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Toasted K250 (very tech) [message #2054]

Fri, 25 October 2002 18:49

WildBill
Messages: 4
Registered: July 2002

Junior Member

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

Junior Member

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 #2056 is a reply to message #2054]

Fri, 25 October 2002 21:04

KustomBlues
Messages: 490
Registered: June 2001
Location: Michigan

Senior Member

Bill, Gary overhauled my 250-1 and put in 2N3055 and they are correct. No problems here! The head does run warmer than most of us would like to see, but I have not had a problem with it cooking eggs or anything.... I ususally keep a household fan blowing at the back of my equipment anyways, it helps. Oh, if you need any of the discontinued IC chips, part number NTE725, I have four of them I will sell.

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Re: Toasted K250 (very tech) [message #2073 is a reply to message #2056]

Tue, 29 October 2002 12:59

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

I had the same thing happen to mine on a gig for no reason, nothing is foever! I would up grade to 2 matched sets of nte181s. they are offered in matched sets but I fotget the number. If yours blew like mine you may need the IC chip and some 2n3904 and 2n3906 transistors on the preamp board.the output transistor mounting strip uses the hole metal of the amp for a heatsink. A fan would not at all be a bad idea, but I left the metal cover off and with just that it runs alot cooler.

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Re: Toasted K250 (very tech) [message #2074 is a reply to message #2054]

Tue, 29 October 2002 13:04

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

PS it may have also taken out the FET transistor on the 2nd chennel. That to can be had from NTE.It might be best to get some of all these parts as spares,one has no idea how much longer these will be around. and these are cheap.

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Re: Toasted K250 (very tech) [message #2075 is a reply to message #2054]

Tue, 29 October 2002 13:04

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

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Re: Toasted K250 (very tech) [message #2076 is a reply to message #2075]

Tue, 29 October 2002 21:42

KustomBlues
Messages: 490
Registered: June 2001
Location: Michigan

Senior Member

Quite a few of the 250 parts are no longer being made, and haven't been for a while. I'm selling some spare new parts, check out the classifieds.

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Re: Toasted K250 (very tech) [message #2093 is a reply to message #2054]

Fri, 01 November 2002 13:04

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

If you are trouble shooting the amp with the preamp boards and input jacks removed from the front panel, once you get the amp working you will have to at least ground the input jacks, or the amp will go into oscillation and you will think the amp still has a problem when it doesn`t

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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

Junior Member

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 #2101 is a reply to message #2098]

Mon, 04 November 2002 14:30

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

the preamps and driver layout are very good in the 250s.for myself I would like a little more mid boost in the preamp with the mid control.With larger filters the power amp will do 130RMS. If you have 2 eiffecent 15" it should be enough to do battle with 2 players with 100 watt stacks! But hey for bass its always great to have more.

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Re: Toasted K250 (very tech) [message #2103 is a reply to message #2101]

Mon, 04 November 2002 21:47

KustomBlues
Messages: 490
Registered: June 2001
Location: Michigan

Senior Member

My completely overhauled 250-1 (thanks again Gary) benched out at 131 watts RMS with new stuff..... With 2 new Carvin speakers in the cab, it's LOUD!!!!!

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Re: Toasted K250 (very tech) [message #2159 is a reply to message #2101]

Wed, 13 November 2002 07:34

stevem
Messages: 4745
Registered: June 2004
Location: NY

Senior Member

Carvin has very good speakers, but they are made by eminence. If you want them order them from parts express, they are alot cheaper and you have many more models to chose from, almost too many!

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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: 490
Registered: June 2001
Location: Michigan

Senior Member

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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