| Toasted K250 (very tech) [message #2054] |
Fri, 25 October 2002 18:49  |
WildBill
Messages: 4
Registered: July 2002
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Junior Member |
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Well last night I was gigging on my K250. Moderate volume level, sounded very good...until the sound started getting a little grainy and then everything went quiet in mid-song. As I punched off the power switch I could see bright light inside the amp (through the holes of the input jacks...)not a pretty sight. Resistor on fire... Anyhow, I looked into it tonight and I've fried (on PC5065) R29, Q8, and Q11. Possibly more but that's all I've had time to test.
Q8 (labelled RCA 38736) appears to cross-reference to a NTE 128, though the RCA has the heat sink pre-attached and the NTE doesn't. Anybody know for sure if the 128 is correct? Also, Q11 was replaced with 2N3055's which brings me to my next question: I've read the 2N3055's don't handle high (40V) rail voltages well, since the possible 80 volt differential can exceed the design max. (I've stiffened up the power supply with 10,000 mfd caps on each rail BTW) Would it be advisable to go up to a 2N6254, in same family as 3055 but higher V rating, beta curve is somewhat higher though so may have to adjust bias, I assume that would be R29 to adjust bias.
I'm also surprised a little that the output transistors are hidden under the amp chassis, instead of outside with a heat sink. Seems like a very good overheating possibility...I am tempted to put in a cooling fan, anybody else had the same thought?
Thanks for the help in advance!!
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| Re: Toasted K250 (very tech) [message #2055 is a reply to message #2054] |
Fri, 25 October 2002 19:16  |
WildBill
Messages: 4
Registered: July 2002
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Junior Member |
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I hate replying to my own post. Rat Shack has a 2N3053 that also cross references to the NTE 128 which supposedly crosses to the RCA 38736 for Q8. Anybody know if the 2N3053 is OK as well?
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| Re: Toasted K250 (very tech) [message #2098 is a reply to message #2093] |
Sat, 02 November 2002 22:31 |
WildBill
Messages: 4
Registered: July 2002
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Junior Member |
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heh heh...yes, I already pwered up without remembering to plug the preamp connector in. Might have gone into oscillation, evidently I still have at least one transistor shorted, voltage readings on one side are way off.
Now I have a thought. Some people may call it sacrilege, but if I'm going to have to replace several of the drivers (with obsolete, hard to find parts), why not just replace the entire power amp board, maybe with one of the projects G. R. Slone has in his book. (High Power Audio Amplifier Construction Manual). Some modifications would be required--like a new power transformer to get 56 volt rails--but hey, I'm using this for bass and I want more power!! Of course, eventually that may lead to replacing the preamp boards too...
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| Re: Toasted K250 (very tech) [message #2165 is a reply to message #2159] |
Thu, 14 November 2002 22:28 |
KustomBlues
Messages: 495
Registered: June 2001
Location: Michigan
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Senior Member |
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Well, actually they are made by eminence, but to the specifications that Carvin requests. They are different. Eminence also makes these special request speakers for Fender, and they put them in their Blues Jr., etc. amps....... Usual differences include watt Rms differences, magnet weight, etc.
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