SP-600 Tuning Backlash
 

Some days ago there was a thread in this group (I think)
about the cause of backlash in the tuning control of an
SP-600. I can't find the thread now.
In any case, I ran into something the other day which I will
pass along. I found backlash could be caused by misadjusting
the ball-bearing load of the gear that drives the tuning
capacitor. This is a large gear about at the center of the
front panel. It drives the tuning capacitor by means of an
arm attached to the gear which has a post on it connected to
the cap. I found that if the bearing load is too light there
is backlash. I couldn't feel any looseness but I suspect
that if the screw in the front is not tight enough the gear
can wiggle a little causing the backlash. I had loosened
this one in order to lubricate the bearings and to
investigate if it was the cause of an overall slight drag in
the tuning system. It wasn't but I discovered the backlash
effect. Tighening the screw back to the paint marks on it
fixed this. Probably the procedure if the marks are lost or
someone has gotten the thing way off is to operate the
receiver with the front panel off (being careful not to
short anything) and ajust the loading while tuning a signal
with the BFO. You should be able to hear when the tuning
becomes mechanically stable.
In any case, this is something else to check. Normally
backlash in the gear system is removed by having some of the
gears mounted on spring loaded "spiders". While
correspondence about the SP-600 frequently mentions weak
springs in the spiders I am skeptical of this. Springs do
not loose strength with age despite the common wisdom. They
loose strength by being compressed or stretched many times
resulting in metal fatigue. The spider springs in the SP-600
are under pretty much constant compression and should last a
very long time. OTOH, I've found that the spiders can be
stiff due to lack of lubrication, or in one case, due to the
MFP lacquer gluing the spider to the receiver frame.
I pass this on FWIW and may be preaching to the choir.


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Richard Knoppow
Los Angeles, CA, USA

SP-600 Tuning Backlash
 

Richard,
I was the guy that had the SP-600 backlash problem.
The solution was indeed the pre-loading on the front
of the tuning cap. Actually, the ball bearing that the
adjusting screw hits was rusty causing a lot more friction than
usual. When changing direction while tuning I could see the
spring spiders flex, which led to backlash.

Steve

SP-600 Tuning Backlash
 

"Steve" wrote in message
...
Richard,
I was the guy that had the SP-600 backlash problem.
The solution was indeed the pre-loading on the front
of the tuning cap. Actually, the ball bearing that the
adjusting screw hits was rusty causing a lot more friction
than
usual. When changing direction while tuning I could see
the
spring spiders flex, which led to backlash.

Steve
Good, the bearing is something that I think is easy to
overlook. I think I should clarify that this is not the
bearing adjustment on the back of the tuning condenser
itself (which I am sure an also cause trouble) but on the
gear that drives it. I ran into this after reading your post
and the thread it started.


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Richard Knoppow
Los Angeles, CA, USA