| Hillbilly-engineered K200B-5 (PC 105 problems) [message #10113] |
Thu, 18 December 2008 11:13  |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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All right folks, this K200B-5 is killing me. Got the output section working no problem...four outputs, four drivers, four emitter resistors, one small-signal transistor, one diode in the protection circuit...piece o' cake. The amp will deliver full power into a resistive load, and clips evenly. However, right after it starts to clip, if you increase the level just a hair, the waveform goes all to hell. I traced this all the way back to the preamps, when I noticed something totally weird...on each of the four preamp boards (PC 105) it looks like someone snipped off a tantalum cap and twisted and soldered the leads together (!!!). I'm not kidding. And all four preamps deliver only a few hundred millivolts of signal before clipping all crazy. I have a feeling this is a truly spectacular case of hillbilly engineering. Anyone got a schemo for PC 105? (Mine's the +/- 12V version if that means anything).
Joe
The King of Nerd-A-Billy
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| Re: Hillbilly-engineered K200B-5 (PC 105 problems) [message #10117 is a reply to message #10113] |
Thu, 18 December 2008 21:08  |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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Bill -
I've got it figured out...the earlier version used a blocking capacitor and 1-M self-biasing resistor on the base of Q103. They later direct-coupled the input of that stage and used a voltage divider in the emitter of Q104 to set the bias (which coupled through the tone control network). Somebody must've noticed that the cap wasn't there on the later schematic and thought they could just jump across it - without realizing that they also changed the biasing arrangement! Why do I always get the weird stuff?
So I put in some 10-uF caps (I figured that cap was either 10 or 33 since they tended to use the same values everywhere) and it fixed it right up. Works great except for a low-level 3-MHz oscillation...probably higher gain in the NTE 128/129 drivers I put in. I did notice that the K250 amps have the same output stage except that they tweaked the output emitter resistors and increased the compensation caps...probably for a reason. Thanks again.
Crazy Joe
The King of Nerd-A-Billy
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| Re: Hillbilly-engineered K200B-5 (PC 105 problems) [message #10122 is a reply to message #10121] |
Sun, 21 December 2008 15:18 |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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Steve -
What, the amp or the problem? Haha...I got the thing from a guy who supposedly bought out a repair shop, and this was among the "unrepairables" or abandoned items. That probably should have sounded some kind of alarm. Ahhh, it was 50 bucks and the sparkly gold Naugahyde stole my heart, what can I say?
Crazy Joe
The King of Nerd-A-Billy
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| Re: Hillbilly-engineered K200B-5 (PC 105 problems) [message #10123 is a reply to message #10122] |
Sun, 21 December 2008 22:34 |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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Hey Bill (I copied you on this in a PM):
You might find this interesting. In trying to tame the 3-megacycle oscillation in my amp, I got out the old tan RCA Designer's Handbook with the original circuit and noticed that they used several output stability networks. I added just the Zobel network consisting of a 22-ohm resistor (I used 5W, probably overkill) and 0.02-uF cap, and it cured the problem stone dead. You never see solid-state amps these days without the output network...amazing that Kustom got away with it.
Joe
The King of Nerd-A-Billy
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| Re: Hillbilly-engineered K200B-5 (PC 105 problems) [message #10623 is a reply to message #10123] |
Mon, 09 March 2009 13:35 |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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OK, here's a bit of a late update...the thing started oscillating again after I fixed the reverb (don't you love weird crap like that) so I figured it must be the higher gain or wider bandwidth of the NTE 128/9 replacement driver transistors, one of the potential problems of using NTE's (besides high cost). Since I didn't have a full set of 40409/10 originals (I think I've got the PNP but not the NPN...isn't that always how it is?) I used a different alternative...TIP31C/32C. THAT works. I've used the amp quite a bit with all kinds of different loads and it works fine. The moral of the story? I guess use the TIP's instead of NTE 128/9 in Kustom amps. The TIP31C/32C is a TO-220 and you have to bend the leads kinda funny, and I used a clip-on aluminum heatsink. But it works!
The King of Nerd-A-Billy
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| Re: Hillbilly-engineered K200B-5 (PC 105 problems) [message #10632 is a reply to message #10630] |
Thu, 12 March 2009 09:42 |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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Steve - Well, that would appear to be the one they cut...I wonder if you have to do both mods to make it work properly, because my amp was so distorted and awful-sounding until I put the caps back in I hardly think the factory would've commissioned it as-is. Very interesting. Thank you for the heads up.
The King of Nerd-A-Billy
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