[Boatanchors] SP-600

[Richard Knoppow]

----- Original Message ----- 
From: "Al Parker" <anchor at ec.rr.com>
To: <telegrapher at q.com>
Cc: <boatanchors at mailman.qth.net>
Sent: Thursday, August 04, 2011 2:44 PM
Subject: Re: [Boatanchors] SP-600


> Hi Larry, et al,
> Sounds a little like AVC problems, the audio may be 
> distorted from
> overload, the RF & IF stages may be running wide open, or 
> close to it.
> Does that change if you switch to manual AVC & drop back 
> the RF gain?
> I think you've eliminated tube microphonics, with your 
> substitutions.
> I'm just finishing up the complete job on an SP-600-JX-14 
> that was kept
> in an un-A/C environment for some yrs.  Of the 55+- BBOD 
> caps that I
> removed, only 6 weren't cracked.  One had a resistance of 
> 58 ohms, and
> had smoked the dropping resistor ahead of it (in the FCU). 
> I've found 2
> mica caps that were bad, one was in an IF can and was one 
> AVC problem,
> the other was also in the AVC ckt.
> As has been mentioned by Richard, and others, the 
> selectivity switch is
> prone to getting cruddy with lack of regular use, a drop 
> of DeOxit on
> each wafer would be good.  The turret pins do get a bit of 
> oxidation,
> but aren't usually much problem, as long as the turret 
> gets some use.
> With all that in mine, it actually was working before I 
> started into
> it.  So, these things are pretty ruggedly designed, to 
> endure various
> "hardships".
> IIRC, there was a prod. det. mod. that used the BFO buffer 
> tube, which
> is that one at the rear corner.  I've also seen really 
> botched up prod.
> det. mods in them that made things really sound crappy 
> (technical term
> there).
> I hope that's a little food for thought.
> 73,

     This doesn't quite sound like the AVC since its 
intermittant and responds to banging on the panel.
     Since you brought up the AVC, common problems are a bad 
6AL5 AVC rectifier tube, and leaky decoupling capacitors in 
the AVC circuit. The filter cap can also cause some odd 
problems when it begins to go bad.
     This sounds like a mechanical problem. Since spinning 
the tuning dial causes it I would take the top cover off the 
tuning unit and examine the wiring to the tuning capacitor. 
This will also allow you to poke around a bit. Watch the 
mechanism while you spin it, you may be able to spot what is 
happening.
     Also, shine a strong flashlight up between the front 
panel and chassis and look at the mechanism there. Its 
barely possible there is a wire touching something. Far 
fetched but possible.

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