Johnson Valiant Low Voltage B+ too high!

[W4JPL]

Greetings,
I'm working on Valiant (I call it the money pit) and have come on yet another mystery. (or maybe it's not a mystery)

The low voltage B+ reads 380V. I measured this in two places; one on pin 4 of the aux plug on the back, and at C98A.
Is this normal, or has anyone a suggestion on how and what I can look for.

Thanks and 73, John W4JPL

[Pete, WA2CWA]

Rather then everyone running out to find a Valiant schematic, why don't you tell us what the voltage is suppose to be or what you think it's suppose to be. Also, what's the AC line input voltage and is it tube or solid state power supply?

[W4JPL]

The schematic states 300V after L44.
I was just coming up from doing some further tests.
At 110V (from my variac) it is 380V.
At 120V it is 414V.

The power supply is standard in the Valiant. No solid state, except the HV replacements for the 866's.

I have been working on the transmitter to the best of my ability; which is why I ask questions when I don't understand what is going on, or if this is a problem or not.

Hopefully someone familiar with the Valiant can give me an answer or point me in the right direction.
W4JPL

[KA2DZT]

If it is a choke input filter for the low voltage supply,  check the choke for it being shorted or maybe it was removed.  Without the choke the filter will be cap input causing the voltage to go way above 300 volts.  Check to make sure a filter cap wasn't added ahead of the choke.

Fred

[W4JPL]

thank you Fred, I'll check that out in the morning!
73, John W4JPL

[K4RT]

I don't have a Valiant. What's the voltage drop for an 866, about 15 volts? I'm assuming HV replacements for the 866s means solid state diodes are now in place. Were dropping resistors added to the power supply when the tube rectifiers were replaced? That might account for some of the voltage increase. A flakey or failing filter cap could be throwing things off.

[K9MB]

The LV transformer has a 310-215-0-215-310 vac winding.
The Valiant has a cap input filter with 15uF on the input side.
Normally, a cap input will give 350-400 volts with a solid state rectifier.

The 5V4 data sheet has a voltage drop of 25 volts at 175mA, but the load graph on the data sheet for a cap input filter and a 300-0-300vac transformer at a small load, an output voltage near 400volts as you might expect, but about 300volts at about 175 mA, given that the series resistance is 100 ohms.

The choke in the Valiant may normally have a 100 ohm resistance and then you may expect the published 300vdc from the circuit when it is drawing the normal current. If you are measuring it when the circuits are not drawing current, then the voltage will be near 400volts.

If the choke shorted, then the normal 100 ohms would not be there and you are left with only the drop across the 5v4 and the secondary winding of the transformer.
I would check the choke to see what the resistance measures.
If all else fails, put a resistor in series with the choke or in it’s place. Might need to increase output cap to limit ripple. If choke is bad, best to find a replacement, however.

https://frank.pocnet.net/sheets/093/5/5V4GA.pdf

[W4JPL]

Thank you for this info!
I'll check it out shortly.
73, John W4JPL

[W4JPL]

Thank you Michael K9MB for clear, easy to understand into!

I lifted one side of the choke, and found that the resistance across the L44 choke is 290 ohms.
I cross referenced this reading with a couple of different meters.
Upon close inspection of the choke, it looks like it might have taken some moisture or water damage, although it was in a corner of the laminations and did not look like the actual windings were damaged.

So I would imagine that the choke needs replacing, unless someone confirms that 290 ohms is within specs.

Does anyone have the specs for this choke? The valiant manual only says "low voltage choke" with no further info.

[W4JPL]

I found that the choke is a 10HY @ 100ma DC (300 WVDC)
I can buy a new one from Peter Dahl for some odd 200 bucks, but I need to find something a lot cheaper!

[K4RT]

Rereading your initial post I realize you were referring to the low B+ and the 866 or it's SS replacement would likely not be involved.

Regarding the needed choke, place a want ad here on amfone.  Otherwise, I have had fair luck finding replacement parts for tube gear at hamfests, eBay, and Surplus Sales of Nebraska.

[W4JPL]

K9MB also mentions adding (or using as a replacement) a resistor instead of the choke, if it is bad.
Can anyone give me a suggestion as to what size and wattage resistor to try?

