Need help replacing Valiant Mica caps

[K1CWS]

I am restoring a Valiant which had vaporized tank circuit Mica caps. It blackend the cahssis and melted the wiring harness. Some of the caps are actually series/parallel combos. I guess to get the voltage rating up. Each cap is 500V. This happened before I got it, so I don't know what caused the problem.

I have some A2 Mica caps that registered a 5 megohm reading on my VOM. I connected a 100Ua meter to 300VDC supply and they read from 1 to 50 Ua. Am I correct is assuming these are NG?

My thinking is to buy new caps where each cap is 1KV rating since the original ones were 500VDC and the blewup.

I found some mica caps at surplusssales.com. Anyone know another source?

[Jim, W5JO]

If those caps blew like that, make a close inspection of the bandswitch wafer where they were attached.  You might just be looking for one of those as well.

[W4RFM]

If the parts in question are the old brown mica jobs let me know what you need, I bought a sack full at a fest last year.

[Ed/KB1HYS]

rebuild that "special" cap too. Gut the switched in caps and go with some good quality RF types. The special cap is prone to failure (mucho smoke).  This part of the system was not very well done (IMHO). I pulled them out and built in "doornob" types of similar values. (exact matche not really required).  No problems since that repair.  I did try to replace them with some expensive ribbon leaded RF caps and they smoked.   Transmitting Micas of the larger size would work, but I found space a little tight and went with the small HV doorknob type.

[K1CWS]

The special cap is one that blew. To replace that, I need a 600pf, 900pf and 1200pf @ 1500V. The other caps I need are two 300pf and one 150pf @ 1500V.

The rear section of the band switch had a burnt contact which I repaired with a contact for another similar Centralab switch.

[Jim, W5JO]

Bruce, I hope you carefully examined that switch rotor between the little rivets.  They are very close together and every  one in line is for the opposite side.  As for the postage stamp size micas, you can use 1KV silver mica there.  No big deal just replace with the same set up.

Do as Ed suggested, for the space allowed.  Being off a few pf or hundred will only make a difference where the fine variable cap ends up when loaded to full output.

[K1CWS]

Hey Jim, you called it! I found carbon tracks between the rivets and the shorting contact. I used a diamond needle point bit in my dremel tool and ground it out. I will want to get a better one, but where?

[Jim, W5JO]

Good find Bruce.  I wouldn't trust trying to clean that switch, I would see if you can find a parts rig somewhere and hope the one in it doesn't have the same problem.  Someone here with a part may be able to help you but if you don't find something to to the QTH mail list for Johnson and post there.

I have what I think is that wafer, but am not sure.  It has been a long time since I looked at a Valiant.  I quit fooling with them mainly because of this very problem.  Should one leg of your antenna be down and you have high reflected power, it can or will cause this switch to arc.  It isn't a matter if this will happen, it is when.

If you have a camera and can take a picture of the wafer, send it to me @ w5jpw (at) yahoo.com so I can compare what I have to it.  If it is the right one, you can have it.  Problem is I don't know if it is good or not.

[Ed/KB1HYS]

Post the photo here too. 

I've got a couple ceramic switches too... not sure about fit though.  IF I get the chance I'll be opening the Valiant soon to do some more work so I'll take a look and compare. Pictures here may be quicker though!

i

[K1CWS]

Here's the sw. It is the rear section on the band sw.

[KC9GMF]

This is what I did to solve this problem.
N-750's for the caps, and 0-80 nylon bolts
for the switch, stable and no arching for
over 2yrs.

Tom
KC9GMF

[w3jn]

N750s are a very poor choice for loading caps.  As they heat up they'll change value considerably (-750 ppm/degree C).  Replace them with NP0s of the same size.

[WA1GFZ]

John,
I don't think NPOs go that high in value for that size. At 100 watts they may stay cool enough to be stable. I think the highest value I have seen in NPO is 270 pf. I've been collecting them for years to build a filter for a SS QRO amplifier. Agree N750s are quite unstable over temperature.

[KC9GMF]

W3NJ

"N750s are a very poor choice for loading caps."

