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Thread: Johnson Viking Thunderbolt

  1. #11
    Beach Bum KA2PTE's Avatar
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    Once again Warren thanks for the very useful info.

    Turns out a new set of hurdles has arose. So plans for the 160 mod are postponed.

    Apparently one of the set screws in a 4-400A heat sink was loose, so that explains why one tube was more eager to glow red
    under low power levels - starving for plate current most likely. So I resolved that unusual issue I believe. But before I noticed that,
    had attempted a tune on 40m, and encountered some xfmr humm and other not sounding nice noises, so shut down.

    Apparently part of some kind of mod, this NP 600V / 0.5u F cap leaked oil -

    https://www.mediafire.com/view/olwny...l_cap.jpg/file

    It was hooked from ground effectively in parallel with C117 near the bias plug jack. Measures a bit high ESR, so tossed it.
    Someone speculated its an anti-humm mod on the screen supply but it is connected on the output of the VR75 regulator tube which I think delivers -75V to the system in certain modes.


    As this Amps been modified, I set out to update the drawing and have an updated version in this schematic:

    https://www.mediafire.com/file/c59nb...2PTE_.pdf/file

    As it stands now in CW or LINEAR mode the voltage regulator tubes go off, which means they are
    encountering an overload. I get just about the entire supply volts as a drop
    reading on R115 (4k) which concurs the tube string is seeing too much load
    or perhaps one or more of the VR tubes is weak.

    Default voltage readings as per the manual are in spec, except when you
    place tubes V1 and or V2 (4-400A's) in place.
    Somehow with them in the sockets, standby current is drawn from the VR tubes, but
    cant trace back to see which way. With this modded chassis, they are actually measuring
    cathode current on the meter now, not plate. At 75mA or so this seems awfully odd.


    Trying to figure out the mode switch is sort of painful - its got (3)
    commons - 1, 5 and 9.

    All tests by default are with only the filamant xfmrs powered, no HV 3400V
    applied.

    Even if the screen volts is making it to the tube pins 4 or 2, cant see how
    the plate mA meter is reading like 60mA.
    They modified the plate meter to be reading cathode current by lifting the 2
    common heater connections from ground.

    From what I have read, with no HV applied the screen is more or less a sub for the plate
    and some current flow through is expected. I'd think perhaps not as much as I am seeing now
    as there has never been this kind of current reading at idle ever. I have (4) tubes, and they
    all read the same thing so I dont think there is tube failure.

    The original owner of the amp said this of the mod:

    "Please note that the 4-400A parameters have been rejiggered for class
    AB1 operation. The original design operated class AB2, grid current on
    voice peaks approximately 2 ma. Absent an input signal the class AB2
    mode resulted in substantial resting plate current. After the internal
    power supply zorched itself, I was gifted a KS-1 power supply which
    puts out 3000V. The original PS ran around 2400V. This was an
    opportunity to switch from AB2 (grid current, distortion, etc) to AB1
    which for all practical purposes runs at zero grid current and requires
    less grid drive power while reducing distortion. The input to the
    T-Bolt gives a choice. Either use the tuned circuit or a 300 ohm
    termination resistor as seen on the diagram. In AB1 the driver doesn't
    suffer from the grid current induced non-linearity of AB2.

    Results: Grid bias increased and screen grid voltage increased."

    I seem to concur seeing this when I had ok results testing the amp before
    something went wrong. Though for some reason, the screen current never ever
    registered anything at all, till this odd situation.

    Now its got that "substantial" resting plate , er cathode, resting current
    he described for AB2 possibly. Could something have changed the config back toward AB2 ?
    That oil cap perhaps?

    Also another question is, the new resistors I found (R116A-B) in the updated schematic,
    do they look like they are modding to AB1 operation?

    Apparently you can swap out the VR75 tube with a VR105 and even a VR150 and that lowers idle current
    in the factory AB2 config.

    UPDATE:

    I concluded the NOS VR tubes must have just went over the cliff on me and cant supply much current under any load anymore. Supplier is taking them back for an exchange, but I have more on the way from another source just in case.

