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Viking Valiant - No grid current




 
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Author Topic: Viking Valiant - No grid current  (Read 3696 times)
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WZ8J
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« Reply #25 on: September 18, 2017, 10:25:00 PM »

No,
The exciter has no effect on the buffer meter reading. Nor does it effect the oscillator meter reading.
I will take the aluminum cover off tomorrow and check out the contacts on the band switch.
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WZ8J
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« Reply #26 on: September 19, 2017, 05:17:41 PM »

I think I figured out the cause of the lack of grid current. There was a capacitor from the control grid (pin 5) of one of the 6146's to the shell (ground) on the 5763. I could not find this cap on the schematic, so I removed it.
Lo and behold, suddenly we have grid current and I can tune it with the exciter.
Well, that joy lasted for a few minutes until I thought I heard a little arc and smelled a rank odor and then a bit of smoke. I fear it may be coming from L-43.
Of course I turned it off immediately.  Undecided
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N1BCG
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« Reply #27 on: September 19, 2017, 08:38:37 PM »

Yikes.

Can you post a pic of the cap you removed and where it was? I'm not clear on what you meant by the shield of the 5763. Also, L43 is the choke in the high voltage power supply. If that smoked then you're drawing tremendous amounts of current.

Lots of things need to be checked before any high voltage is applied...
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DMOD
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« Reply #28 on: September 20, 2017, 12:55:32 PM »

I think I figured out the cause of the lack of grid current. There was a capacitor from the control grid (pin 5) of one of the 6146's to the shell (ground) on the 5763. I could not find this cap on the schematic, so I removed it.

Lo and behold, suddenly we have grid current and I can tune it with the exciter.
Well, that joy lasted for a few minutes until I thought I heard a little arc and smelled a rank odor and then a bit of smoke. I fear it may be coming from L-43.
Of course I turned it off immediately.  Undecided

It doesn't appear your V8 Clamper tube stage is working properly, if the current draw was due to the Finals'.

Were the 6146's checked in a tube Tester? One of more may be faulty.

As Clark said, make sure the LV and the exciter section are working before applying HV.

It is always a good idea to compare the circuitry to the schematics before beginning any mods or upgrades, even before applying any voltages.


Phil - AC0OB
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WZ8J
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« Reply #29 on: September 23, 2017, 07:40:00 PM »

The weird thing is that I have not switched on the high voltage. Only turned the Exciter control on low voltage mode. I did not try to tune up the rig. Never got that far before I smelled whatever is heating up.
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N1BCG
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« Reply #30 on: September 23, 2017, 10:11:54 PM »

If something is heating/burning up then it's time to switch to "safety mode". A simple system involves taking a short three conductor extension cord and intercepting the black (hot) lead. At that point, connect a porcelain lamp socket (like you'd find in a closet) and insert a 150 watt bulb. The Valiant plugs into this cord and the cord plugs into the wall.

Basically, this circuit acts as a current limiter so even a direct short would merely illuminate the bulb fully. A 150 watt bulb will only partially light with the Valiant on (but not keyed) and it makes finding short circuits much easier with less risk to the components.

Of course, all of your voltages will be slightly lower but you'll still be able to get some drive for test purposes. I've found this series bulb circuit invaluable for "first time lightups" of all kinds of gear, particularly in conjunction with a variac.


* IMG_5659.JPG (2022.75 KB, 3024x4032 - viewed 74 times.)
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WZ8J
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« Reply #31 on: September 25, 2017, 03:01:36 PM »

Now there's a station accessory even I can build!
Very clever. I will definite put one of these together.
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KB2WIG
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« Reply #32 on: September 25, 2017, 03:25:03 PM »



Try mine------ it has a switch... much more attractive than that one



klc
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What? Me worry?
WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #33 on: September 27, 2017, 11:29:31 PM »

My Valiant II crapped out.
No grid drive.
Buffer, yes.

Turned out a badly acting 5763 drive tube.
Works fine now...

Might be the problem, might not.

