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Lead III SC schematics, and transistors. [message #19587] Fri, 25 October 2013 22:32 Go to next message
digi2t is currently offline  digi2t
Messages: 2
Registered: October 2013
Junior Member
Recently picked up a Kustom Lead III SC. I found the schematic for it, but the power board schematic is wrong. The board is a PC5065 rev.2. I've corrected it, here it is;


http://i214.photobucket.com/albums/cc196/digi2t/Kustom/PC5065rev2_zps9ef58d11.jpg

I updated the preamp boards, replacing the transistors, op amps, and most of the ceramic caps for metal film. For transistors, I recommend MPSA18's, and PN2907A's. NE5532 op amps are very good subs for the RC4558P's. One thing to be wary of, is the fat 0.47uF film cap. It's oversized, and the lead spacing is tight, so it has a tendency to break it's solder connections. If you have an intermittent tremolo, check this first. Two fixes are either A) hot glue all the caps together, to add support, or B) replace it with a modern, smaller equivalent.

The NE5532 works well for the voicing board, and replacing the two end to end 22uf caps with a 10uF bipolar is a good move as well.

Another thing to be aware of, is the channel 2 board PC5128. The component side of the board is copper clad, and attention must be paid to the legs of the film caps. Again, some of the caps are oversized for the lead spacing on the board. This bows the leads together, and in extreme cases, if the cap gets pushed down, a lead can contact the copper clad. The copper clad is a ground plane, and among other things, this can induce a high pitch oscillation (whistling) into the audio.

I'm in the process of replacing the original driver and power transistors in the power section. Toshiba's 2SA1837 / 2SC4793 combo is very good, along with MJ15003's. I have these in my Ampeg G-212, and they work really well.

Insofar as transistor replacements, I believe these should work well, Kustom part number, followed by modern replacement;

007-6006-00 - 2N2219
007-0009-00 - PN4249 (PN2907A or 2N5087 match pair for power amp input)
007-0005-00 - PN3567 (2N4401)
007-0006-00 - PN3638 (2N3906 or 2N4403)
007-0008-00 - MPSA18

The bias diodes are RCA D1300A (1N3754) can diodes. They are still available, but a TO-126 transistor screwed into the output transistor heatsink, with a trimmer for fine tuning the bias, might work as well if they're not.

I'm also installing input protection, and output catch diodes on the power end. Just a bit of insurance.

[Updated on: Sat, 26 October 2013 06:12]

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Re: Lead III SC schematics, and transistors. [message #19589 is a reply to message #19587] Sat, 26 October 2013 08:45 Go to previous message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
Welcome to the place.
Thanks for the info, your time and the up dated schematic.
One thing to note is the matching on whatever TO3 types you use will be the determining factor in how much off set DC and in turn 120 hz hum the amp outputs to the speaker(s)
Just 6 months ago, after 40 plus years of doing this I made a adapter/jig to let me pop TO3s in and out of the amp and read the current difference.
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