    If the tubes do not work, the raw filtered DC supply volts is applied directly tot he screen. The tubes only rated at 600V for the screen max,
    and I imagine theres likely 700-800V once the filter caps do their thing from the tube full wave diode. Its now acting like the plate and makes
    current draw on the meter of about 70mA or so. The meter is actually the cathode current meter , as per part of the mod, in the updated dwaring...so makes sense. Also tells me the transformer is able to put out ample current into the tube diode, up to that amount of current at least.
    Last edited by KA2PTE; 03-12-2021 at 03:58 PM.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

  2. #12
    'Grumpy old bastid' kb2vxa's Avatar
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    Oh boy, trying to finger out a modified amp I'm getting in a bit over my head, like you not knowing just what was done or why we can only give it a best guess. This is why I ALWAYS documented every step of a build or modification so there was NEVER any guesswork involved. Then there is The Golden Rule of Boat Anchordom, never irreversably modify a classic, for if you do its collector value drops to zero. Now you have a decision to make, whether or not to reverse all the mods based on the fact that electrical engineer(s) designed it to meet FCC certification standards, an unknown changed it. By "unknown" I mean was he an engineer or a hack? The big question is will it meet FCC spectral purity requirements or is it a Dave Made cee bee leenyaar? Hell's bells, it doesn't have any pills!

    Pills = CB lingo for RF power transistors. Jokingly we called them helicopter transistors. Back to hamspeak......

    That cap was an oil filled paper "bathtub" cap of the type that was common in power supplies, especially HV supplies because they never go bad under normal conditions, and they're far more compact than a 450VDC electrolytic string with associated equalizing/bleeder resistors. Johnson actually used one in the choke input HV supply in the Viking, the only transmitter they didn't cheap out on. BTW you really went over my head with ESR, I had to look up that theoretical equivalent value for inductors and caps I never had need to learn not shooting for my BSEE. (;->) C117 being part of a pi section RF filter to prevent leakage into the power supply cable and being radiated I have only a guess why that cap was in parallel with it, a lazy repair. Instead of replacing a damaged C117, just shunt it with anything in the junk box. That being the case I suggest replacing C117 with a .005uF 600V ceramic disc cap like the original. Johnson was famous for those pi section filters on everything except RF connectors, even panel meters in steel shield cans. Oh, that's the bias supply input to the 75V regulator tube, why the dropping resistor is in the supply instead of at the tube socket I have no idea.

    "As it stands now in CW or LINEAR mode the voltage regulator tubes go off, which means they are
    encountering an overload."
    This is where I get lost reading the schematic, the eyes aren't good like they used to be, the rotary switches is where they fail me. Just one of those Mumbai Mumbles helpless desk questions, are the VR tubes all in the correct sockets?

    Whoever changed the multimeter from grid and plate current to grid and cathode current royally goofed. In a grid driven RF power amplifier, cathode current is total tube current, grid, screen and plate. Often in Class C CW and plate modulated AM operating grid bias is derived from fixed bias plus rectified grid drive, at rest the fixed bias is protective bias keeping idling current down to a dull roar. Johnson engineering wired the meter to read grid and plate for a reason! <moan>

    "Though for some reason, the screen current never ever registered anything at all, till this odd situation."
    Screen current? You lost me there, I don't understand the metering logic at all. Ordinarily when tuning up you tune grid for maximum and adjust drive for proper grid current, then dip plate current while adjusting loading for proper plate current. Last if you use CW you should have the correct operating parameters, for SSB you adjust drive so voice peaks barely twitch grid current. That amp is modded weirdly so I wouldn't know what the readings are supposed to be now.

    "Could something have changed the config back toward AB2 ?"
    I think those poor 4-400s are as confused as I am.
    "That oil cap perhaps?"
    No, it didn't belong in there, and where it was had nothing to do with anything really.

    "Also another question is, the new resistors I found (R116A-B) in the updated schematic, do they look like they are modding to AB1 operation?"
    First, I have no idea why they have 3 resistors in series totaling 7.6k off the -75V regulated line. Forget what the guy who "rejiggered" it said (he sounds very unprofessional) and try to find out what Johnson did when upgrading to this model.

