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A while back, I started a thread about replacing the driver transistors for my JC-120. It was a very fruitful thread, and I learned a lot about SS amplifiers. Unfortunately, some of the information I posted about my particular amp was not accurate. I thought it was based on the '79 schematic that is floating around, but it is not. It is also not like the '82 schematic. I think it is somewhere in between.
The board numbers are AP-113 (141-113), AP114 (141-114), and PS-56 (146-056). If anybody has schematics for this amplifier, that would be awesome. It is not any of the three common JC-120 schematics often found.
I made some progress perhaps but the symptoms confound me.
Anyway, I finally got the amp back to my house, and fired it up. A 330 ohm resistor at the collector of the PNP driver was burning up (if you have the '82 schematic, this part of the circuit is identical as far as I can tell). I discovered that the 1/4w 10ohm fused resistor from the collector of the PNP to the base of its corresponding power transistor was burned out, so I replaced it to see if that would do anything good. The amp did produce sound for a while, mainly a loud hum that sounded like it was maybe 120Hz or so. The (fresh) 330 ohm resistor wasn't even breaking a sweat though and I didn't see any smoke or smell anything bad from anything, so I considered that progress.
I also noticed that when power cycling, if I strummed the guitar and turned off the amp, for a split second the hum would disappear and I could hear the guitar clear as day, as everything was discharging - indicating to me that on some level there is a complete signal path.
But after power cycling a few times, I heard a crackle and the fuses popped. This happened as I was pressing down on the switch a little harder than usual, so I'm not sure if this was coincidental or I was making a more (or less) complete connection with the switch which caused the overload.
Anyway, I'm stumped and I don't want to break it more than I have. I'm pretty intimidated by this very complex, high-power amp at the moment, but I want to get it fixed.
Oh, also as far as I can tell the power transistors all work, but they are not TO-3s, they are a package called SIP-flange. The transistors are 2SD736A and they are quite obsolete. If I do have to replace one or more of these, are there any modern replacements which have a compatible package?
The board numbers are AP-113 (141-113), AP114 (141-114), and PS-56 (146-056). If anybody has schematics for this amplifier, that would be awesome. It is not any of the three common JC-120 schematics often found.
I made some progress perhaps but the symptoms confound me.
Anyway, I finally got the amp back to my house, and fired it up. A 330 ohm resistor at the collector of the PNP driver was burning up (if you have the '82 schematic, this part of the circuit is identical as far as I can tell). I discovered that the 1/4w 10ohm fused resistor from the collector of the PNP to the base of its corresponding power transistor was burned out, so I replaced it to see if that would do anything good. The amp did produce sound for a while, mainly a loud hum that sounded like it was maybe 120Hz or so. The (fresh) 330 ohm resistor wasn't even breaking a sweat though and I didn't see any smoke or smell anything bad from anything, so I considered that progress.
I also noticed that when power cycling, if I strummed the guitar and turned off the amp, for a split second the hum would disappear and I could hear the guitar clear as day, as everything was discharging - indicating to me that on some level there is a complete signal path.
But after power cycling a few times, I heard a crackle and the fuses popped. This happened as I was pressing down on the switch a little harder than usual, so I'm not sure if this was coincidental or I was making a more (or less) complete connection with the switch which caused the overload.
Anyway, I'm stumped and I don't want to break it more than I have. I'm pretty intimidated by this very complex, high-power amp at the moment, but I want to get it fixed.
Oh, also as far as I can tell the power transistors all work, but they are not TO-3s, they are a package called SIP-flange. The transistors are 2SD736A and they are quite obsolete. If I do have to replace one or more of these, are there any modern replacements which have a compatible package?
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