I'm Cursed I tell you, cursed!

[ka4koe]

Busted my SB220!

One of the jumpers got shifted around when I was working on something inside the HV cage. Put it all together and BANG!

See attached photos. The red circled area is the cause of all the hatefulness.

[ka4koe]

More photos.

I toasted the meter/rectumfryer board, RFC 6 underneath, and the jumper ain't looking too good either. See attached skeemagic for the fault path.

Hope this didn't fry the meters as well.

Already placed an order with Harbach to replace the rectumfryer board.

[WD5JKO]

 I wonder if that braid was stock Heath, or was there something more stiff in there originally? Some 16awg buss wire inside that braid would hold a bend to keep the clearance well away from that tank coil.

  This incident illustrates what happens in most power amps when the plate bypass, er, I mean coupling capacitor if it were to short out suddenly. The little RF choke across the pi-net output to ground will vaporize. That choke would have to be MUCH beefier to survive. Consider the plate coupling capacitor shorting, the RFC vaporizes, and yet everything works fine...except there is 3KV on the antenna.  

  An antenna with a big balun usually is a DC short, and could prevent this hazard, depending on what else is in line. A tuna with a  'T' network will isolate the feedline B+, but not short it out.

Jim
Wd5JKO

[KA2DZT]

Jim,

You say plate bypass cap.  I guess you're referring to the plate coupling cap?  Bypass cap, I think of the B+ bypass cap on the plate choke.

Fred

[N8ETQ]

   You'll like that Harbach Board..  Diode protect the meter!

/Dan

[WD5JKO]

Jim,
You say plate bypass cap.  I guess you're referring to the plate coupling cap?  Bypass cap, I think of the B+ bypass cap on the plate choke.
Fred

  Fred, thanks for the clarification...I edited my original post. My hands typed something different from what I was thinking. 

Jim
Wd5JKO

[ka4koe]

I also checked the meters carefully and they appear to be okay. So perhaps we'll get off fairly cheaply on this one. The tubes are the original Eimacs and did not arc over the first time I energized the amp. B+ reads (again, before the hatefulness) about 500v low, but that may have been the metering/rectumfryer board resistors changing values over the years.

If the Eimacs were gassy I assume they would have arced over on first application of HV.

I think I'm gornto replace the big red doorknobs with new BEFORE they fail.

I have a new Measures parasitic suppressor kit so that is going in as well (yes, I know there has a been a hot/spirited and sometimes hostile debate on this for at least 20 years). The resistors in the originals look like they've gotten hot over the years, as the color codes are dark and hard to read.

The other bright spot in all of this is the Valiant is working nice and stable. I will NOT claim it is fixed. I don't think these old transmitters are ever really and truly "fixed".  The audio improvement with the original D104 is such that I likely will put off doing Timmy's audio mods for awhile. A friend on-air keeps telling me to install an MFP102 fet to mod the D104? Why is this necessary?? The D104 is already seeing about 9M input Z on the first audio stage.

[w1vtp]

Hand over the heart at we observe the sacrificial passing of RFC-6.  It gave its life for you, Phil. 

Would not using a copper strap be better then using braid?  That's a very close proximity of the braid to tank circuit and braid can shift its position.  I built a SB-220 decades ago and had a heart stopping experience when a short developed from the grid to plate which took out that parasitic choke.  The tube was in warranty so Heathkit sent me a new 3-500 and the choke.  After that turning on the amp was very stressful for me.

Al

[ka4koe]

Any idea what current rating a replacement choke should handle. Fairly stiff, I would think. Only one of the breakers tripped----if it went fault on negative or positive going part of the AC supply, this might explain that as well.

[W2VW]

Those plate blocking caps are not usually a problem.

Research glitch resistor and if there's a next time the fried parts count will be low.

I'd replace the safety choke with the same thing that was in there. A glitch resistor should keep it from destruction.

Late 80s 3-500Zs can  be a problem.

Other than that they seem to live as well as other tubes with similar construction provided someone had the brains to dissipate them hard enough to glow once in a while.

Buy some more older equipment. This is interesting.

[ka4koe]

"Buy some more older equipment. This is interesting."

You know, this made me chuckle......but the statement is almost twisted in a perverse sort of way...



Rationale: Philip has bad luck in general......if he buys more old stuff we'll get to see him blow it up and then try to fix it!

You guys/gals are good sports. I appreciate that.

Philip KA4KOE

[N8ETQ]

Hey Phil,

     Nothing special I wouldn't think. I see regular 2.5 mH stuff
in some pretty big amps. It's just to blow the breakers/fuses
if the "Plate Blocking" cap craps out. (Or gets bypassed!)
An "ARC" is fairly low impedance.

