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Author Topic: heathkit sb200  (Read 3598 times)
M0WAD
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« on: February 23, 2014, 10:30:33 AM »

hi All
 I have a heathkit sb200 amplifier  heard a loud crack and have found that when i switch on it shows 2100 volts in hv when i go to plate instead of it showing 90milliamps it shows 800 milliamps i have replaced the 33ohm 1 watt resistor which had blown but everything else looks ok PLEASE any surgestions as you can tell i dont know much about amps
thanks m0wad
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W8JI
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« Reply #1 on: February 23, 2014, 12:59:29 PM »

The meter shunt for plate current is blown.

When you repair that you should add a clamp diode from negative rail to chassis.

Your amplifier obviously had an anode to grid flash over.
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W0BTU
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« Reply #2 on: February 23, 2014, 05:04:46 PM »

You are very lucky that you didn't destroy the SB-200's meter movement, which is unobtainium. Next time, it may very well blow! You absolutely need diode protection to protect the meter coil, if somebody didn't already add it.

It may not be the best arrangement, but I have back-to-back 1N4007's across that meter in my SB-200 and an anode fuse with current limiter. That fuse blew a couple of times not long after I replaced the tubes.
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W1QJ
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« Reply #3 on: February 23, 2014, 05:34:08 PM »

You are very lucky that you didn't destroy the SB-200's meter movement, which is unobtainium. Next time, it may very well blow! You absolutely need diode protection to protect the meter coil, if somebody didn't already add it.

It may not be the best arrangement, but I have back-to-back 1N4007's across that meter in my SB-200 and an anode fuse with current limiter. That fuse blew a couple of times not long after I replaced the tubes.

Not really, I have plenty of meter movements for SB-220 and Sb-200's.  Anybody need one?
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M0WAD
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« Reply #4 on: February 25, 2014, 03:14:03 PM »

thanks for the info but amp has the harbach power board fitted so its different than one in manual so can anybody tell me were the meter shunt is in the circuit as i have looked and can't find any resistors blown thanks for any help
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W1QJ
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« Reply #5 on: February 25, 2014, 03:41:53 PM »

It is the 1 ohm 3 watt resistor.  Check it with a meter.  Lift one leg.
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W9GB
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« Reply #6 on: February 25, 2014, 04:04:55 PM »

HARBACH ELECTRONICS
http://harbachelectronics.com/downloads/

You can DOWNLOAD the Instruction Sheets & Notes (PDF).

Harbach PM-200 : POWER SUPPLY MODULE INSTALLATION INSTRUCTIONS
http://www.harbachelectronics.com/_mgxftp/pdffiles/PM-200.PDF
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K8AXW
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« Reply #7 on: February 26, 2014, 07:31:57 AM »

BTU: 
Quote
and an anode fuse with current limiter

Will you supply detail on this please??

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W0BTU
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« Reply #8 on: February 26, 2014, 09:26:47 AM »

BTU: 
Quote
and an anode fuse with current limiter

Will you supply detail on this please??



http://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm#PROTECTION
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W8JI
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« Reply #9 on: February 26, 2014, 11:50:09 AM »

BTU: 
Quote
and an anode fuse with current limiter

Will you supply detail on this please??



http://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm#PROTECTION

I don't think a fuse, or the suppressors on that web page, are such good ideas. The rest looks pretty good.

Let's not kid ourselves and other people about fuses. When an AGC style fuse opens at high voltage, which already takes more time than people think at low voltages, it can sustain current through arcing plasma for seconds.

The only reliable or significant damage reduction comes from the resistor, so the focus should be on it.

I'd just add the right resistor, and not worry about a nuisance part that doesn't protect and only tells us the system had a fault.

The closest cheap fuse that affords some protection is a 3AB type.

http://www.littelfuse.com/products/fuses/cartridge-fuses/~/media/electronics/datasheets/fuses/littelfuse_fuse_508_datasheet.pdf.pdf

If people are just sticking an auto-glass fuse in there, they are doing nothing to reduce surge duration time. The fuse is gone after the arc stops and the plasma goes away. During the fault, the fuse is just a low voltage drop plasma ball of vaporized metal and superheated gasses.

http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_312_318_datasheet.pdf.pdf

Fuses stop fires, they don't protect things during a HV fault.



