Back to Home
By John R. Holden N7IQV
|
Up one level Back to Home |
A MASTR II Ferroresonant Power Supply Safety Alert By John R. Holden N7IQV |
|
This is a writeup of a SERIOUS SAFETY PROBLEM in the stock MASTR II Ferroresonant Power Supply discovered by John R. Holden N7IQV and documented at the
request of the Repeater-Builder staff after he posted (on the Repeater-Builder Mailing List) a description of it.
Thanks go to John for writing up his discovery, and for permission to present his work.
|
From: John R. Holden N7IQV Here is a Mastr 2 Power Supply safety note, which all MASTR II system owners might like to check out: During a repair of a "transmitter dead" complaint, inspection of the MASTR 2 power supply showed the "switched" side of power switch S-1 pin 4, had been making intermittent contact to the back wall of the power switch/fuse/outlet box. There was no insulating "fish paper" or Mylar sheet behind the switch and it appears there had been long term (albeit brief) repeated shorts to the chassis. This repeated shorting caused a LOT of arcing (see two photos below). A check of the power supply schematic (see page 7 of LBI30867 or LBI4806 and also in other M-2 books) reveals there is NO protection ahead of the power switch, other than the facility's fuse or circuit breaker. The hot wire of the AC power cord is wired to the utility outlet, to the S1 power switch, then to the fuse holder, and the transformer primary. If lead length and trim at the S1 switch is not kept close, and insulating material is not applied to the back surface, there is a possibility of a short if pressure is placed on the drop-down panel at the front of the power supply. This may include plugging another power cord into the utility outlet on the supply. Given the gauge and length of the supply's power cord, this could present a risk of fire if the input power is not interrupted. MAKE SURE YOUR STATION INPUT POWER LEAD IS PROTECTED APPROPRIATELY! There was no input breaker trip, and luckily no fire, in our instance. Depending how YOUR station is set up, a bit of checking may save you a very unhappy ending. Remember: the circuit breakers and fuses are not installed to protect the EQUIPMENT. They are there to protect the WIRING! Written 03-Jan-2005 by John R. Holden N7IQV
|
Note from WA6ILQ:
Besides making sure that the switch terminals clear the sheet metal, a
piece of "fish paper" like the one you see behind the fuse posts is
the ideal and preferred insulator but is hard to find. A useful
substitute that can be used in circumstances like this is a chunk of an
old credit card. Just use a blob of rubber cement or even a 1
inch by 1 inch square of double-sided foam tape to hold it to the back
panel. Not pretty, but inexpensive and quite functional. And
besides, who's going to see it?
The "real" manual for this supply is LBI4806 or LBI38067. In both you
will find that the DC fuse value varies with the radio power level:
|
So make sure that your used power supply isn't over-fused for the load that you have on it.
End of comment from WA6ILQ
Original text (the white background) © Copyright 2005 by John R. Holden N7IQV
Edited by Mike Morris WA6ILQ
Hand coded HTML by Mike Morris WA6ILQ
This page last updated 05-Jan-2005
The information presented on this web site, in and on these conversion pages is © Copyrighted 1995 - 2005 by Kevin Custer W3KKC and multiple originating authors.
