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VAC "Vacuum Atmospheres Co." GLOVEBOX, DWYER "PHOTOHELIC" PRESSURE CONTROLLER: VACUUM STUCK ON?

NEW FAIL! Sept/2020. A recent model VAC glovebox has loud buzzing, only one soleoid works, and high/low cannot be set. These are the LED versions having front panel pressure display with the curved rows of colored LEDs: HE-63, with a model SSG-5 VAC pressure gauge. Turns out that all the electrolytics on the internal PCB are dried out. Two 100uF 35V on the rectifiers, raw +-14VDC leading to regulators for +-12V, and a 470uF 35V on the 5V. (Yes, you must remove the right-angle brass hose connector. Retract the four screws on the large circular clamping ring, in order to remove the giant "snap ring" just inside it.) Hmm, w/new capacitors, the +12VDC still has AC hum. These two regulators appear to be powered by halfwave DC, so 100uF might be on the low side, depending on the mA draw for the +12V supply. Instead of 100uF, I used 220uF 50V for the +14V raw volts filter, now it works fine: glowing LED dot does not expand into a stripe!

ORIGINAL FAIL Apr/2017. One of our ancient VAC glove boxes (Vacuum Atmospheres HE-493) started spontaenously cycling,turning pressure rapidly on and off.

FYI, usually these Vacuum Atmospheres Co. gloveboxes have problems in the footswitch or the solenoid valves. The wires in the footswitch wander around, get crushed during footswitch use, and short out the 120VAC to ground, which pops the 2A breaker. Unscrew footswitch top and replace burned wire or repair w/heatshrink or electrical tape, and shove the wires securely in the side gap so they won't do it again. Repairing the strain relief would be wise.

That, or often solenoid valve coils will fail open, or the coil will short out, or the rubber seal on the iron slug inside the sealed valve will harden with age so the valve either sticks shut or won't entirely seal. Note that these Red Hat valves come apart from the top, making it easy to replace the coil or the moving parts. Not much plumbers' skill needed. Note that the coils or the "rebuild kit" cost about $100, yet an entire brand new Asco 120Vac solenoid valve at Grainger costs 40% less. We buy the whole valves of course. Here's a 24VAC coil version used in some units.

This particular rapid-on/off failure wasn't the footswitch. It happened randomly, sometimes when passersby bumped one of the gloves, and the problem could be temporarily cured by turning the power off briefly. Apparently the solenoid valve to the vacuum supply was sticking open, so when the Photohelic pressure controller attempts to maintain pressure at 1" water, it constantly cycles the pressure valve solenoid on and off.

WRONG! The vacuum solenoid is fine. It's actually being turned on continuously by the malfunctioning Dwyer gauge. But not always?! I traced it through and see that the meter needle is designed to cover/uncover a pair of photoresistors adjacent to an incandescent bulb. Normally both photocells are illuminated and their resistance is low. Adjusting the manual high and low knobs will apparently move the photoresistor positions. When the gauge needle rises to the upper manual-adjust needle, it covers one photocell, causing its resistance and the voltage across it to rise, which operates a relay via a 2-transistor driver. So, six internal wires connecting to the front gauge assembly:

• Black pair: 4.5VAC supply to the bulb
• White pair: "low" photocell to control the solenoid valve for pressure supply
• Red pair: "high" photocell to control the solenoid valve for vacuum supply

Dwyer Photohelic gauge, front is model 3310 or 43310 for +-5" H2O, rear relay board is HH-117VAC

When the voltage across either photocell rises above ~9VDC, it fires that relay. MALFUNCTION: the bulbs are too dim, and one photocell voltage isn't nearly low enough: 8.9VDC when illuminated! Any slight changes in the bulb brightness or the supply voltage will push this to 9VDC and turn on the vacuum. (The other photocell goes down to 4.5VDC when illuminated: far away from the 9V threshold.) The spontaneous switching problem seems to be caused by slight dimming of the meter's light-bulb brightness: when gloves sag and the pressure relay turns on, the bulb voltage drops slightly, and this incorrectly turns on the vacuum valve at the same time as the pressure valve. Perhaps the low-volt transformer inside the gauge has developed some partially-shorted windings?

CURE: the offending photoresistor inside the gauge (very thin red wires to the hex plug on the gauge) is 8K ohms at the valve-switching threshold, but fortunately it has a 4.7K resistor in series on the PC board on the gauge. So, it actually takes a rise to a total of 12.7K to turn on the vacuum solenoid. No need to remove the PCB; you can find this resistor using a beeper ohmmeter. I solder another 4.7K in parallel across the existing 4.7K, so now the relay won't switch until the photocell resistance goes past 10.5K rather than 8K. Oscillation problem gone.

BLACK BARRIER-STRIP

• [GND] safety gnd green
• [3] "R" pedal, Pressure sol. coil
• [5] "L" pedal, Vacuum sol. coil, via on/off switch to [2]
• [4]
• [6] AC neut.
• Red: AC neut, jumper to [6]