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Author Topic: SB-200 what won't power up?  (Read 4368 times)
WALTERB
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« on: July 08, 2013, 04:40:10 PM »

I picked it up a few weeks ago. at the suggestion of my Elmer, I put a Variac on it, about 4 hours into the powering up process, I walked out of the room, when  I came back, the Variac was going but the amp was dead.  Shocked Slight electrical smell in the air, like something burned up.  I plugged it directly into the outlet, and nothing. 

I assume it is an internal fuse?  Capacitors?  I guess this is my chance to rebuild an amp?  Grin

thanks.
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W8JX
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« Reply #1 on: July 08, 2013, 07:49:22 PM »

There are breakers under top cover in front just right of center. Capacitor and bleeder resistors would be first thing i checked. Harbach makes a complete power supply replacement board with CAPs, resistors and diodes.
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WALTERB
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« Reply #2 on: July 08, 2013, 08:33:00 PM »

There are breakers under top cover in front just right of center. Capacitor and bleeder resistors would be first thing i checked. Harbach makes a complete power supply replacement board with CAPs, resistors and diodes.

thanks.  I'll check and see.
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KE3WD
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« Reply #3 on: July 09, 2013, 05:41:55 AM »

A 100W filament light bulb placed in series with the Line between Variac and Amp would have prevented the smoke during this process.  If the unit under test has a component fail to short, the only thing that happens is the light bulb glows brighter. 

Wiring a 120VAC rated switch ACROSS the light bulb filament such that you can short the lightbulb out of the circuit once your testing is complete is a good idea also. 

One of the easiest pieces of test equipment to obtain, consists of an AC light socket and a switch, build it into a box or otherwise make it safe, the Variac and Lightbulb are actually the piece of test gear that gets used darn near on every repair or troubleshoot here. 


73
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W8JX
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« Reply #4 on: July 09, 2013, 05:55:21 AM »

A 100W filament light bulb placed in series with the Line between Variac and Amp would have prevented the smoke during this process.  If the unit under test has a component fail to short, the only thing that happens is the light bulb glows brighter. 

Wiring a 120VAC rated switch ACROSS the light bulb filament such that you can short the lightbulb out of the circuit once your testing is complete is a good idea also. 

One of the easiest pieces of test equipment to obtain, consists of an AC light socket and a switch, build it into a box or otherwise make it safe, the Variac and Lightbulb are actually the piece of test gear that gets used darn near on every repair or troubleshoot here. 


73

100 watts bulb would provide too much voltage drop. Maybe 2 or 3 in parallel.
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KE3WD
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« Reply #5 on: July 09, 2013, 07:23:09 AM »


100 watts bulb would provide too much voltage drop. Maybe 2 or 3 in parallel.

The point is not to be able to *operate* the amp at full power. 

The point is to be able to bring up an unknown, or form caps in an older unit that's sat for some time, etc. without incurring smoke due to possible component failures. 

Adding more bulbs in parallel would soon defeat the purpose. 

And, I might add, I've use the 100W bulb, even 60 watters at times, for years, on all sorts of equipments, many of which have been linear amps.  Even have a 220 Variac set up, if needed, that uses a 250V filament bulb.  That one in particular has been used to bring up some pretty stout stuff, including motors.  With the one bulb.

Dunno why 8JX is obsessed with my posts these days, but he's wrong once again. 


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KD0REQ
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« Reply #6 on: July 09, 2013, 08:22:11 AM »

the point is that since the 20s, the "filament testbench" has kept a lot of shorted equipment from causing further damage, inside and out, during initial checkout.  the equipment is NOT supposed to run, although lots of tube receivers will operate just over the threshold of deaf in "starved circuit" voltage conditions.  the point is if the bulb glows full brightness, you have a big oopsie in the machine.
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K8AXW
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« Reply #7 on: July 09, 2013, 09:17:28 AM »

OK boys and girls, time to get back to the problem at hand.  The amp has already let the smoke out..... so now is the time to find out what went wrong.

WALTERB:  When you said "dead" that means no HV showing on the meter.....no filaments lit....right?

You can have a loss of HV but the filaments should still light up if the circuit breakers are set.  When you opened the box, does anything stand out as being bad?

More info please.

BTW, a safety note is imperative here.  Even though the linear might appear to be "dead," every precaution should be taken to make sure the individual caps are bled to zero.  It might just be setting there waiting to make you "dead!"
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KE3WD
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« Reply #8 on: July 09, 2013, 11:33:54 AM »

That light bulb also acts as the top resistance in a Voltage Divider. 

Proper use of Lighbulb and Variac can reduce the amount of voltage inside the amp to much lower and safer levels when troubleshooting faults. 

73
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WALTERB
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« Reply #9 on: July 09, 2013, 02:01:16 PM »

OK boys and girls, time to get back to the problem at hand.  The amp has already let the smoke out..... so now is the time to find out what went wrong.

WALTERB:  When you said "dead" that means no HV showing on the meter.....no filaments lit....right?

You can have a loss of HV but the filaments should still light up if the circuit breakers are set.  When you opened the box, does anything stand out as being bad?

