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Author Topic: Heathkit SB-200  (Read 6937 times)
KB9RGD
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Posts: 24




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« on: November 07, 2016, 12:26:12 PM »

Any Heathkit SB-200 geeks out there?
I have an old Heathkit SB-200 amplifier I recently tried to put new tubes that I received from a friend and when I powered up the amp, there was a blue flash under V-1. Since that, every time I apply RF from the exciter the Grid current indicates to the left instead of to the right on the meter. I can't figure it out, after hours of checking parts and reading schematics till my eyes bleed.
Any incite would be helpful.
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W1QJ
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« Reply #1 on: November 07, 2016, 12:56:39 PM »

Check  both 33 ohm resistors in the back on the tube sockets. orange-orange-black.  Lift one end of both and check the resistance.  If the resistance is far from 33 ohms or they are both far apart in value change them.  You may find one is open.  Also check the 1.5ohm grid shunt resistor on the terminal strip underneath the amp where the low voltage supply is.  Chances are that 1.5 ohm resistor could be open too.  Remove those tubes that flashed, the one that flashed did the damage.  Throw it away.
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KB9RGD
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« Reply #2 on: November 07, 2016, 08:09:12 PM »

Thanks for the thoughts.  I will look into those.   Smiley
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K8AXW
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« Reply #3 on: November 07, 2016, 08:14:53 PM »

The SB-200 is an old amp and quite often the resistors drift off and can cause troubles.  In your case I agree with QJ .....you have a tube causing the problem.  It's a simple matter of finding out what it took with it. 

Go over the plates on the TUNE cap and see if it arced.

The SB-200 is a very basic amp and easy to work on. Just take your time and check everything around the tubes.

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KB9RGD
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« Reply #4 on: November 14, 2016, 01:26:20 PM »

I still can't find any bad parts.  I ordered new tube sockets.  The tubes fit too loosely in them for my liking anyway.  Maybe I will find something when I replace them.  I don't get it.  It shouldn't be this hard to find a bad part.  I have a little less than a month till the sockets get here from China.  Then we will see what I find.
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N8CBX
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« Reply #5 on: November 14, 2016, 04:34:46 PM »

I have a little less than a month till the sockets get here from China
MFJ has them in their catalog. I replaced mine from MFJ.... Why China?
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
K6BRN
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Posts: 490




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« Reply #6 on: November 14, 2016, 10:48:55 PM »

Brian:

The symptoms you describe are often caused by failure of the tank circuit bandswitch wafer - "Wafer B" as shown in the SB-200 manual.  Burned contacts in this switch wafer are hard to see and so the problem is easy to miss.  When one 572-B tube flashed over, one or more contacts on this switch may have burned or vaporized, which would cause the exact meter indication you are describing.  Plus, the tube may be history as well.

If you put known good tubes into the amp, and grid current still reads backwards on one or two bands, but not on others, it is likely the bandswitch wafer is bad.  These are sometimes difficult to find and pricey when you do.  Right now Harbach has a direct replacement in stock for $38: http://harbachelectronics.com/shop/misc/sw-100-band-switch-wafer/

 If all bands show bad, check the plate resistor/parasitic suppressor assemblies - PC1 and PC2 (attached to the top of the tube) for an open resistor - easy to miss because they are shunted by a wire winding - partial disassembly is needed to check the resistors.

If you find that the tank band-switch wafer is toast (a pretty common SB-200 problem), it will require significant mechanical and soldering skills to replace, plus patience.  Contact me at my email address on QRZ.com if you need to do this and I can give you some advice on how to proceed.  I just replaced one in an SB-200, which had similar symptoms to yours.

Best Regards,

Brian - K6BRN
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W1QJ
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« Reply #7 on: November 15, 2016, 05:17:18 AM »

Usually a tube arc won't affect a band switch contact.  Tube arcs take out glitch resistors if you have one, grid and plate shunt resistors and short protective diodes . Have you checked the 1.5 ohm grid shunt resistor?  Out of circuit?
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K6BRN
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« Reply #8 on: November 15, 2016, 01:07:55 PM »

Hi Louis:

Does it hurt to look?

Brian - K6BRN
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W1QJ
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« Reply #9 on: November 15, 2016, 04:40:36 PM »

Hi Louis:

Does it hurt to look?