    Changing the resting bias from -75V to -105V and even more drastic a change to -150V with a corresponding increase of screen voltage is driving them toward Class AB1. Without knowing what the TOTAL grid bias under operation is there is no way of knowing where on the curve family the tubes are operating. Your best bet is downloading a data sheet and taking measurements with the beast fired up in CW operation into a dummy load. I found that modern FETVMs are RF sensitive, older VTVMs less so, and my trusty Simpson 460 VOM was immune and at 50k per volt only loaded down AGC lines badly, a VOM would be a good instrument to measure T Bolt operating voltages. Just stay away from the 3KV without a WELL INSULATED X10 multiplier probe.

    I hope I'm not confusing you TOO badly working from 40 year old memories foggy at best. Working from pdf with the + sign clicked on helped my bad eyes, but scrolling around is one PITA. (;->) BTW I doubt Covid 19 is related to tRump Derangement Syndrome, I survived the virus maybe because I never had a case of tRump anything?

    RF helicopter power transistor.jpg
    "The universe is under no obligation to make sense to you."
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    73 de Warren KB2VXA
    Station powered by atomic energy, operator powered by natural gas.

  3. #13
    Administrator N8YX's Avatar
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    Quote Originally Posted by kb2vxa View Post
    ...Then there is The Golden Rule of Boat Anchordom, never irreversably modify a classic, for if you do its collector value drops to zero...
    World + Dog would take major exception to me, the FCC and the NSA for some irreversible modifications we've done to gear over the years.

    A couple cases in point:

    I have two Drake R-7s which came from two different field offices. One (Allegan, MI) even had paperwork from the station chief included in its document set. Both have had 50KHz IF buffer boards and corresponding rear-panel outputs installed (my ex-Drake engineer buddy claims by the factory itself).The Allegan unit now sports a dual-mode squelch, inspired by Mark Mandelkern's published designs. OEM knobs and board interconnects were used. Drake could have included something like this in the consumer offering; the real estate for the extra board was already there (and unused).

    The Icom R-7000 - famous for its role in taking down Pablo Escobar (so the story goes). NSA bought the things by the pallet load for domestic use, as they didn't have to meet the EMI requirements imposed when operating in foreign embassies - particularly, former ComBloc countries. For that, Watkins-Johnson got the nod - at many times the price. I've seen all sorts of factory and aftermarket/spook-shop mods for these receivers. Most of mine have better (CFJ vs CFW) IF filters fitted, which is something of a one-way process as IF Unit modifications including board surgery are required. Icom could have included higher quality filters from the factory as the form factors between the two series really aren't that different but the cost certainly was. Two of mine have internal SGC ADSP units fitted. I did an article back around 2011 on this site for both mods; too bad the ADSP is out of production as it really adds to the usability of the radio. The other things I'm thinking about doing to a couple of the several I own are replacing the wide FM filters (230/180KHz, respectively) with 110/82KHz parts for improved FMBCB DX performance. A variable BFO, AGC defeat switch and tunable IF notch round out the nice-to-haves, and the PLL/VCO unit's phase noise can be knocked way down with some changes to that assembly. I have a couple non-working boards to repair, modify and test.

    Back to the glow-in-the-dark project.

