[QuadList] Ampex VR1200 power supply question

[Don Norwood]

Hi Lee:

Chris covered the basics, but because the supplies are shunt regulated, if you have too LITTLE load on the supply, the excess is burned up as heat.  As he mentioned, if the machine has been reconfigured in any way, the supply may no longer be properly set up.  There are several possibilities, and they may be somewhat differently implemented, depending on the version of your machine.  The standard supply includes a resistor which is shunted depending on the load, and generally determined by a jumper in the machine wiring harness, and that's what may be done in different ways depending on the original factory configuration of your machine.

For high current operation, the jumper (wherever it is in the system) will produce a short across J6-20 and J6-18.  This is easy to measure using the power supply extender cable.  If it is shorted, locate the jumper and remove it.  If the supply will still provide +/-12, then you will find that it will run much cooler.  On some of the machines that we've refurbed, we've used a different resistor to properly balance the load.  The ultimate goal is to have the shunt regulators doing very little so that they run cool and last a long time!

Another thing you may need to check is your line voltage.  The supplies were generally strapped for 115 VAC at the factory, and today, that's probably not correct.

Don Norwood
Digitrak Communications, Inc.
www.digitrakcom.com
  ----- Original Message ----- 
  From: Chill315 at aol.com 
  To: quadlist at quadvideotapegroup.com 
  Sent: Tuesday, June 28, 2011 5:15 PM
  Subject: Re: [QuadList] Ampex VR1200 power supply question


  Lee

  There are a couple of things to check.  

  First are the air filters clean below the power supplies?  Very important for cooling.

  Second, are all the fans turning?  Pull the filters and use a small mirror to check.  These have a tendency to stop or get so sluggish.  Or you can pull all three supplies and see if they are turning.  The first method is easier other than for us old folks having to get down on the floor.

  Third, are the shunt resistors strapped correctly.  If you have added or removed any accessories, this can cause a problem.  So if you have disconnected the Editor, Erase amplifier, or other items, follow the procedure in the manual.

  Lastly, check the capacitors in the supply, signal system, and Intersync to see if you have an issue there.  There might be a load on the +12 volt side that is drawing more current.

  Chris Hill
  WA8IGN



  In a message dated 6/28/2011 3:05:20 P.M. Eastern Daylight Time, lasvegastvengineer at yahoo.com writes:
          I have been working on a VR1200 and noticed that the +12V power supply transistors run noticeably hotter than the -12, 24 and 135 transistors. 

          Is this normal for a 1200? 

          Thanks 
          Lee  


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