The AM Forum

OK

Replaced the modulator tubes with matched pair of 6146 RCAs and their associated 22 ohm plate resistors. Getting good swing at audio gain, position 2 with both D104 and EV638. Audio with EV sounds better. The D104 sounds scratchy.

Need to replace input resistor with minimum 10 Mohm, or higher.

Insulated HV leads where we were experiencing some arcing.

Some hum on the audio, but we're right here in the same room with the cover off the radio.

Probably could put on air, but need to do some other things, like replace chernobyl resistor and calibrate VFO, which is about 80 KHz low on 75m band.

Philip

Next I will check carrier on the Flex bandscope in both CW and AM modes, and may post some screen captures later. I will tell you that the spectral display in AM mode has a lot of "grass" about 50 db down from the carrier peak.

Probably next step is to actually put her on the air and get some reports. I have a dow key relay ready to go with the Hammarlund. The Collins 388 will need a mod to energize the TR relay inside, so the 180 can be put into service without a lot of fuss.

Philip

Much over 4.7M into a 12AX7 could be asking for instability. I settled for the 4.7M in a customers DX-100 and with a D-104 all reports were complimentary and no hum. The customer wanted the audio mods tailored to a D-104 and I wound up using a 12AT7 in order to get the gain pot from 2 to about 6 which smoothed out the response. Those with SDR rigs reported the pattern very clean at about a 8kc BW.

A EV-633 into a 47K right at the mike jack resulted in muffled audio and some hum reports. Most tests were with the TX up on its side and wide open.

There is also a problem with some mikes, especially dynamic,  when the mike and PTT ground is shared.

Carl

No waveforms tonight. Now I'm troubleshooting why it started blowing fuses when tuning up, going from PTT to MAN. Trying to address some intermittents in the ACC plug and VFO jumping as described by Tom Rauch.

On second thought, since I have no plans to use a SSB adapter, I'm going to strip off all the coils and caps on the accessory and hardwire everything. I will keep them, however, if there is a future owner.

While we are at it, is there any reason to keep all the same coils and bypass caps where the power wire enters the chassis?

Philip

May have found a potential source for the fuse blowing I've been experiencing with the Valiant out of the blue.....the nine pin accessory plug socket in the rear of the radio. Are we surprised my fellow old iron afficionados? There are several high voltage spots at the socket in close proximity that route power/audio here and yon. Looks like we may have had a potential short between pins 5 and 6; they were mighty close together and they are not solid....can wiggle a bit. Examining the handy dandy Valiant schematic, it appears there is B+ on pin 6 (600V) to the 6146 finals and pin 5 goes to the primary center tap of modulation transformer T3 (300V). Not good.

Fuses are a good thing. That accessory connector is a time bomb waiting to go off; potentially worse than the undersized power resistor in the VFO compartment.

Philip,

I think you're not the first to have problems with those accessory sockets.  I remember posts about them in the past.  I think many have done away with the caps and coils that are on the pins.  That stuff was there to prevent TVI.  TVI is not much of a problem nowadays.

Fred

They generally don't wiggle unless the Valiant is mounted on a shake table or you're diddling with them. Put an 9 pin plug into the socket; do any of the wired connections touch; yes/no; pull the plug out; do any of the wired connections touch, yes/no; if yes to either answer, separate and/or insulate the wired connections. That accessory socket type wiring is very typical in many transmitters of the 50's, 60's, and 70's.

Yes they wiggle and yes they touch; sorta like the Teletubbies in a bizarre, truly scary fashion!

Seriously though, the whole thing looks like a mess.

Philip

I may have found the problem, and its indeed in the accessory plug. Pin 2 goes to B+. The insulation on this pin was worn down and making contact with chassis ground. I cleaned the pin off and installed new heat shrink tubing as an insulator. There was enough play in this pin to cause issues.

I am going to finish off the work cleaning up the connections around the accessory plug/jack and then throw the "OH S***" switch SW8 and see what happens.

