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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K2GTM on June 10, 2020, 06:58:33 PM

Title: Valiant issues
Post by: K2GTM on June 10, 2020, 06:58:33 PM
Hello all!

A friend of mine recently purchased a Valiant that had some issues and I am helping him get it in good working order.  Here is what I've done so far:

Replaced some out-of-spec resistors. (The "Chernobyl" resistor was already replaced.)
Replaced some bad tubes (0A2 and 6AU6 in VFO, 12AU7, and all five 6146As).
Cleaned all switches and pots.

The set has some extensive audio mods and the Valiant II bias pot mod.  Otherwise, it looks stock.

I am a bit green with the Valiant and I've been going through the test and tune-up procedures in the manual.  Some things seem a bit "off" and I'd like to see if anyone might have any insight.  Maybe I'm just being too picky, but these things seem strange:

- When doing the 80M test on pages 9 and 10, the buffer resting current is about 3mA instead of 6mA.  When I increase the drive to "3", the buffer current only gets up to around 15mA instead of 22mA.  When I set the grid to 3mA and turn on the plate, I can tune the final, but I get WAY more than 80mA on the plate.

- When testing 40M (pg. 10), the exciter tuning pulls the VFO frequency around.  Also, when I try adjusting L17 nothing changes.  (Should be able to peak the grid current.) I also have the same effect when I try tuning the final - I get way more than 90mA.

- When trying to tune up on 20M and 15M, and I have the grid set to 8mA, the HV relay won't switch when I flip the switch to manual.  If I lower the drive to around 2mA, it'll work.  Then, if I tune it up, it'll make power, but won't work the next time I try to turn on the HV.  It's as if something is preventing it from working when the grid is over a particular point.  I haven't tried this on 11 or 10M.  The relay works fine on 160, 80 and 40.  

Any help would be greatly appreciated!

Thanks and 73,
Greg K2GTM  

Title: Re: Valiant issues
Post by: W1NB on June 10, 2020, 09:21:16 PM
I have no direct experience with the Valiant but looking at the schematic the first place Iíd look at is the 300v rail, specifically the electrolytics, especially the first one.  It helps set the working voltage. If the filtering is bad, Youíll have lower DC with more ripple. Once you start to draw a lot of current through the choke I would suspect youíll see a noticeable drop in voltage. If your not drawing a lot of current the voltage remains high enough to pull in the relay. Once itís closed, it will hold with a lower voltage. As you increase the drive, though, itís drawing more current which in turn drops the voltage. Just a theory but low 300V could also be causing some of the other anomalies you described.

Title: Re: Valiant issues
Post by: K2GTM on June 10, 2020, 10:04:25 PM
An interesting idea. I can check that tomorrow.  Thanks!

I forgot to mention earlier that the filter caps had already been replaced and they test good on my cap tester.

Title: Re: Valiant issues
Post by: K2GTM on June 11, 2020, 12:44:52 PM
I found R20 (part of the voltage divider to feed the PTT relay) was bad, even though I'd already replaced it.  It was causing the relay coil voltage to drop too low on higher grid current (as you mentioned).  Might have to increase the wattage on that one.  Still have to check the other issues to see if that helped with those.

Title: Re: Valiant issues
Post by: W1NB on June 11, 2020, 05:47:37 PM
They all seem to be tied to the 300V rail so if R20 value went low it would pull that voltage down, impacting the at rest current of the buffer and the source that feeds the VR on the VFO. If things are still squirrelly, Iíd check the voltages on the buffer and VFO tubes against those listed in Table 1.

Title: Re: Valiant issues
Post by: K2GTM on June 12, 2020, 06:53:32 PM
So, the PTT relay still wasn't pulling in and I investigated further:  The relay had been replaced before and it just didn't have the pull needed to reliably pull the contacts in.  I tried to clean it and tighten-up the pole piece that was loose, but it didn't help.  I am currently working on replacing it with a lower-voltage coil relay.  Need to finish that before I can dig-in further. 

Luckily, I did begin to notice some change when adjusting L17 this time.  It's a pretty broad curve, so I did my best to get it in the middle.

Title: Re: Valiant issues
Post by: WBear2GCR on June 13, 2020, 09:15:26 AM
Unless the relay set up has been changed, it's sending 120vDC out the mic cable!

It was on mine.

