Wednesday, June 26, 2013

Restoring Scott Stereomaster 340B Receiver

I just picked up Scott 340B receiver locally. It lost 2 control knobs and has broken power switch but inside was virgin and every thing was original. Quick check of inside and out side resulted:
1)lost knobs
2)all the knobs lost caps
3)broken power sliding switch
4)arching on left channel OP tube socket pin 2 (heater) and 3(plate) caused discoloration and smoke stain.
5)All coupling caps are bad
6)electrolytics are bad
7)Caned caps are questionable
I ordered Orange Drops from Mouser. There will be no progress on restoration until parts get here. Out put tubes that I swapped with newly restored Sansui 1000a played beautifully. All tubes are in good condition and there are many Telefunken tubes (all 12ax7s'). I am having hard time to wait for the part to arrive.
1) Cleaning up
  I disassembled as much as I can without de-soldering  or cutting wires. I cleaned cases and face plates with Simply Green solutions and put all control knobs in zip lock bag with Simple Green liquids. I left knobs to soak in Simple Green for over night, and rinsed with water.  I cleaned chassis with Simple Green by wetting the cleaning cloth ( my old shirts) and gently rube larger parts and wider areas. For small areas, I used Simple Green soaked Q-tip swaps to clean. Do not clean letter on Tubes or tuner frequency marks and face plate letters. You can wipe them Clean!!  Because old paint will come off if you use Simple Green on them. As you can guess, I really like Simple Green stuff on electronics. I used them to clean electronic circuit boards. Just make sure, you rinse them thoroughly with water and shake or blottem before air dry. Things that I do not use Simple Green is, Pots, switches, tubes, coils. ( Be careful not to jingle tubes together when you remove them from socket this may damage the tube- I broke 2!!!, use Styrofoam to stick the tube on it)

2) Re Capping
 All electrolytic caps  and paper oil caps need to be replaced. Paper oils are round cylindrical shaped caps. I decide to use Orange Drops from Mouser.com. I decide to stuff the old multisectional caps( this means using same or larger valued individual caps and 100% silicon calk from Homdepot  to replace content of multi sectional cans. You just make sure tie all (-) terminals together and isolate (+) terminals (leads) -- Some of cans might have common (+) terminals, look at the can of original multi sectional cap to verify this!!.
Most electrolytic caps can be substituted by slightly larger capacity caps and higher voltage rating, rare exception to this rule is.. if it is for filter such as tone control, feed back control, coupling, you need to find exact capacity cap to replace.
3) Checking and Feeding power to bring it up slowly
I checked for obvious defective parts, cold solder joints, burnt  wires and sockets, parts  while recapping and cleaning.  I am fairly confident to bring the unit up by using my Variac from low voltage to AC level. Before Variac,  I used light bulbs ( 5w,15w 20w etc) to feed unit with roughly 30v to start then go higher slowly by replacing higher wattage bulbs. Lower voltage takes longer time to glow heater filaments. Once it is working with lower voltage you move on to higher voltage. As you slowly crank up the voltage keep your eyes and ears and nose open for any signs of problem.

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