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Viking Valiant - No grid current




 
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Author Topic: Viking Valiant - No grid current  (Read 736 times)
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WZ8J
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« on: September 12, 2017, 01:56:11 PM »

I've got a Viking Valiant - (later kit version) Can't get any grid current on the meter.
I have done checks on the meter and I think its okay. The meter will display to half scale on the oscillator position and quarter scale on the buffer position, but no deflection on grid, nor on plate position.
I can hear the signal in a receiver using either crystals or the VFO.
I can adjust the signal strength in the receiver by tuning the exciter, course and fine coupling and final tuning. Same behavior with a crystal and VFO on either 40M or 80M and on other bands using VFO.
Checked the drive pot and it is operating correctly.
Resoldered suspect looking cold solder joints on the mode and oscillator switches and other points under the chassis.
Checked to make sure the square aluminum cam actuated by the band switch shaft with the notches in the proper position for the band
I have substituted the 6CL6 (Crystal Osc. Buffer) and the 5763 (multiplier) with good tubes (thanks WB8UHZ!).
I subbed the finals for known good 6146B's
I replaced the Chernobyl resistor in the VFO with an 18K 10 Watt.
Recapped the electrolytics.
Someone added a phono jack on the front panel for a mic.
Some of the resistors appear to have been changed using modern components.

So can someone give me a roadmap on where to go next? Would love to get this old gal on the air. She looks very clean inside and out with very little dust and no corrosion.
I have schematics and the manual.

I have VOM's a VTVM and a scope. I know how to work safely around high voltage.
Any advice is welcome. I have not repaired much equipment but did repair a TS 520 and rebuilt an SB-220, so that is the limit of my experience.

Thanks folks!





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N1BCG
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« Reply #1 on: September 12, 2017, 08:04:35 PM »

Try pulling out all three 6146 tubes and see if you get RF on the grid circuit. The concern is damaging these tubes with too much drive if the metering circuit is faulty. It's also possible that the driver tank circuit isn't tuning correctly although you mentioned the signal on a receiver which suggests it is functioning correctly.

Have you checked the RF bias setting? There should be approximately -70 volts on the grids of the 6146 tubes.

None of these tests require the transmitter to be keyed. Lots of stuff to try...
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N3GTE
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« Reply #2 on: September 12, 2017, 08:13:23 PM »

Have a look at the metering switch. Make sure the contracts are clean and tight.

Terry N3GTE
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WZ8J
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« Reply #3 on: September 12, 2017, 08:42:40 PM »

Thanks Clark,
Looks like it's only about -50V on the grids.
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N1BCG
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« Reply #4 on: September 12, 2017, 08:48:57 PM »

The RF bias pot (lower one on right side of chassis) should have enough range to give you -70 volts. If not, check the bias supply for proper negative voltages. That supply affects several things so it has to be functioning properly. Either a weak rectifier tube or bad filter caps can cause low bias voltage.

I also second Terry's suggestion. A great place to start.
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WZ8J
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« Reply #5 on: September 12, 2017, 09:12:32 PM »

I adjusted the bias and now have -70V on the amp grids.
Still no effect on the grid current.
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WZ8J
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« Reply #6 on: September 12, 2017, 09:15:23 PM »

Those tabs on the meter switch contacts look a bit fragile. I am lothe to break them. I sprayed em with some tuner cleaner.
Still no joy.  Tongue
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N1BCG
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« Reply #7 on: September 12, 2017, 09:29:10 PM »

Check the continuity from the meter to the grid circuit. When set to "Grid" each lead of the meter is connected to R57 (5.1 Ohms). You could also connect a VOM to that resistor, and when set to a low voltage range, see if you get a reading.

Btw, switching the Oscillator switch to Zero turns on the VFO and all stages up to the 6146 grid circuit for testing safely.
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WZ8J
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« Reply #8 on: September 13, 2017, 07:30:44 AM »

Okay Clark,
I checked and have continuity from R57 to the meter. The meter on the rig deflects when the VOM is connected across that resistor.
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N1BCG
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« Reply #9 on: September 13, 2017, 10:09:15 AM »

The meter on the rig deflects when the VOM is connected across that resistor.

Hmm.

I made an "RF diode" for testing circuits like this. It's basically a diode detector with a 0.01 cap to filter out everything but the resulting D.C. Measuring the RF voltage at the grid terminal on one of the 6146 sockets should show a pretty substantial voltage that can be peaked with the "Exciter" control.

Analog VOMs set to A.C. can usually do this. The other lead doesn't need to be connected. This won't yield an accurate measurement of P-to-P voltage but it will show relative signal.

The main concern about no grid current is that, if it's true, then the 6146s will conduct way too much and could be damaged. The plate current meter would pin. If that's not happening then the clamper tube is doing its job by sensing the lack of drive and pulling the screen supply down to a safe level of plate dissipation.

Try the VOM test. If the RF level isn't substantial then you might have a bad coupling caps (C97, C98). These are two red mica caps in parallel. The RF level should be the same on both sides of these caps.

Are you sure the 5763 tube is good? Ah! And one more thing... check that the drive control is actually providing 0 - 300V on the center (wiper) lug (be careful!). This is a wirewound pot that often wears out.
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WZ8J
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« Reply #10 on: September 18, 2017, 03:37:27 PM »

Okay Clark,
Again, thanks for the direction. I did verify that the DRIVE pot can be adjusted from 0 to 300V.
I tried the analog VOM on AC at the mica coupling caps. Very low reading on the grid side of the amp a slightly higher reading on the coil side. Changing the EXCITER level did not have a noticeable effect on the RF level measured with the VOM.
So, if these caps need replacing, can I substitute a single 50 mfd for the two 25 mfd caps in parallel? Could I use an electrolytic?