Brian - K6BRN

Not if my wife is near by.
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KM1H
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Posts: 2629




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« Reply #10 on: November 16, 2016, 08:12:52 AM »

Quote
he symptoms you describe are often caused by failure of the tank circuit bandswitch wafer - "Wafer B" as shown in the SB-200 manual.  Burned contacts in this switch wafer are hard to see and so the problem is easy to miss.  When one 572-B tube flashed over, one or more contacts on this switch may have burned or vaporized, which would cause the exact meter indication you are describing.  Plus, the tube may be history as well.

An arced switch is usually caused by hot switching the relay which slows down with age; seriously burnt contacts exacerbate the problem. All the burnt contacts on both wafers are easy to see. Harbach carries new relays and switch wafers.

Other causes are misalignment of the rotor, too much time spent tuning especially on 10 and 11M and a parasitic arc caused by burnt out or high value suppressor resistors. Use Ohmite OY series to replace.

A tube short will take out the 33 Ohm grid resistors and often the mostly useless 200pf mica bypass caps, those can be replaced or paralleled with .01 uF 2KV or better disc ceramics.

Quote
If you find that the tank band-switch wafer is toast (a pretty common SB-200 problem), it will require significant mechanical and soldering skills to replace, plus patience.  Contact me at my email address on QRZ.com if you need to do this and I can give you some advice on how to proceed.  I just replaced one in an SB-200, which had similar symptoms to yours.

That is the first time Ive heard of those special skills being required and it is no more difficult than removing the complete assembly to add 10M to a SB-201. Get the assembly manual and reverse the installation. Study the steps and have the tools needed. What is really needed is the time and patience since there is no instant gratification.Ive done well over 100 repairs and or 10M conversions over the decades the amp has been around.

If the owner feels it is too much work Id be interested in buying it for a 6M conversion which Ive also done in the hundreds.

Carl
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K9AXN
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« Reply #11 on: November 16, 2016, 10:52:41 AM »

I don't believe I'd remove or change the 200pf caps.  They were used in the Collins 30L1 to equalize the grid lead inductance in all of the HF bands, generate negative feedback, and provide equal per band neutralization/equalization.  The .01uf caps leave the grid lead inductive in the upper HF bands. 

Regards Jim
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K6BRN
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Posts: 490




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« Reply #12 on: November 16, 2016, 04:40:07 PM »

Hi Carl (Km1H):

Do we really know what caused the failure?  Probably not.  We only know the O.P saw a flash near the tubes.  Hard to see exactly where with the cover on.  Could easily have been the band switch going.  So... given that the O.P. has stated he's checked all critical components, it makes sense to check the band switch as well.  Burned contacts in the tank circuit are not always obvious.  And a band switch failure WILL result in the symptoms observed.  You would check it.  And disassembly is only needed if it is burned.

Regarding special skills... look at my thread regarding the SB-200 rebuild I performed.  One helpful Ham suggested sawing the band switch shaft in half to remove the damaged wafer - the way he did.  I didn't need to do that and simply slipped the rear input network wafer off of the shaft, by releasing the tuning coils from their mounts.  So, yes, it does take some skill and patiences to do - abilities not all Hams have.

For further proof of the diversity of skills in the Amateur community, just follow this thread:
http://www.eham.net/ehamforum/smf/index.php/topic,110615.0.html

To quote my wife:  Res ipsa loquitur - "the thing speaks for itself"

Best Regards,

Brian - K6BRN

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K8AXW
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« Reply #13 on: November 16, 2016, 06:02:16 PM »

There was a mod to the SB-200 bandswitch to keep HV away from one of the switch mounting posts.  If you're interested I'll provide the change info.

One set of contacts vaporized on my 200 because of this problem.  As for an "easy" fix, that wasn't in the cards for me either.  The guy who built the amp shortened up the wires to the input network switch wafer.

I found it necessary to remove the wires so the switch shaft could be removed.  I modified that shaft by cutting it in two and reconnecting the two parts with a shaft coupler.  Oh yes, to add insult to injury, the input switch wafer contacts were brittle and one broke off.  Had to make a change there too.

I felt fortunate to find a heavy duty wafer thanks to W8JI.

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W1QJ
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« Reply #14 on: November 17, 2016, 04:04:12 AM »

Well it's over a week now and I'm wondering if the OP has fixed the amp yet and what the problem is.  Nothing needing a repair on an SB200 should be more than a couple of hour job.  Total rebuilds take a while longer.
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