[R-390] R-390 PTO Warmup Time

[Renee K6FSB]

in PTO everything still needs to come to equilibrium-tube, caps, coils, 
resistors and the case/hardware.  it is a thermal issue and need to be 
in the final temp environment for final adjustment .  the tube will be 
first all else takes time. Even If SS the tube it will still take time 
due to the rest of the radios heat soaking. I turn mine on about an hour 
prior if I need more stability then longer.....what is there to go 
astray? and an hour is about right.
Renée/K6FSB

On 7/30/26 9:59 AM, SCOTT Barry via R-390 wrote:
> Since I'm doing this with the PTO removed from the radio and the ovens 
> off, I presume this is a product of just the 5749 warming up. Is that 
> correct?  If so, could a solid-state replacement for that tube work 
> here?  I know it's a remote cut-off, etc., but in an oscillator 
> circuit, would that matter that much?
>
> I'd be hesitant to leave the radio on unattended so having an extended 
> warmup like that is pretty much out of the question for me.
>
> Just thinking out loud...
>
> Thanks,
> Barry - N4BUQ
>> Hi
>>
>> Best to simply let the radio warm up for a while. The ovens will heat 
>> things in a way that is not “same / same” as normal warmup. Unless 
>> things are really cold, you probably will not save a lot of time.
>>
>> How long does it take for a radio to stabilize? That depends a lot on 
>> how picky you are and the exact radio you happen to have. Picky folks 
>> just might turn it on a few hours before they need to use it. Very 
>> picky folks might turn it on the day before :) :)
>>
>> If the radio has been in storage, humidity will have had an impact. 
>> It will drift longer than a radio that is used on a regular basis. 
>> Store it in a damp barn for years and who knows how long you should 
>> run it to get it dried out. On some stuff the answer is measured in 
>> weeks. I have gear here that the answer is measured in months.
>>
>> Bob
>>
>>> On Jul 30, 2026, at 12:30 PM, SCOTT Barry via R-390 
>>> <r-390 at mailman.qth.net> wrote:
>>>
>>> While I know the manual states that the receiver should be allowed 
>>> to warm up (30 minutes?), I wonder how long it should take for the 
>>> PTO to stabilize.  I have the PTO out and monitoring the frequency 
>>> from cold and it takes quite a while to stabilize - maybe 15 or 20 
>>> minutes.
>>>
>>> It climbs until it reaches a certain frequency and then starts very 
>>> slowly up and down a few hundred Hz before settling down to a 
>>> reasonably stable frequency.  Before I took it out, I was seeing 
>>> end-to-end about 1,000,001.5 kc so I thought it may need a touch-up 
>>> on that; however, I don't think I had let it warm up sufficiently 
>>> because if I let it warm up a good while, it is much closer to 
>>> 1,000,000kc end-to-end (approximately 1,000,000.300kc), hence my 
>>> wondering about warmup time.
>>>
>>> Any thoughts on this?  Would running the oven for about 10 minutes 
>>> and then turning it off help stabilize it faster?
>>>
>>> Thanks,
>>> Barry - N4BUQ
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