Hi all,
I’ve finally gotten around to replacing the dead outputs and leaked primary side capacitors in my old T707 and it won’t start up.
The power light comes on, a get a sawtooth wave from the tl494 but the output square wave won’t stay constant. It powers up in little burst and then dies down again.
Im pretty sure the output section is fine, any pointers on why this could be?
Would appreciate a service manual if anyone has one.
I’ve finally gotten around to replacing the dead outputs and leaked primary side capacitors in my old T707 and it won’t start up.
The power light comes on, a get a sawtooth wave from the tl494 but the output square wave won’t stay constant. It powers up in little burst and then dies down again.
Im pretty sure the output section is fine, any pointers on why this could be?
Would appreciate a service manual if anyone has one.
Thanks Perry, bias pots have no effect on current draw (at either end of the spectrum, I am assuming that CCW is minimum) from the bench power supply, which is about 200mA.
The rail voltage is likely regulated. Reduce the 12v supply voltage. Does that increase the pulse width of the square wave?
The rail voltage is +-0.6v or one diode drop. I pulled and tested the ps mosfets out of circuit but that seems to imply that there is a fault in the power supply still?
I have the burst of square wave on the mosfet gates, 12v on the drains but nothing beyond some small high frequency spikes on the source.
I have the burst of square wave on the mosfet gates, 12v on the drains but nothing beyond some small high frequency spikes on the source.
No change until about 9 volts, where it drops out completely.
The duty cycle looks to be pretty close to 50% and the burst repeat at around 45hz.
Attachments
I don't understand what you did.
With the supply functioning (all FETs in the circuit), did you reduce the 12v supply voltage while watching the pulse width on the drains?
With the supply functioning (all FETs in the circuit), did you reduce the 12v supply voltage while watching the pulse width on the drains?
We posted at about the same time. The supply appears to be working.
What's the rail voltage? Does the rail voltage remain constant when you vary the 12v supply voltage?
What's the rail voltage? Does the rail voltage remain constant when you vary the 12v supply voltage?
The drain waveform should be swinging 24v. From ground to 24v. Yours doesn't appear to be going to ground.
The pull down resistors measured at around 110 ohms, the mosfets were tested with a gm328 tester and a multimeter.
Nothing seems burned although one of the primary side caps had shorted and scorched the PCB where it sat. I did clean the capacitor residue off the board too.
Nothing seems burned although one of the primary side caps had shorted and scorched the PCB where it sat. I did clean the capacitor residue off the board too.
I had replaced it on your advice a while ago, then parked the project whilst I waited for parts and time.
Completely dead Alpine MRV T707
I should be able to jumper between the collector and emitter to test whether the transistor is bad, shouldn’t I?
It reads 12v on the base, 5.8v on the base and 5v in the emitter.
Completely dead Alpine MRV T707
I should be able to jumper between the collector and emitter to test whether the transistor is bad, shouldn’t I?
It reads 12v on the base, 5.8v on the base and 5v in the emitter.
Q901 tested bad out of circuit, I have an mje172 I think will substitute for it as long as I don’t need the full 160v capability of the 2sa1275.