[TechAssist] Re: Radio designed for B and A batteries: want to convert to ac
- From: Gary McCartney <gary@xxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Wed, 18 Dec 2002 17:14:35 -0500
Sounds like you need 2V dc regulated at a fair amperage. The easist way
of doing this is to use an LM338 regulator IC. It is adjustable via 2
resistors, anywhere from 1.25 to 32V dc and is rated at 5 Amp. If you
need the data sheet on this, I can dig it out for you. If you only need
a couple of Amps, you can use an LM350 (rated 3 Amp), and if you need
less than one Amp , you can use an LM317 (rated 1.5 Amp). All these IC's
operate the same. The result should be that of a lab bench supply if
filtered properly. These IC's also make a simple adjustable bench supply
by using a potentiometer instead of a fixed resistor to set the output
voltage.
Gary McCartney
McCartney Electronics
7134 Fife Rd, RR 7
Guelph Ontario Canada N1H 6J4
Fax: (519)821-1530
email: gary (at) number63.ca
wjstv@xxxxxxxxxxxxxx wrote:
>
> Anyone less than 50 years old should probably delete now.
>
> This is a Sears radio model 2541 using 4 tubes, 1A7, 1N5, 1A5, and 1H5 and
> designed to operate off 2 "B" batteries @45v each (90v in series) to power
> the B+ and an "A" battery @2v to operate the tube filaments. We have the
> schematic.
>
> Very good friend wants us to convert it to ac. Using two bench dc power
> supplies we found that the radio works and sounds good. We made a 90v dc
> supply out of a bridge rectifier and the power transformer from a circa 1977
> Zenith tv which had lots of taps to choose from. (first use we've found for
> these transformers other than two bolted together to make a boat anchor).
> Problem is the 2v filament voltage. If we use a bench power supply or a 1.5v
> battery to power the filaments, radio plays fine. If we use an ac-dc adaptor
> or a little transformer and bridge rectifier to power the filaments we have
> loud ac hum (with volume turned down). Adding additional filter capacitance
> helps but not enough.
>
> Any suggestions appreciated.
>
> Smith TV Service
> 1930 Main St
> Newberry, SC 29108
> 803/276-4946
>
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