[Milsurplus] Fwd: Re: Lowering Receiver B+, Again.

[Bruce Gentry]

-------- Forwarded Message --------
Subject: 	Re: [Milsurplus] Lowering Receiver B+, Again.
Date: 	Thu, 22 Dec 2016 10:46:16 -0500
From: 	Bruce Gentry <ka2ivy at verizon.net>
To: 	David Stinson <arc5 at ix.netcom.com>



If the power transformer is well designed and built, using half wave  to
the rectifier would probably be ok. The issue would be the asymmetrical
magnetic field in the core pushing it closer to saturation, causing
excessive primary current draw and overheating. There is another issue.
Power transformers and circuitry in older equipments were often designed
for 110 volt line power. I have had great success using a 12 volt
filament transformer wired as an autotransformer to reduce the input
voltage to about 109-110 volts. Heating in marginal transformers was
reduced greatly. If you did this with the RME, and changed the filtering
to full wave choke input, the voltages on everything would be reduced
but probably with no effect on operation.

      Bruce Gentry, KA2IVY

On 12/22/16 9:16 AM, David Stinson wrote:
> We've probably covered this particular issue in discussions about
> lowering B+ in receivers to help preserve them, but I can't find it in
> my archives
> and don't remember, so I beg your patience with my aged and leaky head.
>
> I've recently restored a nice RME-45.  Here is a photo of it next to
> the WWII "Liberty Ship" MacKay:
> https://goo.gl/photos/zwz8CeYmGb9EVR1k8
>
> As originally designed, the output of the full-wave
> B+ rectifier (type 80 tube) at the capacitor-input
> filter is a needless 340V and better than 320V gets distributed. I've
> already had an insulaton failure at an RCA "accessory" jack which
> shorted B+ to ground:
> https://goo.gl/photos/So8HJtFZKEeY7mQ29
> (I will be Q-doping the old, oxidizing wire insulation in the future.)
>
> There are nearly irreplaceable band switches which
> could suffer the same fate.   So lowering the B+
> to this set is a priority.  While trouble-shooting,
> I determined that the radio would play
> acceptably on AM with as little as 90V for B+.
> I did not test for BFO operation or higher-band LO
> operation at that level- need to do more testing.
> I don't "chase DX" with boatanchors and hold  "preservation over
> performance," so reasonable
> reductions in performance are acceptable to me.
>
> I know several non-destructive techniques to reduce the B+.
> A "bucking" transformer will not do the job as one which took the B+
> down enough would also take the filaments below an acceptable voltage.
> As a temporary measure, I removed one leg of the
> HV transformer winding from a plate of the 80
> rectifier, converting it into a half-wave rectifier.
> This reduced the B+ at the filter input to 260V,
> giving some temporary "breathing room."
> Performance did not suffer, nor is there any
> notable increase in AC hum in the audio.
>
> Here is my question:
> What, if any, are the pitfalls of converting the
> full-wave rectifier into a half-wave rectifier?
> Overall current draw is reduced in proportion
> to the voltage reduction.  No evident increase
> in transformer heat dissipation.  What are the
> possible long-term issues with this?
>
> 73 DE Dave AB5S
>
>
>
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20161222/1cf55dfe/attachment.html>