GregDunn
Active Member
OK, I've been rebuilding and restoring Dynaco gear for (mumble) years now, and this is something I've never seen. I got this amp essentially for free, but it spent many years in a garage in Florida so I anticipated spending a lot of effort in cleaning it up. However, before stripping it I thought it was a good idea to run some electrical tests so I could see what I needed to order for repair parts - or if it was even worth cleaning up the chassis.
The good news is that the transformer, rectifier, and caps are in perfect shape. These old Sangamo caps that Dyna spec'ed on their power amps are something else: the 10k µF units measured about 11-13k with amazingly low leakage. I was able to re-form them and they are in impressive shape.
Measuring the nominal resistance for the output stage test points gave 3000 ohms pretty much on the nose; so there's nothing shorted or defective on the outputs.
However, the PC-28 boards are obviously in need of some repair; I got too much bias current on one channel, and the other one is OK but has a DC offset which increases as I bring up the power supply voltage. These can be dealt with, I'm sure. But when I started examining the boards for obvious damage, I saw this on one of them.
It looks like someone epoxied ... something ... to the board which subsequently broke off. It resists chipping or scraping, and I'm going to have to remove the board from the amp before I do anything further. The rectangular piece looks like the carcass of a sandblock resistor, but the goop around it is definitely epoxy or something very similar.
Has anyone else had to remove a hunk of epoxy from electronics before? Fortunately, it's on the top side so it won't take any PC traces with it, and I think the grommets for Q213 are the only things I'll have to be careful of. What's the best approach? Grind it off carefully with a Dremel tool? Drill holes and try to break it off in chunks? This is a bizarre experience and I'm torn between amusement and horror.
The good news is that the transformer, rectifier, and caps are in perfect shape. These old Sangamo caps that Dyna spec'ed on their power amps are something else: the 10k µF units measured about 11-13k with amazingly low leakage. I was able to re-form them and they are in impressive shape.
Measuring the nominal resistance for the output stage test points gave 3000 ohms pretty much on the nose; so there's nothing shorted or defective on the outputs.
However, the PC-28 boards are obviously in need of some repair; I got too much bias current on one channel, and the other one is OK but has a DC offset which increases as I bring up the power supply voltage. These can be dealt with, I'm sure. But when I started examining the boards for obvious damage, I saw this on one of them.
It looks like someone epoxied ... something ... to the board which subsequently broke off. It resists chipping or scraping, and I'm going to have to remove the board from the amp before I do anything further. The rectangular piece looks like the carcass of a sandblock resistor, but the goop around it is definitely epoxy or something very similar.
Has anyone else had to remove a hunk of epoxy from electronics before? Fortunately, it's on the top side so it won't take any PC traces with it, and I think the grommets for Q213 are the only things I'll have to be careful of. What's the best approach? Grind it off carefully with a Dremel tool? Drill holes and try to break it off in chunks? This is a bizarre experience and I'm torn between amusement and horror.