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Author Topic: SB-220 madness  (Read 2086 times)
N6AF
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« on: June 17, 2019, 08:37:40 PM »

My SB-220 was running great with nearly 1400 Watts showing on the Bird on 40 meters. Then the other night when I turned on the power it popped its circuit breaker. It is pulling gobs of plate current. I confirmed the bias is correct with +120 VDC on the cathode via the filament transformer in receive mode and + 5 VDC when keyed to transmit mode. After burning up various components I've resorted to adding a temporary fuse in place of RFC 2. As long as I don't connect the plate voltage to the tube plate caps nothing crazy occurs. But if I connect to the plates all hell breaks loose. I've tried three sets of 3-500Z's and the same problem occurs, very high plate current draw. All six tubes do not have internal shorts between plate, grid or cathode. The tubes are fine. The bias is correct. Plate voltage is about 2500 in low setting (CW) and about 3050 in high setting (SSB). I'm at my wits end with this amp. Everything seems to be correct but the tubes just pull gobs of current, even with effectively -120 VDC grid bias when in standby (receive) mode. Who has seen this behavior?  All suggestions highly appreciated. I'm on the verge of parting it out!  I have a second SB-220 that works perfectly. Very very frustrating!
Thanks. N6AF Chuck
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VU2NAN
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« Reply #1 on: June 18, 2019, 01:10:58 AM »

Hi OM Chuck,

Could you check RFC-4 and RFC-5 ? Also contact between 2 & 8 in relay RL1?

73,

Nandu.
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W1QJ
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« Reply #2 on: June 18, 2019, 03:18:12 AM »

It’s time to replace your parasitic suppressors.  You probably had one of the 47 ohm resistors. Open and the amp is oscillating wildly.  Replace both.  You can easily make your own or you can contact me off list and I can get a set to you.  I suggest not even turning the amp on anymore until you get new ones in there.
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N6AF
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« Reply #3 on: June 18, 2019, 06:06:43 AM »

Responding to both replies.

Nandu-there is no RFC3 or 4 because both grids are hard wired to ground. I had already checked the relay contacts and it seems that perhaps two times out of ten occurrences I found that the relay had mysteriously keyed itself into "transmit" mode and was sort of stuck there. Parasitic oscillation "welded"? Strange.

To the other gentleman, yes very interesting.  One of the plate resistors is nicely "baked"!  Conveniently I do have a pair of brand new wire half loop/resistor suppressors I bought from
a guy in France via EBay for another amp project I was staging so great! In they will go this morning and I'll report back. Thanks to both of you!  The whole behavior and symptoms are so bizarre that a high power oscillation sure makes sense. It's frustrating because it happens so quickly there's no way to monitor it. A truly elusive gremlin.


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W1QJ
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« Reply #4 on: June 18, 2019, 06:17:34 AM »

you"ll see.  Once you get a good set of suppressors in there the amp will be fine.  Those horseshoe ones do work so you should be OK.  If for some reason normal operation isn't the same, try an original type suppressor but those tend to work.
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N6AF
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« Reply #5 on: June 18, 2019, 08:02:06 AM »

Well so far so good!  I put in the new suppressors and have a "paranoia component" consisting of a 4 Amp slow blow fuse in place of RFC-2.  Shielded with safety glasses and a set of Bose noise canceling headphones, I flipped on the power switch. There was a quick arc somewhere in there and the relay again "welded" itself in the transmit position even though I had not keyed the amp. But this time the temporary 4 Amp plate fuse didn't blow. Hmmm. So I unstuck the stuck relay and went ahead and tried again switching on the power switch.  Wow, no arc this time! Keyed it to transmit mode via my K3 rig and the installed Harbach keying interface. Bias current around 100 mA. Yahoo!  Just shy of 1400 Watts out on 40 meters. It's alive! It's alive!
Thanks so much! I was beyond frustration. Almost threw away a perfectly good pair of Eimac Y-826's. That's a super high plate voltage version of the 3-500Z. Retired Harris engineer here (36 years). Guess you can teach an old dog new tricks. This forum is an invaluable resource! I will return the favor. Very 73! N6AF Chuck
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N4UE
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« Reply #6 on: June 18, 2019, 09:54:54 AM »

It’s time to replace your parasitic suppressors.  You probably had one of the 47 ohm resistors. Open and the amp is oscillating wildly.  Replace both.  You can easily make your own or you can contact me off list and I can get a set to you.  I suggest not even turning the amp on anymore until you get new ones in there.

Lou gives GREAT advise. When I rebuilt my SB-220 I took a careful look at the 47 ohm resistors. Mine looked perfect. Since you can't measure the resistors without disconnecting one end, I removed both of the originals.
To my surprise, one was open.
Thanks again, Lou!

ron
N4UE
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If you're not the lead sled dog, the view never changes......
VU2NAN
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« Reply #7 on: June 18, 2019, 10:10:45 AM »

Anytime, OM Chuck!

That was real quick, thanks to OM Lou.

