Viking II selenium rectifiers

[Edward Cain]

   I have a couple questions concerning these rectifiers that someone here can probably answer.

1) Why is RCT70, the selenium stack, so replete with cooling fins when it's only rated for 300 mA?

2) Can RCT70, or RCT71 and 72, the 35 mA rectifiers, be tested with the diode test function of a multimeter?

   I ask the last question because my meter shows overload when I test these rectifiers.

Thanks,
Ed

[WQ9E]

Ed,

The forward voltage drop of a selenium rectifier is pretty large compared to a solid state unit so even at 300 mils the selenium stack is generating quite a bit of heat.  Furthermore, the voltage drop tends to change with heat so it was desirable to keep the stack as close to ambient as possible.

For the same reason (voltage drop), I don't believe the diode test function of a modern meter is suitable for selenium rectifier testing.

I have some of the old magazine ads that touted the selenium rectifiers as being a lifetime replacement for tube type rectifiers and then a few years later it turned out the lifetime wasn't as long as predicted. 

If the Viking II is going to be used regularly as opposed to being a "shelf queen" I would advise replacing the selenium rectifiers with modern silicon and adding a small dropping resistor as needed.  I assume that some of the 35 mil seleniums are used for the bias supply and if so it is especially important that they be replaced with something more reliable to avoid collateral damage.  Of course at the same time you should replace the bias filter caps.

The Viking I and II are probably the most rugged and reliable transmitters made by Johnson.  I have a couple of Viking 1's and a Viking 2 and all three get regular use without problems but I have converted all of the tube rectifiers to solid state and added inrush current limiting.  I assume the use of selenium in the CD model was to improve reliability over the high vacuum rectifiers.

Rodger WQ9Eb

[KM1H]

My CDC still has the original 5V4 Johnson labeled tube with a 1961 date code. And the 6AL5 bias rectifier. Maybe they went back to tubes as that is a pretty late production date.

Carl
KM1H

[Edward Cain]

   Rodger, your explanation for the fins makes sense. I had suspected a larger voltage drop as being the reason I couldn't use the diode function of my multimeter but didn't think it could be responsible for all that cooling.
   The large selenium is in the secondary of the modulation xformer and the two smaller seleniums are connected to the two sections of the 6AL5 and feed the microphone jack. I should understand these circuits by looking at them, but I don't.

   Carl, my V II CD also has the same rectifier tube lineup as the regular V II. The selenium rectifiers are in circuits peculiar to the CD as I mentioned above.

Thanks for the comments.

Ed

[WQ9E]

Ed,

OK, I forgot about the modulation limiting circuit in the CDC version.  You wouldn't want the local CD volunteer to blow the mod transformer when he screamed into the mic, "The Russians are coming".

I don't have a schematic of the CDC but since all of the power supply is the same as a regular Viking 2 then I wouldn't worry too much about the selenium units as long as the transmitter modulates OK.  Since the exact characteristics of selenium rectifiers would be difficult to simulate with modern silicon my advice would be to leave these units as they are and don't necessarily rely upon them to be your modulation protection; adjust the mic gain as you would for a standard AM rig.  Assuming the selenium units don't really come into play until you approach clipping, they shouldn't be a problem.  In my experience, the normal failure mode for selenium rectifiers has been increased forward voltage drop.  Again, I don't have the schematic but guessing at what they have done an increased forward voltage drop means that the limiter may not limit as it should but shouldn't affect the circuit with normal signal levels.

If you have a schematic and can scan it, it would be nice to have a copy on BAMA.

I am not sure what the age of your Viking 2 is but fairly early Johnson modified the instructions for grid drive level to protect the 6146 finals and warned users never to exceed 8 mils of grid drive.  Make sure that you follow this warning closely and even 8 mils is pushing it and is probably more than needed.  It is easy to damage the fairly fragile control grid in a 6146 with excessive drive.  Also, make sure the socket and tube bases of your 5R4 series HV rectifiers are clean and dust free.  These tubes are very prone to developing carbon tracks on the socket and the base between pin connections and/or ground if debris is allowed to build up.  I had to replace the sockets in my Viking 1 because a previous owner allowed dust to build up and carbonize.

Rodger WQ9E

[Edward Cain]

Rodger,
   I'll try to upload the entire manual to BAMA later today. Never done this before so I don't know how successful I'll be.
   Looking again at the microphone circuit I mentioned above, I think it is a PTT circuit.

Ed

[WA1GFZ]

My CDC had a stink-onium rectifier in series with the modulation transformer secondary. I deleted it. I replaced all the rectifier tubes with diodes and wired the spare heater windings in series with the LV transformer primary so it runs nice and cool now and the 6146 heater voltage is 6.3 rather than 6.8 volts.
I yanked out the 6AL5  and mounted a fuse socket  so I could fuse the LV transformer. The LV transformer was pretty hot after an hour of on time before the change.