                  _-_-
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WZ8J
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« Reply #34 on: October 01, 2017, 11:25:55 AM »

Again, just want to thank all who provided helpful comments and guidance.
Here's the latest on Val-ain't:
I took someone's suggestion and started checking the wiring vs. the schematic, but then also found the build instructions for the kit on line. This was a great help to confirm my reading of the schematic.

Turns out that whoever installed new HV filter caps hooked them up incorrectly. I fixed that and tried it again, this time with the 3B28's removed. No smells, no smoke and buffer, oscillator and grid currents look good.
I then installed the HV rectifier tubes and tried it again (low voltage only) and still no problems, all seems okay.

So, I guess the next step is see if I can tune up the rig....stay tuned.

Sure learning alot in this process, but that was the intention of getting a fixer-upper.
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N1BCG
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« Reply #35 on: October 01, 2017, 08:33:40 PM »

That sounds great! Incorrectly installed caps in the HV supply would certainly get your attention on transmit. Wondering now if the previous owner made claims of the rig "running great". Hmm.

You might be able to try it out, quickly, first with the meter set to Plate and then again set to Mod. The Drive control should be fully counterclockwise.

If the current goes full scale on Plate, the Clamper tube setting may be wrong.

The current should go to 50-70 Ma on Mod when the modulator bias is properly set.
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WZ8J
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« Reply #36 on: October 02, 2017, 06:43:24 AM »

Hey Clark,
I tried to go through the tune up process after properly installing the HV filter caps, but when I flipped the manual switch to on and the meter to plate there was no reading. A quick turn of the FINAL adjustment yielded nothing. Before I got a chance to adjust the COUPLING, I heard a "braaap" and both fuses in the line cord blew.
Hope the HV transform is okay... Huh

I had thought about going to email with this correspondence, but I am sure someone will reference this thread for leads to solve their own issues as I scoured the net looking for such clues. Or, maybe morbid curiosity will lead them here for entertainment.
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WD5JKO
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WD5JKO


« Reply #37 on: October 02, 2017, 07:42:52 AM »


I have no specific hands on experience here, but generally speaking, installing electrolytic capacitors backwards can only be fixed by replacing them with new ones, and getting the +,- correct. The ones installed before are likely toast. Electrolyte capacitors are NOT self healing.

Jim
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N1BCG
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« Reply #38 on: October 02, 2017, 07:48:22 AM »

Agreed. Check the resistance across those caps and you'll probably find a short circuit. Removing them should raise the resistance to about 20k, the value of the voltage dividervresistors across each cap.
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DMOD
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« Reply #39 on: October 02, 2017, 11:10:59 PM »



I took someone's suggestion and started checking the wiring vs. the schematic, but then also found the build instructions for the kit on line. This was a great help to confirm my reading of the schematic.

Turns out that whoever installed new HV filter caps hooked them up incorrectly. I fixed that and tried it again, this time with the 3B28's removed. No smells, no smoke and buffer, oscillator and grid currents look good.
I then installed the HV rectifier tubes and tried it again (low voltage only) and still no problems, all seems okay.



There should be no HV generated until you close the PTT circuit (KeyDown) or flip the front panel switch.

Let's hope the 3B28 filament to plate resistances saved the HV transformer.


Phil - AC0OB

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WZ8J
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« Reply #40 on: November 04, 2017, 10:58:41 AM »

Started a new job with my company and been traveling extensively so haven't worked on the Val-Ain't lately except that I did pick up some SS HV plug in rectifiers to replace the 3B28's of unknown status. I also installed new 82uf 450V electrolytics to replace the old ones that someone had installed on the incorrect pins of the HV rectifier tubes.

