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Author Topic: Problemactic SB-201?  (Read 3037 times)
W1CEW
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« on: May 24, 2012, 05:02:12 PM »

A 1980-vintage SB-201 just landed in my shack.  Here's the issue I'm seeing:  I get no meter readings on any settings but HV and Grid.  Grid current reading will increase with an increase in carrier drive from my IC-706MkIIg (heavy duty Heathkit tuner in line between load and amp, SWR 1:1).  Nothing on plate, SWR, Rel Pwr, no meter movement whatsoever and my SWR/watt meter confirms nothing is coming out of the amp.  Keying with outboard relay between the rig and amp, as always recommended.  I'm connecting nothing to ALC.  Turning load/tune does nothing to any readings.  If I flip the band switch it does affect grid reading.  Not sure what to make of it, hopefully just bad finals?  Filaments in the 572B's are lit.  Perhaps just bad finals?

[edit] just noted that SWR at rig increases significantly when amplifier is being keyed.

Thanks for any troubleshooting suggestions.

73,

Chuck
W1CEW
« Last Edit: May 24, 2012, 05:22:36 PM by W1CEW » Logged
KA5N
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« Reply #1 on: May 24, 2012, 06:28:31 PM »

If you don't have any plate current, the one OHM resistor in the negative end of the power
supply to ground could be open. 

Since this SB201 "just landed" in the shack.  The best approach would be to take the cabinet
off (after insuring that the amp is not powered and the HV capacitors have discharged) and
checking all solder joints for integrity and all resistors for value.   The tubes may not be in
great shape but even with soft tubes you should see SOME plate current.  The high voltage
should be 2400 volts with no load.  There is a good chance that the dropping resistors (3
4.7 Meg 2 watt carbons in series) have changed value. 
So look for more information and report findings.
Good Luck
Allen
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W1CEW
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« Reply #2 on: May 24, 2012, 06:45:25 PM »

Thanks, Allen!  These are great tips, and I will check out those resistors.  I'm pretty sure I'm not getting that HV reading, but will check. I've got a lead on a local (tested!) SB-220, so I'm almost inclined to put the SB-200 back on Flea-bay for "as-is."  That's only because I work so much, not much time for DIY stuff.

Will dig into it a bit this weekend, with my time off and see what gives, though.  Thanks so much.

73,

Chuck
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W1QJ
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« Reply #3 on: May 25, 2012, 05:23:43 AM »

unfortunately you may have a few problems going on there.  The meter is obviously good but certain functions on the meter do not work.  Could be a problem with the meter function switch, maybe a bad ground.  The metering problem and the no output problem are unrelated I think which tells me you have more than one problem.  If you bougt it off ebay it looks like you got dumped on.  Ebay is a dumping ground for junk.  You may have an open plate choke or no HV getting to the tubes.  That will make the grid current rise with no plate current indication and no output, so you need to check that.
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WB2WIK
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« Reply #4 on: May 25, 2012, 08:27:33 AM »

A 1980-vintage SB-201 just landed in my shack.  Here's the issue I'm seeing:  I get no meter readings on any settings but HV and Grid.  Grid current reading will increase with an increase in carrier drive from my IC-706MkIIg (heavy duty Heathkit tuner in line between load and amp, SWR 1:1).  Nothing on plate,

Assuming your plate current shunt isn't shorted, which would result in no meter reading for Ip, this says it all right here.  When you "key" the amp and hear the relay click, the plate current meter should indicate idling current whether you apply any RF drive or not.  If it doesn't, there's definitely something wrong that will prevent the amplifier from actually working.

Could be any number of things, but as Lou W1QJ pointed out, if the B+ isn't actually reaching the 572Bs that would create exactly what you're seeing.   


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W1CEW
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« Reply #5 on: May 25, 2012, 06:51:54 PM »

*Sigh*  OK thanks folks. Looks like a problem amp.  Boo!  Cry  If someone wants a project, let me know.
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W1QJ
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« Reply #6 on: May 26, 2012, 03:19:12 AM »

*Sigh*  OK thanks folks. Looks like a problem amp.  Boo!  Cry  If someone wants a project, let me know.
That is why the guy dumped it on ebay.  Ebay seems to be the dumping grounds for bad amps.
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AD4U
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« Reply #7 on: May 26, 2012, 05:54:19 AM »

Very first thing you should get a manual and go through the building steps, step by step.  This should not take more than an hour or so.  Since you know nothing about who built the amp or whether it ever worked properly, the first thing you need to do is make sure everything is in the right place and that the soldering is good.

Then check for "bad" components.

The SB-201 is really a very simple amp and trouble shooting is not hard.

Dick  AD4U
« Last Edit: May 26, 2012, 11:03:07 AM by AD4U » Logged
K8AXW
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« Reply #8 on: May 26, 2012, 08:24:45 AM »

CEW: It's really easy to find yourself becoming hysterical when you find yourself with a piece of "problem" gear.

However, in this case, you're in like Flynn because it's a Heathkit and you have the manual....don't you?  If you don't, it's available online.

AD4U is correct.  This is a simple amplifier that was build by someone so it can be 'rebuilt' by you.  This is what I would do. 

I would simply start out at page one of the manual and go through each and every step verifying the components and wires are correct, verify the value of each component, which means disconnecting many of them and measuring their values. Check that the solder joints are good and mechanical parts are tight. 

This is a great old and simple amplifier that should give many more years of service.  It is also a great learning experience.  For example, you've already learned when buying from eBay, you have a 50/50 chance of walking away with one hanging out of you!   Roll Eyes

 
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W9GB
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« Reply #9 on: May 26, 2012, 09:39:14 AM »

Quote
*Sigh*  OK thanks folks. Looks like a problem amp.  
Send the SB-201 to Lou, get this amplifier properly repaired.
Then sell SB-201 on QTH or QRZ, as Lou restored -- with a minimum $$ of your eBay plus his costs.

eBay is filled with estate sale junk, dumpster divers, gov't surplus markups,
pirate radio operators, and golden screwdrivers.
Appliance and Plug-N-Play operators (no time for DIY Repairs) should NEVER shop
on eBay for radio equipment.  You just "bid up" prices for more than parts are worth!!

w9gb
« Last Edit: May 26, 2012, 09:45:20 AM by W9GB » Logged
W1CEW
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« Reply #10 on: May 30, 2012, 05:42:06 PM »

Well, I did end up opening this puppy up and found that the surge resistor wired into the parasitic suppressors (which I believe is a mod) was disconnected from "lug 2."  So the parasitic suppressors themselves were completely disconnected.  Reconnected and now things are working.  Yay.  Testing on 40 meters, I'm seeing a carrier output of just under 200 watts, and can whistle-up maybe just under 400 watts.  I think that's maybe 30-50% shy or more what it should be. Found some relevant thoughts here: http://forums.qrz.com/showthread.php?225896-Heathkit-SB-200-Power-Out.

Thanks all for your input (so to speak!).   Cheesy
-Chuck

« Last Edit: May 31, 2012, 07:44:52 AM by W1CEW » Logged
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