And, if this works, then we can get back to the issue of finding out why my audio has a lot of hum. I even bought some decent oscilloscope probes off Ebay for my Tek 468 to trace the audio chain and see if we have any ripple on the modulator tubes' bias/HV supply.

FEELEEP

Philip,

What does your Valiant show for HV and does it remain steady when modulated?

Nothing at the moment as its blowing fuses when throwing SW8. Didn't get anything accomplished this weekend due to mother's day and honey-do's.

With the transmitter unplugged let it sit for a few minutes and then short the HV line to ground just to make sure the filter caps are discharged.  Then remove the short and measure the resistance to ground from pin 1 or pin 4 of one of the HV rectifier sockets, if you measure significantly less than 40K trace the problem.  If the resistance is OK you should still do a test without the rectifiers in place (make sure the plate caps are well insulated from anything nearby) to see if the fuse still blows.  If it doesn't then the problem is still on the rectified side of the HV, possibly something arcing over.  If the fuse still blows with the rectifiers out of circuit I would temporarily disconnect the primary leads to the plate transformer to make sure the problem isn't there.

If there is something in the rectified/filtered side of the HV circuit causing the fuse to blow instantly you are putting a heavy load on the transformer and it may show its displeasure if this abuse continues.

Makes perfect sense!

You mentioned that the HV PS has been solid stated. I would check out those diodes and or SS plugins that you may have used. That audio feedback event that you've written about in another thread may have taken out the SS devices. Its wise to get a transmiiter like that going first with a minimum of modifications. Have it working in a fairly reliable way as stock as possible and then do some of the mods such as SSing the PS and do it step by step.

Did you add any sort of soft start when you SS'ed the HV PS or the other supplies? Inrush current issues? Check out the bypass caps at the top and bottom of the big RF choke for the final. They can fail/short.

Hopefully the modulation transformer hasn't developed an intermittancy. A replacement mod iron isn't an item that you can order up from AES.

Al VE3AJM

Yes, direct replacement Taylor 866AS from RF Parts. No current limiting added.

http://www.rfparts.com/catalogsearch/result/?q=866as

But, these were replaced after the oscillation event.

I've had intermittents that caused multiple problems (acc plug). I'm not planning on modding anything else at this time until I restore HV, which I think I've located.

If the mod iron is intermittent, then my options are likely very limited.

It is humming with the audio gain turned all the way off, but I've yet to track that down as I need to get the HV fixed first.

Just got a quote from Hammond/Dahl. New modulation iron for the Valiant is a tad over 200 bux. Not obscene, but not cheap either.
Lets hope this isn't the problem.

Philip

HV rectifiers measure almost exactly 40 KOhms to ground.
Checked modulation transformer secondary between pins 2 and 3 and R was less than 100 ohms.

Philip

??

up a bit

With the transmitter unplugged let it sit for a few minutes and then short the HV line to ground just to make sure the filter caps are discharged.  Then remove the short and measure the resistance to ground from pin 1 or pin 4 of one of the HV rectifier sockets, if you measure significantly less than 40K trace the problem.  If the resistance is OK you should still do a test without the rectifiers in place (make sure the plate caps are well insulated from anything nearby) to see if the fuse still blows.  If it doesn't then the problem is still on the rectified side of the HV, possibly something arcing over.  If the fuse still blows with the rectifiers out of circuit I would temporarily disconnect the primary leads to the plate transformer to make sure the problem isn't there.

OK, HV no longer blow fuses. Tx loading up fine into dummy load, 450 mils, 7.5 mA grid, positions CW, VFO.

Here is the waveform with my Flex bandscope. Looks like the grass starts growing around -33 dB from the carrier.

Hear hum on carrier, although given the proximity to the receiver, this may be normal. I wonder if part of my audio problems are due, in part, to the rinky dink connections on the audio plug/socket. Will check the audio tonight. I also chatted with Timtron on 7295 and absorbed a great deal of his wisdom in the process.

Philip

What did you fix to stop the fuse from blowing??

Short B+ to ground on the ACC plug.

A lot cheaper than the mod xfmr.  At least you're making some headway.

