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Author Topic: Valiant LV choke burned up.  (Read 4250 times)
KG6YV
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Posts: 504




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« on: April 05, 2013, 08:55:25 AM »

The low voltage filter choke in my Valiant  ""cooked".... I have a replacement but I need some advice on how to check for downstream shorts in any Low voltage bypoass capacitors before I try it on the variac.  The filter caps have been replaced but the originals checked Ok on my checker (checked OK at 400V across them).  The Valiant manual is not very good for finding the various points where the LV line goes.  If anyone has any suggestions on a methodology to trace out all the loads and all the bypass caps I am all ears....

Thanks,

Greg
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KE3WD
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« Reply #1 on: April 05, 2013, 09:17:59 AM »

Chokes typically fail due to wire insulation breakdown.  Some of the old shellac or varnish insulations are organically based and these substances do indeed crystallize or get invaded by bacteria or other little critters and over time, the insulation loses its insulation properties.  When one turn shorts, the thing will still work, but the heat goes up.  The heat causes other turns to become shorted more quickly, etc. 

I just install the new choke, bring it up on the variac keeping an eye on current draw not going too high, then might make a few DC voltage checks and AC ripple checks and proceed on to burn-in with the new choke. 

73
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AA4PB
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« Reply #2 on: April 05, 2013, 11:03:43 AM »

I agree with the insulation breakdown idea. If anything on the load side of the choke shorted it would likely have drawn enough current to blow the power supply fuse before the choke overheated and "cooked".
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KG6YV
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« Reply #3 on: April 05, 2013, 11:24:20 AM »

Thanks guys, I was suspicious of insulation breakdown since the choke is 50 years old.  I'll solder everything back together and watch closely as I bring up the voltage.  I too thought it odd that the power supply fuses did not blow but the choke sure started smoking at around 60-7VAC on the variac....

Thanks again,

Greg
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K5RT
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Posts: 115




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« Reply #4 on: April 06, 2013, 09:15:58 AM »

Guys, this is the low voltage choke. The OP is asking the right question. I'd suggest understanding the expected load (how much current should he expect to see drawn) and go from there. It's a simple matter to insert a DC amp meter after the choke and bring the Valiant up using a variac.
Check the obvious - check the resistance to ground at the output of the choke. If its under 1k, find the culprit before proceeding to power the transmitter up.

Let us know what you find
VY 73
Paul
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AA4PB
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« Reply #5 on: April 06, 2013, 09:30:00 AM »

Guys, this is the low voltage choke.

Low voltage in a Valiant is 300VDC. It's quite possible to have an insulation failure at that voltage.
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AC5UP
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« Reply #6 on: April 06, 2013, 11:04:58 AM »

I'm as guilty as anyone of taking the occasional shortcut like assuming what's most likely to fail, next most likely, etc.......... 

But............ Never assume what's probably good or what's probably bad.  Your test equipment knows for sure.
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KE3WD
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« Reply #7 on: April 06, 2013, 12:09:29 PM »

...install the new choke, bring it up on the variac keeping an eye on current draw not going too high, then might make a few DC voltage checks and AC ripple checks and proceed on to burn-in with the new choke. 

73

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KG8LB
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« Reply #8 on: April 29, 2013, 08:44:43 AM »

Heyboer transformer in Michigan has or at least had  a service that can help hold off  insulation breakdown to a large degree . In the past they have taken from me , older transformers  . When they get them , they dry them and place them into the vacuum varnish (epoxy ?) pot . This helps fill the voids and restore the insulation effectiveness . It could give some old iron a new lease on life .

   By the way , Heyboer is a first class transformer builder as well . If you find yourself in need of new iron , give Alden at Heyboer a call . Not cheap , but first rate quality and service .Considering inflation and what new transformers cost back in the old days , their prices are very fair . They also stand behind what they build .
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N2EY
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« Reply #9 on: May 02, 2013, 11:09:07 AM »

If anything on the load side of the choke shorted it would likely have drawn enough current to blow the power supply fuse before the choke overheated and "cooked".

Don't count on it!!

Many popular BA transmitters, including the Valiant, have just one mains fuse protecting the whole rig. That fuse has to be sized large enough to not blow under full-yellow YAY-am modulation conditions - which, in a Valiant, involves a sizable amount of amps, most of which go to the HV plate transformer.

A short or overload in the LV supply can easily cook the choke, LV power transformer, rectifier, etc., and never come close to blowing the mains fuse.

I strongly advise any ham running one of these multi-power-transformer rigs (Viking 1 or 2, Valiant, DX-100, Apache, probably many others) to install additional fuses to protect the iron.

Yes, I learned this the hard way, on a Viking 2.

73 de Jim, N2EY
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K5RT
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« Reply #10 on: May 02, 2013, 07:50:17 PM »

Jim this is a fantastic idea! It's easy to implement, under the chassis so it doesn't detract from the radio and easily reversed if desired. Just use a plastic in line fuse holder in series with the primary of the transformer is all there is to it. I think I'd fuse any large chokes as well. It also simplifies troubleshooting by isolating the fault somewhat. I wish I'd thought that one up. Write it up for Hints And Kinks in QST before someone else does!

Vy 73
Paul
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N2EY
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« Reply #11 on: May 03, 2013, 07:46:53 AM »

Jim this is a fantastic idea! It's easy to implement, under the chassis so it doesn't detract from the radio and easily reversed if desired. Just use a plastic in line fuse holder in series with the primary of the transformer is all there is to it. I think I'd fuse any large chokes as well. It also simplifies troubleshooting by isolating the fault somewhat. I wish I'd thought that one up. Write it up for Hints And Kinks in QST before someone else does!

Thanks for the kind words. I may just do that!

The one thing about inline fuse holders is that they'd be difficult to get to. I'd look for some kind of fuseholder that would be more accessible. A V2 has a hinged lid, so a couple of fuseholders on a bracket could be a no-holes mod. A DX-100B has a trapdoor lid. Valiant poses a bit of a challenge.

Another consideration is a time-delay circuit which prevents applying juice to the HV plate iron until a certain time delay has elapsed. Rigs with MV rectifiers (like the Valiant) need this!

73 de Jim, N2EY
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WA1RNE
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« Reply #12 on: May 04, 2013, 10:43:32 AM »

Quote
The one thing about inline fuse holders is that they'd be difficult to get to.

The iron shouldn't blow a fuse often enough to worry about accessibility.

It took 50 years for this one to pop.....I'd install a chassis mount fuse holder wherever it will fit.  Cool
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KE3WD
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« Reply #13 on: May 04, 2013, 05:42:09 PM »

The famous Hammond B3 and C3 organs came from the factory with no fuse in them. 

The fuse was taken out of the production line by Laurens Hammond himself, when, during the first year production run of the "A" series organs, a customer call about a dead organ was answered by the owner himself and it turned out to be nothing more than a popped fuse. 

So he ordered the fuse to be removed in all future production runs and it stayed that way. 

I've often wondered about certain church fires...

And, I also put an AC fuse in every Hammond that passes thru my hands that does not have one. 

As for multiple fuses to better protect sections of boat anchors, be careful about that, for example, a fuse in a Plate Voltage circuit is very likely to be working way outside of its voltage rating. 


73
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KG8LB
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Posts: 234




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« Reply #14 on: May 05, 2013, 05:20:16 AM »

 Seperate fuses could be costly in the event you lose the plate supply and the screens are still powered . Perhaps a better scheme would have the fused circuits also tied to relays that would drop all power if any one of the fuses opens .
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