Viking Valiant modulator destruct

[WB5IRI]

Thanks. C93 and C99 are original, and will be replaced. I'll have to order them. Gone are the days when you could just go pick them up.

Doug

[WQ9E]

I use Mouser for just about all of my component purchasing, there is no minimum order or handling fee but for small orders it is cheaper to specify USPS instead of the default UPS.  You can safely use the common standard value of 22uf in place of the original 15uf bias capacitors.  The original parts were not precision components and it isn't unusual to see a tolerance spec for older electrolytics specifying the actual value will be within +80% and -40% of the marked value.

[AB2EZ]

Doug

The bias supply capacitors are: C93 A&B (two halves of a dual 15uF capacitor). You can replace them with two separate capacitors, each rated for at least 350V, and each having 35uF or 50uF of capacitance. Make sure these capacitors are installed with the correct polarity for a negative voltage supply. I.e. for both capacitors, the + terminal is connected to ground.

Note: I am having trouble finding R63 in the schematic of the bias supply of a Johnson Valiant. Is your transmitter a Johnson Valiant or a Johnson Valiant II?

Step 2

Keep the 12AU7 audio driver tube unplugged for now.

Measure the DC voltage between each modulator tube's grid pin and ground. It should be a negative voltage in both cases.

Measure the DC voltage between each modulator tube's screen pin and ground. This should be around 200V (more or less).

Please clarify:

Is the hum that you are hearing coming from the Valiant itself (acoustic noise due to vibration) or are you listening to you transmitter's output RF signal with a local AM receiver, and hearing hum on the output of the receiver?

Stu

[AB2EZ]

Doug

Looking at the schematic for a Viking Valiant II, I can now find R63.

If the capacitors C93 A&B have lost their capacitance, that would explain why the modulator resting current is so high... even with the bias pot wiper at the maximum resistance end.

If the hum you are hearing is acoustic (vibration), perhaps that is coming from the mod transformer or the modulator tubes due to 120Hz on the bias voltage being converted to plate current in both modulator tubes. The two currents would cancel with respect to producing a voltage across the secondary of the mod transformer, but each would produce mechanical vibration, and the audible hums from those vibrations would probably add.

Stu

[WQ9E]

Looking at the schematic for a Viking Valiant II, I can now find R63.
Stu

Stu,

That change occurred later in Valiant 1 production which provided a much easier way of setting the correct resting current for the modulator tubes and the finals (when running in SSB mode) as opposed to setting the screen voltage via bleeder taps.  It makes the modulator bias largely independent of the final drive level which is necessary when using the Valiant with the Johnson 6N2 transmitter and the 6N2 manual contained information for making this required modification to early Valiant production so that they could be used with the 6N2.

Interestingly enough the Valiant II manual (at least the manuals that came with my Valiant II) show an incorrect value for the plate DC blocking capacitor.  Early Valiant production came with a 500 pf cap which was far too small for 160 and was also problematic on the low end of 80; later production used and the manuals indicate a pair of 1,000 or 1,200 pf capacitors here.  Both of my Valiant II transmitters are factory wired and use a pair of 1,200 pf in parallel but the manuals show the original value of 500.  But the accuracy of the manual is still pretty good, at least compared to Hallicrafters

[Jim, W5JO]

Any chance you have the bias caps in backards?  Easy to do.

[AB2EZ]

Rodger

Interesting!

Thanks

Stu

[WB5IRI]

Thanks everyone for the comments. Stu, the hum is not acoustic, I am hearing it on a nearby AM receiver. I will order caps tonight, probably have them by Monday or Tuesday.
I'll repair the VTVM this weekend and do some voltage checks.

Again, thanks everyone for the assistance.

Doug

[AB2EZ]

Doug

I know this sounds silly, but:

Turn off the AM radio, to see if you still heart the hum.

