Pre-Power Inspect

 


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Last Edited:
12-Sep-2004

 


!CAUT!ON!
No matter how anxious you are to "fire 'er up" and start enjoying your newly acquired vintage H.H. Scott equipment, please DO NOT PLUG IT DIRECTLY IN TO 120V AC without following these recommended procedures.

Failure to follow these results could result in personal injury and the unit could suffer catastrophic damage. If you want to gamble, visit Las Vegas!

  1. Beg, borrow, buy or draw a schematic. If you don't have a schematic trace out the point-to-point wiring (try that with modern printed circuit boards!).  Become familiar with the locations of major components and their basic functions. I find it helpful to draw or obtain a component layout diagram with both top and bottom chassis perspectives.
  2. Carefully remove the case (if present) and then remove the bottom cover. Look for anything that looks to be missing or broken. Check for the mechanical soundness and tightness of all fasteners. You don't want a heavy transformer falling off! Look for any obvious signs of over-heated or badly burned resistors or leaking capacitors or transformers.
  3. If you remove tubes at this stage, be sure and notate their respective locations. On octal-base tubes, note the location of the key-pin on the tube. If it is missing or broken off, be sure to carefully orient the tube correctly in the socket. (Many tubes will use the same socket-type but you could damage the tube, amplifier, or tuner by not returning tubes to their the proper location). Nothing is more irritating then to realize that half the tubes have their numbers worn off, or to find out that the tube placement sticker is missing and the receiver doesn't have the tube numbers stamped in the chassis next to the socket. You'll also want to verify the tightness or tension of the tube sockets with a dental pick.
  4. You'll probably want to do a light cleaning of the chassis and transformers by brushing and vacuuming it. Use caution not to rub off tube legends,  faceplate or tuning dial numbering!
  5. Removing heavy oxidation or pitting from the chassis is probably not a good idea at this stage. Refer to the page on Controls for details on cleaning contacts, switches, and potentiometers.

Still with us? Don't forget these additional important preliminary power transformer safety tests:

  1. Power transformer primary-side continuity check. Connect an ohm-meter across the UN-PLUGGED AC power cord plug or accessory AC outlet. Turn the unit's power switch on and read and note the resistance. A reading above 2 ohms, (and below about 20 ohms) means the primary windings are probably OK and the power transformer's primary windings are probably not shorted. An open or high resistance indicates a possible blown fuse or open power transformer primary winding.
  2. Power transformer secondary-side continuity check. With the unit UNPLUGGED; connect an ohm-meter across the high-voltage secondary.  One set leads to the rectifier tube and another set goes to the bridge rectifier. (The low-voltage tube heater filaments are likely to give readings of 1-ohm or less). Read and note the resistance. Typical readings range from 40 to 200 ohms for the high-voltage secondary windings.

!CAUT!ON! Remember: If your unit does not pass these simple power transformer safety tests you should not proceed to the next step. 

 

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