[R-390] Power Transformers

[Bob Camp]

Hi

Well here are a couple of ideas. As far as I can tell they are not 
really needed at this point. We don't seem to have very many failed 
power transformers out there. At the moment there also seem to be a 
reasonable supply of replacements at reasonable prices. Seems like this 
may be something the list will need to deal with in another few 
decades.

Here are a couple of ideas 20 years ahead of the need:

1) Since we don't run the heaters on the crystals any more (oven switch 
always off) we don't need as much current into the transformer. We 
should be able to drop the main line fuse to about 2/3 of it's 
specified value.

2) The mod you mentioned is one I had forgotten about. Fusing the high 
voltage is a good idea. If the fuse was in the return tap it should 
blow when the high voltage arced inside the transformer.

3) A MOV on the primary, or on the high voltage secondary is a cheap 
way to reduce the insulator flash over failures. They are cheap and 
fairly small. I don't see a down side in using one.

Transformers are amazingly good at handling short duration over current 
/ over power situations. Long term overload will over temp the 
insulation and once the insulation goes you are in trouble.

There's my ideas. We'll know if they are needed in a 20 or 30 years. 
See you then !!

		Take Care
	
			Bob Camp
			KB8TQ



On Jul 25, 2004, at 11:08 PM, David Medley wrote:

> Hi Bob,
> I have at least two bad R-390 and one R-390A transformers among my 
> junk here. In later model R-390 radios there was a mod to double fuse 
> the high voltage circuitry. This is described fully on my web site 
> (http://davemed.com).
> Dave
>
>
> Bob Camp <ham at cq.nu> wrote:
>  Hi
>
> One of the things that there "are no more of" is the power transformer
>  in our favorite radio. Given the way they made these we are unlikely 
> to
> get together and make up a batch of them. Obviously this is not the
>  only part in the radio that might be in this category ten or twenty
>  years from now. Let's just focus on the power transformer right now.
>
> I have never seen a R-390 with a blown power transformer. How common is
> power transformer failure in these radios? Certainly it is not in the
>  top ten or even the top 20 observed failures. That alone is an
> encouraging thing. It is especially encouraging when I compare it to
> the number of computer power supplies I seem to see.
>
> There are things we can do to lengthen the life of the transformers we
>  have *if* it's an issue. All of these things have an impact so you
>  really don't want to fix one thing only to break another.
>
> So, has anybody ever seen a blown power transformer in one of these
>  radios? If so do you have any idea what caused it to fail?
>
> Thanks!
>
> Bob Camp
> KB8TQ
>
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> Sincerely
> David
>
> Please download this really helpful book as my gift for reading my 
> email.
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