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Author Topic: Viking Valiant: DOA - blows fuses and diodes  (Read 871 times)
W0OPW
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« on: August 16, 2004, 07:59:15 AM »

I am in the process of restoration of a Valiant. I have no information as to whether this ever worked. I replaced all the electrolytics and installed a pair of 1n5408 diodes in the HV. THe HV electrolytics are 150 uf. With the 866's removed, I turn on the filaments and let the tubes warm up. Applying HV using the PTT on the D104 mike, the power cord fuses and the 2 diodes both blow.
 ANy suggestions how to track this down?
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KA5N
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« Reply #1 on: August 16, 2004, 10:27:35 AM »

My first guess is that the 1N5408 3 amp 1000 volt PRV is not suitable for replacement of 866's unless you have 2 or 3 in series for each leg.  Next guess is that you have a short somewhere since the fuses and diodes don't blow until rig is keyed.
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W4PTO
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« Reply #2 on: August 16, 2004, 05:46:24 PM »

I have to agree with KA5N. Putting a series of 1N4007's is probably better than a single one per let. However, it sounds like a bigger problem. Are the filter caps any good? Or perhaps the old 866's or 3B28's provide a "softer" start on transmit on the Val? Cal, N6KYR.
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W0OPW
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« Reply #3 on: August 16, 2004, 05:55:52 PM »

I have gone thru the tube resistance checks and they seem to be ok. I did find one wiring error. Pin 4 and 5 on the 5v4 were reversed!. Seems it must have been a half wave rectifier.  I also found some unsoldered connections in the rf section.  I have one(1) unconnected red wire floating around. I will need to find the kit wiring direction sheets to see if I can figure out what it is.  So far it is still dead.
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KA5N
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« Reply #4 on: August 16, 2004, 08:01:53 PM »

The PRV rating of an 866 is 10,000 volts and the voltage drop through it is around 60 volts.  The Valiant has a choke input filter, but the electrolytics would probably do better with an additional 20 ohms or more of series resistance between the rectifiers and the caps.  On the schematic the plate voltage for the 6146 finals is 600 volts but the turn on surge with just a single pair of solid state rectifiers could blow the fuse and a reverse voltage spike of over 1000 volts could short the diodes.  Since the primary is switched by the PTT relay that means that the electrolytics are setting there looking like a short circuit many times during a QSO.
There are a lot of 40-50 year old parts in this fine transmitter.
Allen
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W8JI
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« Reply #5 on: August 18, 2004, 07:47:20 PM »

I have Valiant's, and went through all of this a few times. My main radio has diodes in the HV supply.

The surge current and voltage of the supply is primarily limited by the choke, so the addition or subtraction of a few ohms is meaningless. So is the difference in drop between the tubes and semiconductor diodes.

You will need at LEAST three 1000V diodes in series. The reason why is the peak reverse voltage of the diodes is not only the secondary diode to CT peak voltage of 1100 volts, there is some "kick" from the inductance of the choke as the relay chatters or bounces.

I've been sucessful with 3 1N5408 diodes, but two in series will almost always fail over time. I use FOUR, just in case! Better to spend $.20 more.

Other common problems are 866 sockets arcing to the chassis, the small terminal strips arcing from pin to pin and to the chassis, and even arcing from the filament winding of the 866 to ground in the LV power transformer.

You could of course have other problems.

I'd be really careful about hooking wires back up in case they were pulled for a reason like I just mentioned.

What I would do is connect a 100 watt light bulb in series with the 120V lead of the HV transformer while troubleshooting. This will allow you to work without constantly blowing fuses and subjecting the relay contacts in the HV transformer primary to severe arcing from overloads.

I'd start by using the correct number of diodes. If that didn't do it, I'd yank the plate cap leads off all the 6146 tubes and try again. If that didn't work, I'd kill power and cut a HV lead just past the filter caps and bleeders to see if the fault stops.

Eventually you find something you reconnect that causes the fault to come back. It might even be a filter choke fault.

73 Tom
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W8JI
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« Reply #6 on: August 18, 2004, 07:51:27 PM »

I'm sorry I made one error.

The peak voltage across the diodes is TWICE the peak secondary to ground voltage of about 1100 volts. So it is over ~2200 volts PRV without ANY choke kick, and that is why I have failures  with two diodes and am uncomfortable with only three.

 



 
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W0OPW
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« Reply #7 on: August 19, 2004, 09:24:34 AM »

Thanks Tom. THat all makes sense. Yesterday I did remove the HV ouput leads and then reconnected one by one.  I found arcing across the HV secondary leads on the terminal strip.  I am back to using 3B28's instead of diodes. But, one of the tubes does not have the blue glow so it may be bad.  At least I am not blowing fuses anymore.  
 Will continue - Pat W0OPW
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N6AYJ
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« Reply #8 on: August 21, 2004, 08:46:56 AM »

Hee, hee!  I just love it when W8JI advises people how to fix their Valiants.  I print his answers out and save all of them in my "Valiant repair" file.  Maybe someday, thanks to Tom, I'll be able to fix anything that goes wrong with my Valiant.  Thanks, Tom!
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W4PTO
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« Reply #9 on: August 23, 2004, 06:21:14 PM »

Is the unit kit built or factory? You can tell if most of the tube sockets are held in with rivets. If they are not  (that is, they are held in with screws), them it is a kit built Valiant. Double check that wiring!!!! It might be as simple as miswiring (as was previously mentioned)>
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W8JI
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« Reply #10 on: August 24, 2004, 12:35:09 AM »

Swap the plate caps on the rectifiers and see what happens. If the other tube glows blue, you have an open secondary half. I really doubt this would happen, but it is possible.

You should also ten swap tube sockets.

I've seen some tubes fit the 4-pin sockets wrong if someone at some time forced one into the socket wrong. Be sure the two fat pins line up with the big holes.

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