Thanks, Bill.
That idea also gets around the issue of using a potentially RF-noisy DC
motor, as pointed out by K7RDX.
Short of the complications and overkill of a single-phase VFD, a couple
of speeds with a switched capacitor might be the best compromise.
Color me a heretic, but I don't like the stock 2-speed tapped-resistor
scheme used in the '77 at all. Mine runs at high speed all the time
anyway, so I'm assuming the clothes dryer thermostat is shorted. Or
something equally annoying...
73,
Jim, N7CXI
Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Sun, 05 Jul 2009 15:28:27 -0700, Jim Barber <audioguy@charter.net> wrote:
>
>> All this talk about seals, temperatures and sensors has me wondering
>> what would be a reasonable, conservative guessestimate of the maximum
>> safe exhaust temperature on an Alpha 77? (single 8877, grounded grid)
>>
>> I really need to rebuild the blower anyway; a potential project might be
>> a temp sensor peeking into the silicone-rubber chimney and a small board
>> with a uP and husky PWM fan speed controller. It would require
>> converting the blower over to a (potentially larger) DC unit, but I've
>> seen that done before. It could stand the addition of some rubber
>> isolation gaskets while we're at it, too. (transmits a lot of vibration
>> through my station desk)
>>
>> Any thoughts?
>
> REPLY:
>
> I don't have data on the 77, but two other Alphas I have owned, a 91b and an
> 89,
> both had exhaust temps that stabilized around 190 F after prolonged brick on
> the
> key TX. I suspect the 77 would be in the same ballpark.
>
> My current homebrew 8877 amp with a 91 CFM blower stabilizes around 140 F
> under
> the same conditions. 91 CFM is overkill for a single 8877 but it makes me feel
> good. :-)
>
> One thought on your proposed blower change: If you get a blower which uses a
> "run" capacitor, you can switch in different values to change speed. For
> example, my EBM-Pabst blower requires a 4 mF cap for full speed. If I had a
> one
> or two mF it would run slower, and I could switch in the extra capacitance
> when
> needed. Just an idea.
>
> 73, Bill W6WRT
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