[Hammarlund] Fw: Hq170 hum

[Edward B Richards]

Hi Craig;

A cheap way to improve the hum would be to put a pad in the speaker
leads. For every db of attenuation you install you will reduce the hum by
the same amount. Of course you will have to run the volume a little
higher, but most receivers have plenty of audio. A TEE pad made of three
1 watt resistors with 1.7 ohms in the series legs and a 2.2 ohm in the
parallel leg will provide a loss of 10 db which will reduce the hum to
1/10 the former level. 1 watt from the radio will provide 1/10 watt at
the speaker, usually plenty for normal listening with an efficient
speaker. A more common value of resistor is one ohm. Two of these in
series will make two ohms. Using two ohms in all three legs gives a small
mismatch, being 3.44 ohms instead of 3.2 ohms; negligible. This gives a
loss of 11.5 db with an output of .07 watts for 1 watt in. Good luck.

73, Ed Richards K6UUZ


On Sat, 21 Feb 2004 15:57:10 -0500 "Craig Roberts" <crgrbrts@verizon.net>
writes:
> Hello Lee,
> 
> I agree with you -- absolutely. The increased audio bandwidth 
> afforded by my
> new output transformer is quitre possibly at the root of the hum 
> problem. I,
> however, have no idea of what the factory "normal" passband was, 
> since the
> radio -- as I got it -- had been retrofitted with a low-end 
> universal type
> transformer. The output stage wiring had also been modified and the 
> audio
> was plagued by a limited bandwidth and a lot of noise. I had to do 
> something
> to remedy this.
> 
> I didn't arbitrarily install a "high-fidelity" transformer in dumb 
> 'n doomed
> attempt to transform a '60's communications receiver into an 
> audiophile
> set -- it's simply what I had available. I must admit, though, that 
> while
> the new transformer seems to pass audible hum, it's also cleaned up 
> the
> sound considerably.  If possible, I'd like to attenuate the hum 
> without
> unduly compromising the new intelligibility.
> 
> Barring that, I'd like to find an original output transformer for 
> the
> HQ-170A (the late xfmr with the dual 3.2 and 500 ohnm secondaries), 
> but
> these are understandably scarce. I'd also like to find out how the 
> factory
> leads were routed so I could replicate the original wiring. At least 
> then
> I'd know what the radio USED to sound like. I'd be happy jus tto 
> know that.
> 
> Anyway, thanks again for the wisdom, Lee. And thanks to everyone 
> else who's
> responded. I'll keep you posted.
> 
> 73,
> Craig
> W3CRR
> 
> 
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