26-11-2016, 07:43 AM
Update. I did as you suggested Mark and popped a 7912 in, this fixed it. No sooner I had I done this and put it all back together when the new 7915's turned up from CPC. Amp apart again, power board back in and alls well. I can't figure out how no negative rail would result in the led on.
You make a good point about why 78/79 3 pin regulators fail. The supply is higher after I fitted new mains tfmr's which have 24v AC secs instead of the original 15v AC secs. That puts the input a bit closer to the max IP/OP differential. For the LM79** that's 30v, I'm using MC7915CTG's which have a slightly higher IP/OP diff of 35v. The old regulator was definately pushed, I'd fitted a LM7905 by mistake, must get new glasses, so with a -35v supply rail that's 5v over the 25v IP/OP diff.
Re idle current, when I tested the amp yesterday I got 150w out of the left channel and 200w out of the right. They should be capable of 265w into 8 ohms for an input of 1.5v RMS. My distortion meter and sig gen don't go down to the stated 0.08% of the amplifier. So I had bit of a play, setting the idle to get 260w whilst keeping an eye on the temperature of the OP Q's and heatsink. I found that 300mv set the OP at 260w whilst the temperature of the OP Q's and heatsink was just warm to the touch, you were smack on Mark. : )
I tested them on dummy loads for about an hour at 200w, all fine, but the fan is noisy. I'll give them a good long soak test later, probably running it at 100w to start with.
Thanks for your help Mark, Andy.
You make a good point about why 78/79 3 pin regulators fail. The supply is higher after I fitted new mains tfmr's which have 24v AC secs instead of the original 15v AC secs. That puts the input a bit closer to the max IP/OP differential. For the LM79** that's 30v, I'm using MC7915CTG's which have a slightly higher IP/OP diff of 35v. The old regulator was definately pushed, I'd fitted a LM7905 by mistake, must get new glasses, so with a -35v supply rail that's 5v over the 25v IP/OP diff.
Re idle current, when I tested the amp yesterday I got 150w out of the left channel and 200w out of the right. They should be capable of 265w into 8 ohms for an input of 1.5v RMS. My distortion meter and sig gen don't go down to the stated 0.08% of the amplifier. So I had bit of a play, setting the idle to get 260w whilst keeping an eye on the temperature of the OP Q's and heatsink. I found that 300mv set the OP at 260w whilst the temperature of the OP Q's and heatsink was just warm to the touch, you were smack on Mark. : )
I tested them on dummy loads for about an hour at 200w, all fine, but the fan is noisy. I'll give them a good long soak test later, probably running it at 100w to start with.
Thanks for your help Mark, Andy.