    /out
    Last edited by N8YX; 03-13-2021 at 04:52 PM. Reason: Grammar
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  4. #14
    'Grumpy old bastid' kb2vxa's Avatar
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    I can take a fair guess about you, however I'm reasonably sure neither the FCC nor the NSA sold boat anchors to collectors. I bought a National NC-303 with a sweet AGC mod and matching speaker from the Bell Labs ARC when they upgraded their station. It was fully documented with a partial schematic, and was reversible, but I didn't because it improved SSB and CW reception. BTW I completed the package with the luckiest ever hamfester find, the matching converter cabinet with all 3 converters, 2M, 1.25M, and 70cM. It was a steal because the 1.25M converter is rare as hen's teeth and alone costs many times what I paid for the package. Until I moved and had to downsize I kept my entire collection in perfect physical and electrical condition, like the TV show the price was right both buying and selling. No, I'm not like some hamfester sellers that think their shit is made of gold when it's really made of shit. I also restored 2 R-390s and had to replace the open circuit selenimum rectumfryers with silly cone dynodes, but they are for the DC relay coils and are hidden inside the shield under the power transformer module where nobody knows (except us now) there is anything in there. The actual mod in one is very noticeable but easily reversed with no tools, 2 tubes with 6V heaters replaced with 12V versions and the extremely rare 3TF7 ballast tube replaced with a jumper plug, the tube kept with the receiver so it wouldn't get lost. The idea was should it burn out a replacement costs the proverbial body parts IF you can find one not sold out. The crystal heaters can be switched off for fixed operation in a heated operating position, and the ballast tube was used for 24VDC mobile operation and not needed on our stable 117V mains. Leave it to the Army to call anything like the mating BC-610 transmitter that could only be moved with a forklift mobile equipment.

    Oh no, every citation CBers got for "shootin' skip" came from the Allegan Point, MI monitoring station, why only that one we never did understand when "skipland" was only Texas. Well on channel 18 the Black channel they all were in Noke (Newark, NJ) when most were in Hillside and a few in Rahway.

    Ah yes, the Icom R-7000 was legendary being legally sold only to specially identified government agencies and employees as well as industrial that showed proper ID creds. The first one I saw was in an FBI monitor van at the infamous Waco incident, a news camera caught it through the open back door that was quickly closed. The second I saw in operation up close and personal while talking to an Atlantic Electric engineer tracing sources of RFI in West Creek. The engineering truck was well equipped to say the least! Thanks to a complaint from Phil K2PG that day he found 2 broken insulators, 2 arcing tree branches, a burned out HPS lamp, and touch control lamps in 2 neighbors' houses. They were the worst noise makers spitting out static and harmonics of the touch plate oscillator, one quickly resolved and the other we had fun with. One so easily resolved by trading the touch lamp for one with a switch, that one got smashed to bits, and I set about finding "usable" harmonics from the uncooperative neighbor's lamps. Those fell within the CW portions of a couple of ham bands, clear the decks for action! Come nightfall we saw the lights on and Phil tuned his Drake TR-7 to the best "usable" harmonic and sent CQ in slow CW, the house looked like Frankenstein's lab with the lights flashing low, medium, high, off, repeat. ROTFLOL! He unplugged the lamps, no noise for a few days, then the cycle began again and again each time he plugged in the lamps. It took a couple of months but he FINALLY got the message, get rid of the lamps.

    IIRC analog FM channels are 100KHz wide and transmissions occupy a bandwidth of 75KHz with a 12.5KHz guard band on either side. A 110KHz filter would do the trick, 82KHz better, but now I'm wondering what the attenuation curves are, just what do they look like? If you attenuate sidebands you'll hear annoying distortion leading to listener fatigue. That's extremely undesirable these days since the half hourly ID with callsign requirement was eliminated and there are countless stations using The Hot 92 logo. You can listen for hours and not figure out what station you're listening to, that's why I gave up on broadcast DX years ago.

    "Back to the glow-in-the-dark project."
    Some think real radios glow in the dark and even wear T shirts with it silk screened on. So why don't you sell T shirts with my version on them? 12 volts is for wimps, real radios can KILL you.

    Watkins-Johnson at many times the price I'm sure, they made the best general coverage HF marine radios on the planet. I remember a few years ago several fleets upgraded their radio rooms and used WJ receivers went on the market. I could hardly believe old receivers going for tens of thousands of dollars... EEK!!! I imagine a few hams were missing arms and legs.
    "The universe is under no obligation to make sense to you."
    Neil deGrasse Tyson

    73 de Warren KB2VXA
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  5. #15
    Beach Bum KA2PTE's Avatar
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    Open choke

    Ok so got in a nice set of NOS 150V reg tubes with GE logo, made in Holland. They look almost new,
    and was told they are a matched set.