Stu

[WB5IRI]

Yes, there is some mechanical hum to the thing, always has been. What I am hearing through my headphones, though, is LOUD and wasn't there before. No microphone in line for these tests, either, just using the transmit switch in AM mode. In CW mode there is no hum -- well, OK, a very faint hum on the signal in CW mode, very high pitched when zero beat, but that could be because the receiver is within 2 feet of the transmitter and might be getting some front end overload, despite having the receiver antenna connection shorted and 18dB of attenuation in. However, if you have any ideas about how to totally eliminate what mechanical hum IS there, I would love to hear it. Doesn't bother me, I hardly notice it, but if it's a sign of something else about to go kerflooey, I want to know.

Went to EPO in Houston today and bought a couple of 50 uF 450V electrolytics to replace C93 A&B with. Impatient, and can't wait for delivery from JustRadios. I'll put those in this weekend and see what happens. When my parts order arrives, I will recap the entire transmitter.

I also notice a flash from the relay when the rig is keyed. Never saw that before because the relay is under the chassis, but with the thing on its side the flash is visible. Any advice on damping the arc in the relay?

I really appreciate all the advice and encouragement I am getting from you guys.

Doug

[AB2EZ]

Rodger

There is something I noticed in the Valiant II schematic that could be the problem:

The cathodes of the modulator tubes connect to one end of R59: a 0.404 ohm meter shunt resistor; but the other end of that resistor does not connect to ground.

Instead, the other end of the 0.4 ohm meter shunt resistor connects to two things:

A) The center tap of the high voltage power supply transformer (which connects to other tubes' cathodes, but which has not direct path to ground.

B) A different meter shunt, R58, 0.202 ohms, whose other end goes to ground. Since the cathode current from the modulator tubes flows into R58... causing the cathode voltage on the RF output tubes to change... if the modulator tubes' cathode current has hum on it (due to bias supply hum), then that will modulate the RF output.

A test for this would be to temporarily ground the end of the 0.404 ohm meter shunt resistor, R59, that is opposite the end that is connected to the modulator tube cathodes, and to see if that has any effect on the hum coming out of the local receiver.

Stu

[WB5IRI]

Replaced C93 A&B, and that cured the problem. Modulator bias now normal and adjustable, no detectable hum on the signal in the monitor receiver. All I could find at EPO in Houston was 100 uF at 400V, but they seem to be working fine. When my parts order comes in, I will replace those with something closer to the originals, unless you guys think the 100 uF is OK. It's just filter caps, which shouldn't be all that critical.

Lesson learned: when next I see someone has replaced several old caps in a rig, I will not assume the ones left in place are good. I will replace them all.

BTW, Stu, mine is a Valiant I. Thunderstorms in the area tonight, so I won't be putting it on the air until later in the weekend.

Again, thanks all for the expert advice.

Doug

[AB2EZ]

Doug

I'm happy to hear that the new bias supply capacitors fixed the biasing problem and the hum problem.

It's hard to argue with what works, but it is still very puzzling to me as to why the push pull modulator would produce hum on the carrier... and why the hum decreased when you removed one of the modulator tubes.

With a push pull modulator I still would not expect hum on the modulator bias supply to produce high levels of hum on the carrier... and I would expect the removal of one modulator tube to significantly increase the hum on the carrier caused by hum on the modulator bias supply.

It is almost as if both modulator tube plates are connected to the same side of the modulation transformer's primary winding... which would result in only upward or only downward modulation of the carrier (and some carrier level shift).

Anyway... the biasing and hum problems are fixed.

Stu

[WB5IRI]

Me, too, Stu. I can hear no hum at all on AM, but on CW I do still hear a very light amount of hum, as described earlier. I suspect I may have another bad cap. Once my bag of goodies arrives, I will recap the whole rig. We will see if that takes care of the problem.

Again, thanks to you and all the others for help with this. And my hat is off to everyone who right off the bat said replace the bias caps!

Doug

[WQ9E]

Doug,

If you have a crystal check to see if you have CW hum on both the VFO and crystal.   Does it appear in the spotting position also or just during full power transmit?  Overload of your monitor receiver will give misleading results.  Common hum producers in the Valiant (along with the 500 and Ranger) are the keyer and VFO tubes, even very slight heater to cathode leakage will create the problem.  I also experienced hum with my Ranger when a clamper tube developed severe heater to cathode leakage.

