BOOOM!!!! Valiant Goes Tits Up In Spectacular Fashion

[ka4koe]

Well, something just went POP, heard a sizzling sound, and then lots of smoke. Toldja it would break within a week of putting the case back on.

Hope it was an electrolytic and nothing major. Didn't smell like copper or carbon, or that funny ozone smell.

Almost sounded like quenching a hot tong in water.

Opened a window to get the smoke out of the shack. I may look at it later. Anyone want to take bets on what it was? Didn't blow the fuse on the HV transformer as I was scrambling to kill power to it.

I was sending CW on 20M when the fun occurred.

Actually, in a perverse sort of way, its funny, so I guess I am enjoying this nonetheless.

Philip

[ka4koe]

One of the 600V 100 UF electrolytics in the HV section went. Likely either a defective cap or the B+ is higher than 600V by a substantial margin.

[KA2DZT]

Philip,

You're lucky it was only a cap.  When I read your first post I was thinking it may be a xfmr.  You need to use two caps in series with some resistance across each cap.  Two 450V caps will work.  Problem is you can never trust some of these new caps so maybe use two 600V caps.  In my HB xmtr I use all 1000V oil caps in the 650 volt supply.

Fred

[KA2DZT]

Philip,

You can use 100K across each 600V cap.  You may need a little higher wattage resistors.  These will act as bleeder resistors.  V*/R  600V*/200K = 1.8watts.  So about 1 watt on each resistor.  Use at least 2W resistors.

Fred

[ka4koe]

How much resistance across each one?

The fuse didn't blow. Man it stinks in here.

[wa3dsp]

Not familiar with the Valiant but if it has a bleeder then divide that resistance across the two caps. If not or if you don't want to mess with that then 50-100K is fine. I used 68K 3W's in my Ranger but 100K 2W's would also work. Its just to equalize the voltage across the caps. The Ranger is choke input so some minimum current is needed to keep the regulation reasonable.

Also if your line voltage is high you may want to use some bucking on the primary. Those rigs were not designed for 120V or more on the primary. Check the filament voltage with a good AC digital meter. If it is much more than 6.3 you should use some bucking. The tubes and the rest of the components will last longer.

[pa0ast]

The lekage current of two electrolitic  capacitors is never equal, so You need two resistors connected  over them to divide the voltage aprox.in two. So the same Voltage over both capacitors. Normally 2 times 330 kOhm will do. If the  leakage current from the capacitors differs a lot, then You can lower the resistor value in order to compensate for the difference in leakage current. But that  will produce more heat and the transformer is extra loaded. That extra heat close to the elca's will dry them quickly out. The result : Shorter life span and capacity loss and the series resistance of the capacitors goes up.( Higher tangens delta and ESR ) The life of an elca is short , just 600 hrs over 105 deg celsius. and doubles with each 6 deg lower. Exept if you have a type designed for 105 deg. The water inside with the acid vaporizes . If an elco is becoming to hot , it starts to boil and pressure occurs,  as result a big boom. There is not much water inside so it can happen very quickly...from my own expirience.
RGDS Anton

[WQ9E]

Make sure the Valiant is wired as per the original circuit in this section.  There should be two caps in series with a 20K resistor across each cap.  The original caps are each 80uf @450V, a pair of 100uf @450V are fine as replacements but make sure to replace both and check the equalizing resistors.  If someone before you changed to a single capacitor that was a VERY bad idea and the junction of the two equalizing/bleeder resistors provides part of the reference voltage to the clamper circuit so it should not have been changed.

My guess is there are two of those "600V" caps in series.  I have never dealt with capacitor are us but you can never go wrong buying proper quality capacitors from Mouser.  I sent you a link via separate email to the proper Mouser caps and it is only $7 for a pair.  Again replace both and if there are any other suspect brand new electrolytic caps (like in the B+ and/or bias supply) go ahead and order replacements now with your Mouser order before something else goes up in smoke.

[W4RFM]

I have some oil filled 1KV if you need them.

[Rob K2CU]

I think both Digikey and Mouser have "hours" in the filterable items in the capacitor selection process. you should be able to find 2000 or 3000 hours rated parts. worth the extra bucks.

[ka4koe]

Photos. MIEC caps. Yeah, I know. I shouldn't have bought them. Good thing is the guy is refunding me for the two biggies (100 UF). The unswollen one does appear slightly bulged on the end opposite of the valve. Replacement Xicons on the way from Mouser. This shack still stinks like cooked capacitor.

[WBear2GCR]

Just checking, the polarity was correct??

[VE3AJM]

"There are very few problems in life that cannot be solved by the application of high explosive (HE) artillery rounds"

This Valiant project seems to have become quite a frustration for you. I'd say that those electrolytic caps almost did the job.

Al VE3AJM

[ka4koe]

Yessir. Checked and double-checked. I'm also gornto check the bleeders and see if they are within spec.

This odor is hard to get rid of (the capacitor, not me).

[ka4koe]

"There are very few problems in life that cannot be solved by the application of high explosive (HE) artillery rounds"

This Valiant project seems to have become quite a frustration for you. I'd say that those electrolytic caps almost did the job.

Al VE3AJM

Nah. Would I have learned anything at all if I had obtained a pristine unit......? No.

I actually chuckled after it smoked. It was so dramatic (me scrambling for the off switch), what else could I do?

Do the wirewound bleeders/dividers ever go bad? If one of those was faulty, then one of the caps could've been overloaded due to an imbalance. Is this common? They look old, not burned.