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W0BTU
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« Reply #10 on: February 27, 2014, 08:24:38 PM »

I appreciate your advice, and I read your similar thoughts at http://www.eham.net/ehamforum/smf/index.php/topic,95048.msg739244.html#msg739244 .

Right or wrong, in my legal limit amp I have an AGC2 fuse and two 10 ohm resistors, all in series. In both amps, I blew several AGC fuses not long after I installed new tubes. In each case, the dim blue flash (when the fuse blew) was of very short duration.

I am fully aware of fuse open time vs. current curves. My thinking --admittedly not based on any math or tests-- was that the 10 ohm resistor would limit the current so that the fuse wouldn't arc or explode. (In other words, the series resistor effectively increases the voltage rating of the type AGC fuses by limiting the arc current [preventing the fuse from developing a sustained arc].)

I've also heard stories of cheap resistors exploding, whether or not the fuse was there. I did have one resistor (one of those cheap white square ones) open when the fuse blew in my homebrew 2x833C amp, and so I have some brown Ohmite vitreous enamel resistors here now in case that happens again.

I know a guy who built an amplifier, who just has a large high voltage fuse in series with his power supply and his 3-500Zs. I could never convince him to add a 20 ohm resistor. His reasoning was that he's blown more than one HV fuse with no other damage.

If I were using tubes with delicate grids, I sure wouldn't have the fuse in there without some means of disabling the drive when it blew. :-)

Maybe I ought to jumper the fuses. But having them there makes me feel better.  Grin

EDIT: I'll look at the 3AB.

73 Mike

BTU:  
Quote
and an anode fuse with current limiter

Will you supply detail on this please??



http://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm#PROTECTION

I don't think a fuse, or the suppressors on that web page, are such good ideas. The rest looks pretty good.

Let's not kid ourselves and other people about fuses. When an AGC style fuse opens at high voltage, which already takes more time than people think at low voltages, it can sustain current through arcing plasma for seconds.

The only reliable or significant damage reduction comes from the resistor, so the focus should be on it.

I'd just add the right resistor, and not worry about a nuisance part that doesn't protect and only tells us the system had a fault.

The closest cheap fuse that affords some protection is a 3AB type.

http://www.littelfuse.com/products/fuses/cartridge-fuses/~/media/electronics/datasheets/fuses/littelfuse_fuse_508_datasheet.pdf.pdf

If people are just sticking an auto-glass fuse in there, they are doing nothing to reduce surge duration time. The fuse is gone after the arc stops and the plasma goes away. During the fault, the fuse is just a low voltage drop plasma ball of vaporized metal and superheated gasses.

http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_312_318_datasheet.pdf.pdf

Fuses stop fires, they don't protect things during a HV fault.

« Last Edit: February 27, 2014, 08:43:43 PM by W0BTU » Logged

W8JI
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« Reply #11 on: February 28, 2014, 04:51:55 AM »

Right or wrong, in my legal limit amp I have an AGC2 fuse and two 10 ohm resistors, all in series. In both amps, I blew several AGC fuses not long after I installed new tubes. In each case, the dim blue flash (when the fuse blew) was of very short duration.


I'm not saying the fuses don't blow. They make wonderful indicators.

The thing that limits the surge is the total ESR and impedance of the loop path that includes the arc. If you have enough impedance in that loop compared to capacitance the voltage can drop nicely.

All I'm saying is the idea of a thin wire or a fuse to limit fault current is misplaced. I don't have a fuse now on a 7.5 kV supply and when a tube faults, it just goes "tick" and opens the breaker. I have a 50 ohm 200 watt carbon bar resistor about a foot long. 

The AL12 series added a 2 inch carbon bar, ~25 ohms, for the fault (because it happens so often with Chinese tubes) and the AL8 mainframe now has a 20 ohm made from two 10 pulse rated resistors in series. That equipment has about the same range of additional impedance of 10-15 ohms plus a little inductance.  If it had 12 ohms through the path the resistor would add another 20 for 32 ohms.  This gets you into the 90 amp range or less for fault current. Without the resistor it is closer to 250 amps, which works out to be well over eight times the surge power into the components in the fault loop.

All of the protection is in the resistance. The fuse is too slow and unreliable to save most things.
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