[Hallicrafters] Hallicrafter S-77

[Glen Zook]

It is possible to replace a 2-wire line cord with a 3-wire line cord in metal cased AC/DC receivers.  Also, a fuse should be added to comply with NFPA NEC (National Electrical Code).  A 3-wire cord should help with the hum problem and, much more important, get the metal cabinet at true ground potential.

As indicated on the schematic, there is a "ground buss" which keeps the AC power basically from the chassis.  The only connection between the AC line cord and the actual chassis is through a 270K resistor and a 0.05 mfd capacitor.

The AC line connections need to be modified when the 3-wire cord is added.  Connect the 3-wire cord as follows:

Black wire:  Through an added fuse to the off / on switch.

White wire:  Directly to the ground buss in the receiver.

Green wire:  Directly to the chassis.

For additional safety, I would replace the 270K (R-37) resistor and the capacitor in parrallel 0.05 at 600-volts (C-59) with a 0.047 at 600-volts, or better.

If the manual on the S-77 is needed, one can be downloaded from BAMA at

http://bama.edebris.com/manuals/hallicra/s77/


The manual is in DJVU format.  If a DJVU reader is needed, do a "Google" on "WinDjView" and that will take you to the reader, for free, which is, in my opinion, the best DJVU reader around.

Glen, K9STH


On Monday, August 4, 2014 3:53 PM, radiomanL <radiomanL at verizon.net> wrote:
 


A low level hum is a normal condition for line operated  receivers . it generally is not audible over 12" from the speaker unless there is a problem in the power supply filtering or a HK short or leakage in the final audio tube . The hum level is higher that what you hear in a transformer operated receiver due to the use of a half wave rectifier . a larger electrolytic at the point where the B+ connects to the 25L6 plate may help but be careful not to exceed the maximum current rating of the rectifier .  in a quiet room with no other background noise it may be audible . reversing the line cord may help .  BE AWARE that some , not all line operated receivers use the chassis as B - . if one that does is connected the wrong way there will 125 vac on the chassis and if the chassis to case insulation is not good it will appear on the case if  metal  . a small 300 watt isolation transformer is a good safety investment . 

andrew
kd5pnt
______________________________________________________________
Hallicrafters mailing list
Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Hallicrafters at mailman.qth.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **


This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html