jasoncauthron
Active Member
I picked up a little Realistic SA-500/ST-500 combo for cheap. I knew it had a problem, but I always thought these looked cool, I have kind of a Realistic collection anyway, AND I figured as long as it had discrete outputs and drivers, I could fix it regardless. Well, long story shorter, I couldn't find a schematic for it and it DOES have an STK-2230 amplifier. I wouldn't have bought it if I had known, but like I said, it was cheap enough I don't feel too bad... as long as I can fix it.
It's kicking my butt, though. My first mistake (or second, if the first was buying it!) was that I ASSumed that the problem with the right channel distortion was a bad STK module. Out of sheer good luck, I happened to have an STK-2230 that I know to be new and genuine, so, after checking the power supply voltages (because I'm not a complete dummy!) I soldered in that new module and powered it on, expecting the problem to be fixed. Didn't change a thing... Same problem as before, and that problem is that after powering on, (relay clicks after 2-3 seconds) the unit seems to work normally for about 10 seconds, and then the right channel becomes distorted. I believe that I have acquired the very unit that was mentioned in this thread https://audiokarma.org/forums/index...eo-amplifier-right-channel-distortion.909033/ . Power the unit off and back on, and you get basically 10-15 seconds that sound pretty normal, and then it's as if something switches and the right side becomes distorted.
Anyway, I have become kind of determined to fix it now. If I input a sine wave at the tape-in, scope on the speaker outputs, the left side looks normal. Right side, immediately after power-on also has a normal looking sine wave, but it's offset about -.45V. After the 10-15 seconds, the DC offset goes away, but so does about half the sine wave. Like asymmetrical clipping. Tracing the signal from the input jack, I find that it first goes to the selector switch, then to the volume/balance control, then to the tone control board, and then on to the differential amplifier. Signal looks normal going to the tone control board, but it's clipped on the right side when it comes out of the tone board. It's a pretty simple-looking circuit, with only passive components except for the opamp. I think I've checked all the caps and resistors. The original opamp was a JRC-082D. I switched it out with a TL-072 but there was no change. The supply voltage seems to be about 15 volts on the positive alternation, and -13.2 on the negative side. On the opamp, the right channel inputs are pin 2 (which is clipped) and pin 3 (which looks normal). I'm kind of stumped. The rest of the circuit up to the STK seems to be functioning and, in fact I have swapped most of the semiconductors left to right, trying to make the problem move. The STK is amplifying what appears at its input, and seems to be working correctly. There's some other junk: There's a muting circuit that mutes the signal for a few seconds after switching inputs. It seems to be working and the problem doesn't seem related to it as far as I can tell, although the delay is about right. There's a couple of microphone inputs that can be mixed in, but they appear to be amplified by a separate opamp and that output is mixed in on the tone control board. There's an echo control.. As I said, I'm stumped. If anyone has any ideas (or a schematic for this thing!) I would appreciate the help.
It's kicking my butt, though. My first mistake (or second, if the first was buying it!) was that I ASSumed that the problem with the right channel distortion was a bad STK module. Out of sheer good luck, I happened to have an STK-2230 that I know to be new and genuine, so, after checking the power supply voltages (because I'm not a complete dummy!) I soldered in that new module and powered it on, expecting the problem to be fixed. Didn't change a thing... Same problem as before, and that problem is that after powering on, (relay clicks after 2-3 seconds) the unit seems to work normally for about 10 seconds, and then the right channel becomes distorted. I believe that I have acquired the very unit that was mentioned in this thread https://audiokarma.org/forums/index...eo-amplifier-right-channel-distortion.909033/ . Power the unit off and back on, and you get basically 10-15 seconds that sound pretty normal, and then it's as if something switches and the right side becomes distorted.
Anyway, I have become kind of determined to fix it now. If I input a sine wave at the tape-in, scope on the speaker outputs, the left side looks normal. Right side, immediately after power-on also has a normal looking sine wave, but it's offset about -.45V. After the 10-15 seconds, the DC offset goes away, but so does about half the sine wave. Like asymmetrical clipping. Tracing the signal from the input jack, I find that it first goes to the selector switch, then to the volume/balance control, then to the tone control board, and then on to the differential amplifier. Signal looks normal going to the tone control board, but it's clipped on the right side when it comes out of the tone board. It's a pretty simple-looking circuit, with only passive components except for the opamp. I think I've checked all the caps and resistors. The original opamp was a JRC-082D. I switched it out with a TL-072 but there was no change. The supply voltage seems to be about 15 volts on the positive alternation, and -13.2 on the negative side. On the opamp, the right channel inputs are pin 2 (which is clipped) and pin 3 (which looks normal). I'm kind of stumped. The rest of the circuit up to the STK seems to be functioning and, in fact I have swapped most of the semiconductors left to right, trying to make the problem move. The STK is amplifying what appears at its input, and seems to be working correctly. There's some other junk: There's a muting circuit that mutes the signal for a few seconds after switching inputs. It seems to be working and the problem doesn't seem related to it as far as I can tell, although the delay is about right. There's a couple of microphone inputs that can be mixed in, but they appear to be amplified by a separate opamp and that output is mixed in on the tone control board. There's an echo control.. As I said, I'm stumped. If anyone has any ideas (or a schematic for this thing!) I would appreciate the help.
