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K50 Schematic & Info Quest [message #15231] Tue, 08 February 2011 11:45 Go to next message
El Músico Loco is currently offline  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

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Re: K50 Schematic & Info Quest [message #15233 is a reply to message #15231] Tue, 08 February 2011 12:27 Go to previous messageGo to next message
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.

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Re: K50 Schematic & Info Quest [message #15234 is a reply to message #15233] Tue, 08 February 2011 12:52 Go to previous messageGo to next message
El Músico Loco is currently offline  El Músico Loco
Messages: 12
Registered: February 2011
Junior Member
Hi Chicagobill,
Thanks so much for the info. I was able to get a reading off of the knubs of the remaining leads and came up with .784 on my diode check setting so I am going to try a 1N4004 right now and see if it works. Would this diode being broken cause the amp to behave as if there is too much gain or does it only function in a thermal bleed? The amp works but is very hissy. The volume knob does not engaged until the last third of the turn radius so I need to replace it too before I can tell what is going on. I will let you know how I get along.
Cheers,
Chris

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Re: K50 Schematic & Info Quest [message #15235 is a reply to message #15231] Tue, 08 February 2011 15:54 Go to previous messageGo to next message
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.

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Re: K50 Schematic & Info Quest [message #15238 is a reply to message #15231] Wed, 09 February 2011 09:21 Go to previous messageGo to next message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
Do you know the PC board # your looking for? You can check this link to see if its in there and print it out.
http://schematics.number26.org/index.php?dir=kustom/

Good Luck. Steve C

Steve C

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Re: K50 Schematic & Info Quest [message #15239 is a reply to message #15231] Wed, 09 February 2011 10:15 Go to previous messageGo to next message
El Músico Loco is currently offline  El Músico Loco
Messages: 12
Registered: February 2011
Junior Member
Hi Chicagobill,
I replaced the diode with a fat SY228 that had about the same .784
that the original had and it does not get warm. I replaced the volume pot and now everything seems normal except the hiss is very loud when I turn up the treble. I can search some of the other threads now for help but any advice you have is great. I don't have a scope to check the stages and I used a freeze spray on all the transistors but was unable to reduce the hiss at all. Any ideas. I also have not found a schematic for this yet either.
Thanks for all the help.
Chris

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Re: K50 Schematic & Info Quest [message #15240 is a reply to message #15238] Wed, 09 February 2011 10:34 Go to previous messageGo to next message
El Músico Loco is currently offline  El Músico Loco
Messages: 12
Registered: February 2011
Junior Member
cassent5150 wrote on Wed, 09 February 2011 09:21
Do you know the PC board # your looking for? You can check this link to see if its in there and print it out.
http://schematics.number26.org/index.php?dir=kustom/

Good Luck. Steve C


Hi Steve,
My board is a PC5026 - not on the list and I cannot find another model there that is similar enough to this one. I found another old thread here and the email of a guy who has not logged in for 5 years who said he has the schematic so I will send an email in hope he is still around. Thanks for the help.
Chris

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Re: K50 Schematic & Info Quest [message #15241 is a reply to message #15231] Wed, 09 February 2011 13:22 Go to previous messageGo to next message
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.

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Re: K50 Schematic & Info Quest [message #15245 is a reply to message #15241] Wed, 09 February 2011 16:26 Go to previous messageGo to next message
El Músico Loco is currently offline  El Músico Loco
Messages: 12
Registered: February 2011
Junior Member
Hi and thanks for the pointers. I now realize that the tremolo/vibe function is not working. I will go over it slowly and see what I can come up with. I still have not been able to find the PC5026 schematic for this free or for sale.
Chris

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Re: K50 Schematic & Info Quest [message #15246 is a reply to message #15245] Thu, 10 February 2011 09:54 Go to previous message
El Músico Loco is currently offline  El Músico Loco
Messages: 12
Registered: February 2011
Junior Member
Hi All and thanks to pleats for passing along a schematic. Really appreciate the kind gesture. I will report back with how I get along.
Chris

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