Fred

Fix one problem find another....

Got HV fixed. Tried rig on CW. Plate idle current pegs meter on key up.

Clamper circuit now?

RF bias found at -40V. Set to -70V. See new tx waveform. The grass doesn't start now until about -55 dB. Looks cleaner. Still working on the clamper.

Can only get clamper plate idle down to around 100mA with key UP in CW position. Will swap out 6AQ5s with a couple of spares laying around and see what happens. Adjusting the grid final bias to the specified value seems to have cleaned up the o/p waveform and reduced hum greatly, as stated in an earlier posting.

Hi Phillip,

The single CW carrier looks pretty good.  It's hard to see much deeper without test artifacts creeping in unless you work at the test set up.

Now the real test...When you have the modulator working, put in a 1 KHZ audio tone and modulate to 98% negative.  You will now see the harmonic distortion created by the modulator.  If the second lower audio peak (2nd harmonic) is at least -25 dB down from the center carrier, you will generally be OK. (about 5.6% THD) The 3rd, 4th, etc audio peaks should diminish farther down (-30, -35 dB) as they go away from the center freq to maintain this figure. You can also hear these tones on a remote receiver and can use the S-meter as an indication. Look for  X2, X3 X4 multiples, etc., of the 1 KHZ audio tone running above and below the center freq..

Post the pic when you get to this point and we'll take a look.  This is a good indication of potential splatter and will teach you the optimum settings for the rig to produce the cleanest signal under modulation.

T

I will cautiously say that the Valiant is working. Still needs some more work before I button it up, but the big stuff seems to be okay for the moment. Remaining tasks include:

1. Calibrate the variable frequency oscillator (tuning).
2. Replace a power resistor in the VFO compartment that is prone to fail in these transmitters.
3. Clean 50 years of gunk off the chassis and front with a 50/50 mixture of ammonia and 409.

Modulating ok, no sparks, no carbonized wiring, and no glass shrapnel in my skin from exploding vacuum tubes.
In other news, not shorting out anything. Cannot get clamper idle current below 50 mils (tried three tubes). Listening on a companion receiver and its nothing to get excited about, quality wise. Sounds a little scratchy on voice peaks. May be overmodulation on my part. The manual says to run 330 mA plate, 8 mA grid, and modulate no more than 165 mA.

OK, I'm not sure I can get a precise measurement of 98% negative. The best I can do is a smidgen before the bottom flattens out. All I have is a Tek465/DM43, HP5326 counter, and the bandscope on the Flex.

Doesn't one have to be careful injecting tones of this nature with respect to duty cycle? I don't want to burn up another set of 6146's and their associated spur suppression resistors.

Yes, limit the time modulating with a sine wave.  Few seconds on and more off.

I meant to get as close to -100% as possible, but not over. -98% is just a way of saying that. Anywhere from  -90 to -99% is fine.  Heavily modulated will produce worse THD performance than lower modulation, so it's an acid test, worst case.

I don't see why any plate modulated AM rig worth its salt can't take a modulated sine wave. I run them thru my rigs quite often.  Going over negative 100% can break things, but if everything is running right and we limit the modulation to under 100%, it should be fine. If anything, keep an eye on the tube color.  If the 6146's get excessively red, then maybe you need better cooling or need to load the rig a little lighter. (More C2 loading capacitance to produce less carrier output)

T

Measurements using Daiwa CN101L/50 ohm dummy load

Plate 460 mil
Grid 7.5 mil
PO ~ 170 watts CW

Power output rises after tx heats up to about 480 mils ~ 180 watts, grid @ 7.5 mils.
Wave clean on scope

On 160m cannot get VFO higher than 1980 khz when calibrating.

460ma?????????  The rig has three 6146s in the final.  Don't think you should be loading the finals much above 300ma.  Check the manual for the correct amount.  I think most folks run the Valiant below 300ma final current.

Fred

 " I think most folks run the Valiant below 300ma final current. "

Johnson loads their VK2 to 300 mA........ Its got 2 - 6146(s).

