TheRadioBoard

I'm building a VFO..

So, first I do the power supply. I put a transformer in the chassis, then dug into my junk boxes for a set of suitable diodes for a bridge. I have 4 matching 1N255's. Rated at 380 volts @ .4 amps, they will handle what I'm building and since they are in the junk box they will do quite nicely.

I've never used Stud type diodes, but no problem, mount 2 of them on them on the chassis and two of them on stand offs. Mount a choke and a pair of cardboard wrapped axial lead electrolytic capacitors.

I'm sure someone already knows what I did wrong, but to continue, I wire everything up.

Then, before I go any farther, I smoke test the power supply. I should get ~ + 185 volts, but instead I have ~ -70 volts. So I do a quick visual to make sure I have the meter connected right. About that time ...

KA-POW

Sounded like a cherry bomb. One of the caps exploded. Oil everywhere. I shut off the power strip. What did I do wrong???

I assumed (without checking) that the stud end of the diodes (where they mount) is negative which would make the chassis negative. Wrong, the stud end is positive. Which means I wired up the electrolytic caps backwards.

Clean oil off the chassis. Replace 2 blown electrolytic caps. Mount all four diodes on standoffs and make double sure the chassis is negative. Now it works.

So.... If you want to know what happens when you push 185 volts backwards through an electrolytic cap rated for 450 volts, you don't have to try it, I already did and I can tell you it turns them into cherry bombs.

Don't Try This At Home.

_________________
Secret of Electronics - keep the smoke _in_ the wires.

Ah, yes, the old backwards connected electrolytic capacitor. It sure gets your attention. And, it makes you realize why you should always wear eye protection when working around prototype electronic circuits.

But, don't feel too bad. I've been there and did that, too. With me, it wasn't connecting it in the circuit backwards, but rather using a full wave bridge rectifier instead of a center tapped full wave rectifier off the transformer secondary. Thus, instead of the 400 Volts I expected, I applied closer to 800 volts across the pair of 100 uF/450 Volt electrolytic capacitors. KABAM!

By the way, that's not oil, but a salt/water solution, usually a Borax salt in the old days, but who knows what it is now. It's not oil, since it's designed to be conductive.

Also, note that there's some ambiguity in the specifications of the 1N255 diodes, depending upon the manufacturer. One manufacturer whose datasheet I found listed them as 400 Volt/1 Amp diodes, while another listed them as the 380 Volt/.4 Amp diode.

Also, note that the 1N255s are available in either standard configuration or reverse configuration, which is designated by an "R" suffix on the part number (e.g., 1N255R).

Another issue to consider is the surge current rating of the diodes when you turn the supply on. Back decades ago, when I was building my general purpose bench B+ supply, I kept blowing "top-hat" diodes. It seems that the combination of the beefy transformer (with low leakage inductance), the pair of 100 uF capacitors, and the general heavy-duty wiring I was using was causing the surge current rating on the diodes to be exceeded, which was popping them at turn on. There are, of course, several solutions to this. Since this was decades ago, I switched to using a 5U4 vacuum tube rectifier. But, adding a bit of series impedance to limit the surge current would have worked also. Anyway, it's something to consider.

Good luck on the VFO. There are several tricks to building a stable VFO. The main tricks are to keep the voltage stable, and to run the oscillator stage at as low of a power as you can, and use one (or more) buffer stages to get the output to a reasonable level.

Dave
WA4QAL

Microbes, now I know why your picture looks the way it does.

Good story. Been there, done that, Really, my empathy runs deep. Life is rich, isn't it!

Macrohenry

Congratulations! i have a few anecdotes regarding electrolytic capacitors, including a few self-inflicted spontaneous explosions. Somewhere around here I have Flying Pigs certificate #2 for smoking something by reverse-connecting the power. You qualify for one, also. Can't find the link on their site, though.

73,

FP-835

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

[singing]
Young Microbes had a bench.
Eee-ay-ee-ay-away-blown.
Boom-boom there. Ka-pow-pang here.
Eee-ay-ee-ay-away-blown
[/singing]

Ho ho. Only kiddin'. Who doesn't recognize themselves here Micro.

Isn't that part of the fun of home brewing?

Gus

It's not fun when those capacitors going boom cost the poor student a week's wages. Ouch! That was an expensive lesson.