I'll defer to the Experts, but why do these work so well?
I have a box full of caps I tried they all would drift...(heat)
mica's, ceramic, etc.

"As they heat up they'll change value considerably"

Well, these don't! The originals did, so did everthing else I
tried in this crkt. Keyed down on 160M they seem just fine
plate dip never changes, "like a rock", cool to the touch.
They are rated for ~3.8A@1Mhz and ~ 7A@10Mhz,
and they fit in place nicely.

What Did You use in Your Valiant???
(please no VV's or AV's it's a Valiant)

GMF

[KC9GMF]

oh, did I mention current?

GMF

[Ed/KB1HYS]

Nice work there. Looks a lot better'n mine that's for sure.

The current rating is key.  I tried some expensive "RF" type caps that were supposedly rated for high curretn applications.  They had ribbon leads about 3/8 or so wide. When one of THOSE smoked I pulled them all and used doorknobs similar to what you've got. Not a problem since...  I was luck enough that the switch didn't get a sorch so I left it as is (maybe not a good idea).  All that was maybe two years ago.

I'm also using a much better tuna now so maybe the system isn't getting beat up as badly as before. (always sees a 50+/- ohm load)

[ke7trp]

what values of door knobs did you use and where can I purchase them?  Some of you guys helped me through this exact same issue with my valiant. The back switch section was arcing to the rivet.  I got a new one and replaced it and then checked the caps. THe caps where good but I think I will go ahead and install the door knobs. That looks much better.

Now If I can figure out why R39 SMOKED while at idle.   Sitting here on the computer and smoke POURED out of the valiant. I switched it off and on closer inspection R39 is Toast.

It takes a valiant effort to keep a Johnson running!

Hooked up the ranger1 and it works perfectly at 45 watts LOL. 

Clark

[W1EUJ]

N750 capacitors, BY DEFINITION, have a -750 ppm change in capacitance per degree C. The small ones are used in temperature compensation. The large ones (perhaps because they were cheaper to make) were only used where tolerance was not an issue, such as for decoupling. They are a poor choice for anything where you are depending on the capacitance staying put. Is that what it's being used for?

Why has it worked so well? I don't see your circuit, but it is perhaps (a) you didn't let it warm up long enough (b) the tolerance at that particular point does not need to be that great (c) you didn't have the right conditions for a failure (d) all of the above.

[Steve - WB3HUZ]

Go with 850 series ceramics.

[KC9GMF]

"what values of door knobs did you use and where can I purchase them?"

I got them from RF Parts, back then about 1/2 the price.
580170-7  170p 7.5kv
and
580140-5  140pf 5kv

Bought some 140's 160's and 170's to try and get the right
value needed, ended up with 2)170's in parallel and one 140.

once again, I tried  big fat bad ass ceramics and they got hot
I'm sure there may be a better cap but space a the end value
kind of play into this too! You just don't pick up phone and say
I need XYZ @ Xkv

This Valiant lives on 160, and keyed down for 5-8 min, they stay cool
more than I can say for the old "iron" which has seen a change too.

Tom

[KD6VXI]

"what values of door knobs did you use and where can I purchase them?"

I got them from RF Parts, back then about 1/2 the price.
580170-7  170p 7.5kv
and
580140-5  140pf 5kv

Bought some 140's 160's and 170's to try and get the right
value needed, ended up with 2)170's in parallel and one 140.

once again, I tried  big fat bad ass ceramics and they got hot
I'm sure there may be a better cap but space a the end value
kind of play into this too! You just don't pick up phone and say
I need XYZ @ Xkv

This Valiant lives on 160, and keyed down for 5-8 min, they stay cool
more than I can say for the old "iron" which has seen a change too.

Tom

The temp coefficient doesn't play all that much if you aren't heating (too much I) them up.

If they can get to operating temp (ie, the temp controlled oven we call a boat anchor), and you are running marginal current through them, compared to the rating, then they won't get all that much warmer as you key down.

So, if you have large enough current ratings on them, you will be fine.

I used TV style doorknobs in my Valiant in the 80s.  No problems, and I believe it is still functional and running today.

--Shane