Hi, I'm working on a 222 SX amp here and am not too familiar with it. Measured the voltage rails to be about +/- 16V, is this right? Seems low, but the 7812s and 7912s used for the OPAMP power supply seem to agree with the voltage rails (I'm guessing they would be 15V regulators otherwise). Also it appears that the switching transistors are BUZ11 MOSFETs, looks like the amp was modded to work with these. Right Now I have some DC voltage on the outputs and the clip or protect lights are coming on, one of the OPAMPs is running considerably hotter than the rest, think I will start there.
not a 222, but i had a prolem with a 2150sx that had a bad mylar capacitor (or was it a tantalum, always get them mixed up) which was tied into the supply leg of an opamp and made it throw out a dc signal, the resistor connected to it was also bad.
another problem i had on a 225hcca was a bad resistor that was tied into one of the regulating transistors, causing it to put out only half of the waveform...simulating dc
id check to see if the outputs are putting out dc, positive or negative then you can track it from there.
another problem i had on a 225hcca was a bad resistor that was tied into one of the regulating transistors, causing it to put out only half of the waveform...simulating dc
id check to see if the outputs are putting out dc, positive or negative then you can track it from there.
grjr said:
The voltage rails should be close to the large capacitor ratings on the output side of power supply. Also check to see if any of the outputs have direct shorts between the collector and emittor. Sometimes they're even shorted between the base and collector or emmiter. If you find a shorted transistor or more, remove them and power it up without them installed to try for the DC voltage again. Be sure to have the transistors clamped to the heat sink though to keep from frying the good ones.
ok, so I've checked a few things out and the outputs appear to be good along with their drivers. The rail caps are rated for 25V, so I'm guessing the rail voltages are fine. I found a pair of bad MPSA56 BJTs along with a couple of open 2.7 ohm resistors in the power supply section, they appear to be part of the drive circuitry for the switching MOSFETs. I've also found a couple of bad 2N5639 MOSFETs that shorted Drain to Source. They are connect to the inputs of one of the OPAMPs. I don't have replacement parts on hand so I'll be waiting until I can get parts.
If the 2.7 ohm resistors were in series with the collectors of the MPSA06 power supply drivers, replace the MPA06s also.
The rail voltage is set by the zener connected to the base of the 2N6488 standing in the supply near the transformer. There may be another diode in series between the zener and the base of the 6488. If the zener is a 15v zener, the rail voltage was correct. The 225 of approximately the same vintage had an 18v zener.
The amp will play without the 2N5639s (jfets, not mosfets). They are part of the muting circuit.
The rail voltage is set by the zener connected to the base of the 2N6488 standing in the supply near the transformer. There may be another diode in series between the zener and the base of the 6488. If the zener is a 15v zener, the rail voltage was correct. The 225 of approximately the same vintage had an 18v zener.
The amp will play without the 2N5639s (jfets, not mosfets). They are part of the muting circuit.
hmm, I stole some transistors from another amp to power this one up, I am getting DC voltage at the outputs. I probed the outputs of the OPAMPs and they have DC voltage on their outputs so there is something wrong with the preamp section. I noticed some corrosion on the pins of the OPAMPs near the DIN connection. Any recommendations?
If you have all of the power supply voltages (±rail, ±regulated), look at the polarity of the voltage on the outputs of the op-amps in the power amplifier. If the polarity of the DC on the output is the same as the voltage on the output, it's probably being driven high by something in the preamp section. The newer amps have DC blocking caps between the pre and power amp sections. I'm not sure if these have them.
It should be easy to see where the DC offset enters the signal line. All of the outputs of the op-amps should be near 0v.
It should be easy to see where the DC offset enters the signal line. All of the outputs of the op-amps should be near 0v.
Thanks, I'll do some more troubleshooting. Here's a couple pics of the amp for reference.
http://www.kernclassifieds.com/forumpics/diyaudio/
http://www.kernclassifieds.com/forumpics/diyaudio/
The board is blue, the camera just makes it look green. It's a MPSA56 (PNP) with the collector tied to ground, it's connected to a GE D40D8. There is one of each on each side. I'll circle them in the pic that I posted.
http://www.kernclassifieds.com/forumpics/diyaudio/DSC07579.JPG
http://www.kernclassifieds.com/forumpics/diyaudio/DSC07579.JPG
In most of their other amplifiers, they use Pulse Amplitude Modulation instead of Pulse Width Modulation. Since the regulator seems to be different (absent?), I was trying to determine if they're using PWM in this amp.
In the other amps, they use something similar to the attached image. If the collector of the NPN driver (D40D8) is connected to B+, it's likely that it's either unregulated or uses pulse width modulation.
After you replaced the defective parts, did the rail voltage increase?
In the other amps, they use something similar to the attached image. If the collector of the NPN driver (D40D8) is connected to B+, it's likely that it's either unregulated or uses pulse width modulation.
After you replaced the defective parts, did the rail voltage increase?
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