[Boatanchors] SX-25 Troubleshooting Help Needed

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: <hallicrafters at mailman.qth.net>; "boatanchors" 
<boatanchors at mailman.qth.net>
Sent: Friday, June 24, 2011 5:43 PM
Subject: [Boatanchors] SX-25 Troubleshooting Help Needed


I picked up a beat-up SX-25 today and need some help.

After changing the 'lytics, the broadcast band (Band 1) 
plays beautifully. The set, however, is completely dead on 
all other bands. There isn't even static. There is no sound 
when I disconnect or conntect the antenna. Jiggling the 
bandswitch does not help. I have plate and screen voltages 
on both the 1st and 2nd RF tubes (6SK7s).

I did signal injection. On Band 2, I got an RF signal from a 
signal generator through the 2nd RF with some amplification. 
(Thanks God for tube extenders!). I got an RF signal through 
the 1st RF with no amplification. Even the amplification I 
got on the 2nd RF was far less I got on Band 1 (BCB). On 
Band 3, I couldn't get any signal through either RF stage.

The set is pretty original. There are signs of a few caps 
being replaced in the '40s or '50s. The only modern cap is 
one orange drop used as a bypass cap. I'd like to 
troubleshooting this systematically, rather than simply 
blindly replace caps and hope I hit the problem.

Any ideas or troubleshooting tips?

As always, thank you for your help.


Joe Connor
_____________________________________________________________

     I think the first RF stage is by-passed on the 
broadcast band, the antenna tranformer connecting directly 
to the second RF stage. It sounds like there is a problem 
with the first RF stage. If you get all voltages OK it could 
still be a bad bypass or decoupling capacitor. I am more 
suspicuous of the band switch and would try cleaning it 
thoroughly.
     If you have an oscilloscope it will be helpful to trace 
the signal around the first RF (and elsewhere). Its possible 
its a bad tube, try exchaning it with the second RF. If 
signals go away on all bands you have a bad tube.


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