Finally - I have no way to check the 5763, I did get a replacement and swapped it, but no impact on the grid drive.
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N1BCG
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« Reply #11 on: September 18, 2017, 04:09:01 PM »

Changing the EXCITER level did not have a noticeable effect on the RF level measured with the VOM.

There's the first clue. The exciter control is actually the tuning control for the 5763 plate / 6146 grid circuit so you should see the voltage peak at resonance. The "Drive" control varies the output of the 5763. Where was that set? Most of the time they're around the 10-11 o'clock area.
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WZ8J
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« Reply #12 on: September 18, 2017, 04:42:45 PM »

I had the drive at about 3-5 (9-11 o'clock). But adjusting it does not move the meter noticeably.

Also the mica caps are marked 25 MMF not MF as I mentioned in the previous post,
so I guess we are talking pico farads rather than micro farads so never mind about the electrolytic substitution question.  Roll Eyes
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N1BCG
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« Reply #13 on: September 18, 2017, 05:00:42 PM »

Try this on 160, 40, 10 with the VFO and using a crystal (be sure it's in the right pair of connections on the 8 pin socket and that the Oscillator switch is set correctly. This will allow you to see if any of the circuit combinations produce drive.

Also be sure that the Mode switch is set to "AM".

The meter switch will show the cathode current for the 5763 when set to "Buff" and reads "13" (bottom scale X2) on my Valiant. The Drive control will have a large effect on that reading. For comparison, my Osc reading is "24" (bottom scale X2).
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WZ8J
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« Reply #14 on: September 18, 2017, 08:06:05 PM »

Okay,
Let's see how many pages we can extend this thread to!

So I set the mode switch to AM
I only have two Crystals one for 40 and 1 for 75 but I put one in the socket anyway. Fortunately a previous owner dabbed some green ink on the correct pins.
OSC to "zero"
and here are the meter readings (bottom scale X2) for the bands:
160 Buffer 4, Oscillator 20
80  Buffer 6, Oscillator 20
40 Buffer 8, Oscillator 15
20 Buffer 6, Oscillator 15
10 Buffer 5, Oscillator 16

So inspector who do you think is the likely culprit? Are we getting closer to solving the mystery?
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N1BCG
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« Reply #15 on: September 18, 2017, 08:22:49 PM »

I swear, Valiants are responsible for most of my gray hair...

Does the Drive control have any affect on the Buffer readings?
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WZ8J
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« Reply #16 on: September 18, 2017, 08:25:28 PM »

Oh yes, it adjusts it quite a lot in fact.
I should have mentioned that all of the buffer and oscillator readings were done with the drive all the way down.
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N1BCG
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« Reply #17 on: September 18, 2017, 08:35:38 PM »

OK, well you're going to need some amount of drive so what you had around 9-11 o'clock is reasonable for testing.

With that set and the VOM on AC and connected to one of those mica cap leads, adjusting the Exciter (tuning) knob should result in a peak in VOM reading.

If not, then I suspect a bad connection on the wafer switch which selects the proper tap on the driver tank coil (it's the vertical inductor behind the VFO cabinet).
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N1BCG
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« Reply #18 on: September 18, 2017, 08:59:04 PM »

Do you measure high voltage on the driver tuning cap (C7)? And, because I have to ask, is the shaft slipping or does the knob turn the cap fully?
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WZ8J
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« Reply #19 on: September 18, 2017, 09:10:01 PM »

Yes of course the shaft slips. I seem to have to really tighten the set screws, but if I snug em up good and only trn the knob clockwise, it doesn't slip.
I haven't tried to measure anything on that air variable cap. What voltage should I find there?
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N1BCG
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« Reply #20 on: September 18, 2017, 09:16:35 PM »

By slipping I meant that the knob is actually turning the cap. It just seems like you're not able to get an RF peak on the meter which suggests that the tank circuit isn't getting tuned.

That cap will have 300 volts on it, full 5763 plate E!

We're basically testing the tank circuit connections...
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WZ8J
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« Reply #21 on: September 18, 2017, 09:17:47 PM »

Okay Clark,
I show 356 volts across C7 in AM on "ZERO" position and 377 volts on AM on "VFO" position.

I noticed that issue too, that cap is kinda stiff to turn, but yes, it is opening and meshing the plates.
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N1BCG
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« Reply #22 on: September 18, 2017, 09:24:14 PM »

That's about right for voltage. I should have mentioned that the Oscillator switch should be set to "Zero" in order to get proper RF at the mica caps. 
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WZ8J
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« Reply #23 on: September 18, 2017, 10:03:56 PM »

Yes thanks, I did that in fact tried all the positions for the RF reading at the mica caps but no change. I wonder if I should try it with the oscilloscope?
You think the micas might be the suspects? - I think I saw them on the freeway in a white bronco.
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N1BCG
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« Reply #24 on: September 18, 2017, 10:13:41 PM »

Does the "Buff" reading change on the Valiant meter as you tune the Exciter control? It's just as if the driver tank isn't getting to resonance.

Are the wafer switch contacts clean for the bandswitch switch under the aluminum shield?  It should have several wires of different colors connected to it from the driver tank coil.
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