[Amps] Restoring DX-100

[Glen Zook]

If you are getting almost half of the full legal peak power from a DX-100 on AM then your are grossly exceeding the specifications of the 6146 final amplifier tubes.

Although Heath specified an input carrier power of 160 watts for the DX-100, this is definitely in excess of the RCA specifications for the 6146 or the 6146A/8298.  MAXIMUM input power ICAS to a single 6146 when used for Class "C" AM phone operation is 67.5 watts and therefore a pair should not exceed 135 watts carrier.  Assuming an efficiency of 60% then you should not see more than 81 watts output for the carrier.  Even at 65% efficiency you should not see more than just under 88 watts output for the carrier.  Then assuming the 4 times peak power output at 100% modulation, that means that you should not see more than 324 watts peak output at 60% efficiency or more than 350 watts output at 65% efficiency.

The life of the 6146 will be greatly extended if you drop the maximum input power to no more than 120 watts and even more if you drop the input power to around 100 watts.  This means a carrier output power of 72 watts at 60% efficiency and 78 watts at 65% efficiency at 120 watts input or 60 watts carrier output at 100 watts and 65 watts output at 65% efficiency.

Frankly, the station on the other end won't know the difference at 120 watts input and probably won't know the difference at 100 watts input.  However, the final amplifier tubes definitely will be able to tell and the life of the final tubes will be much longer.

The Johnson Viking II is one of the few transmitters that the manufacturer actually recognized the limitations of the 6146 and that transmitter is spec'd at 135 watts maximum input for AM.

As for "reforming" the electrolytic capacitors:  New capacitors are VERY cheap and I recommend replacing the original capacitors rather than trying to "reform" them.  Sometimes the "reformed" capacitors turn out fine.  However, in the majority of cases if they do "reform" the life expectancy is not that great.  Therefore, to be on the "safe side", I recommend replacing the capacitors.

Also, on the "safety" side, the 2-wire power cord needs to be replaced with a 3-wire cord and a fuse needs to be added in the "hot" (black wire) side of power cord.  If you want to drill a hole in the back chassis then you can even use one of these fuse holders from Radio Shack:

http://www.radioshack.com/product/index.jsp?productId=2062230

If you don't want to drill a hole, then use one of these

http://www.radioshack.com/product/index.jsp?productId=2062257

Of course, the same fuse holders are available from numerous places, I have just given Radio Shack as a potential source.

Glen, K9STH

Website:  http://k9sth.com


--- On Sat, 1/15/11, Ron Youvan <ka4inm at tampabay.rr.com> wrote:

In several months I shall be doing the very same thing. The procedure I have used in the past is to put a VOM on the high Voltage line and watch the 5R4s and the meter as you increase the AC Voltage, (on the 100 or 200 Volt scale) when you see the meter start to move stop and wait until the DC plate voltage Voltage quits raising, like 1/2 an hour. Slow bring the AC up until you get about 30 Volts wait a few hours, you are drying out the transformers also.  Slowly bring the AC up until you get about 60 Volts wait a few hours. Slowly bring the AC up until you get about 100 Volts wait a while. Slowly bring the AC up until you get about 200 Volts wait a while. (100 Volts at a time Etc.)

 You must monitor the inside of the 5R4s for any sign of spark no matter how feeble. That is a sign of way to much electrolytic (S) leakage current.
 
This will be my second time for this unit.
 
I have several comments about this fine unit.

1. The 5R4s are just fine for this transmitter there is no reason short of a fetish to replace them with solid state replacements.

2. The 5R4 sockets should be lowered two nut thicknesses *below the chassis with longer bolts, as the rivets on the sockets tops arc to the chassis.

3. The 6146 sockets should be lowered two nut thicknesses *below the chassis with longer bolts, to allow some air flow up through the bottom of the cabinet to cool the tubes better.  (hard to do)

4. The shield between the two 6146's and the 12BY7 (I think) should be soldered to the chassis at 4-6 places on each side of the shield to provide a good electrical connection.  I used a Weller GT-7. *  1/4" spacers would be perfect

There is an AM net in the Tampa area I want to check into, and the DX-100 will put out almost 1/2 of the maximum power allowed in addition to the fine cw the rig does.