All of the reading that I have been doing suggests that in years gone by, chokes were cheap and capacitors were (relatively) expensive.
Now the reverse is true, and and I have seen several threads that state that a resistor will do the same thing as the choke.

This is way above my experience!

Thanks and 73, John W4JPL

[N1BCG]

I'm curious what you are trying to fix (aside from the LVB+ being too high). Does the Valiant work normally? It may be that the power supply doesn't have enough loading which would allow the voltage to increase significantly. Do all of the currents look correct?

[Pete, WA2CWA]

I agree with BCG. Loading of the low voltage supply will change that "300 volt" reading. Switch positions might also change the reading. Before diddling with the low voltage supply or buying a new choke when you don't really know if it's bad, I would refer to the voltage and resistance tables in the manual and check all those values under the conditions that Johnson specified. Once you have verified all those readings (to high/to low/right on, etc.) then you can move to finding the why if need be.

Does the rig work??

[K9MB]

I agree with BCG. Loading of the low voltage supply will change that "300 volt" reading. Switch positions might also change the reading. Before diddling with the low voltage supply or buying a new choke when you don't really know if it's bad, I would refer to the voltage and resistance tables in the manual and check all those values under the conditions that Johnson specified. Once you have verified all those readings (to high/to low/right on, etc.) then you can move to finding the why if need be.

Does the rig work??

I am going to agree with this, given the measured resistance being 290 ohms. Since it is rated at 10H at 100mA, this choke is likely ok.
The next thing you can try is putting a 10 ohm power resistor in series with the choke and measuring the voltage drop across the resistor. 100mA (0.1Amp will produce a 1.0 volt drop, so you can confirm what is likely- that the load current is too low to control the voltage. Of course, you could also put a milliammeter in there, but a resistor is safer for your meter. Start with a higher scale and drop it intil you get enough voltage to calculate current.
Be careful, though. Clip the voltmeter on before turning power on and discharge caps fully before touching anything later.
I do not remember if anyone has said whether this Valiant is working or not. If it is not working, check the wafer switches that carry this LV to the curcuits. You may have burned contacts and open wafers, so the power supply is not seeing a proper load. My idea about the 10 ohm resistor and voltmeter can give great clues about this and then trace the wafer switches to see if there are voltage drops across contacts, etc.

[W4JPL]

First, I want to thank everyone for their input.
Yesterday, I put my second valiant up on the bench, to comparison to the other unit.
The LV B+ was even higher, some odd 420V at 120V input.
I did not measure the choke in this rig. (I'll call it Valiant #1)
Valiant #1 seems to work fine. I (with the help of W1ES) put a lot of work into it. Loads up fine, and has (hopefully!) good audio.
I put it into transmit for a good 5 minutes into my dummy load, and checked for anything getting hot or warm. The transformers were cool to the touch.
I was getting some odd 135 watts out on 75.
 
The only mod that I will put in it will be the 10 watt 5K resistor in series with the drive pot, as recommended by HLR and others.

Valiant #2 is semi-working; I have to do a bit more work to the output network. The doorknob cap connected to the finals is only reading 300pf, and the output is low. I ordered a 20KV 1200 PF cap.
1200pf 20KV D709T3M122M20KV High Voltage Ceramic Disc Door Knob Capacitor SR07A
Bought this off of ebay.

I also have a bit more work to do with the coupling capacitors.

My main concern with Valiant #2 was the drive pot was trashed (the windings in it were broken, etc) and when I replaced it with a 25K 5 watt I noticed that it was getting hot. It was suggested that I measure the voltage, and this is when the questions on the 400V started.

On both rigs, the voltage change is just a few volts, between standby and transmit.

Since both rigs have the same voltages in the B+, I am going to assume that this is normal.

I'm also going to put the 5K resistor in series with the drive pot on Valiant #2.

My final note is that I intend to run either valiant (on the air) through one of my variac's to keep the voltage at 110V.

Comments?
Thanks and 73, John W4JPL

[MikeKE0ZUinkcmo]

Here are the transformer/choke data from the manual.   Note that the LV choke, L44, has a 300Ω resistance.

[W4JPL]

I missed that.
Thank you!
73, John W4JPL