I found the construction manual for the Valiant kit on line. This is a helpful guide.
Did the suggested resistance checks and got the following results:

PIN 3 of XV20 Resistance should be 10.8K, got 56.5K
PIN 1 of XV12 Resistance should be 250K, got 314K
PIN 6 of XV12 Resistance should be 60K, got 109K
PIN 1 of XV14 Resistance should be 60K, got 67K  Smiley
PIN 2 of XV18 Resistance should be 20K, got 40K
PIN 5 of XV21 Resistance should be 12.2K, got 9.7K

Seems okay on LV. No attempts at HV at this stage.
Want to be confident before I throw that switch.
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WZ8J
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« Reply #41 on: November 05, 2017, 11:34:16 AM »

Managed to tune up the Valiant last night  Cheesy
Low output. Only getting about 45 watts out into a dummy load on CW. Probably have soft tubes.
Can only get about 225ma plate current.
Haven't checked the audio stages, don't have a microphone plug for the two prong jack. Will either need to replace that or find a male plug on ebay.
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N1BCG
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« Reply #42 on: November 06, 2017, 11:44:53 AM »

If you're getting sufficient grid current (7.5mA) and the final bias voltage is correct (-70vdc) then it's likely you have one or more slacking 6146 RF tubes. Also be sure they're all lighting. I've heard a case where one of the tubes' heater wasn't lit due to dirty pin connections. One reason or another, someone's not putting out.

Another possibility is one or more shorted loading caps. This would limit how little loading can be set and artificially cause too great a dip. Check that confounded turnstile cap and each of the micas in series between the AUX loading switch and the chassis. Conversely, plate current that can't be dipped low enough is indicative of open loading caps.

A clever way to check the output tuning network is with an antenna analyzer such as what MFJ makes. With the transmitter unplugged and all caps discharged, clip a 2200 Ohm (or close) resistor between the plate cap of one of the RF tubes and the chassis. Connect the analyzer to the antenna connector via a short coax jumper and set it to the center of a band that the transmitter can operate on. With the analyzer set to display resistance and reactance, tune the transmitter as though it were on. You should be able to find a nice 50Z +/- 0j match. If not, there's something rotten in the output circuit. Try this on each band, particularly 160 as that band requires all of the loading caps to be working.

** The output tuning and loading controls are merely an impedance matching circuit from about 2000 Ohms (the three tubes) to 50 Ohms (the antenna). It works both ways!
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WZ8J
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« Reply #43 on: November 07, 2017, 09:09:10 AM »

This was an "aha" for me Clark. Thanks for that last post:
Another possibility is one or more shorted loading caps. This would limit how little loading can be set and artificially cause too great a dip.
I noticed this artificial deep dip and saw that the power on the Bird watt meter went down, so I knew it was not a resonant point.
Also - since I last posted I subbed in a known good 6124B in the final and now showing 178 or so watts out on CW!
I achieved this by tuning for max output power on the watt meter not by the dip in the plate current.
I will have to check those loading caps for shorts as you directed.
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N1BCG
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« Reply #44 on: November 07, 2017, 09:30:21 AM »

--- N I C E ! ---  aka  _.   ..   -.-.   .   !  since you're using the CW mode.

That deep dip shows tank circuit resonance, but little coupling with the output, so a low output power indication is expected. That's where the loading caps come in. The value is gradually reduced to the point where maximum power is transferred to a 50Z load.

Of course, maximum power is also limited by the tube ratings. I use 300mA of plate current for AM and CW. While it's a bit lower than the manual specifies, it's easy to see that the pointer is on "300" and it's gentler on the modulator and finals. The transmitter was designed in the 1950s when line voltages were lower, so lowering the current helps compensate for the higher resulting plate voltages.

You can use a wattmeter for tuning, but be mindful that the plate current doesn't soar. The traditional method of tuning is to lower the drive level somewhat to prevent excessive plate current while dipping and loading (never adjust the AUX switch while keyed up). The drive level is then gradually increased to a max of 7.5 mA of grid current and the dip at the proper plate current reading set with the loading controls.

Sounds like a lot, but ops back then had a pipe in their mouth and a wife to bring them a Rhinegold or somesuch beverage to aid on the process.
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w1vtp
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« Reply #45 on: November 07, 2017, 10:08:04 AM »

If something is heating/burning up then it's time to switch to "safety mode". A simple system involves taking a short three conductor extension cord and intercepting the black (hot) lead. At that point, connect a porcelain lamp socket (like you'd find in a closet) and insert a 150 watt bulb. The Valiant plugs into this cord and the cord plugs into the wall.