    Upon putting them in noticed not all lit. Swapped a couple around, then they all lit. That made me inspect the sub chassis
    they were mounted on. Most the 1959 solder looks still great but Johnson used stiff buss wire on everything here and one wire was straight against the sharp edge of the terminal of the wire next to it , on 2 of the sockets. The heat from that pin
    over the decades, melted a groove into the insulation, and the terminal was loosely touching the exposed wire as shown in the arrows


    Now all the tubes stay lit very solid, but still I had the odd idle current on the cathode of the 4-400A's in CW or LINEAR mode and the VR tubes would extinguish in those modes as I measured a little over 100mA draw on the dropping resistor
    which the tubes being only 40mA rated clearly would not tolerate.

    Removing the tubes restored proper screen voltages. So clearly the tubes were biased, though my standby voltage was being generated ok, but is it making it to the control grid?

    Sure enough , no.

    After tracing why, I found L8 , a 2.5 mH RF choke open.



    The wire on one end is completely vaporized - I guess a souvenir from the unstable condition that oil cap had , perhaps
    making the grid bias too unstable.

    Without at least -150V on the grid, I guess these tubes can still behave like a forward biased diode from screen grid to cathode.

    Now to locate a new or good used choke. The Tbolt used these same ones in 3 different parts of the chassis.
    Last edited by KA2PTE; 03-15-2021 at 08:44 PM.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

  6. #16
    'Grumpy old bastid' kb2vxa's Avatar
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    Now you see the value of visual inspection, by scrutinizing What Is And What Should Never Be (a cool Led Zeppelin song) you isolated two problems, odd behavior of the VR tubes, and the open RF choke. I've seen plenty of 4 pie 2.5mH chokes, but never one sprayed with Cosmoline. That BTW is that brownish stuff that looks like spar varnish the military moisture proofed radios with. You wouldn't want to work on one with that gunk covering everything under the chassis, BELIEVE ME! I see the wire is blown open, melted on the end, but what made the pie become displaced when it was "varnished" in place I can't imagine. No big deal wondering how or why, all that matters is you need a new choke. No problem there, ePay is littered with them, 3 and 4 pie with and without Cosmoline. Oh, in addition to pushing the shorted wire off the socket connection opening the short I'd lift R112 well away from the chassis, it looks like a short waiting to happen. If there's one there is more, time for search and destroy, kill them all and let God sort them out. (;->)
    "The universe is under no obligation to make sense to you."
    Neil deGrasse Tyson

    73 de Warren KB2VXA
    Station powered by atomic energy, operator powered by natural gas.

  7. #17
    Beach Bum KA2PTE's Avatar
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    Ok Warren roger that. I did order a few new ones soon to arrive.

    Was told that usually when these chokes go theres a bad HV tank cap somewhere. Since I replaced all the doorknob plate tank caps, set out to check C10 (0.001 / 2.5-5KV) and it was breaking down under load. So were the rest of the coupling caps that SW2 shorts out to help you tune up. Was wondering why I had to have it always set to MAX coupling to get 80m to dip. With the switch fully shorted, only had about 395pF max capacitance on the array. One was only wrapped on the switch solder terminal, no solder - and you could tell that connection was overheated quite alot.

    So as these are obsolete, I am gonna use doorknob's because also the KV rating will be much overkill but that makes me feel ok because this is
    now a modded amp, with 3400V, not the factory 2400.

    I started dry fitting a bracket nearby the switch to hold the door knob caps, and it ought to work real nice, as the leeds will only be a little longer to reach the new location.

    Theres also 1 new hole you can see now available on the bottom of the bracket because the old capacitor array was using it as a mount. Theres also another up higher in this photo - so I could place 2 more door knobs possibly, to add more coupling if I need to try and mod for 160m.




    So possibilities look good over there, however in the fan compartment where C10 is, its alot tighter, and it means I probably have to mount (2) door knobs in parallel to get 0.001 uF.....but its possible.

