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Author Topic: SB-200 Loading Problem  (Read 3161 times)
W9CW
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Posts: 109




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« on: January 29, 2013, 10:12:19 AM »

I have a SB-200 that I built in the mid-70s.  I recently updated the SB-200 with all of the Harbach mods (power supply, soft start, soft key, and fan).  Prior to the addition of the Harbach upates, it had set on the shelf for around 5 years.  The tubes are not the originals, but I replaced them about 6 months prior to placing it "on the shelf" with USA made NOS Cetron 572Bs.

I know the proper procedure for tuning the amp, as to the plate (Tune) and Loading controls.  I always start with the Loading control at the "4" position on the dial, which is around the 10 to 11 o'clock position.  I always get a good dip with the Plate (Tune) control.  I generally use the Relative Output position on the SB-200's meter, and my wattmeter for peaking the output.

Here's what's happening:  While tuning into a known-good 2kW-rated Bird 50 Ohm dummy at full drive with an Ten-Tec Omni, and attempting to peak for max Relative Output (SB-200 meter while keeping grid and plate currents in the safe region of course), the loading cap wants to be fully meshed, e.g. under-coupled, on 80 and on 40 (BTW... it acts the same way on resonant dipoles).  On all other bands, the tune-up procedure is fine.  I can never peak the output on 80 or 40, as I'm running out of loading capacitance.  Under-coupling an amp is bad in many ways, so I'm trying to figure out what's causing this problem.  The 1,000pf and 100pf doorknob caps are original.  I do get normal, or close to normal, output on all bands, but again, the loading cap is fully-meshed on 80 and 40, and I'm not able to truly peak the output on those two bands, unlike on 20, 15, and 10.

I remember having a slight problem before on the low-end of 80 years ago, but never on 40.  If it was only on 80, I would certainly look into the 500pf mica padder that's in the loading circuit on 80, but why on the problem on 40?  Any thoughts would be appreciated.

Tnx,
Don W9CW
 
« Last Edit: January 29, 2013, 10:19:28 AM by W9CW » Logged
WB2WIK
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Posts: 20669




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« Reply #1 on: January 29, 2013, 11:10:59 AM »

What happens if you apply more RF drive?

Can you stay within the safe grid current level and still apply more drive?

If so, I'd try that first. 

Almost every amp I've ever owned wanted to have a lot more loading C when used at reduced drive levels.
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AD4U
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Posts: 2186




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« Reply #2 on: January 29, 2013, 11:54:53 AM »

I built my SB 200 in 1970 and I still have it and I am still running the original Cetron 572B tubes.  I have never had the problem you posted.

Granted the 500pf padder cap is only switched into the circuit on 80M.  Since you are having the exact same problem on 40M, where the 500pf padder is not in the circuit...........

Maybe the wipers on each section of the loading cap need a squirt of DeOxit and a bit of exercise.  I guess it is possible that there is some dirt or miniscule corrosion on the wipers on one or more sections of the loading cap making those sections intermittent or effectively taking them out of the circuit all together?Huh?

If so it may not matter much on 20-10 meters but it would show up most on 80M and 40M.

Just a thought.  Other than that or a bad solder joint, I have no idea.

Dick  AD4U
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W9CW
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Posts: 109




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« Reply #3 on: January 29, 2013, 12:22:06 PM »

This SB-200 typically requires around 70 to 75W drive for full output, but I normally drive, and load, the amp with the full output of the Omni (100W).  The grid, and plate, currents are always in the safe area.  I do have to watch the plate and grid currents when driving it with 100W, but as previously mentioned, I never allow either to go above the safe levels.  Nevertheless, the Loading control always wants to be fully meshed, thus I really can't peak the output, regardless of the drive level from 70W to 100W. The output does increase when I fully mesh the Loading cap of course, but I never see a peak.  I was always instructed back in the early '60s to drive an amp with the exciter at full output for the tune/load procedure, and I've always adhered to that tenet.  But, then again, I've always owned grounded-grid linears.

Last night, I did carefully clean all switch wafer contacts with careful use of DeOxit.  No change...

I do have a 500pf @ 3kV cap in my back stock.  I think I will remove the existing 500pf padder, solder in the replacement, and see if this affects the loading on 80.  But, that still doesn't explain what may be going on with 40.

I must admit, this one has me a bit stumped!  Thanks for the advice.
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AD4U
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« Reply #4 on: January 29, 2013, 01:33:49 PM »

I did not suggest cleaning the switch wafers, although that is always a good first thing to do.  What I suggested was cleaning the wipers on each section of the loading variable capacitor.  Each individual movable section of the loading capacitor (I think there are 3 or 4 ganged together) is "connected" to ground via a spring tensioned wiper. 

Since the amp sat on the shelf for a number of years, I suggested that the wipers on the rotatable part of the loading capacitor may not be making good contact with ground because of small amounts of dust, dirt, or corrosion.  If one or more sections of the loading capacitor are not making good contact with ground via the wipers, that section(s) of the loading capacitor is (are) effectively removed from the circuit.

