Good thing curiosity got the best of me. I revived the PSU anyway and now run it inside the original enclosure that comes with a fan. It's been heating 4 6C33 tubes for about 90 minutes now.. I will now sleep (4:56AM..) and hope that i will still see it working when i wake up.
I had to replace the mains rectifier diodes and the puny 1 ohm base drive resistors. The chopper transistors were fried as well. Fortunately, the transformer is still intact.
One thing still bothers me: the voltage output dropped to 11.4 volts which is about 1.2v away from the optimum heater voltage of 12.6v. Attached is the basic schematic for the PWM driver (EST7502). We can see the feedback for regulation goes to pin 16. However, there are two voltages to that pin: 5V and 12v. Can i skip the 5v by removing R4? I would like to increase the output a bit just to reach the 12.6v goal.
I had to replace the mains rectifier diodes and the puny 1 ohm base drive resistors. The chopper transistors were fried as well. Fortunately, the transformer is still intact.
One thing still bothers me: the voltage output dropped to 11.4 volts which is about 1.2v away from the optimum heater voltage of 12.6v. Attached is the basic schematic for the PWM driver (EST7502). We can see the feedback for regulation goes to pin 16. However, there are two voltages to that pin: 5V and 12v. Can i skip the 5v by removing R4? I would like to increase the output a bit just to reach the 12.6v goal.
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my experience with atx is that the more you load the 5volt rail, the higher the 12volt rail becomes...adjusting vr1 gives you some tweaking of the 12volt rail....
Woke up at 7.30AM and it's still running.. Took the car for a spin and returned to it still working. This is it.. it's ready. I then moved the PSU board to the chassis, installed the fan as well and made some holes on the chassis for ventilation. Put the chassis cover, turned it on and ten minutes later.. POP! it died (again) You can cook eggs on the heatsink. Definitely the problem is lack of proper air circulation under the chassis for heat dissipation.. The heat from the tubes didn't help the situation either. So now i'm back to square one.. I need to decide whether to keep the ATX PSU (which proves to be sufficient when it's inside the original enclosure) and improve the air circulation further more or go with my own sturdier SMPS design. Linear is also getting interesting 🙂
I think it's about regulating the 5V line under load. To maintain the output at 5V when loaded, duty cycle has to be increased. Subsequently, this would increase the voltages on the other outputs including 12V line. I did succeed in increasing my 12V line. It's as simple as installing a trimpot parallel to the lower voltage divider resistor (on my diagram it's VR1 while on my ATX PSU it is a resistor). Reducing the value of the lower divider will increase the output voltage.
an observation i had from working with computer hardware....
testing the atx psu outside the pc, i observed lower 12 volt rails,
now when this psu is installed to power the pc, i noticed that
the 12 volt rail was actually higher.....
yes, that is it....and congrats on your experiment,
although i would suggest you get a better atx psu for long term reliability...
I have just finished reviving the PSU for the second time (i knew those old PSU i gathered would prove useful one day) and just drilled more holes on my chassis for the ventilation (it looks closer to cheese than an amp chassis now). It's currently powering just a pair of tubes.. I'm too afraid of plugging the other pair 🙂 Don't want another dead PSU.. I'm considering just running half the heater for the OTL amp. So instead of getting 25W per channel, it would only be around 6W. It might be enough for my bedroom. Unarguably it's good for the health of the PSU right now. Haha.
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