    I believe all these caps that are shot probably contributed to the issue I saw as I was trying to load on 40m just before the choke blew. There was for sure not enough capacitance coupling, and C10 being almost invisible, probably does something bad for the tubes output plate capacitance by the looks of it.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

  8. #18
    'Grumpy old bastid' kb2vxa's Avatar
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    OH @#$&(#!!!!!!!! I have WiFi problems you wouldn't believe in addition to it being barely above dial up level (damn cheapskates) and severely overloaded even before adding a few med carts that bogs it down even worse. "They" changed several channels bungling it more, now I get disconnected several times a day and struggle to get reconnected, meanwhile I lose what I was doing, this time my nearly completed reply.

    Trying to pick up a few of the pieces, you're on your way for the most part, so I suggest whipping it into shape before you even THINK of a 160M mod that for one thing isn't needed, 100W is quite enough to get you where you want to go. Working under supervision not the least bit needed except for legal reasons I worked from 160M up through 6M with 100W, less above that, and worked the world. Remember that 160M is MW like AM broadcast (HF begins at 3MHz) and is clear channel bottom to top and coast to coast. 100W is like BS in business, a little spread thinly goes a long way. Just for fun I worked Georgia, USSR on 10M AM with a re-tuned CB set, no BS.

    You're running those 4-400s like they're supposed to be run with a proper power supply, and better when you bias them for Class AB1 or Class AB2 linear and Class C CW operation like the factory specs call for. I hope you downloaded a pdf page from the RCA Transmitting and Industrial Tube Manual I had once upon a time, or the sheet from Eimac that will guide you on the path that you must follow. I noticed a couple of transformers that need attention, T103 LV & bias is connected for 110V mains because that was nominal when the amp was made, put it on the 120V lead because that's today's nominal. Then there is only one way of dealing with T102 powering the 4-400 filaments that require 5V +/- .25V because they're very fussy about that. Being run out of spec shortens the life of a couple of very expensive tubes, price them, you'll flip! I suspect voltage to be a bit on the high side with higher mains nominal now, so if it is first remove the tubes and meter one of the secondaries no load for reference. Then try wiring the unused secondary winding in series with the primary and measure again to see which way it goes, up or down. If it goes up reverse the winding, in phase boosts, opposite phase bucks. The idea is to buck out some of the primary input to reduce secondary output, with a bit of luck you'll wind up with filament voltage under load within specs. If not it's time to get out the big guns, but we'll hold that idea for later only if needed.

    Why do you think you need doorknobs to replace C10 that maybe shorted and put the full output of the bias through L8 and blew it open? All you need there is a new 2.5mH 4 pie RFC with or without Cosmoline (;->) and a .001uF 600V ceramic disc cap. Looking at the 4-400 spec sheet the maximum voltage to be expected at that point assuming under load in AB1 plate 3500 and screen 750 is -145, and AB2 plate 3500 and screen 500 is -75V so the 600V cap should be sufficient. With those parts replaced and before waking the sleeping dragon it's a good idea to check for shorts, something else may be providing a DC path to ground.

    "I believe all these caps that are shot probably contributed to the issue I saw as I was trying to load on 40m just before the choke blew."
    AHAAA, so it was YOU who blew the choke! Then maybe it's not because C10 shorted, but no harm in replacing it anyway, now it's looking tube related, maybe a grid short. That's easy enough, with voltages all wrong grid dissipation shoots skyward, they can overheat and sag against other electrodes, fizz, zap, BANG! BTW there's a mistake on your schematic, that dot at the filament common point needs a ground symbol.

    "There was for sure not enough capacitance coupling"
    Possibly correct, it could also be a parasitic VHF oscillation problem.
    "and C10 being almost invisible, probably does something bad for the tubes output plate capacitance by the looks of it."
    No, it's a bypass cap that grounds the cold end of the grid tank for RF while leaving it float for DC bias voltage. At the end of the day you'll probably have to neutralize any parasitics by adjusting C2, you'll need the manual for that. Parasitic oscillation can cause tube overheating, and some really weird tuning and loading effects, like maximum output on one side of the dip in plate current for one thing.

    Until next week, same bat time, same bat channel. Don't try it, only bats can sleep upside down.

