[R-390] R-390A Power Transformer, going SS and turn on surge (was R390A Audio Solution)

[Ken Harpur]

Good morning list,

I am thinking seriously about solid stating the power supply on my R-390A, adding a B+ dropping resistor and a slow turn-on circuit. I've been thinking about the long-term preservation of my radios and I'm still young enough to get another 40 or so years of use out of them. Up until this point I had been reluctant to touch the power supply...I figured the rectifier tubes would give a good enough slow turn-on or 'soft start' for the equipment.

But...browsing through the archives has led me on a different train of thought.  About the stresses on the transformer (and other components for that matter) at turn-on...about a faulty rectifier tube taking out a transformer winding...about people that solid state the power supply without adding the B+ dropping resistor therefore the increased B+ generating more stress and heat...etc. A lot of it I already a little about, but the reading gave me a good refresher on the subject. Also the recent conversations here regarding soft start circuits, well...it all got my gears turning.

The 26Z5W tubes in my radio do get very hot...to the point of charring the nearby chassis wall. This can be removed very easily with a good cleaner but it shows that a lot of heat is being generated in this area. Then I started to become concerned for the safety of the Power Transformer...I've already had one 26Z5W fail taking out a winding in the past - rather foolishly, I threw that Transformer away. Anyway after seeing the visual evidence of heat in this area that's where my first thoughts of going solid state came from. The recent discussions here about soft start circuits planted another seed in my mind and got me out and about in Google...I stumbled across a circuit that I really like the look of...it uses 24V from the secondary of the PT to power the relay and a small timing circuit. The relay's contacts are across a resistor in series with the PT primary and when the timing circuit operates the relay the resistor in the primary is shorted out allowing full line voltage to the PT. There is no circuit diagram or description for the circuit other than a PCB layout but it looks very simple and easy to reverse-engineer to fit one's needs. 

So the more I think about all this the more it's making good sense to go ahead and do it...as far as long-term reliability goes.  Getting some heat out must be a bonus too. Well, OK the B+ dropping resistor would put some heat back in but I guess a lot less than the 26Z5W's.

Kind Regards,
Ken
ZL3AA


On 4/01/2013, at 10:55 AM, David Wise <David_Wise at Phoenix.com> wrote:

> I kept the 3DW7 quiet by not generating hash in the first place.
> The German guy who was (is?) selling another SS 3TF7, I think he's doing what you say.
> 
> Dave
> 
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Drew P.
> Sent: Friday, December 28, 2012 9:45 PM
> To: r-390 at mailman.qth.net
> Subject: Re: [R-390] R390A Audio Solution
> 
> 
> Chuck "60 Hz" Rippel wrote (sorry Chuck, I couldn't resist):
> 
>> There are already spare parts issues with PTO's and power 
>> transformers.   I doubt that Hammond will make an R390A 
>> power transformer; perhaps it will be Walt or someone 
>> like him that will come up with a workable solution 
>> benefiting us all.
> 
> I can envision a Dave Wise design which puts an off-the-line 
> switchmode-type power supply inside a gutted transformer can, 
> along with a metric buttload of caps and inductors to keep 
> the hash IN the can.
> 
> Drew
> 
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