[Johnson] Johnson Viking II repair

Fri Apr 21 07:45:52 EDT 2017

Mitch,

R13 is often damaged by users who don’t use sufficient care when adjusting this resistor.  It is an adjustable tapped resistor which allows setting the screen voltage (and thus idle current) of the modulator tubes.  So it should have a screw tightened clamp with a wire attached that slides on the resistor.  It is critical to release clamp tension and move the clamp carefully to avoid damage to the rather fine resistance wire.

When replacing capacitors the oil filled HV cap should normally be fine unless it is leaking from the seals.  These oil filled capacitors have a very long life so I would not replace it unless it is leaking oil.  The oil is probably “spiked” with PCB given the age so if it is leaking wash your hands carefully after handling.  The filter caps for the B+ and bias supplies along with the 10 uf cathode bypass caps in the audio section should be replaced; be sure to observe proper polarity for the bias supply filter caps which are installed positive to ground since it is a negative voltage output supply.

The 5R4 tubes used as HV rectifiers are very prone to developing carbon paths on the socket and base of the tube if not kept clean so remove any buildup there.

I have restored a couple of Viking II transmitters where there was damage to the mechanism that drives the 160 In/Out switch.  If the rotary switch itself is sticking the drum that drives it will shear off.  Put a drop of oil on the switch bushing and make sure that it turns easily.

Audio driver transformer T3 is a pretty high failure rate item and this is due to corrosion of the very fine wire used in the winding and not from a transmitter fault.  This same issue occurs in many vintage AM transmitters of the era.  Antique electronic supply is one source of a suitable audio driver transformer if the one in your transmitter has failed.

R25 acts as both the B+ bleeder and to adjust drive via adjusting screen voltage of V5.  Although not a high failure item they do fail and high wattage pots of this type are becoming more difficult to find.  Some people modify this circuit by using a horizontal output transistor from a TV set along with a standard low power pot as a substitute for the high power pot.  The basic circuit devised for the DX-100 can be found here and can easily be applied to the Viking II if needed:  http://www.amwindow.org/tech/htm/drivepot.htm

6146A, B, or W suffix along with other special rated versions of the original 6146 will work fine as replacements in the Viking II irrespective of internet lore about later variants not working to replace the original 6146.

Rodger WQ9E

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From: Mitch<mailto:mskobier at charter.net>
Sent: Thursday, April 20, 2017 11:28 PM
To: johnson at mailman.qth.net<mailto:johnson at mailman.qth.net>
Subject: [Johnson] Johnson Viking II repair

All,

                Hi, my name is Mitch KJ7JA, and I have acquired my first
Johnson transmitter that is in need of repair that I plan on using as the
basis for a future AM station. I have a Hammarlund HQ-150 that I will be
using for the receiver. First let me say, I am a noob when it comes to these
old transmitters. I do have some tube gear experience, and am fairly capable
at troubleshooting and repair. I am also very careful when working around
high voltages and have all the necessary HV probes and other necessary test
gear to make this all happen. But I have started down a new road, and joined
this group to hopefully gain some insight and possibly give something back
to the group.

Anyway, here is where I am at. While going over the Viking, I noticed a few
things that are not correct. First off, one of the original filter
capacitors has been removed. Fortunately, they left enough of the original
wire to figure out where to hook the replacements. I have already purchased
a complete cap kit, so that is on the list of things to do.

The big HV resistor (R-13) appears to have been damaged somehow, and a PO
used a piece of metal and a spring as a jumper over the bad section. I have
not removed it yet, but it says on the resistor that it is rated for 35
watts. I did a little math using the plate voltage and the 20K ohm rating
from the manual and came up with approx 20 watts for the amount of power the
resistor dissipates. I have already located a suitable replacement that is
rated at 50 watts. It is physically the same size as the original, so it
should work with no problems. The attached picture shows the damaged R13

There is another resistor that shows some damage, but this one is from heat.
It also does not look like it is original equipment. I will be sorting that
issue out too.

But before I put too much effort into replacing things, I will be checking
out the transformers to ensure they are not burnt out. The fuse in the ac
line is blown. The plan is to remove all the tubes, and using the VTVM,
compare the resistance reading across the various windings to what is listed
in the manual. If that all checks out, I will apply power through a metered
variac and apply voltage and see if anything doesn't look or act right. Then
once it is up to full voltage, I'll check the ac output of the transformers
and again compare them to what is in the manual. I do fully realize that
there is lethal voltages exposed while it is powered up.

Anyway, I plan on taking the circuits back as close to factory as I can.
That way, I know what I am starting with and it should make it a lot easier
to get it back on line. From a cursory look, there does not appears to be
any mods, other that the repaired resistor.

Anyway, thanks in advance for any and all help.

Mitch KJ7JA

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