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    "The universe is under no obligation to make sense to you."
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  9. #19
    Beach Bum KA2PTE's Avatar
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    Got the new RF choke today

    Hi Warren



    Ok interesting comments. Yes, they lifted that ground on the heater middle connection between the 2 tubes, and ran a
    black wire down to the plate mA meter - see diagram again.

    Also updated the drawing, as they put 0.01 Capacitors to ground where that connection to gnd was.The meter already had
    one, plus the terminal they used had C105 already there so you have (4) caps in parallel across the meter. Not sure if they are figuring cathode resistor bypass cap(s)?

    While I was in there saw one of the terminals the control grid goes to on one tube was not soldered, and was really loose
    so fixed that.

    C10 was an original Sangamo molded mica, 2500WV / 5KV max. It was mounted inside the fan compartment, so maybe they thought the heat on the tube sockets would shorten its life and they went overkill?

    I like using a doorknob for it. Yes way overkill but since this is no longer factory plate voltage and is AB1, I rather have higher rated in case of a catastrophe.

    Yes its neat to ponder the 160m mod while I am in there. Today while tearing up an old desktop pc for parts, found a nice
    metal bracket they used I think for shielding that fits way better than my other and is stronger.



    Can easily fit 6 or more doorknob caps on it plus the octal relay in case I can do the 160m mod later.
    The one doorknob in the lower right corner near the bottom is dry fitted on the bracket that held the string
    of bad molded mica output coupling caps. I can mount 2 there at 500pF and make a new C10 of 0.001uF
    as original. Turns out it goes right to the huge neutralizing cap thats very close by, so thats a great spot for it.

    I read neutralization was rarely needed on the Tbolts, but as I am into new territory here, would be worth looking into
    and try to avoid the parasitics.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

  10. #20
    'Grumpy old bastid' kb2vxa's Avatar
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    12,937
    I'm not happy with that meter modification, you can get a dip in cathode current when adjusting the tune variable, but you can't read plate (anode) current with screen current added to it. It would also be nice to read screen current after all those mods to be sure it's the proper value, at least until operating conditions are settled on and everything is proper. At the end of the day (except for neutralization that may be needed now) you can throw away the manual and write a new one, that's no longer a Johnson ANYTHING you have there, more like a KA2PTE Firedrake (fire breathing dragon). If you can make a sticker, or better yet a decal and put this in that blank spot above the tuning control it would make a mean finishing touch. JIGGITY JIGGITY! Yeah, in addition to an airline pilot Glenn Quagmire is a ham fond of boat anchors with tubes in his basement. Heh, Godzilla finally set King Ghidorah on fire! ('->)

    I see the metering change, confusing at best, I can't make sense out of what it was originally, the switch selects "screen" & HV, but in Class AB1 it was originally set up for max screen current is 28mA for 2 tubes, but why measure it? I'm used to tuning the grid circuit, so grid current was metered, I'm lost here. Then there's a watts scale on M2 lower scale with plate current on the upper, why I have no idea because to get plate DC input you multiply the upper scale by M1 lower scale (KV) provided calibration is correct and that depends on R101-105 being all within 1% so there could be no more than 5% from nominal worst case scenario. Would Johnson spring the bux for precision resistors when they cheap out on everything else? Collins on the other hand used a single 1% multiplier in the 20V I repaired, but there's a gulf as wide as The Grand Canyon of Mars here. (;->) You can't have enough bypass caps though, the Ranger and Valiant had them up the wazoo and there was a shield can on the meter, but when they designed the Valiant they cheaped out on the most critical parts, the modulator and final. Mod iron on both was puny, and the Valiant used 6146s that suck as audio tubes when 807s shine. Then they paralleled 3 6146s in the final and at the voltage and current it ran at the plate impedance is only around 500 ohms. That leaves a 10:1 typical transformation ratio in the pi network, not high enough to adequately tune out harmonics and spurs, plus it was a BITCH to neutralize, more shit going up the pipe to the signal spitter. UGH! That's where my Drake TVI filter came into play, without it my TV went nuts, likely the neighbors' too. Oh, if your TV goes BSOD when you key up the Firedrake there is another Drake that can fix it, they made a KW filter too. Cathode resistor bypass caps? No, there are no cathode resistors, it's not cathode biased, it's an RF output amp, not an audio power output stage.