Not the best plan.
It's something of a PIA to build up a circuit to replace the keying relay.
I just did it on mine.
Mine also did not pull in properly. (3rd time replaced - no more)

I'd suggest finding a suitable 6vDC relay if you can. The rig only has 6.3v
available on the filaments... then you'll want to make that into DC for the relay.

Not sure what those "3ma" figures come from?
They way to run it is to look at the "Buff", "Grid" meter positions, and the Exciter
and Drive controls.
So, the Buff (iirc) should DIP with the Exciter control, the Grid (iirc) PEAKS at the
same place more or less. Then the Drive control sets the "Grid" - normally 8ma..
(EDIT: had those two backwards... now corrected)

The Drive control is known to fry.
There's a circuit over in the box to the upper left on this page called "Technical Info"
and it's hiding somewhere in there, it's a mod that uses a pass transistor and a low
power pot.

Also, the tubes need to be good... always check the tubes.


Tuning: Gotta watch which scale you are reading!

Plate ought to bang to the right when keyed and when not dipped.
Proper tuning/dip is the needle straight UP.
So you'd dip the Plate, UNKEY then turn the loading SWITCH, re-dip, repeat
depending on which way the switch needs to be.

NEVER turn the loading switch when KEYED!!!

A little LESS plate current than the "book" calls for is a good idea, as
the modulator, unmodified is straining to make 100% positive.

You might want to consider the 3 diode negative peak limiter mod also...

Title: Re: Valiant issues
Post by: K2GTM on June 15, 2020, 01:17:12 PM
Hey Bear,

This one is definitely still using the old 120VDC keying setup. 

I'm planning on doing just that.  A previous owner had installed a 6.3VAC transformer to keep the 6AU6 VFO filaments lit 24/7, and I have already removed that "mod".  I'll use the transformer to make up a 6VDC power supply for a new relay.

I was talking about the buffer current not being as high as the manual says I should see.  I wasn't sure how critical that was.  Could be an issue with the shunt, but I don't think it's that big of a deal since I can get plenty of grid current and power output.

I've heard stories about the drive pot.  This one tests OK when watched on a Simpson 260, but I can feel some "iffy" spots while turning the knob. The owner wants it to be reliable, so it might not be a bad idea to make that mod as well.

The tubes either test good, or have been replaced.  All of the 6146s were toast and resulted in practically no power output.

Indeed - I've got a reminder card made up to remind me of the scale for each switch position.

OK on the loading switch.  I can imagine there being some damage done if switching it hot!

I'll let the owner know to not push the plate too hard.  The difference in power output just isn't worth the potential damage.

I'll check out the limiter mod.  The audio section has already been heavily modified and I haven't dug into it yet.  I do know that T4 has already been upgraded and the clipper has been removed.

Any other mods that aren't too invasive, but will make this old girl more reliable?


Title: Re: Valiant issues
Post by: DMOD on June 15, 2020, 07:50:52 PM
I usually rip out the harmonic generating clipper and use these schematics as upgrades:


Title: Re: Valiant issues
Post by: MikeKE0ZUinkcmo on June 16, 2020, 11:28:15 PM
You mention you're getting good output but the plate current is high.   THe meter shunts in several of the EFJ transmitters are simply a piece  of wire, and they drift high, resulting in precisely the situation you describe, full power, but at elevated current levels. 

I've replaced 3 or 4 of them in the past.   The values are somewhat non-standard, but 1% resistors have been close enough.   The power dissipation is pretty low due to the resistance values, so we aren't talking any 2 or 5 Watt units here.

I make it a point to check the meter shunts on all EFJ transmitters that are on the bench for the first time.

you need a good Ohm meter with sufficient resolution to accurately measure fractional Ohm resistors, on the low scales, at least 1/100th of a Ohm.

Title: Re: Valiant issues
Post by: WBear2GCR on June 17, 2020, 11:50:41 AM

you need a good Ohm meter with sufficient resolution to accurately measure fractional Ohm resistors, on the low scales, at least 1/100th of a Ohm.

I may be all wet on this, but I think one does not need a super great ohmeter?

Just set up a PS to run current through a known resistor... measure the resistor accurately,
use ohms law to take the measured voltage across the resistor and convert to current...
then put the shunt in place (series with the resistor), connected to the meter. Look at what the meter says, adjust the
shunt to read properly!

Presumably the amps scale on a DVM is "good enough" to do the same thing directly, and
skip the ohms law calculation?

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