[Milsurplus] 125 VAC Line? Make Your BA's Happy, Cheap.

[George Babits]

One thing you should take a look at before going through all the trouble of 
bucking or bosting is that a many of these old sets have power transformers 
with tapped primaries.  Usually you will see primary taps for 105, 110, 115, 
120, and sometimes ever 125 volts.  This is especially true of shipboard 
gear where the AC mains could be anywhere from 100 to 130 volts.  As for me, 
once I know something is working, I just plug it in and don't worry about it 
very much about the line voltage.  Here it runs about 118 to 120 volts and 
I've never had anything let the smoke out..

73,
George
W7HDL


----- Original Message ----- 
From: "Bruce Gentry" <ka2ivy at verizon.net>
To: <Milsurplus at mailman.qth.net>
Sent: Thursday, January 14, 2016 5:24 PM
Subject: Re: [Milsurplus] 125 VAC Line? Make Your BA's Happy, Cheap.


> There are many rigs both surplus and otherwise that were designed for 110 
> volts. I have a surplus morale receiver built by Crosley that I restored, 
> and the power transformer was shorted and obviously overheated. Once I 
> found a scrap receiver with a good transformer, I finished the restoration 
> and was surprised and concerned about how hot the power transformer got in 
> a short time. I noticed the rating being for 110 volts, and reducing the 
> power to that level with a Variac completely eliminated the heating 
> problem.  I didn't want to use the Variac all the time, so I took a 50 VA 
> control transformer with two 120 volt primaries and two 12 volt 
> secondaries, mounted it under the chassis, and connected it as an 
> autotransformer. The morale receiver transformer has dual primaries to 
> allow 220 volt operation, the control transformer with it's dual windings 
> allowed each primary on the power transformer to be supplied seperately 
> and allow the radio to be operated on 240 volts if needed. The 
> autotransformer connection gives 108 volts out to each power transformer 
> primary for 120 volts in, and the power transformer only gets warm. I 
> think a lot of rigs with 115 volt ratings could be marginally overheated 
> by 120 volts, and may be  why I see so many National NC-183 and HRO 50/60 
> receivers with fried or substitute power transformers.
>
>   Bruce Gentry, KA2IVY
>
>
>
> On 1/14/16 12:00 PM, David Stinson wrote:
>> Got 125 VAC + on you shack's AC power buss?
>> Ronnie, W5SUM and I worked this problem.
>>
>> There must be a zillion old, broken Uninterruptable
>> Power Supply (UPS) units laying around.  These typically
>> have an AC cord going in and AC sockets going out plus circuit breaker, 
>> etc.   In 99% of the cases, the
>> transformers in these units are still good and are perfect
>> for building a "line bucker."
>>
>> We found a couple of dead 12 Amp APC units.
>> The transformers had a 120 V primary,
>> a lo-current 15 V and a Hi-current 15 V windings.
>> The lo-current winding is not used, unless you want
>> to light a pilot light or something.
>>
>> Pulled the "hot" leads from the dead circuit board
>> and directly powered the transformer through
>> the circuit breaker.  Wired the Hi-current 15 V
>> winding in the "bucking" configuration with one side to the junction of 
>> the incoming "hot" and the other side going to the "outgoing Hot" sockets 
>> on
>> the back.  Left all neutrals connected together
>> as normal.
>> If the voltage is boosted instead of bucked,
>> swap the 15V secondary wires.
>> Mine drops my line voltage from 125 V to 110 V
>> and my boatanchors are happy.
>> 12 A service will run a lot of stuff and broken UPSs are cheap, so you 
>> can add more.
>>
>> Important Tip:  A transformer "line bucker" acts as
>> an "autotransformer."  To measure the output voltage
>> accurately, you must connect a load- even a small one.
>> The load current sets-up the magnetic field the autotransformer needs to 
>> work.
>>
>> 73 Dave AB5S
>>
>>
>>
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