[AB2EZ]

Just to be sure:

When you put the capacitors in series, the negative side of one capacitor connects to the positive side of the other capacitor. One balancing resistor should be across each capacitor. Therefore, where they meet in the middle you should have one lead from each capacitor and one lead from each resistor... all soldered together. There should be nothing else connected to this midpoint. Is this how you connected them?

From the picture, each capacitor is rated for 600VDC (interestingly... these are smaller in physical size than I would have expected for a 100uF and 600V rating). In series, with equal balancing resistors, the series pair should have been able to handle 1200VDC.

Assuming you have measured the balancing resistors with a multimeter... and assuming they have roughly equal resistance... I wonder if the soldered connections were good. If one of the resistors was not making a good connection (not that unusual a problem if the lead isn't cleaned well with steel wool before soldering), essentially all of the voltage would appear across the associated capacitor.

Stu

[ka4koe]

I removed two and replaced them with the two new ones in exactly the same places. However, the balancing resistors are not exactly across the caps; they are located on a nearby terminal strip. I will be checking this wiring during Round 2.

[N8ETQ]

Hey Phil,

   Just going through the same thing here with a big
amp.  Last winter I did up a BTI LK2K amp for a cust.
It has a 3X1 in it at 4.4 KV (10 Cap). It made amazing
power and worked Flawlessly. So whats the guy do?
Brings some POS P.Sup  over here and the amp for me
to swap out the nice P.Sup for this POS.  Anyway, I
told the guy "I'm not "Flaming" that unless your here".

    His eye's were 4 inches in diameter when he left.
  Fireball..

/Dan

I removed two and replaced them with the two new ones in exactly the same places. However, the balancing resistors are not exactly across the caps; they are located on a nearby terminal strip. I will be checking this wiring during Round 2.

[WBear2GCR]

Electrically across the caps, not physically.

Something is odd that they would blow, unless they are bogus caps and can't handle anything near the rated voltage. These days there are quite a few parts coming in from asia that are counterfeit and repackaged to be something they are not...

Also, I doubt that the lack of balancing resistors would even cause modern caps to breakdown... nothing wrong with the balancing resistors though. Use them.

                               _-_-

[AB2EZ]

Bear

I respectfully disagree

You can model each of the two real capacitors as an ideal capacitor (no leakage) having value C (farads), in parallel with a large resistor (representing the leakage resistance) having values R1 and R2 (ohms) respectively. When the B+ in initially turned on, half of the voltage will appear across each capacitor.

However... with no added balancing resistors...  following the initial application of B+, the ratio of the voltages across the two capacitors will approach R1/R2 with time constant: R3 x C

The value of R3, in this time constant, is given by: R3 = (a value which depends upon R1 and R2; and which is between the values of R1 and R2... much closer to the smaller of the 2 values if one is much larger than the other)

You can simulate this using (for example) LTSpiceIV.

In the attached simulation, C= 100uF, R1= 1 Mohm, and R2 = 2 Mohms, V(n003) is the voltage across the lower capacitor, and V(n002) is the voltage across both capacitors in series.

In the example, twice the time constant R3 x C is the time when the voltage reaches 386.4 volts... which is approximately 266 seconds. Therefore, in the example, R3 x C  = 133 seconds (approximately). This implies that R3 = 133 seconds / 0.0001F = 1,330,000 ohms (which is between R1 and R2, but closer to R1)

Since R1 and R2 are the respective leakage resistances, it is risky to assume that they are approximately the same.... only that they are both larger values than the selected pair of balancing resistors.

Stu

[ka4koe]

I have a confession to make. I hope my friends here on AM Fone can forgive me. I checked the bleeders and they were 20K per spec. I pulled up the digital photo of the original caps prior to beginning work.....and did a double take.

I had installed on of the new caps backwards. I could've sworn I installed them correctly.

So perhaps the 600V rating did its thing and kept the thing from blowing immediately.

I'm amazed the radio worked at all like that.

Gibbs (NCIS) slap me in the back of the haid, please.



Philip

[W2VW]

I have a confession to make. I hope my friends here on AM Fone can forgive me. I checked the bleeders and they were 20K per spec. I pulled up the digital photo of the original caps prior to beginning work.....and did a double take.

I had installed on of the new caps backwards. I could've sworn I installed them correctly.

So perhaps the 600V rating did its thing and kept the thing from blowing immediately.

I'm amazed the radio worked at all like that.

Gibbs (NCIS) slap me in the back of the haid, please.



Philip

You sir are human and honest.

[KA2DZT]

Philip,

If you have two 20K bleeders connected in series across your 650 volt supply, you should have no problems with equalization of voltage between the two caps.

Make sure you have the midpoint of the two caps connected to the midpoint of the two bleeders.

I would think your 600V caps should have worked FB.   Connecting one cap backwards probably caused the failure.

Reinstall your new caps (correctly this time) and give it another test run.

Fred

[Steve - K4HX]

Forgiveness? For what? Do you think none of us has ever blown something up due to improper wiring, maladjustment or some other mistake we made?  Welcome to the club!!! Glad you figured it out.   

[AB2EZ]

Philip

I agree with Steve, and I'm sure many others who use this board.

I've made my share of mistakes... including hooking up an electrolytic capacitor in the wrong polarity... even though I try to be careful.

We don't need Gibbs... we do a good enough job slapping ourselves on the back of the head when this sort of thing happens.

Stu