sure why it ran away like that, I guess one of the biasing trannies might be going wonky but it played fine at the high-power setting for something like 30 hours without going into protection mode again.
This is a magnificent amplifier but most certainly not the easiest one to work on.
-F_D
So this bad boy started misbehaving again - playing perfectly when set to the low-power position, but cutting out and going into protection when switched to high-power. In order to preserve the meter lights (expensive and snotty to replace), I normally run it in the 30w low-power mode which switches the lights off when running. The other day I wanted to show off the luvverly illuminated VU meters to someone and switched to high-power mode, and the lights came on displaying the true beauty of this magnificent beast. Five seconds later the amp went into protection mode. Back to low-power mode, runs perfectly. High-power mode - protection.
Opened it up and took bias measurements - all was fine. Unsoldered the 10k resistor which I had fitted to the input of the protection IC and the amp worked perfectly, all measurements were still in line. Left it on for four hours or so and took measurements again. Immediately noticeable was the left channel heatsink which was running hot, not too hot to touch, but much warmer than the right channel. Measured the voltage of the Bias settings control pot that controls the quiescent collector current of the output transistors and it was running very high at 65mV (should have been 15mV), resulting in the over-heating. So it had drifted. Resetting this to 12mV seems to have cured the problem. Not sure why it ran away like that, I guess one of the biasing trannies might be going wonky but it played fine at the high-power setting for something like 30 hours without going into protection mode again.
This is a magnificent amplifier but most certainly not the easiest one to work on.
-F_D