I don't load my VK2 more than 200mA on phone.


klc

I think the manual calls for 450ma on CW, 330ma for AM.

300 mA on phone? You must be reading a different manual.   ;D

"  300 mA on phone?  :

Gack! It doesn't mean Clock Wise??.

Yeah, the manual says 300mA for CW and 230mA, max, for Phone (p12, Johnson Manual). I feel so cheep. But I still have value.

klc

OK. 300 for CW. This is the AM Forum. Why are we posting CW numbers?  ;)

Have the cover off the VFO compartment. I can see why there is much wailing and gnashing of teeth with respect to R3, 18K large resistor. Its gonna be a booger to get at and replace.

Philip

Clip it out and put it under the chassis with wires connecting.  Use a 5 watt or, at very least, a 3 watt.

Very hard to read but try enlrging the page view

(http://i10.photobucket.com/albums/a112/daedalusnj/Valiant_zps5d65f64a.jpg) (http://s10.photobucket.com/user/daedalusnj/media/Valiant_zps5d65f64a.jpg.html)

My manual for the Valiant sez 450 ma for CW and 330 for phone with 8 ma drive, modulator at rest is 60 to 80 ma, peak to about 165 ma.....Al/W8FAX

Here ya go.

We're cool on loading procedures. I have both a hard and digital copy. I find it is easier to pull up the PDF viewer than deal with my bound hard copy.

However, since this discussion seems to have morpherized to a log of my progress, and not one dealing with a specific problem, perhaps the Moderator(s), in their Profound Amplitude Modulated Full Carrier Wisdom (PAMFCW), may wish to move this chain of postings into the QSO section.

Chernobyl resistor measures 9.5 Kohm out of circuit vs. 18Kohm specified. Working on its replacement now...this is truly hateful job.

Replaced resistor. VFO working after a fashion. Will likely need to replace 0A2 regulator. However, getting some really severe backlash with the dial. Can someone tell me how to break this thingy down without breaking something?

Cannot get end points on 160 LO adjustment and HI adjustment to line up. This may be related to tube damage (0A2, 6AU6) as R3 was found to be only 9.4 Kohm. Also, the aforementioned backlash is not making things easy.

Before Natasha visits you, you may want to read this. I don't have a Val, and I don't know fer sure, but I think you should "Beware the flexible coupler" if you take the panel off.


http://amfone.net/Amforum/index.php?topic=15634.0


klc

Hey Phil,

    If you have that much Backlash, Chances are
it's already Broke...

GL

/Dan

Could be the phenelic shaft coupler is broke.  Take a close look at it while turning the dial.

Checked modulation on a scope. I am hearing my modulation and seeing it on the bandscope, but the oscilloscope/wattmeter are a different story. Modulation peaks on an analog meter go up to about 150 watts with carrier at 100 watts, final loaded to around 300 mils with grid at 7.5 mA, resting mod current at 70 mA. Does this seem right?

That does seem OK for the loading and grid current, resting mod current is also about right.

The meter you're using may not respond fast enough to see peak power.  You need a peak reading meter.

Well, I will tell you this Flex has spoiled me. It has a speech compandor, adjustable bandwidth audio, etc. With the amplifier in line giving 100 watts carrier, it swings really well on the meter, while staying in the correct ALC zone. I've set it up using W1AEX's online procedure; the audio simply rocks and usually garners unsolicited compliments (not pertaining to on-air content; that's another story, HI).

I have read the Valiant's modulator (iron and 6146's) is not up to the task at hand. I did see some improvement when I swapped out the 12AX7 in the first audio stage.

If this is as good as a bone stock Valiant will do, then I'll stop for now and work on the VFO and the female microphone chassis jack (worn out), and a few other things. When I get the classic set up where I want it with the R388, then I'll consider doing the audio mods.

Frankly, I've been working on this transmitter for over two months and I want to play with it a bit and solicit harsh, cruel comments with regards to my on-air audio...."Gee Philip, you sounded so much better with the Flex......). 

