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K400 Repair Help [message #17917] Tue, 18 December 2012 13:31   davohilts Messages: 40 Registered: August 2006 Location: Portland, OR Member Hello, Thanks to Bart I now have copies of PC704A and B, PC403 and PC303 which are 5 of the 5 PC boards in the amp. The one I'm missing is PC504. This amp burned up durning use and a friend of mine has given it to me to try and repair. As far as I have been able to tell, the problem is in the power amp on the PC704A. When the amp failed the 100 ohm resistor associated with an onboard diode completely burned up making a real mess. No fuses blew so the amp continued to burn until the board went dead. This amp has two power amp boards (PC704a and b) an can be operated in either stereo or mono. I have disconnected the PC704A and the amp operates without issues thru both the PC403 and PC303 preamps. What I found is that Q1 and Q3, CR703, CR702, R722 and Q706 were destroyed in the event. The driver transistors Q704, 705, 708 and 709 are also questionable. I replaced CR702 (1N3754) with a NTE 5801 and used heat conducting grease to tie it to the clip on the board as I was unable to find a metal can radial lead replacement. As for the driver transistors, I replaced them with NTE 128/129 and installed them tight to the clip on heatsinks on the board and again used a conducting grease. I also replaced Q706 (2N3567) with a NTE 123 and the blown diode CR704 (FD111) with a new one. I also replaced the 2 matched pairs of power transistors (Q1-4, 36892) with NTE 130 match pairs. I connected the board back into the circuit and wired it thru my current limiter (lightbulb type) and switched it on and off quickly. The lightbulb glowed brightly indicating that I haven't solved the problem. Can anyone with some experience with these amps give me some advice on where to go from here?? I do not have very much experience working with solid state amps, most of my work is done with tubes. Thanks for any help Dave Report message to a moderator

Re: K400 Repair Help [message #17918 is a reply to message #17917] Tue, 18 December 2012 15:53 chicagobill Messages: 2005 Registered: April 2003 Senior Member I don't have a copy of the 704A schematic, so I can't really give you any specific direction. It sounds like you've pretty much replaced all of the typical things that go bad when one of these blows up. This past summer I had a 400 that was so badly burned, that I just made a new power amp board for it and got it back up and running. Active devices like transistors and diodes are always the first things to check, but you also should be certain that there are no other stressed or open resistors on the board as well. They may not look burned, but they can be really off value. Be certain that you have replaced all of the dead transistors with the correct types, NPN for NPN and PNP for PNP, etc. Also be certain that the replacement transistors have the same basing as the originals or are correctly oriented in the pc board. Same thing for the diodes, be sure that they are installed with the correct polarity. Also if you have removed or moved the board a lot, check to see that all of the connecting wires are still solidly connected and for that matter are connected to the right places. If all of this is correct, power up the amp with no speaker connected and your light bulb connected and start taking voltage readings. The first thing to test is to see if there is any dc voltage on the speaker output. If there is, then based upon whether it is positive or negative, you may be able to tell what side of the amp is either working or not working. Keep us informed as to how you progress. Report message to a moderator

Re: K400 Repair Help [message #17922 is a reply to message #17917] Tue, 18 December 2012 17:17   stevem Messages: 4733 Registered: June 2004 Location: NY Senior Member it would be a good idea to pick 4 inline fuse holders and install one ( butt splice) in each red and green wire feeding the output boards and then load them with 1 amp fast blow fuses, that can be up`ed to 1.5 amps when things are right. Report message to a moderator

Re: K400 Repair Help [message #17923 is a reply to message #17922] Tue, 18 December 2012 19:38   davohilts Messages: 40 Registered: August 2006 Location: Portland, OR Member thanks for the tips, i will do as you guys have directed and report back hopefully with some progress. Dave Report message to a moderator

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