FIXED: Power Designs 2005A repair

[motocoder]

That will work, and so long as the power device is rated for 50V or more it will do.

I have some IRFZ44N, N-channel Power MOSFET. They are rated at 55V.

[motocoder]

I made some progress on it this weekend, but didn't finish. Here's where things are at.

• Got the thermistor installed and measured the R vs T curve.
• Tested the heater.
• Used some potting compound to seal the holes I had to cut to solder the new heater wires to the connector.
• Created an LTSpice model for the heater control circuit (a simple comparator, thermistor, and MOSFET with some supporting circuitry like a 12V linear regulator).
• Removed the no longer needed components from the PD circuit board, and rewired things to source power from 26VAC.
• Replaced DS1 with a red ultra-bright LED and verified it is visible when powered by the rectified 26VAC signal.
• Added some standoffs to hold a little circuit board that will hold my temperature controller and the power MOSFET that switches the heater current.
• Soldered up about 75% of the little protoboard.

During this process I discovered that Q4 is bad. It was right under a hole I was drilling to mount the board, so I checked it with the diode check setting on my meter and discovered open circuit across base-emitter and base-collector. I think this transistor just drives the overcurrent lamp circuit (haven't quite puzzled out the circuit yet, but I suspect it just provides some delay to keep the lamp on long enough to see even if there is just a very brief overcurrent condition). This would explain why this lamp was not lighting when I initially tested the unit. I am planning on replacing this with a 2N5401 that I had in my parts cabinet. It looks to be a fairly close fit.

Also, I discovered that the if I run the heater coil, heating up to a thermistor setting corresponding to 50C, and then cut the power to the heater coil, there is enough thermal inertia to run the thermistor up to a resistance corresponding to around 54C. I have some hysteresis in my controller circuit, but it's sort of a moot point when the temp continues to drift up 4 degrees after power is cut. I am debating moving the thermistor off the circuit board inside the oven, and closer to the oven wall to minimize this effect. Thoughts?

This thing is definitely going to be a Frakenstein's monstor when I am done. And BTW, that Microprocessor-based PID loop makes a lot more sense to me now. It would be about the same component count as what I have now, and would do a better job controlling temps.

[motocoder]

Can one of you explain how transistor Q4, and the surrounding circuitry is working? This transistor (a 2N4888) was bad, and I've replaced it with what I think is a reasonable equivalent (a 2N5401). when I pull the current set switch out, I measure about 85v across collector/emitter of this transistor. When the current is not being limited, this transistor is in saturation and there is very little voltage across CE.

The current limiting function seems to be working, but the limit lamp is not lighting. it could be something as simple as a burnt out lamp. I'd like to understand how this part of the circuit works before i delve deeper.

[motocoder]

It's all working now!

I verified that the current limit lamp was bad. I replaced it with a yellow LED, with a 1N4001 in parallel (opposite polarity) to prevent any large reverse-biased voltage blowing the LED. The thermostat works fine, although I'm not 100% sure about the temperature. It definitely heats up a lot faster than the 10 minutes described in the manual. I'll have to take some temperature measurements later this week. If it's heating too fast, that might cause excessive temperature overshoot. I could fix that by just putting a power resistor in series with this thing to limit the heater coil current. Have to noodle on that one - might be best just to leave it be.

At 10V output, the output voltage seems to cycle over about a 100uV range, and is clearly correlated with the oven cycles. This seems to be well within specs. I did a quick zero volts and 20V calibration, and then cycled through the voltage range using the knobs. The DeOxit definitely seems to have resolved any issues there - the thing is pretty close to spot-on.

[SeanB]

Well done on giving it another half century of potential.

[motocoder]

Well done on giving it another half century of potential.

Thanks. For the stock unit, do you know what is the typical cycle time for the heater (initial warm-up, and then on/off cycle time once it's warmed up)?