K50 Schematic & Info Quest [message #15231] |
Tue, 08 February 2011 11:45 |
El Músico Loco
Messages: 12 Registered: February 2011
|
Junior Member |
|
|
Hello,
I found this forum looking for a schematic for a Kustom K50-2. Lots of great info on fixing the hiss on these models too! I have a few questions if I may. I see a can type diode (I think) that is mounted to what looks like one end of a fuse holder on the heat sink for the power transistors. Both wires are broken off of the can. I need to know if this is a diode and what can I replace it with? If so which way is it oriented? See why I need a schematic! Also, the volume knob engages only in the last third of the turning radius so I need to replace it also. Once these problems are sorted I can chase down the hiss. Thank you for any help.
Cheers,
Chris
|
|
|
Re: K50 Schematic & Info Quest [message #15233 is a reply to message #15231] |
Tue, 08 February 2011 12:27 |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
Welcome Chris!
I don't have the schematic that you need, but someone else here may be able to help out.
The part that you are talking about is a diode. Kustom part number 007-6006-00, which is an RCA 1N3754 diode. The leads break off of these very easily, so yours is not unusual. It is part of the power amp's bias circuit and it is mounted on the heatsink of the output transistors so that it can reduce the drive to the output transistors when they start to get hot.
Now for the bad news...these diodes have not been made since the 1970's. What you can do is replace it with a basic diode like a 1N914 and find a way to mount it in contact with the heatsink surface. One way is to remove the original clip and replace it with a small terminal strip. Wire the diode across two terminals with the body positioned in contact with the metal surface and then run wires to the PC board. As for polarity, it should be wired in the same direction as the two other diodes in the string.
I've been meaning to run some tests with various diodes, to see how they compare with the original 1N3754 as far as temperature compensation goes, but just haven't gotten to it yet.
|
|
|
|
Re: K50 Schematic & Info Quest [message #15235 is a reply to message #15231] |
Tue, 08 February 2011 15:54 |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
The bias string sets the power transistors idle operating point, so maybe yes to the too much gain question. With the bias string open the outputs will turn on more than normal and I would expect them to get warm if not hot.
Any silicon diode will work for testing, so go ahead and try the 1N4004.
As for the hiss, try and isolate the noise. Is it coming from the power amp or the preamp? Both? If the preamp, before or after the tone stack?
As for the volume control, the circuit is different from the normal voltage divider. It grounds out the signal and changes the gain of the stage as well. Your pot may or may not be the problem.
|
|
|
|
|
|
Re: K50 Schematic & Info Quest [message #15241 is a reply to message #15231] |
Wed, 09 February 2011 13:22 |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
I wouldn't expect the diode to get hot, my heat comment was regarding the output transistors. Without the bias diode the outputs would probably run hotter than normal.
As for the hiss, if the treble control changes it, the source may be one or more of the 2 or 3 transistors that precede it. Not having the schematic, I can't give you anything specific to try, other than the general things like the transistors, noisy resistors or bad tantalum caps.
|
|
|
|
|