[AB2EZ]

Doug

Do me a favor. Take a look at the wiring between the modulator tube plates and the mod transformer's primary winding.

The mod transformer's primary winding probably has three insulated leads ... one red and one black (these are the opposite ends of the winding)....and one insulated lead (the center tap) having a different color (not red and not black).

One modulator tube plate cap should be connected to the red transformer lead, and the other modulator tube plate cap should be the black transformer lead (i.e. opposite ends of the winding).

The center tap (not red and not black) connects to the HV B+ supply via a path the probably includes a switch... but don't bother checking that.

Stu

[WB5IRI]

Rodger,

Yes, in CW the hum is there with a crystal and in VFO spot. I might pull the VFO tube when I work on the VFO -- got a loose or broken coupling to the tuning knob inside the VFO box, and what a pain that's going to be getting to. Got to take the entire front panel off to get to it! Can still tune the rig to a particular frequency, but it's a tedious chore, so once I get on a frequency, I tend to stay there. Fortunately, after half an hour of warm up, the VFO is quite stable and only drifts a few cycles.

Stu, I checked the mod transformer connections, and they seem OK.

Having fun on 7160 today, and people tell me the old gal sounds great. Got so many compliments, I may not do the Timtron mods after all. Using an Astatic DN-HZ with the original element.

Doug

[WQ9E]

Doug,

If the hum is also there using a crystal then the VFO tube is not the problem.  It is quite possible the keyer tube is at fault but any tube in the RF chain, including the clamp tube, is possible.  Not all tube testers are good at indicating minor heater to cathode leakage so substitution is the best test.

The VFO coupler is easily damaged whenever the Valiant is turned upside down, it tries to hold the panel in place and fails.  PM me your email address and I will send you a short document I put together with some Valiant hints and cautions although it is too late to avoid coupler damage.  That uses two different shaft sizes which is specific to Johnson gear so your best bet is to repair the existing one, many people have used the plastic from an old credit card which is perfect for the task. While you are  at that level of repair you should also clean and lubricate the ball reduction drive.

A stock Valiant can sound very good, particularly with a decent dynamic mic.  The typical D-104 will emphasize the  already present bias towards highs in the Valiant.  Normally the clipper control should be set fully clockwise (no clipping) but when conditions are bad a  little clipping can be dialed in to help.

[WB5IRI]

Rodger,

My email is Douglas.rowlett@gmail.com. I look forward to your info. I am pretty sure the coupler damage happened when I was trying to get the cabinet off. Set the rig on its face on carpet, thought it would be safe, and finally managed to get the cabinet to let loose and slide off. Live and learn.

Since it's sounding good now with a quality mic, do you think I should do the Timtron mods anyway? I did substitute a 12TH7 for the 12AU7 audio driver, since the latter had one weak section anyway. Clipper is all the way to the right. I've heard so much about how useless it is that I just haven't used it.

I will recap the rest of the rig, and if that doesn't get rid of the hum I'll start on tube substitutions.

Just ordered Bill Orr's Radio Handbook. Need to get up to speed on theory and practical application!

Doug

[WQ9E]

Doug,

I sent the info to your email address.

There are a lot of opinions on mods for various rigs so the best I can suggest is read about them, thoroughly understand what they are intended to do (and also actually do), and then make your own decision.  My personal preference is I enjoy using vintage rigs as designed so I am not a big fan of modifications.  The only changes I make are those that I feel are needed to protect the operator and the rig.  For example some early rigs were not equipped with a fuse and I will always add one (using an inline holder, NO new holes).  Others have obvious safety issues like my Hallicrafters HT-19 transmitter that brought the 1,500 volt plate supply out to exposed binding posts on the back of the cabinet and I made a protective cover for these posts.  Other areas are a little more gray.  I am not a big fan of mercury vapor rectifiers because if not properly treated they can flash over and take out the plate supply and in general I replace those with solid state plug-in units.  I use inrush current limiters on most rigs to avoid stress to components from high starting surge currents and the use of these often allows you to fuse the rig closer to actual running current which provides better protection when actual failures do occur.  But I don't try to make a ham rig sound like a broadcast transmitter, I have a Gates BC-250 for that , and I accept that vintage receivers like my Hammarlund Comet Pro, early Breting and Patterson receivers, etc. will never perform like my later SRT CR-91 and Telefunken E-1501 military/commercial receivers.  For me a major attraction of vintage gear is experiencing a rig like it performed when it was new while for others the attraction is personalizing a piece of gear to personal tastes.  There are some well thought out mods and some that are truly bad because the modification designer didn't understand or consider the impact of his changes on other aspects/operation of the complete circuit. Ultimately the decision is up to the owner.