73,

Nandu.
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K6AER
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« Reply #8 on: June 18, 2019, 10:56:44 AM »

" I confirmed the bias is correct with +120 VDC on the cathode via the filament transformer in receive mode and + 5 VDC when keyed to transmit mode. "

Should not the bias be negitive?
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VU2NAN
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« Reply #9 on: June 18, 2019, 11:28:26 AM »

" I confirmed the bias is correct with +120 VDC on the cathode via the filament transformer in receive mode and + 5 VDC when keyed to transmit mode. "

Should not the bias be negitive?

Sure, OM Michael, the grid is biased negative with respect to the cathode!

73,

Nandu.
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AK6RF
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« Reply #10 on: June 18, 2019, 09:25:05 PM »

To Chuck N6AF (the OP): I'm glad you got the amp going again, situations like that can indeed be frustrating. To others in the group I offer the following question. What type of parasitic suppressor have you found to be most effective for a 2 x 3-500Z amp? I have read many lengthy threads on the topic (on this forum and several others) and the general consensus seems to be: "Just recreate the stock ones using modern resistors, they work fine." Some people advocate the use of the half-turn / horsehoe shaped units but others say to avoid them. People also say it is better to use 2 x 100 Ohm resistors instead of a single 47 or 50 Ohm. Thoughts? I have restored several SB-220 amps in the past and have just duplicated the stock suppressors using modern metal oxide resistors and never had a problem, so that's probably what I'll continue to do. From reading the aforementioned threads I've also run across the whole "nichrome vodoo" subject so no need to go there except to say it's not needed and can actually cause problems. I guess my main question is: is there a specific type of suppressor (other than the stock type) that is known to work well and improve amplifier stability? I know there are many knowledgeable people here with a lot more experience than I have. Thanks in advance.
Matt AK6RF
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W1QJ
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« Reply #11 on: June 19, 2019, 04:54:24 AM »

I don't like to second guess the engineers unless they goofed and failures occur across a line of amps that a repair shop will see on a regular basis.  Bad parasitic suppressors in an SB-220 amp is very very rare.  The originals work great and the old carbon resistors are about as close to non inductive as you will get.  In normal use they will last many years so after 30+ years you slap in new ones.  For HF amps like the SB-220 using 3-500's the simple coil wrapped around the 2 watt resistor works very very well even on 10 meters. 

Unless that design fails for some reason I would stick with them.  Other designs work well also but this type is simple to re create if necessary.  Nichrome type suppressors should be avoided at all cost and should be replaced if they are present.  The whole idea of them was flawed and the supposed cause of parasitics  was mistaken for other problems.
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N6AF
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« Reply #12 on: June 19, 2019, 08:03:50 AM »

Exuberance short-lived.  The problem came back this morning.  Now at least I can turn on the SB-220 in receive mode without it pulling excessive plate current but once I key it into transmit mode (no RF drive) the plate meter pegs and the 'temporary' plate fuse I have in place blows sky high.  I checked the 5 V bias Zener and it appears to be fine so I am again at a loss with this amp.  Popped fours fuse in a row.  Cannot key into transmit mode.  I figured the bias Zener was open thereby not presenting any negative bias but nope, it is likely OK.  I think in order to absolutely confirm it's not lack of bias in transmit mode, I'll substitute a string of silicon diodes in place of the Zener and that will eliminate that from consideration.  This amp is so blatantly simple it's driving me nuts.  A pair of triodes with -5 Volt bias effectively on the grid.  What the heck?!
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VU2NAN
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« Reply #13 on: June 19, 2019, 09:03:48 AM »

Hi OM Chuck,

Now you need to check R 27 (100 K) and the contact between 5 & 8 in Relay RL 1.

Adjustment for good contact between 2 & 8 in RL 1 could result in bad contact between 5 & 8. Adjustment of change-over relay contacts is a bit tricky.

If RL 1 is the cause, replacement may be the better option.

73,

Nandu.
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N6AF
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Posts: 86




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« Reply #14 on: June 19, 2019, 10:43:58 AM »

I sure appreciate your help and interest Nandu!

Well it's just about impossible to get a clear look at the contact area at 5 and 8 on RL1 but the voltmeter on 8 shows 120 V in receive and (now) 4.8 V in transmit.  The 4.8 V is with the substitute diode string which, as an experiment didn't help, thus confirming the 5.1 V Zener is OK and not the issue.

Ohm meter indicates about 0.0 Ohms between 5 and 8 but the "pad" area of 5 looks a bit burnt.  I tried cleaning it with contact cleaner and careful wiping with clean paper to remove residue.

But still the amp is drawing gobs of plate current when I key it to transmit.

Maybe I'll just bite the bullet and put in a new relay.  Heaven forbid that if I buy that, wait for it, install it, that I again see the same problem.  There doesn't seem to be any way to easily determine the root cause.

Again I say this is indeed "SB-220 madness".  How much simpler can it be than to have a pair of known good 3-550Z's with 2700 Volts on the plate and -5 V bias on the grids!?  Crazy.  I have parasitic suppressors in there and my testing is on 7 MHz.

Guess I'll order the new relay and give it a try.
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