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Viking Valiant - No grid current




 
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Author Topic: Viking Valiant - No grid current  (Read 3609 times)
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WZ8J
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« Reply #50 on: November 17, 2017, 01:15:16 PM »

Again thanks for all the helpful instruction I hope to soon graduate with my degree from Valiante University  Cool

I used my MFJ antenna analyzer to check the tank circuits per your recommendation.
I found that I could not get a decent match on 160M. I did on 80M and 20M. Looks like I need to check those loading caps.

I will also try adjusting the clamper again. Last time I did this I don't recall getting much adjustment.
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N1BCG
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« Reply #51 on: November 17, 2017, 01:29:37 PM »

I will also try adjusting the clamper again. Last time I did this I don't recall getting much adjustment.

That's a clue. Under the setup conditions, the plate current will slam if R13 is adjusted too far in one direction, and it will never be able to rise sufficiently under normal operating conditions if adjusted too far in the other.

The clamper threshold, R13, is set much like a squelch control. You should definitely see the effect during the adjustment process!
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WZ8J
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« Reply #52 on: November 17, 2017, 06:29:55 PM »

Well that was it. I didn't realize the pot for the clamper could be turned more than 360 degrees! Doh!
I got it adjusted to 10 ma and now have plenty of plate current.
Moved on to the operational checks in the manual after clamper is set but did not pass the first check:
Oscillator current should read about 24Ma - I've got 28 ma - close
Buff should read about 6 ma - I'm showing 4 ma
Switch meter to grid and tune exciter to max current with DRIVE in position 3
Adjust grid drive to 3 ma (bottom scale)
Turn meter to PLATE and throw SW8 to "MAN" and tune final for Minimum current - Should get 80ma, I GET 390ma
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N1BCG
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« Reply #53 on: November 17, 2017, 06:58:37 PM »

Well, grid current should be 7.5mA, but if it's low, the plate current should be low also. Double check the clamped setting. It can be tricky to get just right.

R13 turns completely around? It might be damaged.
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WZ8J
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« Reply #54 on: November 17, 2017, 07:30:50 PM »

Yep the Ig is fine 7.5ma
It's the Ip that's high.
I'll check the clamper adjustment again. It calls for 10ma which is a very fine reading on that top scale for plate current.
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N1BCG
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« Reply #55 on: November 17, 2017, 07:34:53 PM »

Also try a higher band. You may not have enough loading capacitance to get a sufficient dip.
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WZ8J
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« Reply #56 on: November 17, 2017, 08:02:44 PM »

Okay,
Looks like it tunes up good to about 125 watts out on CW on 40M!

I'm starting to think the C47 I think multi stage cap is a little knackered.
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N1BCG
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« Reply #57 on: November 17, 2017, 08:20:05 PM »

Ah, nice!! Yep, that "turnstile cap" is notorious for failing. You can replace it with 300, 600, 900, and 1200pF caps (I think). Check my memory...

I expect to hear you soon on 40M!
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WZ8J
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« Reply #58 on: November 22, 2017, 09:13:14 AM »

I was able to load up the finals to the proper plate current on cw on 20m, 40m, and 80m. Cannot get proper loading on 160m which I suspect is a loading cap issue.
I was able to adjust the mod bias to -55, but this was a rather touchy adjustment and it has a tendancy to easily bounce around. Wonder if it wasn't fully warmed up or if there is a component in the circuit breaking down a little,

Either way should be able to get this rig on the air soon. Cool Cool Cool
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WZ8J
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« Reply #59 on: November 22, 2017, 10:45:37 AM »

Spoke too soon...
Disaster strikes. I had a studio mic with a phono jack which I plugged into the front mic jack some previous owner installed and pow- fireworks and some smoke. I fear I may have shorted the mod transformer  Cry
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N1BCG
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« Reply #60 on: November 22, 2017, 12:11:45 PM »

Typical Valiant, unfortunately.

What do you think happened? Did the fireworks happen when you spoke into the mic or when you plugged in the mic?

It's a good time for that series 150W bulb in the power cord...
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WZ8J
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« Reply #61 on: November 22, 2017, 01:18:08 PM »

It happened when I plugged it in. SW8 still on PTT.
I have a variac, won't that accomplish the same thing as the 150W bulb?
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N1BCG
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« Reply #62 on: November 22, 2017, 01:33:06 PM »

Trying to piece this together... so the transmitter was set for "AM", on standby, and just needed to be keyed? Then the mic was plugged in and BANG!  Right?