Basically, this circuit acts as a current limiter so even a direct short would merely illuminate the bulb fully. A 150 watt bulb will only partially light with the Valiant on (but not keyed) and it makes finding short circuits much easier with less risk to the components.

Of course, all of your voltages will be slightly lower but you'll still be able to get some drive for test purposes. I've found this series bulb circuit invaluable for "first time lightups" of all kinds of gear, particularly in conjunction with a variac.

My series bulb tester / current limiter includes a variac. My first AL-82 power transformer had an internal zorch. I could tell as I brought up the variac when the zorch occurred the bulb would flash brightly but no blown fuses / breaker panel trips.

Should work with the Valiant. Maybe use an even higher wattage bulb.

a


* AL-82 Transformer test (comp).gif (886.92 KB, 1500x994 - viewed 65 times.)
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WZ8J
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« Reply #46 on: November 07, 2017, 06:46:57 PM »

I'm not believing my plate current readings. They appear to be way low, especially since I am getting close to 200W out on the Bird watt meter. Shocked
What is a safe way to check the plate current near the meter with VOM? I don't want to damage the meter on the rig, but I need to see if the meter is reading accurately.
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N1BCG
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« Reply #47 on: November 08, 2017, 05:41:52 PM »

Begin testing at R58. It's 0.202 Ohms made from a short length of resistance wire. That value isn't easy to measure, but you could measure the voltage across the meter contacts when it's set to "Plate". With that setting, the meter is indicating the voltage drop across R58. Incidentally, I believe the panel meter reads 5mA FS.

A lower than expected plate current reading could be caused by dirty meter switch contacts. Does rocking the switch cause the meter to read differently?
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WZ8J
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« Reply #48 on: November 17, 2017, 07:34:27 AM »

Okay,
I knew the output power was suspect. I changed the short coax jumper between my watt meter and the dummy load and found that I had a defective coax. The power readings were distorted because of the reflected power.

With a good jumper in place I am getting 50 watts out on 20M and about 20 watts on 80M.

I tried subbing in some new final tubes I had from my TS520. No real change in output. I think this means one of two things:
either I still have a bad tube in the trio of 6146's amongst my selection, or there is something else limiting the plate current.

Other anomalies:

cannot achieve normal levels of plate current during tune up. The highest plate current reached is tuning up on 160M and getting 250ma but only about 12 watts output.

best output is on 20M at about 55 watts but only at about 120ma IP.

Get very low meter readings on buffer and oscillator meter positions when oscillator switch is on VFO.
 
Get more normal readings on both buffer and oscillator when oscillator switch is set on "zero"

I can only get Ig when I advance the drive pot a little, but I think that is pretty normal.

Rechecked the bias voltage and it is now at 72v. Not sure why this doesn't stay put at 70V or if it is that critical.

I am wondering if the buffer or oscillator tubes are weak, or if there is some component in those circuits not operating correctly.
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N1BCG
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« Reply #49 on: November 17, 2017, 10:07:12 AM »

I use 160M and 40M for tests since there's no frequency multiplication on those bands and the lower frequencies put more of the output network to the test (requiring more caps in the circuit). From your tests, it looks like you're getting about 8% efficiency on 160M and 76% on 20M. Are you getting 7.5mA of grid current on the finals without setting the drive control past 12 o'clock?

This sounds like low 6146 screen voltage / improperly set 6AQ5 clamper circuit. The clamper circuit is a protective system that prevents destructive 6146 current with drive loss. When active, the 6AQ5 tube conducts and drags down the 6146 screen voltage. R13 is used to adjust the threshold (check the manual for details).

The efficiency discrepancy may be caused by faulty caps in the tuning circuit and you're actually tuning the output to a harmonic of the intended frequency.
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