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1978 carvin st2500

Started by sempri_fi, November 21, 2007, 02:21:55 AM

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sempri_fi

ok everyone, i've taken on a new project, that is, to restore a carvin st2500 amp to its former working condition. the amp uses 4 2n6254 transistors for power and currently only partially works.

the problem appears to be the age of the amp, being almost 30 years old now, and is quite tired. most of the pots have moderate to severe crackle or will cut out all together. the amp also appears to be stuck in 'distortion' mode, the reverb does not work, but instead adds a tremendous amount of noise to the signal, as does the vibrado effect. the 7 band eq would appear to be functioning normally as it was replaced before the amp cut out. the amp also has a few stuck rocker switches, such that the power drive feature is always on the second channel, and the ground is always lifted.

the extreme distortion and noise appear to be happening in the preamp from what i can tell, though, im not sure if the power amp is bad...

so
i was wondering, is anyone familiar with such an amp? or at least might know something about the RCA 2n6354 transistors? and what normally goes bad or wears out in amps like this one? and anything else that might be helpful is most welcome.

i also plan to replace the horrible 'gain/overdrive' circuit with something with more of a tube screamer type feel
and hopefully carvin will email me the schematic for it as i have inquired, i will post it here for you all to have a look.

thanks in advance
Jacob t. 

joecool85

Quote from: sempri_fi on November 21, 2007, 02:21:55 AM
ok everyone, i've taken on a new project, that is, to restore a carvin st2500 amp to its former working condition. the amp uses 4 2n6254 transistors for power and currently only partially works.

the problem appears to be the age of the amp, being almost 30 years old now, and is quite tired. most of the pots have moderate to severe crackle or will cut out all together. the amp also appears to be stuck in 'distortion' mode, the reverb does not work, but instead adds a tremendous amount of noise to the signal, as does the vibrado effect. the 7 band eq would appear to be functioning normally as it was replaced before the amp cut out. the amp also has a few stuck rocker switches, such that the power drive feature is always on the second channel, and the ground is always lifted.

the extreme distortion and noise appear to be happening in the preamp from what i can tell, though, im not sure if the power amp is bad...

so
i was wondering, is anyone familiar with such an amp? or at least might know something about the RCA 2n6354 transistors? and what normally goes bad or wears out in amps like this one? and anything else that might be helpful is most welcome.

i also plan to replace the horrible 'gain/overdrive' circuit with something with more of a tube screamer type feel
and hopefully carvin will email me the schematic for it as i have inquired, i will post it here for you all to have a look.

thanks in advance
Jacob t. 

The first thing I would do is replace ALL of the pots AND switches.  The reason is this, a lot of your noise is probably coming from them and there is an ok chance that after replacing that stuff the amp may work properly.  Plus they should be cheap.
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