solidstate

Modifying the R390A Power Supply to Solid-State

Modifying the R390A power supply to solid state is a controversial subject and one that has been thoroughly discussed on the R390A Reflector. There are legitimate pros and cons to be considered before performing the procedure. At the bottom of this page, I have included a discussion on managing the additional voltage resulting from the modification.

Pros:

Eliminate the 2 hard to find 26Z5W rectifier tubes
Significantly reduce the overall heat production of the receiver
Virtually eliminate a rectifier based failure in the power supply

Cons:

Increased the B+ voltage will additionally stress some aging coils and capacitors
When turned on, full B+ will be applied to the receiver before the tubes can warm up and begin putting a load on it. This will cause the B+ to even higher until the cathodes warm and tube conduction begins.

Solid stating the power supply is a listed government Field Change. The original authorizing document can be found on this WWW site by going to the Technical section, R390A Field Changes. It is item #6.

Here is the procedure I have developed. It is easy, reliable and easily reversable to 100% original.
The R390A power supply is removeable and located on the underside of the receiver.

Unplug the receiver from power before attempting this procedure ! You will need:

    (2) 3A, 1KV diodes
    Solder
    A 40W soldering iron

Begin the removal process by unplugging the single connector then loosening the 6 green headed, captive screws that hold the power supply into the main chassis. There are 3 in the front which are easily seen but the 3 rear must be acessed through 3 holes in the tube deck.

Lift the power supply straight up and out to a clear work space. Be careful, it is fairly heavy!

Making sure the tubes and shields are cool, remove the tube shields from V801 and V802, the 26Z5W rectifier tubes. In case you want to return the receiver to original, store these safely.

Turn the power supply upside down and note the bottoms of the tube sockets. It is here the diodes will be installed.

Bend the leads of the diodes into a "U." Review the installation location below. You may want to trim 1/4" or so off the leads so the diodes will clear the bottom of the tube socket. Do not remove so much that either of the diodes are "nested" in the socket pins. I like to have mine off the tube base so that they are in plain view for easy identification and removal.

Install each, one per tube socket so the cathode (banded end) is soldered to pin 3 of the tube socket and the anode, to pin 1. Do NOT reverse the polarity of the diodes; review your work before re-installing the power supply.

Reinstall the power supply, secure the 6 green headed captive screws and plug the wiring harness connector back into the power supply.

Check the value of the main fuse. Verify that it is an AGC 3A fast blow fuse and change if necessary. For newer chassis so equipped, It also would hurt to also verify the values of the other 2 fuses, 1/8A and 1/4A respectively.

This completes the procedure. First make sure the receiver has a proper ground then plug it in and power the R390A up. After warm up, it should work as before. Should it not, review you work and check for an open diode.

Should you ever wish to return the receiver back to original. Simply remove the diodes and re-install the tubes and shields. I strongly suggest that if your receiver does NOT have IREC heat dissipating tube shields installed on the rectifier tubes, do so. These tubes run quite hot and are hard to find. Their life can be enhanced with the black tube shields. Click here for additional information concerning IREC Tube Shields.

Voltage Management

The B+ voltages will be higher after making this change. This is further compounded by current higher A/C service voltages. As noted earlier, this will certainly put additional stress on aging components and can result in premature failures.

Before performing this mod, be certain that C-553, the .01ufd plate blocking capacitor for V-501 that isolates the mechanical filters from DC has been upgraded to a 600V Orangedrop. Also, C-549, the blocking capacitor for V-507A has been likewise changed to a .01ufd 600V Orangedrop. You will also want to have the 2 electrolytic filter capacitors, C-603 and C-606 rebuilt. Click here for additional information about filter capacitor rebuilding.

There are several methods to manage the additional voltage. I far prefer using a variac. Not only can you reduce the B+ voltage but also the additional filament voltage caused by higher power company supply voltages. Note that the soild state modification will not affect filament voltage.

Connect the receive to a variac pre-set to output 120V. Then remove a convenient tube and insert the leads of an accurate digital VOM set to read AC voltage to the sockets of where the tube filament leads plug in. Turn the receiver on and allow it to warm up for 30 minutes.

Adjust the output of the variac so the VTVM reads 6.30VAC of filament voltage.

Reinstall the tube and use the receiver.

Although a little more involved and not as accurate, a large filament transformer wired configured to "buck" the supply AC will also reduce the input voltage.

Page prepared by R. Charles Rippel

E-mail to: Charles Rippel
Revision: 04 July 6, 2003
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