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Sb-200 weird thing happening

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    alternatively you might hang a temporary 10m dipole and test into that. A 10m dipole can be cobbled together in no time as it's just approximately 16' of wire, some kind of center insulator and coax. The antenna doesn't even need to be hung up high in an ideal permanent position, just up off the ground enough to present a reasonable 50 ohm match for testing purposes as that's one of the problem oscillation bands.

    Now that you've identified and remedied at least a few major issues (extra output network, parasitic suppressors and missing HV bypass cap at the anode choke) have you tried testing with the backup set of tubes you mentioned? They might both have oscillated originally but you might see differing behaviors now that a few things have changed. Also you might double check the T/R relay contacts and burnish them with something like a dollar bill if they're not shiny clean.

    Adding a 10 ohm 10 to 15 watt, non inductive swamping (aka feed back) resistor between the input tuning networks and the caps that drive signal to the tube cathodes could also help, but really I'm grasping at straws and trying to think of any ways to decrease the loop gain under no-drive conditions that lead to oscillations. I do have a swamping resistor like what's described in my input network but I didn't add it for oscillation reasons but if your feedback loop includes any stray resonances back through the input tuned networks on the higher frequency bands then perhaps that would help. Writing that, it also occurs to me you might try re-tuning the input network on one of the problem bands to see if that does anything to quench the parasitic oscillation.

    BTW, I'd search around and try to source some carbon comp resistors for those parasitic suppressors as your use of metal film isn't ideal since many film resistors are spiral trimmed for value tuning during manufacturing which can leave them inductive and you're looking for purely resistive loads in those suppressors.
     
    KD2ACO likes this.
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    I do have an Antron 99 fiberglass vertical that is up on my tower that my antenna analyzer said was still good the last time I checked.....it’s the last thing left of the cb hobby after I got licensed. 10 meters still had some openings and it worked ok through my matcbox down to 20 maybe so left it. It hasn’t had a feedline connected to it in years though......I stole it for my 2 meter antenna! I guess I need to climb up and swap the coax back to it. It’s only 50ft but I’m scared of heights! I do have a true modern climbing harness for safety thanks to my brother in law whose been a lineman all his life. I could probably throw a dipole together quicker though, than I can put that harness on lol.

    It had a set of Taylor tubes installed inside when I first picked it up. I’ve got another set of really nice looking Cetron 572b tubes also that appear to have full output. They came in the deal and I swapped those in to test when I first got it, but haven’t reinstalled them since. The Cetrons did the same thing on the high bands I believe. I will put the Cetrons back in today and see if that has changed. The Cetrons are more valuable, so I took them out and put the Taylors back in.
    I’m pretty sure the ham I got this from installed all of The Harbach stuff including a brand new relay verifiable by the different value resistor that’s in series with the coil. I actually disassembled it and looked at it closely. The contacts appear to be absolutely brand new with no signs of film or corrosion.....so I don’t think that’s the problem.
    I slightly heated the inductor on the 10m input and adjusted on it a bit but that’s it. It didn’t seem to have much affect on the oscillating. I’m sure it changed the input SWR some, but it’s kind of hard to tell when the only way to apply drive is using the tuner on the rig. When this is happening, the antenna load and plate caps are so sensitive that finding resonance is almost impossible. On 10 meters, when applying drive without the rigs tuner on, there is absolutely one spot for the load and plate caps to be in without making the tuners meter go full out, full swr. And it’s almost impossible to find. When that spot is found, it will put out power, but only like 350-400 watts max. Even when this sweet spot is found, if the rigs internal tuner isn’t on, the SWR meter in the tuner still goes full tilt when keyed on ssb. During these conditions, amp keyed and no drive, the plate current appears to be a few ma higher than on the lower bands. Maybe 100ma or so compared to about 90ma ZSAC. Following W8JI writings, I checked to see if it showed any grid current under these conditions.......keyed and no rf. Rotating the plate tune cap while checking idle current, shows a slight amount of grid current.....barely a wiggle of the meter, but I can see it. When the same thing is done on 40 and 80, plate current is exactly what it should be @ about 90ma and there is zero grid current no matter where the plate cap is. My apologies for being long and drawn out, but I figured I would make one more last ditch effort to describe what is happening one last time. Maybe one of these other details will help someone solve this for me.

    I’m gonna try to locate some carbon resistors online to use in the suppressors. I did read W8JI writings on how to figure out the correct inductance/resistor combination but I’m not sure I grasp it well enough to go messing around with the original design which obviously works just fine in most of these amps. The bandswitch contacts are dirty lookin. I could probably stand to pull it out and give it a good cleaning along with the air variables. I have checked the input from the SO239 to the tubes and the tank circuit through the bandswitch to the output with an ohmmeter and it all shows continuity where it should. I hate giving up but I’ve done it before lol! I may have to throw in the towel on this on!
     
    Last edited: May 18, 2019
    K7TRF likes this.
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    You do not need to disassemble the band switch. Apply Deoxit spray to band switch.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Arc Juice. o_O
     
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    No arcing after application. DeOxit does not leave a trace of residue behind. Just take a few minutes to let the Deoxit dissipate away.
     
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Agreed. There are a 1/2 dozen threads from the past year that detail how to clean the switches properly.
    Spraying dirtball compound around isn't one of them.
     
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    Agreed if the switch is very dirty only resolution is to mechanically clean it or replace it.
     
    WQ4G likes this.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    The spray gets on everything. A Q tip is a better applicator.

    How does it protect the contacts (As it claims) if it leaves zero residue ?
     
    Last edited: May 18, 2019
  9. AA5CT

    AA5CT Ham Member QRZ Page

    Between the tubes, the plate choke, missing B+ cap after the Plate choke - those are the major players/'components' (with characteristics that change over frequency) that work for or against amplifier stability, ESPECIALLY on the higher bands like 10 meters.

    I'd be tempted to never operate the amp on 10 meters, myself, chalking it up to strange "voodoo" and using it on the lower bands where its really needed anyway. But, that's just me.
     
    KN1M likes this.
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It sounds like you're on the right path and checking all the right stuff.

    This one is definitely a puzzle, but you really should be able to solve it.

    One more thing to check might be the grounded center tap on the filament transformer. Make sure that really is well grounded, perhaps back off the screw to the chassis and retighten it to make sure it has a good ground connection. Actually that's a pretty good procedure for all screws used for grounding in older rigs as oxides built up between the ground lug and chassis can lead to funny RF problems. I'm thinking of all the grounded lugs back behind the tube sockets that could be introducing gremlins.

    If you really run into a brick wall I still wouldn't scrap the amp, I'd either get deep into diagnostics trying to figure out what breaks up the oscillations (e.g. hanging temporary non inductive load resistance across the cathode or grid circuits as a diagnostic measure to see what you can do to reintroduce stability on 10m and 15m). That or I'd bite the bullet, hard ground the grids and alter the bias and metering circuitry as necessary which isn't a hard set of mods but dramatically changes things like VHF parasitic behaviors.

    I guess if none of that sounds like it's worth the effort you still have a good 80m, 40m, 20m amp which is pretty useful especially in our current sunspot doldrums.
     

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