Hi all,
After I assembled my K8020 pre-amp I powered it up, and it seemed to be working correctly, but didn’t crank out the volume that I thought it would. It ran for about an hour or so, and then the the sound faded out slowly, and the unit powered off. I disconnected the power and noticed an odour of melting plastic (generally not a good thing). So I decided to open the unit up and have a look. Everything appeared to be normal, other than the melted plastic smell which seemed to be eminating from the transformer. The transformer was extremely hot to the touch ~90˚C, hot enough to cause a burn. Warm=OK, Hot=bad. So I let the unit cool down over night, and went over it to try and spot any obvious mistakes. Bingo! I had reversed the polarity on diode #13 which, according to the schematic, is a part of the power circuit. The odd thing is, the fuse never blew :?: . To be on the safe side, I went out to my local electronics supply and got some new diodes of the same type (and a spiffy new DVM890F meter, does everything but tell the time), and replaced the diodes in the power supply cicuit (correctly polarized this time). Now the unit will power on long enough to light the LEDs, and then promptly blows the fuse.
The good people a QKits could't give me a definitive answer as to whether or not I cooked the transformer. My electronic diagnosing skills are limited at best, and I have no idea how to test the transformer: which points to test across or what values I should get. Or am I just as well off ordering a new transformer and just replace it outright? Where to go from here?
The K8040 power amps came together without a hitch, even having them running off a cheap, semi-crappy Pyramid pre-amp, they sound awesome. Shook the whole house with the soundtrack to "Bladerunner", I was thoroughly impressed :D . I can't wait to hear them with the K8020 up and running. One question: the output meter barely moves at all, even with the volume cranked, does this have something to do with the amps running off a low-end pre-amp? But, I digress, any and all ideas are greatly appreciated.
BTW: A parametric equalizer kit to go with these units would be really cool.
If you take a good look at the circuit diagram, you will notice that by reversing diode D13, you’ve shorted the HT secundary of the transformer every half cycle. The primary fuse did not blow because it was not a dead short, so the primary current never got too high, however, the secundary current exceeded the specs. Remove the transformer from the board. Measure both the HT winding and the primary windings. There is a chance one of them shows an open, shorted or intermittent circuit. If this is the case, you can order a spare transformer from your distributor. Make sure to check all HT rectifier diodes aswell, as some of them had to endure the short circuit current.
Sounds like a plan. In addition, I have replaced all diodes related to the power cicuit. You will have to define “HT” for me. I think I can figure which contacts on the transformer to test across from the schematic. Now that I understand that this is a secondary circuit, the fuse not blowing makes a lot more sense. As I said, my understanding of electronics is limited at best. Nothing like a small disaster to teach a fellow something. Thanks much for the help :).
“HT”, got it, I thought as much but wanted to make sure. The transformer is difinitely fried. I removed it and the plastic that the coils are encased in is cracked (hence the melting plastic smell), and I have to wiggle the pins to get a closed circuit. Live and learn. Thanks again for the pointers
Ok, ordered a new transformer from www.qkits.com (where I originally purchased this kit) many moons ago. Qkits says: “we have heard nothing from our supplier.” Now I’m stuck. Are there any other avenues to order replacement parts?
I believe Qkits has been a little overwhelmed lately, as they have moved locations. My order may have slipped through the cracks. Many thanks, greatly appreciated.
Due to a logistical problem, this order has not been processed yet. Qkits is looking into the possibilities to get this sorted out as soon as possible.