However, when asked why would I work on a transmitter when I already own a first class exciter, the answer is simple. How does one learn hands on electronics by clicking a mouse on a computer display????

I have learned more practical electronics by building my theremin, restoring the R388, and fighting with this Valiant.....and I have a degree in this stuff.

The sad truth of the matter is many of our top line engineering schools are turning out kids who don't know which end of a soldering iron to grasp.

Radio is definitely modulating. the sweet spot seems to be around 290 mils plate, 7.5 mA grid. Better swing on the scope than before. The only thing left is maybe install a different fuse holder in the radio (drill round hole in back type). I have a flat screw on type inside currently....in retrospect, if the fuse blows you likely have a problem anyway and will have to disassemble it.

Also, the VFO needs work of course. Chernobyl replaced. The circuit board around the tubes looks fine, so I don't think it had gone into runaway yet. Just not sure if I am going to go the trouble at the moment. I'm tired of working on it.

I will rewire the bypassed clipper and see how it does. Just so I can make an educated decision myself after seeing how much worse it will make my PW signal sound.

Philip

File of work to date...

Now the chassis connector for the two pin microphone plug is intermittent. A brand spanking new Amphenol 80-PCF2 is on the way. I'm sure this connector has played a part in my journey through the hollow state wonderland.

In other news, the Dow Key is wired up and ready to go. The sucker makes a very satisfying CLICK when energized.

If I'm lucky, I'll be on the air prior to the long weekend....I hope you fine folks aren't tired of my multiple postings. I DO appreciate all the kind advice and encouragement.

FEELEEP

Don't be afraid to step back and rest a bit as well. If you work past the point of exhaustion/interest you might find yourself less able to pay attention to details and to do the best job possible. A fresh mind results in better work.

Got the clipper rewired and its working. Found a couple of pins on the third audio that may have been touching. Traced audio clean through the entire chain. Looked at RF output on a scope. It appears to be modulating 100% modulation or better.

Audio sounds sweet and clean....not bad at all. Aside from the bad mic jack, the transmitter is ready for prime time.

"THREE CHEERS AND A TIGER FOR ME, I HAVE WON!!!!"

The genie with the light brown hare, in WB Cartoon " A Lad and His Lamp".

Link here

https://www.youtube.com/watch?v=LvGsfXXXCMo

Philip

Well done, Mr. One-L! It has been educational as well as entertaining following your exploits.

BTW - What mic are you using on the Valiant?

Jack
WA2OLZ

Still need to make up jumpers for coax and install that. Have both an EV638 and D104. We'll use what sounds best.

Drifting VFO problem appears to be solved by replacement of 0A2. However, the waveform looks junky on 40m. May replace the oscillator tube if I can find one here in the shack.

Is there an easy way to get the VFO cover back on so that all the little adjusters line up?

Easy. Remove the adjusters. Keep the hardware. Use a long handled insulated tuning tool to adjust the VFO.
Rig back on the air with new VFO tubes and a microphonium jackum. Works good now, although I wonder if the final tubes are a little soft, as I have trouble getting up to 7.5 mA grid whilst in the AM mode.

I think we can likely close this chapter out and move on to the next challenge. I need to modify the R388 to switch to a mute with a simple relay closure (there is no voltage internally with the radio, you have to provide 12VDC in series with the mute switch). Then align the receiver, and we can move on to the HQ180 after that.

PHILIP

Determined that the Valiant doesn't work good on AM is the switch is in SSB, no matter what band one is on.

Philip

Phillip,

  Get that Valiant on 14330 tonight, about 7 pm EST. Both Ralph KD6OS/5 and myself plan to be on.

Jim
WD5JKO

Will have to be another evening. Been ill all day.

Can anyone tell me a good place to find doorknob caps for the loading padders?

  Phil,

   I hear you as I write on 14,325 as about an S7, and Q5...

The audio is a bit muffled, but quite copyable.

Jim
WD5JKO

Muffled mic was EV638. Swapped over to D104. Anyone has a Valiant here is a tuning aid. I use a manual tuna and logging settings helps out the finals.

Philip