Tuesday, October 13, 2009

Better Safe Than Sorry!

I should have known better. I service high voltage equipment all the time at work and the safety precautions are thoughtfully and carefully layered so no one gets hurt. High voltage insulating gloves, voltage sensing wands, locking/grounding disconnects, rubber floor mats, etc. Exacting procedures tie together all the safety apparatus. You do things EXACTLY correctly because you don't want your children to grow up without a father. So why should things be any different with amateur equipment?

My amp has issues. Nothing major, but it's old and sat in an oceanside warehouse for many, many years before I rescued it. Just little things. Like all of a sudden it wouldn't bandswitch properly on any band except 20 meters. Now that I was putting up an all-band vertical antenna I was pretty sure I was going to want to use the amp on other bands. So, while I was putting up antennas and dabbling in the California QSO Party, I decided to take a look at the bandswitch. Oh yeah, and while I'm at it I need to have a look at the meter. The HV metering also seems to have quit...

This is not a decision to be taken lightly. First of all, the amp is mounted in the equipment rack. I can just squeeze in along the side of the rack and reach behind to unplug the amp and disconnect the coax and control cables. The next trick is getting it out of the rack. This ain't no 30 pound Tokyo Hy-Power solid-state wonder. This is a piece of heavy-duty, mid seventies commercial equipment and it weighs 350 pounds! Fortunately, the procedure is fairly well developed. Slide it out of the rack onto a piece of carpet and from there, with a bit of grunting, you can slide it to where you want it. Now that its all disconnected and clear of the rack you can open it up. Two dozen screws to get the top cover (with electrical interlock) off then several more screws and pieces to come off to be able to slide the bandswitch control out a bit to take a look at it.

So I'm happily disconnecting wires and removing screws in the RF compartment when *WHAM* something bites me AND HARD! I'm sitting there rubbing my hand and starting at the amp with the same sort of disbelief as when the loving family pet up and bites you. What the heck? It's not plugged in. The cord won't even reach the outlet unless it's all the way into the rack. The capacitors couldn't STILL be charged. The HV hasn't been on for hours! But obviously, that's exactly what was going on. Fortunately, I only had one hand in the works or I might not be telling this tale.

I pulled out the schematics and started looking at the power supply diagram. Oh, the HV metering is in series with the bleeder resistors. Starting to make sense now. No bleeders, no HV meter, no discharging the capacitors. Oh, and the interlock on the cover only short circuits the primary (low voltage) side of the HV transformer. I sacrificed a few high value 1/4 watt resistors to discharge the capacitor. This thing has an 8 uF oil-filled capacitor which would easily discharge enough current to blow the end off a screwdriver if you shorted it directly to ground when it was fully charged. After I had blown up a few small resistors then I felt I could safely zap it to ground with a screwdriver. Just as a final precaution, I checked it with a meter. The voltage was rising again! There must be another capacitor in the circuit that was back feeding this one and slowly charging it again. It was rising at almost 50 volts per minute. I discharged it again and quickly clipped a ground wire onto the capacitor to hold the voltage down and bleed off the other cap. Once I was sure everything was finally dead I proceded to change out the open bleeder resistor and all was well. The metering worked properly and when you shut the switch off the HV would go from 4000 to zero in 10 seconds.

So, the moral of the story? DO IT RIGHT AND BE SAFE.

EVERY time you open up an amplifier or other piece of HV electronic equipment ALWAYS ASSUME that the high voltage lines are still ENERGIZED until they are SAFELY and SECURELY grounded.

Oh, and the bandswitching problem? I had band program card plugged in upside down!

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