With the short removed, the transformer just barely gets warm, its only about 12 degrees above room temperature.
The hot smell went away when I removed the ribbon cable.
I'm sort of assuming that its a 15-10-15 transformer, if so its pretty much right on the money voltage wise.
If I had a transformer, I'd probably swap it but I'm leaning toward leaving it alone since it really shows no signs of damage. The windings aren't discolored, its not drawing excessive current, and its no longer getting hot.
I was able to jumper out the CD path through the chip socket. Its been playing for about three hours now with the selector set on CD, and two jumpers across the open socket enabling the CD RCA's. It looks like the LC7818 just acts like a gateway for the selector. using transistors as relays somehow.
The rear board almost seems redundant since its got a selector switch on the panel. Tape Monitor, Mute, and Bypass functions are all that can't be achieved without LC7818 in place.
The hot smell went away when I removed the ribbon cable.
I'm sort of assuming that its a 15-10-15 transformer, if so its pretty much right on the money voltage wise.
If I had a transformer, I'd probably swap it but I'm leaning toward leaving it alone since it really shows no signs of damage. The windings aren't discolored, its not drawing excessive current, and its no longer getting hot.
I was able to jumper out the CD path through the chip socket. Its been playing for about three hours now with the selector set on CD, and two jumpers across the open socket enabling the CD RCA's. It looks like the LC7818 just acts like a gateway for the selector. using transistors as relays somehow.
The rear board almost seems redundant since its got a selector switch on the panel. Tape Monitor, Mute, and Bypass functions are all that can't be achieved without LC7818 in place.
It was not a good idea to run the transformer in a short for more than a few seconds. The mains fuse should have dropped out.
It wasn't loading the primary windings of the transformer enough to make heat or blow the fuse, it was just enough to make the secondary coil warm.
Its only got a 1/2 amp AGC fuse, so my guess was it wasn't a complete short or enough to do any serious damage.
The fact that the guy who I got from said it stopped making sound and he shut it down likely saved it from any serious damage.
I would think that if the transformer was damaged I'd have a voltage issue or shorted windings affecting its output. It never got hot enough to discolor anything or melt the resin, it was only 90 or so degrees. The loss of sound likely came before it got hot.
The pins that were dark in the pics I posted are the pins that are related to the muting function, apparently muting is timed through the chip. Its hard to tell what the failure was but that was the corner of the LC7818 that was getting warm fast.
It looks to me like the whole preamp was designed around the LC7818.
I likely won't have the new IC's till the end of the week or so, I ordered them from one of the bigger suppliers and they won't likely process the order till Monday, then it'll likely take a week for UPS to get them here. For now, its sitting on the shelf while I continue to go through the rest of the lot I got from the same guy.
I've also got a half dozen speakers to refoam from the same lot. I had originally gone there to grab a few things and that 'few' things turned into two trips with my pickup.
Its only got a 1/2 amp AGC fuse, so my guess was it wasn't a complete short or enough to do any serious damage.
The fact that the guy who I got from said it stopped making sound and he shut it down likely saved it from any serious damage.
I would think that if the transformer was damaged I'd have a voltage issue or shorted windings affecting its output. It never got hot enough to discolor anything or melt the resin, it was only 90 or so degrees. The loss of sound likely came before it got hot.
The pins that were dark in the pics I posted are the pins that are related to the muting function, apparently muting is timed through the chip. Its hard to tell what the failure was but that was the corner of the LC7818 that was getting warm fast.
It looks to me like the whole preamp was designed around the LC7818.
I likely won't have the new IC's till the end of the week or so, I ordered them from one of the bigger suppliers and they won't likely process the order till Monday, then it'll likely take a week for UPS to get them here. For now, its sitting on the shelf while I continue to go through the rest of the lot I got from the same guy.
I've also got a half dozen speakers to refoam from the same lot. I had originally gone there to grab a few things and that 'few' things turned into two trips with my pickup.
The fuse seems to be too strong.
Failure of LC7818 and the like can easily happen with static discharge (while plugging in RCA cables)
and thunderstorm over-voltage conditions.
Failure of LC7818 and the like can easily happen with static discharge (while plugging in RCA cables)
and thunderstorm over-voltage conditions.
I'm thinking its the factory fuse since when I first got it there was a small paper tag over the fuse holder door that read 'Use only 1/2 amp fast blow fuse'. I also think that its more there to protect against fire, and if that sort of short were to occur, even a 20a fuse would blow. If they wanted to protect against a secondary failure they should have put a thermal switch on the transformer or a fuse at the power supply. The transformer would have to pretty hot to pop almost any size fuse. The input wires on the transformer are marked LL34848, which is 16ga HV Hi Temp wire. My guess is .5a is likely the rating of the transformer. The transformer itself would have to fail on the primary side for it to blow the 1/2a fuse.
If the new LC7818 gets it going 100%, chances are I'll never know for sure what caused it. The pins that were heated up and darkened were 17, 18, & 19. They're identified as TMCTL, INIT, and MIN on the chip diagram in the book.
I ordered a bag of 10 of them, just in case, and since I've seen these before, it won't hurt to keep a few spares. I also know the minute those show up I'll stumble on the extras I bought before.
If the new LC7818 gets it going 100%, chances are I'll never know for sure what caused it. The pins that were heated up and darkened were 17, 18, & 19. They're identified as TMCTL, INIT, and MIN on the chip diagram in the book.
I ordered a bag of 10 of them, just in case, and since I've seen these before, it won't hurt to keep a few spares. I also know the minute those show up I'll stumble on the extras I bought before.
I just realized that I never came back here with a follow up on the BK preamp.
Its been about 6 months since I swapped out the LC7818 IC on this thing.
I installed it with a socket so if its ever an issue again, it'll be an easier fix.
Its got a good bit of use on it and I've paired it with a half dozen different amps.
I'd say its got close to 100 hours or so of use on it now and so far, so good.
It paired best with an LSR&D Leach amp I've got but it worked well on several pro amps as well, especially since its got XLR connectors for an easy match. It sounded really good feeding a pair of Carver PM600 amps I have.
I thought about putting a main power switch on this thing since the transformer is hot any time its plugged in but after thinking about it, its really only powered up when the amp is turned on, on those amps with switched outlets. On others, I'll put the whole rack on a master power switch. I've been using a late 70's store display rack from Pioneer that's got a high amperage master switch and the ability to switch amps and speakers. I got it when the dealer that I bought my SX-1980 from remodeled in the mid 80's. I'm working on building a copy of it with fans built in for my living room.
Its been about 6 months since I swapped out the LC7818 IC on this thing.
I installed it with a socket so if its ever an issue again, it'll be an easier fix.
Its got a good bit of use on it and I've paired it with a half dozen different amps.
I'd say its got close to 100 hours or so of use on it now and so far, so good.
It paired best with an LSR&D Leach amp I've got but it worked well on several pro amps as well, especially since its got XLR connectors for an easy match. It sounded really good feeding a pair of Carver PM600 amps I have.
I thought about putting a main power switch on this thing since the transformer is hot any time its plugged in but after thinking about it, its really only powered up when the amp is turned on, on those amps with switched outlets. On others, I'll put the whole rack on a master power switch. I've been using a late 70's store display rack from Pioneer that's got a high amperage master switch and the ability to switch amps and speakers. I got it when the dealer that I bought my SX-1980 from remodeled in the mid 80's. I'm working on building a copy of it with fans built in for my living room.