Is that added jack definitely for a microphone and does it include a contact for keying? If so, what might have happened is the -180 Volts used for keying got into the audio input circuit and that would have sent a thunderous pulse through the modulator. Is the studio mic wired with a PTT circuit? Usually those mics are balanced 600 Ohms where two leads are for audio and the third is grounded. I'd be surprised if it were wired for PTT if it had a 1/4" plug.

Where did the smoke rise from?  I'd trace the leads from that added jack to confirm its purpose. Variacs are very handy, especially if they have a current meter.
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KA2DZT
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« Reply #63 on: November 22, 2017, 01:37:48 PM »

It happened when I plugged it in. SW8 still on PTT.
I have a variac, won't that accomplish the same thing as the 150W bulb?

Not exactly,  the light bulb prevents a heavy short from damaging the xfmrs and other parts.  A Variac doesn't do anything to prevent damage from shorts.

Fred
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WZ8J
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« Reply #64 on: November 22, 2017, 02:35:43 PM »

I think you're on the right track Clark. The studio mic has no ptt.
The jack is wired to the audio pot and to the hot side of SW-8
The smoke looks like it came from arcing at C98 (Electrolytic cap on the 5V4 to ground. C98 came unsoldered from the ground lug.
The leads from T-3 mod transformer are all connected to a terminal strip.
I measured resistance from each lead to each of the others and found them all shorted.
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N1BCG
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« Reply #65 on: November 22, 2017, 02:49:16 PM »

Don't necessarily be concerned about the mod xfrmr resistances. Those windings have relatively low D.C. Resistances. So far, it just sounds like a cap failure.

Got replacements for both? A shorted 5V4 could have done that.

Also, only use a stereo (3 conductor) plug and keep the tip and ring leads well separated. One is for low voltage audio, the other has -180 volts on it for keying when connected to ground/shield.
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WZ8J
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« Reply #66 on: November 22, 2017, 03:35:42 PM »

You mean all might not be lost?
I do have replacements for the C98 dual cap.
I'm going to check the pin out for the 5V4 and check for shorted tube with my VOM
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N1BCG
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« Reply #67 on: November 22, 2017, 03:48:49 PM »

You might be fine. Put your voltmeter on either + lead of that cap and to the chassis to monitor LVB+ as you bring up the Valiant on the variac.
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WZ8J
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« Reply #68 on: November 22, 2017, 05:48:17 PM »

what should the LVb+ be 300V?
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N1BCG
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« Reply #69 on: November 22, 2017, 06:21:29 PM »

300 Volts approximately. Mine shows a bit more because line voltage is higher now than in the 1950s. Anything from 300-340 should be ok.
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WZ8J
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« Reply #70 on: November 22, 2017, 06:34:32 PM »

with the caps replaced, all seems well bringing the voltage up.
I have 125V AC line voltage here. If I turn the variac up all the way, it goes to 367 volts. Is this kinda high?
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N1BCG
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« Reply #71 on: November 22, 2017, 07:49:44 PM »

That's a bit high but not critically. You could always use the variac to re-create 1957 voltage :-)
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WZ8J
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« Reply #72 on: November 22, 2017, 09:04:14 PM »

Ha! yeah. Well what do you think the next step should be. I am a little HV shy but everything on LV is looking normal.
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N1BCG
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« Reply #73 on: November 22, 2017, 10:20:26 PM »

I think the whole AMfone community is on pins and needles over this. Time to take a deep breath and go for it (but keep that mic unplugged). The failure was in the low voltage supply and that's been fixed.
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WZ8J
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« Reply #74 on: November 23, 2017, 09:19:55 AM »

Okay,
I tuned it up on 80m, 40 and 20m.
No smoke or sparks - yeah!
The rig is tuning differently now. Here are my observations:
On 80m, L43 buzzes noticeably on key down
Power is lower than before got about 90 watts out when before I was getting 125
Get very small dips in plate current at advanced loading settings
Loads better on 40m but seems to take more advanced loading to get to full (450ma plate current). Power about 100 watts now.
I am guessing the stress of the "event" stressed out components in the loading circuit that were already weak?
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