/Dan

Any idea what current rating a replacement choke should handle. Fairly stiff, I would think. Only one of the breakers tripped----if it went fault on negative or positive going part of the AC supply, this might explain that as well.

[Pete, WA2CWA]

The other bright spot in all of this is the Valiant is working nice and stable. I will NOT claim it is fixed. I don't think these old transmitters are ever really and truly "fixed".  The audio improvement with the original D104 is such that I likely will put off doing Timmy's audio mods for awhile. A friend on-air keeps telling me to install an MFP102 fet to mod the D104? Why is this necessary?? The D104 is already seeing about 9M input Z on the first audio stage.

Over the years I've noticed that there are some operators that just never sound good or great on AM no matter what audio mods they apply to their rigs, microphones, etc. That's why CW, SSB, or other modes are still popular.

[KB2WIG]

First, it's lightning on the eL.
 Then, its the "Death (and resurection) of the Valient'
   And then, he makes his own lightning.

    "So you get what we had here last week, which is the way he wants it. Well, he gets it. I don't like
      it any more than you men."

         KLC

[K1JJ]

Now that the amp is apart, might as well add some mods:

1) As already suggested, a 25 ohm, 10 watt resistor in series with the HV lead will soften future short events.

2) Wind up some #22 insulated wire on a cigar-sized PVC (1/2" to 3/4"  diameter form) for safety in case the coupling cap shorts. This is placed at the 50 ohm antenna  SO-239.  Replace that little limp-wristed 2.5 mH widow maker.  It doesn't take a lot of inductance to be invisible there.  You want this choke to wihstand a direct short from the HV power supply without opening.

3) Consider winding your own plate choke with larger wire. I've wound all my own for years and have rarely had a problem, either in operation or when the other parts go south around it.   If they blow, we can just laugh and wind another - no big deal.


T

[W2VW]

"Buy some more older equipment. This is interesting."

You know, this made me chuckle......but the statement is almost twisted in a perverse sort of way...



Rationale: Philip has bad luck in general......if he buys more old stuff we'll get to see him blow it up and then try to fix it!

You guys/gals are good sports. I appreciate that.

Philip KA4KOE

Most people who have the skill set to keep this equipment alive have been through a lot of similar problems. Some can't remember it.

[ka4koe]

They can't remember because they have "become one with the snows of yesteryear?"

[ka4koe]

Consider winding your own plate choke with larger wire. I've wound all my own for years and have rarely had a problem, either in operation or when the other parts go south around it.   If they blow, we can just laugh and wind another - no big deal.

---have you analyzed any resonances? If we have one in a band, the sucker will become an M80??

[W2VW]

A friend on-air keeps telling me to install an MFP102 fet to mod the D104? Why is this necessary?? The D104 is already seeing about 9M input Z on the first audio stage.

No need for an MPF-102 in your case with the existing high load impedance.

[K1JJ]

Consider winding your own plate choke with larger wire. I've wound all my own for years and have rarely had a problem, either in operation or when the other parts go south around it.   If they blow, we can just laugh and wind another - no big deal.

---have you analyzed any resonances? If we have one in a band, the sucker will become an M80??

First, they must be analysed in circuit with the tube in place to simulate all stray capacitance and inductance added in.  You can analyse them fairly easily... why not?

Personally, no, I don't usually analyze my chokes.   I feel it's VERY difficult to actually hit a ham band with a resonance. Like spinning a roulette wheel and winning, but harder.  So let probability work and if it blows, it will probably rip the winding apart and no more. They tend to implode the winding.   Wire is cheap and try again with less or more turns.  There are also some existing hefty designs out there that can be duplicated without the guesswork.

I tend to over-rate my parts cuz I get tired of making dumb tuning mistakes when I'm spaced and paying the price.

T

[N8ETQ]

LOL  !!


  "cuz I get tired of making dumb tuning mistakes when I'm spaced and paying the price."

[ka4koe]

One of my theremin friends in France emailed me this fuse guide for future use.

Philip

[ka4koe]

Finally found some time to fix the amplifier. I cleaned up the scorched mess and installed a new Harbach rectumfryer board and RFC6. Readings are now:

CW/TUNE

RX: 2550V TX: 2450V Ip: 100 mA

SSB

RX: 3200V TX: 3050V Ip: 160 mA

[ka4koe]

The SB220 is putting out about 1200W on high power with the original Eimacs. I busted the W1AW/KP pileup on 7173 KC last night on the first time (yes SSB). This is an addicting experience.

[Pete, WA2CWA]

The SB220 is putting out about 1200W on high power with the original Eimacs. I busted the W1AW/KP pileup on 7173 KC last night on the first time (yes SSB). This is an addicting experience.

Which: getting a rig fixed and working, or busting a pileup??