But, that 3 foot diameter Borax solution steam cloud was pretty impressive (and stinky, too!). :*)

Dave

Congrats on blowing the caps...been there done that...the story
is pretty impressive..it wasn't me (really it wasn't), but I WAS there.
It wasn't caps though. About 20 years ago, I was working in the GE
facility that tested CT scanners before they were shipped to hospitals,
on second shift. We had a policy of any time a tech was checking the
high voltage inputs to their scanner, another tech had to be present,
a few feet away by the contactor, to shut it down in case of a problem.
The tech in the next bay was going to check the 440 3 phase power and
went to get an analog volt meter from the tool crib. Came back, hooked
up the meter to test the first leg. The meter was laying on it's back
about 1 foot in front of the tech doing the test. Before I get to the next
part I should say the building was built like a warehouse, with 20 foot
ceilings. So the meter was attached, the tech stepped back, I threw the
big switch, then pressed the "start" button. "BOOOM", the glass front
plate of the meter blew out of the case in several fragments, most of
which hit the 20 foot ceiling and rained down on our heads. Of course I
hit the stop button and pulled the switch... Now you know what happens
when you put an analog AMMETER across two legs of a 440volt three
phase line! The other tech had not confirmed what was handed to her.

Another time, one of the other techs was temporarily flash blinded when
a ground lead fell off a test point hitting a 3/16inch thick, 3/4 inch wide
copper buss bar, vaporizing a section about 1/4 inch wide and 3/8inch
long down the center. All that was left of the lead was the handle and the
banana plug on the other end, at least as far as we could find...

I noticed that. Not being sure who made these I went with the more conservative figures. Kind of strange to me that the same part number would have ratings that far apart.



I'm glad I'm not the only one who's pulled this trick.

At least I have enough sense to hook the meter up and then step back away from something like this before I power it up for a smoke test.

What I hope to use the VFO for is 80/40 meter AM and/or SSB unit since I now have a General Class ticket (KJ4JVB) . I have a SSB unit about 1/2 built, but I'm having a hard time locating a 2Q4 audio phase shift network so I put that project on hold and will work on an AM unit next. Since you are in Florida (I'm up in the middle of the state, Citrus County) maybe I'll chat with you when I have something on 40 meter up and running.

The only thing I have up and going right now is an old Clegg FM-88 2 meter rig. It seems to be an ok radio, and I can use it on the most used repeater around here with it, but I do need to build a tone generator for it before I can hit them all. (not a big project)

But 2 meter isn't a DX band...

_________________
Secret of Electronics - keep the smoke _in_ the wires.

Don't overlook the Central Electronics PS-1 phase shift network if you can not find a 2Q4. Definately fun things to play around with, along with a quad of 6V6 tubes.

I once found a 2Q4 in a box of metal tubes that I paid a dollar for at a hamfest! To all others, it was just another metal tube, like a 6SK7. But when I saw it in the box, I knew it was coming home with me! HI!

I have a CE 20-A exciter and a modified BC-458 for a VFO and it is a fun thing to play with on the 40 and 20 meter bands. It was fun to check into the 14.286mc memorial net in the evenings on AM and after the net was over, the slopbucket operators would take over. So I would simply slowly pull the carrier out from under the AM signal and join them.
Curt

_________________
Curt, N7AH
CW forever
Connoisseur of the cold 807

Love it FT.
I can't help relaying another incident that has stayed with me through 40 years.
As a first year apprentice in the mid sixties, my boss was going to show me how to do an full IF alignment on one of those diabolical US inventions, the portable, transformerless, valve TV. Yes, the ubiquitous POPE 14P10.
Joe powered up the B&K TV Analyst, connected the signal lead from the analyst to the aerial terminal and then attempted the clip the earth gator clip to the chassis. Big Mistake! It was a good quality heavy Utilux clip and the fuse in the meter box was much larger than it should have been.
The clip evaporated in one blinding flash. We found only one half of one jaw of the clip and Joe sported a very burnt thumb and finger.
At least I learnt a valuable lesson. Don't forget the isolation transformer!

Love it FT.
I can't help relaying another incident that has stayed with me through 40 years.
As a first year apprentice in the mid sixties, my boss was going to show me how to do an full IF alignment on one of those diabolical US inventions, the portable, transformerless, valve TV. Yes, the ubiquitous POPE 14P10.
Joe powered up the B&K TV Analyst, connected the signal lead from the analyst to the aerial terminal and then attempted the clip the earth gator clip to the chassis. Big Mistake! It was a good quality heavy Utilux clip and the fuse in the meter box was much larger than it should have been.
The clip evaporated in one blinding flash. We found only one half of one jaw of the clip and Joe sported a very burnt thumb and finger.
At least I learnt a valuable lesson. Don't forget the isolation transformer!

Hello WA4QAL,
there were no real changes since the 30ties:
Water, borax or boric acid, mannite, ethylene glycole or diglycole.
If You want to make experiments, a plain solution
of sodium bicarbonate will do.
Georg

Yes, for the few times I've played with building my own electrolytic capacitors, I've used 20 Mule Team Borax:

http://en.wikipedia.org/wiki/20_Mule_Team_Borax

as the electrolyte solution.

Dave