    OH RATS! The music stopped, the bloody WiFi disconnected AGAIN! Since "they" added several more mobile laptops to this already overloaded wireless network it has been acting funky. Then instead of upgrading it like should have been done a long time ago the cheap bastards changed several channels around and made it much worse. They've also created a medical situation in the process, the nurses' meds carts mobile laptops aren't working, oh lovely. I wish they would do like I do, I have an iron clad rule; IF YOU DON'T KNOW WHAT YOU'RE DOING DON'T F***K WITH IT!!!

    There, finally I got it connected again, and just in time for one of my favorite songs on one of my favorite albums, I Talk To The Wind on the King Crimson Court of the Crimson King. It's such a beautiful song and in my key too, I loved harmonizing to Robert Fripp before I lost my voice. I'm pretty sure you'll like Radio Caroline too, Europe's top pirate radio ship gone legal; http://www.radiocaroline.co.uk/#home.html Rant mode off, so ON WITH THE SHOW.

    "While I was in there saw one of the terminals the control grid goes to on one tube was not soldered, and was really loose
    so fixed that."
    Keep looking, again you were reminded of the value of close visual inspection. If I found a terminal without solder in a Collins 32V2 transmitter (the sweetest audio I ever heard in CB in our AM Gangstas group) I was repairing I can't imagine how many you'll find in a Johnson. (;->) At least your job is pretty easy, Collins is infamous for being technician and broadcast engineer unfriendly.

    "C10 was an original Sangamo molded mica, 2500WV / 5KV max. It was mounted inside the fan compartment, so maybe they thought the heat on the tube sockets would shorten its life and they went overkill?"
    No, just a convenient place to mount it. I had dozens of those things and put them to good use replacing fixed loading caps in those miserable Johnsons. I've heard the expression among hams "Happiness is a big Johnson" and I'm sure they mean the ones they were born with.

    "I like using a doorknob for it. Yes way overkill but since this is no longer factory plate voltage and is AB1, I rather have higher rated in case of a catastrophe."
    You're in love with those TV doorknobs, but I'm sure you'd like a cat ass trophy addition to your shack, you can order one on line. (;->)
    https://www.etsy.com/market/cat_ass_trophy
    Jim N2EIY, W2RXR, and whatever he changed it to now, overkill is his middle name. I needed a power supply so he gave me one he had kicking around, a Tripp Lite 12V regulated 60A unit that could run 3 2M FM rigs transmitting continuously without getting warm, overkill. Then there's the bracket scrounged from an old computer, an octal plug in relat, and MORE DOORKNOBS, overkill. One of my favorite scrounge spots was behind the local TV repair shop and besides tubes that I tested and added to my collection, quite naturally I collected doorknobs. When I built a medium size Tesla coil I saved lots of work making the HV capacitor out of glass plates, aluminium foil, and glue I wired al of my doorknobs in parallel.They sorted themselves when I turned it on, some had a defect or another and exploded like the one in the ATU.

    "I read neutralization was rarely needed on the Tbolts, but as I am into new territory here, would be worth looking into
    and try to avoid the parasitics."
    It's not a matter of new to the territory, I'm more of an egg spurt than an ex spurt, it's a matter of the amp no longer being a Thunderbolt, Arthur Pendragon had a wizard named Merlin who changed it into a Firedrake. How's that for a new kingly tale a bored knight named Lance A Lot came up with? Don't mind me, this f***ed up WhyFry is driving me mad...........

    Here's a thought to ponder, there's a grounded grid mod for the wee beastie. With a grounded shield between input and output you don't need neutralization, just 80-100W to drive it vs. IIRC 5-10W grid drive two tubes.

    Family Guy Glenn Quagmire ham.jpg

    Firedrake.jpg
    "The universe is under no obligation to make sense to you."
    Neil deGrasse Tyson

    73 de Warren KB2VXA
    Station powered by atomic energy, operator powered by natural gas.

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