[w1vtp]

Doug

I highly recommend that you not type out the "@" in your email when giving it out over the internet.  Instead use a space then (at) and then space so the auto bots can't pick up your email for problems such as spoofing, phishing and all the other attempts to do harm to your internet security.  So an email might be "myname (at) yahoo (dot) com"

Second, how are you determining your hum?  If it's over a receiver it might be as simple a just some sort of ground loop.  Are other folks out there saying you have a hum?

Did you actually get things working again?

Al

[AB2EZ]

Doug
et al.

I think I found the source of the hum on the output RF envelope that was previously being caused by the hum on the bias supply

I realize that I may be the only reader of this thread who cares, at this point.

What was puzzling me, up to now, is how common mode 120Hz current pulses flowing into the primary winding of the modulation transformer could produce modulation of the RF output.

What I finally realize now is that this current is flowing (via the center tap) out of the HV B+ supply.

With an average modulator cathode current of 190mA, the average modulator plate current was about 180mA

This average modulator plate current consisted of pulses of current that flow I20 times per second (the frequency of the hum at the output of the bias supply)

Therefore, each pulse of current, flowing from the HV B+ supply into the center tap of the modulation transformer, had an area of 180mA / 120 pulses per second = 1.5 millicoulombs.

Since Q=CV, each pulse of common mode current, flowing out of the HV B+ supply and into the center tap of the modulation transformer, caused a dip in the HV B+ of 1500V/C, where C is the value of the HV B+ supply's output side filter capacitor, in microfarads.

For example, if C equals 50uF, then each dip in the HV B+ will be 30V. If C=25uF, then each dip will be 60V. In a stock Valiant, the capacitance is nominally 40uF (two 80uF capacitors in series), but it could be much less if the capacitors are old.

This 120Hz modulation of the HV B+ will cause 120Hz modulation of the RF output. The level of modulation will be reduced if one modulator tube is removed (cutting the 120Hz current being drawn by the modulator from the HV B+ supply in half).

Separate note:

The 120 Hz hum on the "fixed" portion of the grid bias voltage on the RF tubes, which was provided by the noisy bias supply, did not produce significant hum on the RF output, because the "self biasing" effect almost completely cancels out the effect of the hum on the "fixed" bias.

Stu

[WB5IRI]

Thanks, Stu.

If I am understanding you correctly, I should up the capacitance value from 40 uF to reduce the AC on the B+. I'll try that and see what happens. Going to recap the entire thing anyway.

Thanks everyone for your responses. Rodger, thanks for the tips on Valiant do's and don'ts. If you have more, please send them on. I'm thinking about whether to leave it stock or not.

Doug

[WQ9E]

Doug,

You can safely increase the value of HV filter caps (C91 and C92) and the B+ output filter cap (C96A) but do not significantly increase the value of the B+ input cap (C96B) because this will increase the voltage output of the supply and it greatly increases the repetitive charging current resulting in increased heating of the rectifier and the power transformer.  Going from 15uf to 22uf is fine but if you increase this one a lot you will create new problems.

In general with choke input power supplies (like the plate/HV supply) you can safely increase filter capacitor value but doing the same with the input capacitor in those using a full pi filter (like the B+ supply) can lead to trouble.  I believe you also increased the filter caps in bias supply and the current here is low enough it shouldn't create trouble but do check your bias voltage at the junction of R63 and R23 and if it is above -300 volts then you should probably reduce the value of input bias filter cap to something closer to stock.