516F-2 Service Bulletin 1 Instructions

You should be familiar with the 516F-2 power supply and competent to perform the modifications. If you have questions please send email to barry.w0iy@gmail.com Unassembled kits may be returned for a refund (no refund on shipping charges).

We request your feed back on the kit and the instructions.

1. Options

Before you begin, you need to make some choices:

• Continue to use the vacuum tube rectifiers OR
• Use solid state rectifiers on the PCB and put filament windings in Buck with T1 primary
• Continue to use C1 to tune the L1 choke or use a new C1 on the PCB
• Use the relay on the PCB to switch the high current to the transformer primary

Rectifiers

Either type of rectifier is acceptable, it just makes a minor difference in wiring the chassis to the PCB. The instructions provided are for the solid state diodes on the PCB. The original schematic and the modified schematic should be consulted before proceeding. All connection terminals on the PCB are clearly labelled to facilitate partial modifications.

Regardless of which rectifiers you use for HV and LV, you should remove the old bias selenium rectifier with the diode. When the selenium dies they smell horrible and smoke is likely a carcinogen.

C1

There have been instances of shorted C1 (original component). This changes the power supply topology to a capacitor input type and raises the output voltage. This condition may not be obvious. Measure the HV (final plate voltage) and see if it is excessive. This may be caused by shorted C1 (original coaxial cap).

After unplugging the power supply, wait a few minutes, then short the HV and LV to ground. C1 can be disconnected and tested for a short circuit. If the original C1 is reused, remove the alternate C1 from the PCB.

Relay

The relay is the only significant modification in SB-1. The On/Off switch in the radio is NO LONGER AVAILABLE (unobtainium). Connecting the relay will prolong the life of the existing switch, which now only carries current to the relay coil. This requires minor rewiring under the PS chassis and is fully described in the mod. Refer to the original and post-mod schematics to understand the change.

Update 2015-12

The relay used when the PCB was designed had an armature and a normally open contact. The distributor dropped this model and it went to a 2500 piece minimum quantity. A similar relay was selected which had Form-C contacts. This requires cutting the pin off the relay for the unused Normally Closed contact. If the relay has an extra contact, cut off the unused pin.

Link to Schematics: Original PS, Sb-1 PCB, 516F-2 with Sb-1 Installed Write to me if you have any difficulty with the download and I will email the pages.

2. PCB Assembly

Consult the packing slip included with the kit. Verify all components are included.

1. Install the diodes noting the polarity shown on the PCB silkscreen. If you are retaining the tube rectifiers, skip this step.
2. Install the resistors.
3. Solder the diodes and resistors.
4. Wearing eye protection, Clip the excess leads.
5. Install the caps noting the polarity on the PCB and solder them into place.
6. Wearing eye protection, Clip the excess leads.
7. NOTE - although there are several caps of the same physical size, check the values and voltage ratings before installation.
8. Install the relay and solder.
9. Wipe the PCB and large caps with an alcohol swap.
10. Using a small amount of RTV or hot melt glue to secure the bodies of the caps to the PCB.

3. Chassis Preparation

The following steps remove the unused components from the chassis. Determine which apply if you are NOT installing the full modification. Refer to the original schematic to identify component reference designators. The TB designators are not original, they are used as an aid in performing the chassis wiring.

1. Unplug the power supply from the AC outlet.
2. Remove the tubes from the sockets.
3. Cut off the AC line cord at the outside of the chassis.
4. Remove line cord clamp by squeezing it with a pliers and gently pulling it out.
5. Note: Inside the chassis, leave a stub of the black, white and green wire ends in place until new cord is installed.
6. Many power supplies have very poor condition line cord insulation.
7. On the bottom of the chassis, remove (clip the leads) on all electrolytic caps.
8. Clip R1-3 and discard.
9. Using a small drill, remove the cap mounting clips. Drill the rivet head (do NOT drill all the way through the chassis).
10. Remove the stud mount Bias rectifier and the 100 Ohm resistor.
11. Unsolder and remove R8 and R10. On the PCB, both have been replaced with 5.6K as the new diode is more efficient and one R value needs to be changed.
12. Remove the four bolts mounting the power transformer to the chassis. This will allow enough separation between the transformer core and the chassis to remove the rivets that secured the capacitor clips and insert four flat-head 4-40 screws in the holes. The flat-heads will be below the transformer core.
13. Inside the chassis install a lock washer and 4-40 hex post on each screw. Tighten the posts.
14. Note: you can use a 3/8" wide strip of cardboard (cut up a cereal box) with a small hole in one end. Place the flat head screw in the hole and use the cardboard to position the screw. When captured by the threaded post, just pull the cardboard strip out.
15. Re-install the transformer mounting bolts and tighten.
16. Mount the PCB to the Hex posts using 1/4-in. 4-40 pan head screw with a flat washer atop the PCB.
17. Install the new line cord and retaining clip.
18. The line cord black wire should go to the fuse holder.
19. Wrap the exposed terminals of the fuse holder with electrical tape or heat shrinkable tubing.

The following photo contains reference designators for various terminals which are not identified in the schematic. These references are used to aid in the wiring list provided below.

All AC wiring, with the exception of the relay connections, should be complete before proceeding with the PCB installation.

There is an issue with the line cord clamp. The clamp comes in 2 sizes: too big and too small. The clamp which fits the cable is does not fit the hole in the chassis and the clamp which fits the hole is a bit to large for the cable. The solution is to wrap a few turns of electrical tape around the cord and then install the clamp. Heat shrink (maybe 2 or 3 levels) also works.

Effective Feb 1, 2015 the kit includes 2 pieces of heat shrink which should be cut to about 0.75" long and placed over the line cord where the clamp is located. These should be heated and shrunk. Do the smaller first, then the larger over the top of the first.