[Hallicrafters] SX28 hot & blue 6V6s

[Richard Knoppow]

----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "Guido" 
<laffitte at prtc.net>; <hallicrafters at mailman.qth.net>
Sent: Friday, August 26, 2011 5:47 AM
Subject: Re: [Hallicrafters] SX28 hot & blue 6V6s


>
> ----- Original Message ----- 
> From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
> To: "Guido" <laffitte at prtc.net>; 
> <hallicrafters at mailman.qth.net>
> Sent: Friday, August 26, 2011 12:00 AM
> Subject: Re: [Hallicrafters] SX28 hot & blue 6V6s
>
>
>>    Well I agree with Carl that the grid coupling caps 
>> could
>> also be bad. I had not thought of that at first because
>> _both_ tubes are overheating but both caps could be bad.
>
>
> Any and all paper caps in that radio are guaranteed to 
> have leakage and many resistors will be out of tolerance.
>
> If the cathode bypass electrolytic was shorted it would 
> run the tubes wide open but that isnt a typical occurance. 
> What does happen is that the cap becomes a high value 
> resistor which will affect the 200 Ohm R-42 very little.
>
> Carl
> KM1H

     I agree with this but thought that it should be checked 
in case it had a very low resistance or was shorted. I 
should have suggested the coupling caps because they don't 
need a lot of leakage to cause the bias to be wrong. If old 
enough its quite possible both would be bad.
     I also agree that its worth just dumping all the old 
paper caps. One thing that kills paper caps is absorption of 
moisture. The older wax coated types absorb a lot more than 
the later plastic encapsulated ones, but none lasts forever, 
and most are way past their original life expectancy. The 
SX-28 almost certainly has the old wax impregnated caps so 
probably none of them are good any more. Also, modern film 
or ceramic caps usually have much better characteristics 
than the original paper caps had when new.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com