This may not (probably won't) show up on 20-10 meters but it will definitely show up on 80 and 40 meters where you are having problems.

Dick  AD4U
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W9CW
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Posts: 109




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« Reply #5 on: January 29, 2013, 02:21:56 PM »

Dick,

Good advice, as I missed that step!  I will clean the wipers on each section of the loading variable tonight.

I should have been more specific when I used the term "wafers."  I cleaned all of the switch wipers and the contacts on each wafer as best I could with DeOxit - not the spray, as it always supplies too much, but the pen - and, very, very carefully, a long Q-tip.  As best as I can tell, all contacts are now clean.  After I'm sure there's no residual voltage in the tank circuit, I will double-check for continuity at each switch point with my Fluke 77 also tonight.

As an ex-Navy Leading ET, who worked on surface and air search radars, as well as communications gear aboard a destroyer, I have keen experience with a "shorting stick" and working with one hand tied behind my back when HV is present.  Smiley  One can never be too careful.

Tnx,
Don
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W1QJ
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Posts: 1522




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« Reply #6 on: January 29, 2013, 04:40:58 PM »

with low drive power the sb-200 and many amps will exhibit a lack of load C.  Once you increase the drive power near 70 to 100 watts the load line changes and the available load C is just about sufficient.  Granted, the end result is very close to full C on 80 meters.  I have notice that the SB-200 alos has hign C load on 40 as well.  Try loading at a higher drive level.  Don't worry about momentary grid or plate current.  Tune for maximum power output quickly and the levels will drop right away.  This is the only way to get past the problem.If you load with too little drive you will loop back to not enough load C and get frustrated.  Lou
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W9CW
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Posts: 109




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« Reply #7 on: January 31, 2013, 05:11:21 PM »

Dick,

You were spot on with your assessment!  I cleaned the wiper of each capacitor bank of the Loading cap carefully with DeOxit, and all is well now.  I am now actually able to peak the output throughout 80 and 40 - even at the bottom end of 80 and 40 without running out of C.

Thanks for the tip... that's one gremlin that I must admit, I didn't think of - ashamedly.  Angry

73
Don W9CW
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AD4U
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Posts: 2186




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« Reply #8 on: January 31, 2013, 06:01:12 PM »

Dick,

You were spot on with your assessment!  I cleaned the wiper of each capacitor bank of the Loading cap carefully with DeOxit, and all is well now.  I am now actually able to peak the output throughout 80 and 40 - even at the bottom end of 80 and 40 without running out of C.

Thanks for the tip... that's one gremlin that I must admit, I didn't think of - ashamedly.  Angry

73
Don W9CW


Thanks for letting us know the happy out come.  I took a shot in the dark.  Even a blind hog finds an acorn every now and then.   Grin

Dick  AD4U
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W1QJ
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« Reply #9 on: February 01, 2013, 12:53:39 AM »

Cleaning the conatct on the 80m padder does not explain the 40m problem going away.
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AD4U
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Posts: 2186




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« Reply #10 on: February 01, 2013, 05:20:26 AM »

Cleaning the conatct on the 80m padder does not explain the 40m problem going away.

He did not clean the contact on the 80M padder.  As I recommended he cleaned the spring loaded contacts that "ground" the 4 rotor sections of the loading variable capacitor.  If one or more sections of the loading variable capacitor are not making good ground contact via the spring loaded contacts, those sections of the loading capacitor are not in the circuit.  This problem will usually not manifest itself on the higher bands, but it will make it hard or impossible to "load" 80M and 40M.

Dick   AD4U
« Last Edit: February 01, 2013, 05:59:56 AM by AD4U » Logged
W1QJ
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Posts: 1522




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« Reply #11 on: February 01, 2013, 09:06:07 AM »

yes you did, what was I thinking.  I guess it was too early in the morning.  I suppose that cap had been exposed to some humidity and crud.
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AD4U
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« Reply #12 on: February 01, 2013, 09:10:14 AM »

yes you did, what was I thinking.  I guess it was too early in the morning.  I suppose that cap had been exposed to some humidity and crud.

Agreed!! I try not to post in the morning until I have my second cup of coffee!  HI   Grin
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W9CW
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Posts: 109




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« Reply #13 on: February 01, 2013, 10:40:31 AM »

As per Dick's advice, I did clean the spring-loaded contacts, or wipers, that ground each section of the Loading capacitor with DeOxit.  Interestingly, the SB-200 was covered with a custom cover during the 5 years of storage on a shelf near the ham shack.  All temp and humidity controlled... I guess some contamination can occur even with the temps at 70 to 78 degrees, and 30 to 40% humidity.  I even had a couple of desiccant bags beneath the custom cover during those 5 years.  Things just happen...  the old adage "use it or lose it" also applies to electronics.

Nevertheless, all is now well - working on all bands FB.  Plus, I now have an almost inaudible cooling fan! But, that's another topic all together.

Tnx to all.

73
Don W9CW
 
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