Rick,
I am guessing that you will be fine going ahead with ceramics since most of
the SP-600 circuitry won't care. That said, there are differences between
capacitor types, and within the ceramic family you have Z5U, Z5V, X7R, COG
(NPO), and maybe some others. I listed those from worst to best. It is hard to
find the COG (NPO) types in higher values above about .001 uf, and that is
where X7R is the next best choice.
So what are the differences? In short it is temperature stability, and
voltage stability along with the ESR. With high impedance circuits where the
bypass capacitor is .01 uf and greater, it might not make a whole lot of
difference which type you choose. Then again we see a big difference if the
impedance is lowered where we then need to bypass some RF current. Here is a
case in point:
On the Central Electronics 20A the mixer L-C tank is high Q, and is bypassed
with a .005 silver mica on the B+ end. To lower the Q, a 10K resistor is across
the coil (L). On later 20A's, the bypass is .005 ceramic Z5V, and for the same
L-C tank Q, the parallel tank resistor had to be increased to 22K. If you take
the newer version with the 22K, and replace the ceramic .005 uf with a silver
mica, the Q increases to the point that instability occurs.
I played a lot with that circuit on one of my QRO 20A projects, and tried every
type of capacitor I had to see how they behaved where ESR was important due to
the high Q of the tank circuit. Interestingly polypropylene had the lowest ESR,
and therefore the stage had the highest gain. The difference was easily 6 db
between the ceramic and the poly with the mica in-between. Mouser and others
carry polypropylene and they are quite inexpensive. I also had RF floating on
the unshielded B+ wire to the VFO, and a polypropylene capacitor knocked that
down better than any other type I had. It is interesting to bridge existing
bypass capacitors to see the effect on stage gain, or circuit Q. Sometimes
there is gain stage regeneration from another stage where the B+ is not well
filtered. This might be normal and then somebody arbitrarily starts changing
the bypass capacitors. The circuit behavior might change, and maybe for the
worse. Be careful!
Back to the SP-600, you should expect to need a re-alignment after re-capping,
and you might re-cap one stage at a time to see if anything bad happened. I got
my SP-600 from Ozona Bob, W5PYT. He re-capped this one with Sprague
Orange-drops all except two above the RF Turret. Those failed later, and I
realized why he skipped them over. Quite a job to get at those dudes. There are
several types of Orange Drops, and I believe that some are polypropylene.
Regards,
Jim
JKO
----- Original Message ----
From: Rick Brashear <[EMAIL PROTECTED]>
To: [email protected]
Sent: Saturday, December 30, 2006 9:57:24 AM
Subject: [AMRadio] Caps
Thanks to everyone for your suggestions and advice on the paper versus
ceramic capacitors in the SP-600 rf strip. It seems the majority advise
the use of ceramic and I'll most likely go that route. I also agree
with the thought that anything I put in there will be better than what I
have. :-)
Thanks guys!
Rick
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