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Electrical Safety Advice

Safety Advice For Working On Organs and Related Electrical Items

Starting with advice from Indianajo.

Analogue organs are rather simple electronic systems to learn electronic repair on, but certain safety rules are key to having a pleasant time learning the craft, if not simply surviving it. It is not that hard, I've been doing it since I was 8 without serious injury, but learned some of the rules the hard way because safety warnings were not a high priority in service manuals and tube manuals in 1958. Now we have the internet, so anybody new to electronics should read and heed these.

  • Current at any voltage into a ring can burn the flesh off your fingers, and dangly chains or other conductive jewellery or accessories (like ear buds) can conduct current and voltage in an unintended fashion. Don't wear jewellery while doing electronic repair.
  • Mains electric power can cause severe burns and death. Do most work on an electronic device with the power cord unplugged. Verify with your eye unplugged status every time you restart a session after being interrupted. An extension cord is useful for putting the plug in front of your eye. If multiple people are working together, lockout of the plug with a safety device and two personal locks is required by OSHA, and certainly a good way of communicating that the organ is safe. Certain debugging steps, especially voltage measurement, must be done with the power on, see the one hand rule below.
  • Electricity across the heart can stop it, especially if above 25 VDC on dry skin. Lower voltage on wet skin can lower the voltage that will put 25 ma across the heart between two hands and stop it. Use ONE HAND AT A TIME both when replacing components, and when measuring with a meter or other device. To do this measuring, use an alligator clip lead on the meter negative of the DVM (digital volt meter) to chassis in most cases. Some special devices may have an analogue ground separated from the chassis, but most use the chassis. Alligator clip leads in a pack from Radio Shack have been useful, about the only part I recommend from them as adequate.
  • William Miller, Senior Electronics Technician at the US Naval Base in Norfolk, Virginia, has commented on the 25VDC figure mentioned above: 25 VDC will not stop your heart (thus kill you). The lowest voltage known to have killed anyone is 50 VAC- that's why the National Electrical Code (USA) requires grounding above 50 volts. (I remember when it was 150 volts.) BTW, that 50 volt electrocution was with the victim wet. I work on 27.8 VDC emergency signaling equipment and can't hardly feel it let alone be injured by it.
  • Large capacitors, especially electrolytic ones, can store dangerous amounts of charge even with the power off. Any piece of metal below the chassis (and tube cap clip on organs pre 1958? organs that use top cap rectifier tubes) should be measured at <25 VDC to chassis, before you touch it. Usually the circuit will discharge the caps itself, but faults like burnt resistors can keep this from happening, so verify it is safe (after unplugging from the AC power) before you touch anything. If you find excessive voltage on a point, especially capacitors, the best way to discharge it is with a resistance discharge tool. I made my discharge tool out of a 4700 ohm 10 watt resistor with sturdy solder tabs, an old meter probe, another piece of 600 v rated wire with an alligator clip on the end, and some heat shrink tubing over everything in the middle. Heat shrink is not rated 600v, so I keep the resistor in a little cardboard box when I am using it. Any resistor between 470 ohms and about 8200 ohms would be convenient, and any sort of 600 v rated (insulation) wire. Note the old probe I used was surplus from a broken meter that had a 600 VAC scale. Simpler cheaper meters do not have 600 v rated wire on the probes. Look for the CE/VDE/UL/CSA rating on the meter back.
  • When unsoldering components, use safety glasses; solder splashes and you'll need your eyes again. True also for soldering, but I have less trouble with that.
  • Very old organs may have cotton insulated wire in the power transformer. While few organs have the chassis connected to one side of the power cord (old televisions and guitar amps often do) dangerous currents can leak through the transformer to the chassis if the cotton has deteriorated. You need to test for this. Download the Hammond organ A, A100, BA, BC etc service manual from archive.org and look at the diagram at the beginning. Known earth ground is usually the third (round) pin of your power plug, but you should verify that the house wiring is correct before trusting this. (I rented a house once where the big blade of the power supply was the hot side. WRONG). The 10 k resistor and .01 mf (uf these days) capacitor between organ chassis and safety ground will turn the leakage current from the transformer into a voltage you can measure. Hammond requires < 4 VAC as a safety check, else the transformer or other parts need to be replaced. This test should be done on any old organ before you "assume" the chassis is at a safe voltage (ground) as above.
  • Some old guitar amps and televisions have a capacitor between the power input and the chassis. This is called a "death" cap for a reason. If it fails, it can put 20 Amps of line AC voltage on the chassis. This should be detected by the test above. If you find such a part, remove it, and ask questions on the forum about a replacement if symptoms occur. The Hammonds and Wurlitzers I have seen (5 organs) do not have this part.

Most of these rules will apply to transistor organs, but as geoelectro points out, he uses a debug technique on transistor organs involve touching two places to use the body as a capacitor. As long as voltages on both points are under 25 VDC and the skin is not broken, this is okay. Although I would be more likely to use an actual capacitor with clip leads, since many things I have worked on are totally blown up. Tube (valve) organs have lethal voltages everywhere, whereas transistor organs have voltages above 25 VDC mostly in the power amp that actually drives the speaker. Warning, faults in the power amp can allow voltage above 25 to blow back through all the transistors and parts to the input terminals, so I recommend measuring any metal you intend to touch at <25 VDC to chassis or analog ground before you touch it.

And from David Anderson, some safety advice when working on leslie cabinets.

I thought I'd post regarding what is by far the most common way I've gotten shocked when working on Leslie amps in the Hammond family (30A, 31A, 31H, 21H, 32H, 22H, 122, 142, 122A). All these amps supply B+ to the organ, whether needed or not, via Pin 5 of the 6-pin input plug on the amp. If the Leslie cable is unplugged, this pin is exposed, and if you happen to touch it while touching another part of the amp chassis before the power supply is fully discharged, you get a shock. I've made this mistake a few times after working on and bench-testing these amps. I don't touch anything inside the amp chassis, but I forget that pin 5 is still connected to a charged capacitor. Fortunately, it's through a voltage-dropping resistor, which limits the current. Leslie tube amps in the 147/251 families do not share this "feature."

We would very much welcome any input from our professional technicians/engineers and our own experienced members. This article is protected to prevent unauthorised editing. If you have some information that you'd like to add, please contact andyg via private message and I will sort it out.

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