[Premium-Rx] Bucking Transformer

[Charles Steinmetz]

Mac wrote:

> is a variac not considered.. mac/mc  w5mc

That is a perfectly fine solution, with one possible down side.  With a 
variac, there is always the chance that the knob could be inadvertently 
bumped or turned.  For many of us with stable households, and putting 
the variac in an out-of-the-way location, there is very little risk.

With some users, however, (kids around, people who are prone to 
"adjusting" things at random, housekeepers who might disturb the 
setting, etc.), perhaps not so much.  The HF-2050, in particular, has an 
extremely poorly designed mains power supply that is well beyond safe 
thermal operation even at rated line voltage.  If one were to be fed 
higher than rated mains voltage, the results could (very likely, 
*would*) be disastrous in short order.

While the regulated *outputs* of the existing power supply can be turned 
down, that *will not* cure the problem.  The problem is due to the 
excess "headroom" of the AC-DC power supply -- the difference between 
the raw (unregulated) DC voltage and the regulated voltage(s).  That 
voltage headroom, multiplied by the current drain of the receiver 
circuitry, determines the amount of heat the power supply heatsinking 
must dissipate.

There are two ways to mitigate this: (i) by reducing the raw 
(unregulated) DC voltage of the power supply (such as by using a bucking 
supply or variac, as discussed above), or (ii) by improving the power 
supply efficiency (such as by replacing the existing analog power supply 
with a switching power supply).

I am no friend of switching power supplies in general, and particularly 
not inside the cabinet of a radio receiver, but given the various 
tradeoffs and choices, using a switching supply is what I settled on for 
my own HF-2050.

After much research, I selected the Integrated Power Designs (IPD) Model 
SRW-65-3003 with a chassis cover (i.e., not open-framed).  This is a 
triple-output supply with nominal outputs of 5v, +15v, and -15v.  You 
would set the output voltages to +5.2v, +15v, and -15v under load.

I find that in actual use the HF-2050 with the IPD Model SRW-65-3003 
power supply works FB.  It completely solves the heat problems and does 
not add objectionable switching noise.

Now, if only the HF-2050 worked and sounded well enough to make me 
tolerate using it....

Best regards,

Charles