More info please.

BTW, a safety note is imperative here.  Even though the linear might appear to be "dead," every precaution should be taken to make sure the individual caps are bled to zero.  It might just be setting there waiting to make you "dead!"

The variac shows life,  120 volts on the meter.

The SB-200 appears as if its unplugged.  no lights, no sounds, nothing.  I also plugged in directly to the wall outlet just to see what happened.
again,  nothing. no life at all.
  
As far as making sure the capacitors are bled out,  I've been told this for years, but nobody ever tells me how to or how long to wait.  Grin

I have a lot of honey-dos lined up in front of me for the next few weeks.  fixing this amp wasn't on the schedule, and I doubt the XYL will bump her stuff to get it in there.  Grin

Anyway, for future reference, how can I make sure all is safe when I play under the hood (other than making sure the amp is unplugged)?

thanks.
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K4RVN
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Posts: 794




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« Reply #10 on: July 09, 2013, 02:58:53 PM »

Did you check and try to reset the circuit breakers that W8JX mentioned? Did you check the circuit it was connected to during the variac procedure to make sure it is still hot? If so, with the amp unplugged, check the circuit breakers to make sure they have continuity using an ohm meter. After that check the power switch for continuity. Even with open diodes, no HV except for a short in the h.v. transformer you should have filaments and the fan would run or it would indicate that the circuit breakers are tripping.
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WB2WIK
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« Reply #11 on: July 09, 2013, 03:34:34 PM »


As far as making sure the capacitors are bled out,  I've been told this for years, but nobody ever tells me how to or how long to wait.  Grin




If the front panel voltmeter works, when it drops down to zero, the caps are fully discharged and that's that.  Should take a minute or so, assuming the bleeder resistors are intact.
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W8JX
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« Reply #12 on: July 09, 2013, 04:41:49 PM »


100 watts bulb would provide too much voltage drop. Maybe 2 or 3 in parallel.

The point is not to be able to *operate* the amp at full power. 

The point is to be able to bring up an unknown, or form caps in an older unit that's sat for some time, etc. without incurring smoke due to possible component failures. 

Adding more bulbs in parallel would soon defeat the purpose. 

And, I might add, I've use the 100W bulb, even 60 watters at times, for years, on all sorts of equipments, many of which have been linear amps.  Even have a 220 Variac set up, if needed, that uses a 250V filament bulb.  That one in particular has been used to bring up some pretty stout stuff, including motors.  With the one bulb.

Dunno why 8JX is obsessed with my posts these days, but he's wrong once again. 


No, see I look at what amp needs to just power up. A sb 200 needs 50 watts just for filaments, then 10 or 15 watts for fan,  and likely another 100 for power supply and bleeders and then transformer losses. See i do the math and a single 100 watt bulb is not enough you want 200 watts or more just to power on and get at least 60 t0 70% of hv voltage to test it. Not gonna happened with one bulb. Even three would not let it make full high voltage and protect it from a short. 

No obsession on my part but maybe yours...
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KE3WD
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« Reply #13 on: July 09, 2013, 07:40:17 PM »

Again, not talking "full high voltage" at all, which actually is the point. 
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K8AXW
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« Reply #14 on: July 09, 2013, 09:31:16 PM »


Quote
  RE: SB-200 what won't power up?
« Reply #11 on: Today at 03:34:34 PM » Quote 

--------------------------------------------------------------------------------

Quote from: WALTERB on Today at 02:01:16 PM
As far as making sure the capacitors are bled out,  I've been told this for years, but nobody ever tells me how to or how long to wait. 

If the front panel voltmeter works, when it drops down to zero, the caps are fully discharged and that's that.  Should take a minute or so, assuming the bleeder resistors are intact. 

WIK:  Two words jump out of the screen OM.  The first one is the IF and the second is the ASSUMING.

The problem in this case is the amp owner smelled "a slight electrical smell in the air."  This one thing is what worries me..... did the smell come from a burned trace on the power supply PCB?  If so, then there is the possibility that some of the filter caps didn't bleed down.

Remote possibility?  Sure but who can say for sure that this isn't possible?

WALTERB:  When you get the time to get into this problem, you're going to have to see if you can find something that might have given off that smell.

I'd check the AC line into the amp, the continuity of the breakers and the front panel power switch and then the transformer primary winding.  If all of these things check out OK, then it's time to get into the secondary.

Before even starting here, make a "chicken stick."  This is a piece of dry wood....like a 1/2 or 3/4" wood dowel rod about a foot to 16" long.  Form a bare hook in a piece of insulated wire and tape it to one of the dowel.  Put an alligator clip on the free end. 

You use this thing by clipping the alligator clip to chassis ground FIRST, putting one hand in your pocket and with the free hand use the chicken stick to touch each capacitor, holding it there for several seconds.  This will discharge the capacitor.  I personally prefer to solder a 1 to 5k, 2w resistor to the end of the wire.  I then put the hook in the free end of the resistor.  I use a 2w resistor just because it uses heavier wire. 

The reason for the resistor is because I have a major problem with sparks.

Come back when you're ready to play.

 

 

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