A MASTR II Ferroresonant Power Supply Safety Alert

Up one level
Back to Home A MASTR II Ferroresonant Power Supply Safety Alert
By John R. Holden N7IQV
Prepared by, and with additional info at the end from Mike Morris WA6ILQ

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 a description of it on the Repeater-Builder web group. Thanks go to John for writing up his discovery, and for permission to present his work.

From: John R. Holden N7IQV
Date: 03 Jan 2005
Subject: [Repeater-Builder] A MASTR II Ferroresonant Power Supply Safety Note

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 and also in other MASTR II 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!

In my case 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 LBI-4806 or LBI-38067 for 60 Hz units, or LBI-30868 for the G3 and G6 (50 Hz) models. You will find that there are six models of the power supply, any of which can be wired for 120v or 240vAC (and can be field converted):

19D430272G1 30 amps DC without hum supression
19D430272G2 18 amps DC without hum supression
19D430272G4 30 amps DC with hum supression
19D430272G5 18 amps DC with hum supression
19D430272G3 and G6 are 50 Hz.

The "hum supression" option added a relay that was keyed by the station PTT lead, and reduced the audible buzz from the supply when in receive mode. The LBI contains enough information that the hum supression circuit board could be duplicated and installed if necessary.

In addition to the 18a or 30a for the transmitter power amplifier each supply has a highly filtered output for the receiver and the exciter.

The three fuses on the front panel are wired as follows:

F1 is the AC input to the power supply transformer. The GE LBI specifies a Bussman ABC10 for either the 18 or the 30 amp supply.
F2 is between the AC source and the convenience outlets on the front panel. The GE LBI specifies a Bussman ABC8 for either the 18 or the 30 amp supply.
F3 is a 5a fast-blow fuse and feeds pins 1, 2 and 3 of J601 to the station, and is used for the receiver, exciter and antenna relay (if present). The GE LBI specifies a Bussman NTH5.
F801 (the high current fuse for the PA deck) is located on the rear of the supply and the value varies with the radio power level (see the table below). In all cases the fuse is specified as a Bussman NON (amperage), for example the 15 amp fuse is a NON15. The 35 amp fuse is 3 inches long while the 15 amp fuse is 2 inches in length. This fuse holder length is the fastest way to tell if you have a high current or low current supply.

Band RF
Watts Transmitter
PA DC Fuse
Amps

Low 50 15

Low 70 20

Low 100 30

High 35 15

High 65 20

High 110 30

UHF 20 10

UHF 40 15

UHF 75 20

UHF 100 30

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

Up one level
Back to Home

This page last updated 05-Dec-2006

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.