[Hammarlund] Rebuilding a derelict SP-600 receiver

[Roy Morgan]

> On Sep 12, 2020, at 2:31 PM, Roy Morgan <k1lky68 at gmail.com> wrote:
> 
> NOTE! !!:  THE filter choke may be leaking to ground.  One suggested repair by insulating it from chassis is NOT a good idea.

It can go un noticed that the filter choke has leakage to its case from the winding.  We can assume this is from degradation of the potting material (tar?).  Detecting this can be done in two ways:

1) unbolt the choke from the chassis, elevate it from the chassis and measure with a milliammeter any leakage from the now-isolated choke case to the chassis with normal B+ applied

2) unhook the wires to the rest of the radio (including the filter cap) and from the input to the choke from the rectifier, and measure the current into the choke.  (One of those octal test adapters with a little cotter pin at each pin makes this easier.)

DO NOT follow the “cure” published many years ago to lift the choke case from ground and leave it this way.  You may kill an unwary later owner.


> NOTE2!!  DO NOT “bring it up slowly on a variac”. You risk frying the transformer.  Details later.

Briefly: with old filter caps, the cap MAY reform under your “bringing it up slowly” procedure to a point AND NO FARTHER, at which point it turns into more or less of a short circuit.  Increasing the line voltage more adds dangerous current to the rectifier and transformer high voltage winding.  YOU MAY NOT KNOW THIS TILL IT IS TOO LATE.  And when the rectifier filament gets to conduction heat, the B+ provided may be above what the filter cap(s) can stand.

Solutions to this problem can be:

1) with filaments cold, feed B+ from an *external* variable supply with current monitoring to the filter cap, keep the current AT OR BELOW 5 ma per filter cap section (FIVE)** and increase the voltage slowly.

2) install a solid state rectifier substitute, monitor filer charging current (with an adapter as above, or lift the high voltage winding center tap.  A one-ohm resistor inserted there gets you a millivolt per milliampere, easily read on a digital multimeter.). Trying this with some discarded old filter caps can be very educational.

I have a more complete diatribe on this topic (reform.txt) which I will gladly share with any one interested.  Other diatribes are:

  -powercordsandbypassing.txt  about safe use of line cords and line bypassing,
   and how a fused line cord plug can make a widow out of your wife
  -capleakagetesting.txt  about testing capacitors for leakage
  -linebucking.txt  about reducing line voltages


** the military Mil STD dealing with electrolytic caps tells a procedure based on a possibly WRONG assumption: tu wit that all the caps being reformed in parallel are drawing the SAME reforming current, and none of them gets to a low limit voltage.

Does the SP-600 have a 120 volt tap on the power transformer like the BC-775/SP-400?  If so by all means use it AND do use a three wire grounded line cord (and outlet).

Roy Morgan
K1LKY since 1958
k1lky68 at gmail.com