[Hammarlund] Re: Im recapping a SP600

[Roy Morgan]

At 08:20 PM 12/1/03 -0500, "Charles Mellone" <cmellone@ntia.doc.gov> wrote:
>Just one more question. Is it better to use ceramic caps, like .01 @600
>in the rf pods and use Orange drops on the bottom? If so what ceramics
>do you suggest.

Chuck, (and other Hammarlund list readers),

Here are my opinions about caps and other things:

1) Ceramic caps are fine everywhere except perhaps in audio coupling (audio 
amp). The later models of the SP-600 used ceramic caps for all bypass 
applications.  Any type of  modern ceramic cap (or even old ones) will be 
fine.  Even in the audio coupling spots, ceramic caps would work well and 
you'd likely never hear the difference.

2) Orange drops by brand are not necessary.. ANY, repeat ANY modern film 
cap of the same or greater value or voltage will work fine.  The reputation 
and lore about using "Orange Drops" is way over rated, in my 
opinion.  Granted, they cost only a little bit more if you buy them wisely, 
and there certainly is no harm in using Orange Drop caps.  (In certain 
radios in certain spots, the appropriate Orange Drop is a bit too big to 
fit physically.. this may not be the case for any caps in the SP-600.  It 
is the case in one or more spots in the R-390A.)

   - In a very few, rare, specific circuits in older radios, the loss and 
inductance characteristics of old paper caps were important for good 
functioning of the circuit.  These radios were generally designed with 
serious cost limits and had marginal designs, poor shielding and grounding 
methods, and minimal parts counts.  I expect these conditions do not occur 
in the SP-600.

  -  In AGC circuits, tone controls or places such as across a modulation 
or audio output transformers, the value the engineers chose may be 
important.  Close is close enough in these spots.  (In the Valiant and 
Ranger modulator, they should be removed, say some owners.)  For bypass and 
audio coupling applications, using a cap of higher capacitance will likely 
do no harm at all.

3) Using caps of higher voltage rating than originals is probably not 
necessary.  However,  getting such caps will likely cost only a little bit 
more, and among some types of caps, there may not be lower voltage rated 
caps available.. Do not hesitate to put a 630 volt cap in place of a 400 
volt original, but don't search the world for higher voltage caps for 
"extra safety margin".  Modern film caps will probably withstand three to 
four times rated voltage indefinitely. Whereas old caps might not have even 
when new.  Modern caps inherently have far lower leakage and far higher 
likely breakdown voltages than similarly rated old caps.

4) Any claims that "Orange Drops" or "Vitamin Q" capacitors in radios sound 
better, or even different, are probably pure imagination.  It *may* be that 
in high end audio systems, such caps do in fact sound different, although I 
have never had a chance to hear any such differences. The SP-600 is NOT a 
high fidelity thing in this sense.  (I have read that using polystyrene 
caps in FM demodulator circuits, and particularly the stereo systems, does 
improve the sound, but I have not verified that.)

5) Claims that metal film resistors sound different than the old carbon 
composition ones in radios are also probably pure imagination. Again, in 
high end hi fi applications, they may in fact sound different in some 
spots.  The high end audio folks say they do.  I've never been in a 
position to hear the difference.

6) The use of resistors in impedance-sensitive RF attenuators is a special 
case, and for attenuators operating at mid-HF frequencies and up, the 
resistors must be of the right sort to perform properly.

7) Claims that metal film resistors, because they are made with spiral 
paths of resistive material on ceramic bodies, exhibit inductance that can 
cause trouble are almost always baloney.  True, such inductance can be both 
calculated and measured. But, almost no resistor in any of our radios 
actually operates with significant RF currents.  ("Stopper" resistors in 
grids of modulator or high fi power output tubes, and parasitic suppressor 
networks in the plates and grids of RF transmitting tubes are the only 
exceptions I can think of.  Even there, the resistance values used are 
unlikely to cause any trouble even if they do have a bit of inductance.)

8) About electrolytics:

  - "Bringing a radio up slowly on a variac" to reform the electrolytics is 
a serious invitation for trouble. DON'T DO IT!

  - Most old electrolytics, even if they are reformed successfully, or are 
already operating well, may degrade or fail soon. Replace them all while 
you are at it.  Modern axial lead electrolytics are cheap, easy to get, and 
will last a lot longer than did the old caps you are replacing.

  - Use higher values of capacitance than the original if it's 
convenient.  Input capacitance in B+ filters in some circuits should not be 
increased a lot.  The danger is that the instantaneous peak current in the 
rectifier will be higher than is wise or needed.

  - Can electrolytics are hard to find and expensive if new (See AES for 
examples of newly made can capacitors.)  Rebuilding them with new axial 
lead caps inside the old can is relatively easy to do yourself and the cost 
is low. In the SP-600, the B+ filter caps are below the chassis, so keeping 
the can for looks is not needed.  Install some terminal strips, wire up new 
axial lead caps, and listen to your radio for decades with  no further trouble.

9) On line voltages and inrush current suppression:

  - Many variacs can apply ONE HUNDRED FIFTY FIVE volts to your radio that 
was designed to run on 125 volts max. This can happen from a mere laps of 
attention in combination with the variac being set up for overvoltage and 
the common modern line voltages of 120 to 130 volts.  Be VERY careful if 
you insist on using a variac to control line voltages.  In my opinion, 
"bringing it up slowly on a variac" is an invitation for disaster.

  - The SP-600 power transformer has a tap for 125 volts. USE IT! This will 
lower the B+ and filament voltages through out the radio and make it last 
longer. Tubes will last longer. The radio will work just as well. Use line 
voltage bucking transformers with other radios if you are concerned about 
the high modern line voltages.
See  http://www.r-390a.net/ for details, specifically:
http://www.r-390a.net/faq-HiVolt.htm

  - Inrush Surge Suppression is a good idea. For info see:
http://www.ametherm.com/Inrush_Current/welcome.html for a manufacturers info
and: http://www.r-390a.net/Pearls/inrush-current-FAQ.pdf for a collection 
of emails on the topic,
and: http://www.skirrow.org/Boatanchors/TechTalk3.pdf for Jan Skirrow's 
article on how to do it.

10)  The SP-600 Choke Leakage problem: An article in The Hollow State News 
some years ago recommended a dangerous fix for a leaking filter choke.  The 
tar in the potted choke apparently degrades in some units and causes an 
electrical leakage path from the B+ to chassis. The suggested fix was to 
mount the choke on insulating standoffs and happily run the radio that way. 
DO NOT DO IT. This presents an undetectable, unexpected shock hazard.  Find 
a good replacement choke, or put in a substitute choke of a different sort 
that does not leak.  Do not install a dangerous, unexpected situation for 
yourself or some other person to find later on. By the way, this leakage 
puts a load on the 5R4 rectifier and power transformer that may cause 
damage before you find it.  It is hard to detect, unless the choke is 
getting hot.  You can't find it unless you disconnect the B+ load, and 
insert a current measuring meter or resistor in the 5R4 cathode circuit or 
high voltage center tap.  The easiest way may be to lift the choke case 
away from the chassis and carefully insulate it temporarily.. Then  look 
for voltage from the choke case to ground.


I hope this gives you some food for thought.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --