High Voltage AM Transmitter equals FUN!

hi everyone and welcome to another episode of mr. Carlson's lab not long ago I was visiting a ham radio swap meet I was walking around and I spotted this really neat looking old transmitter on a table I figured hey that would make a really neat video to share with you guys so I bought the transmitter I talked to the fellow that owned the transmitter for a while and it turns out that he watches this channel he had a whole bunch of neat things for sale on his table so I purchased the transmitter and today we're gonna go through this transmitter and make it transmit again so let's get started this is a 1950s era globe Scout model 40 a transmitter so there is no receiver in this all this thing does is transmit way back when you're expected to have a separate receiver and on the back of this thing is a bunch of terminals that allows you to connect this thing to a coaxial switch which will switch between the receive and transmit when you click this up and down so when you click it in to transmit mode it basically opens the coax to the receiver and just allows a path for this to go out your antenna when you click it back to standby again it disconnects this and connects the receiver back up to the antenna so this was all separate from this unit here this is an AM and CW transmitter this doesn't do sideband or FM or anything like that it's just very basic AM and CW and AM is a very enjoyable mode to chat in to operate something like this you need a crystal or a VF o so VF o stands for variable frequency oscillator so what that will allow you to do is move around each one of the bands you can see that this is a band selection switch here for 160 80 40 20 15 and 10 so basically if you want it to move all around the 80 meter band you would attach a vfo to this and you can move the VF all around and basically it's going to move your transmit frequency around so that is a little bit more desirable in some cases than in having a crystal in the front of it because whatever frequency you want to talk on you have to have a crystal for it so I'm gonna look through my box of rocks here in a little bit and see if I can find a crystal for this thing and see if we can fire it up some band I've got a whole bunch of crystal so find something for some band here and we'll see if this thing puts out any power and if it's even working here in a little bit what we're gonna do is open this thing up and first of all see what kind of condition it's in and go from there if it's all original then all that kind of stuff what we're gonna end up doing is you know doing some recapping inside of it and you know making the thing safe to operate I don't know what's inside this thing I haven't opened it up yet so it could be all original or this thing could be filled with rocks for all I know I really don't know so this is where you attach the microphone so this says filament and gain so filament is if you were to have this in the off position basically the globe Scout is just off when you click this like this now it's on so basically there's the filaments inside are energized so what's happening in there is basically all the tubes are just lighting up now when you click the transmit switch this of course is going to apply plate voltage to the final tube into that area there and of course we're gonna have some transmit happening here so that's the difference when you see filament and plate so the red light usually means that high voltage is energizing on more tubes inside so now depending on how this is wired I haven't looked at the schematics for this thing yet sometimes some of the tubes do have high voltage on them all the time or in some cases when you click the fill the transmit switch from stand by to transmit it just energizes everything with high voltage so what I'll do is I'll dig up a schematic here and a little bit in will take a look at that schematic figure out what they're doing in here this is the phone in CW switch so phone is basically AM so you can hook a microphone to it and talk through this thing I don't know what the modulation scheme is inside this thing we'll also take a look at that CW means that you're gonna hook a key up to this thing and you're going to be sending Morse code with this this is the oscillator tune so the oscillator would be attached to this section here so if we're gonna stick a crystal in here we need to tune the oscillator and we're gonna be looking for that on the final grid no I don't see if yeah that is present in the camera so final grade here so this is for the final amplifier and this says final plate and then once we have grid current and then what we're going to do is tune it up and see how many watts we can get out of the model 40 a on the back side of the transmitter we have quite a few connections and some of these connections are extremely dangerous and I'll tell you about those connections here in just a moment we'll start with a side first so this connection right here is just for the radio station grounding system so you can ground this to all your other radios and safely ground the case of the transmitter here we have two so2 39 connectors so this is marked antenna and on the top here it says doublet so since we're going to be testing this thing into a 50 ohm system here in a little bit we'll see how many watts this makes and we'll see if it even modulates we'll be using this jack right here that's just marked antenna chances are I'll be using that Marconi 2955 back there to do that this here is the antenna relay connections here now here's the thing with most of these older transmitters if you recall that switch on the front that went from stand by to transmit well when you click it in to transmit it takes the AC from the line cord and puts it directly on these two jacks right here so for example if you were to reach around the back side of this thing while this was plugged in you would be in for a very nasty shock and that's just how things were back in the day you were expected to know that when you own this transmitter this was built back in the 50s an era where people took responsibility for their own actions and that's how they built things most of all of these brilli of these older style transmitters had some form of high voltage or something exposed on the back even the Johnson radios they had a crystal socket on the back which is you know normally for plugging crystals in but they use that crystal socket for the same idea if that's not bad enough this jack here is usually known as an accessory jack in most of the time they bridge the filament and the high voltage in this plug here so this should come out so there you go basically like an eight oh seven type pin out right here it's like basically what this is an eight oh seven socket so that's usually what happened here now I'd be very surprised in this thing if this was some form of low voltage they're usually full b-plus so whatever the high voltages in this thing that's usually what is in these sockets now some of the times the sockets had a little cup on the backside here that would protect this so that you couldn't touch this but a lot of the times they were just left exposed like this as well so line AC right here full rectified b-plus right here very very scary and this really really hurts if you come across this that's all I've got to say now the b-plus inside these or the high voltage I should say it's called b-plus and these older things alright so the high voltage in these older transmitters has a lot of current behind it because it has to power a transmitter so when you're working on an older radio receiver they have high voltage in them as well but usually at a much lower current this thing is really heavy and I'm imagining it has a pretty large transformer inside this so again I haven't opened this thing up we're gonna take a look inside this thing it is very very heavy and what I'm going to do is dig up a schematic for this thing as well and we'll take a look at the schematic and we'll see if this is all wired like this as I say most of the time it is I'd be very surprised if it's any different so I've worked on so much of this older stuff it's all the same line cord views here and the line cord itself which looks to be in very decent condition so not a problem there and that's what this all is on the back now this looks like somebody's added this some form of an RCA connection here I do not know what this is or where this goes or anything about this so we'll have to find out this could be attached to this or actually I don't think it would because these are isolated from the chassis and this is connected to the chassis so this is going to be some form of them and an input maybe vfo or something like that I really don't know at this point so we'll check this out when we take a look inside and see what this thing is connected to now I notice that there's two holes drilled on the back this is very common because this is where people would normally attach the relay that would switch the coax is they would just attach it directly to the back of the case and chances are that's what these two holes were for so it probably was on an angle here so they could get the coax is down here and things like that now that relay is long since gone and I don't think I have one of those kicking around here I usually use an electronic form of switching and I'll go over that here in the future as well in fact chances are together we're going to end up designing a bunch of things together here we'll design an electronic switch for older transmitters and things like that as well so what I'm gonna do now is get this thing turned around again and I'm going to remove this from the case so I'll take the case off of this and we'll take a look inside and see what we're up against hopefully it's a nice condition getting the transmitter out of its case was very easy I just removed all the screws on the case and this whole thing just slid right forward it was just that easy everything is marked on the chassis here all the vacuum tubes are marked so it makes this thing really easy to figure out we know that this section here is the audio chain so we can see we have two audio preamplifier tubes this is a six l6 here a metal 6l6 we have a power transformer really big power transformer so this is making up the bulk of the weight these included so we have this power transformer here we have two filter capacitors we have a rectifier tube so we know that we're gonna have our high voltage in this section we have our final two which is the sixty one forty six and this is a six piece six over here I'm imagining that this is most likely going to be the crystal oscillator tube right here so a 6v 6 vacuum tube in modern equipment like a guitar amplifier they're used for audio the 6v6 is still a very very common tube as well as the 6l6 this is just the metal version of that 6l6 now where these things differ from radio receivers is in radio receivers usually when you see a cap on the top of a vacuum tube that cap is usually where the signal goes in so there's usually no B plus present now I say usually in a transmitter when you see a vacuum tube like this this is where the RF is coming out of so there's full B + or full high-voltage on this right here and this is all high-voltage stuff right in here sometimes they place high voltage across the meters and the pilot lamps of high voltage on them these things are full of high voltage and it's exposed everywhere so if you're ever to work on one of these things keep that in mind if you're unfamiliar with working on something like this do not do it if you get a shock from one of these things it can be lethal very very painful at the least it'll be very very careful so that's the rundown of the top side here let's take a look at the bottom side and see what that looks like well surprise surprise somebody's been in here and replaced all the electrolytic capacitors and they did a really good job to all the new electrolytic capacitors are put on this perf board here none of them are touching they have lead out wires that run to where they're supposed to tie into the perfboard is stood off the chassis with some standoffs that is a really nice job now I've worked on a lot of these older transmitters in usually the jobs that I find in you know on the underside of the chassis are just no are really bad let's just put it that way this looks really really nice so the fellow that I met at the swap meet if you're watching this and you did this job you did a great job that looks really really nice nice and clean so this thing is pretty much ready to apply AC to the line cord here and see how many watts this thing makes see if it modulates and all that stuff as well so we'll take a look at some of the components on the underside here just before we go about taking a look at the schematic and firing this thing up so for those of you that aren't familiar with point-to-point wiring this is what point-to-point wiring is these things here are an early form of IC so you can see that they've got four lead out wires this one has four and this one here looks like it has six lead out wires and there's a whole bunch of resistors and capacitors inside this little block here and that's what they did to make things smaller back in the 50 so you can see that they were already starting with a basic type of I see in these units right here so there's two of them in here basically resistor and capacitor combinations in these little cards that you see here and unlike what they do nowadays on the schematics they told you exactly what was inside these in most cases they didn't just put the numbers on they show what was inside them so two early forms of ICS here we can see the audio jack on the front here this is a wire runs into this tube so we can see that this is definitely the audio chain a bunch of large resistors here some chokes all ceramic disc style capacitors there are no paper capacitor so there's really nothing to worry about this thing and it looks like this piece of coax from the backside here where that RCA jack is is just running directly up to the front crystal jack so yeah they're just using this as an external vfo input very very clean nice construction so let's take a look at the schematic see what their idea was when they put this thing together and then let's plug the thing in and see if we can get any power out of it let's tune it up here's a pretty decent schematic for this transmitter somebody's written on it but it's still very understandable I like the design of this transmitter it's really nice and simple so let's start here where the microphone attaches to the first microphone amplifier here so this is the microphone jack on the front the audio signal will go directly into the control grid from the microphone this is a pentode so we're gonna get quite a bit of gain in this tube and our amplified signal is going to be present on the plate here the plate attaches directly to one of those little cards that we looked at on the bottom side of the chassis or one of those early type of ICS if you'd like to call it that and as you can see they've drawn the resistor and capacitor combinations on the inside which is really nice as well so again the audio signal path in the control grid it gets amplified amplified signal on the plate that signal travels through this capacitor here down into this variable resistor which is on the face that is the mic gain control the wiper of the variable resistor so the wiper is the part that move along the carbon path runs directly to the control grid of this six c5 triode so the signal gets amplified again it's present on the plate goes down this path here into another one of those cards or those early ICS through this capacitor down here and into the control grid of the 6l6 so this is the modulator tube here so you can see the plate of the six l6 ties directly to the plate of the 61 46 giving our mixing action as you can see the B+ line here goes through that plug on the back so it does go through there high voltage comes out here to here jumps to this side here goes down here through this modulation reactor so this is a form of heisting modulation right here goes through the modulation reactor here and then of course we get our B plus or our high voltage on both plates and we get our mixing action so this is creating RF and this is creating audio down here or amplifying the audio that's being created by you and being spoken into the microphone this is our oscillator tube here this oscillator tube is basically controlled by a crystal or if you wanted to you could plug in a vfo into this point right here so whatever frequency the crystal is this is going to oscillate and then we have a coil here which basically Tunes to our proper band these two are ganged together now in a lot of these schematics they don't tell you what end is which and there's a very easy way to figure this out the further you get away from the plate of the final tube the lower the frequency so because there's more coil in there so this would be the 10 meter 15 20 40 80 and 160 meter tops as you go along here and it's the same for this here as well so as you're moving this along here as you get closer to the plate so you can see here we're closer to the plate side here this is going to move down like this and as you can see the arrow is drawn at the same positions so it shows you that they're both at that end at the same time when I say gang together that means that when you turn one shaft both of these move the same time so these two are getting together this is known as a PI Network one very easy way to remember a PI network is you have a coil in the middle and you have two capacitors you can see that but the both bottom ends of these variable capacitors are tied together into ground so if you were to take the bottom of this capacitor and tie to this bottom and put a ground here you would have a pie shaped symbol very easy way of remembering a pie Network that's what that is this is a parasitic trap on the top here parasitic suppressor they have POS dots or parasitic suppression that's that coil around the resistor at the top side of the 60 146 the reason that they have that there so you have the coil and the resistor is basically so this tube here doesn't turn into its own oscillator we don't want that to happen so this stops any type of parasitics from forming parasitic oscillations see what else can I tell you here oh yeah remember how I told you that there's AC on that relay jack well it turns out there actually is so it's pretty common with all of these transmitters so this is the plug and as you can see we have a switch so this is going to connect across here so as you can see we have a path here down here through the standby and transmit switch this is going to connect to here runs up to here over to here and directly to our jack on the back somebody's written 47 on the bulb here that is incorrect this is not a 47 type ball before t7 bulb is a low voltage type bulb if you did put a 47 there it would just flash very brightly so this would be the 47 up here if anything and then this would be just a standard you know 120 volts style bald like a Christmas light bulb in here so we can take a look at that here again when we start tuning this stuff up so the standby and transmit switches is basically a double pole switch here so single pole or double pole single throw I should say so you can see here the center tap of this transformer runs down to this switch here you can see that it's when you move the switch down to the transmit these two arrows are going to connect so this arrow will connect here this arrow will connect here the center tap of the high voltage winding will be attached to ground so that's what they're doing to cut the high voltage they're just breaking the center Tao this is known as a full wave rectifier here you can see both ends of the winding are attached to the plates so we're gonna get full wave rectification since this is a directly heated vacuum tube it has its own filament winding the filament windings are attached directly to the filaments here because there is full DC on your so the B plus is present on this so we have the AC from the filament winding lighting the filaments and we also have DC present because of the rectification happening in the tube so we have filters over here so we have two capacitors here that are in series so they're doing that just to up the voltage of course the capacitance drops in half and the voltage doubles when you have that and then we have a filter choke right here and another two capacitors in series and we have a bleeder resistor across that just in case you pull this plug out so if you pulled this plug out here these capacitors would stay charged because there's no path for them to bleed off so that would be a bleeder right there and then our high voltage goes through a jumper in this plug and over to this side and it feeds the plates and the screen grids of the tubes over here the two larger pins are the filament connections you can see the filament is tied to ground pilot light is across the filament there's going to be a jumper from the large circle to the large circle here and we're gonna have filament power for all of the vacuum tubes so it's gonna light up all of these filaments the filaments are the little arrows under the cathode here so what else can I tell ya here about this so we have high-voltage on that plug on the back we have full AC on that relay jack it's high voltage everywhere in these things and on the outside as well so if you ever work on anything like this you know definitely study up on this be very very careful and again if you're unsure of working on stuff like this definitely don't do it there is a lot of current present here so we definitely don't want to have anything bad happen be very very careful let's see what else can tell you about this I think that's pretty much it for now well this is a k1 is the CW Jack so you would plug the CW or your key in there and it basically breaks the cathode connection of this oscillator so that's for the Morse code key gets plugged in pretty sure this crystal circuit would draw a little bit of current probably gonna warm up some crystals on this thing it says six f6 here I'm pretty sure that this here is an addendum this is probably an earlier version because in this on the chassis it's clearly marked six B six so they've probably made some changes over time phone in CW switch here so these arrows are basically going to move down like this so just depending on what position you have this in these here are meter shunts so we have a meter shunt here for the plate so the B+ here is present on this line so if we want to measure the plate current of the 60 146 here we need a shunt and you can see C and D is written here and if we look down here we see C and D and this is the meter on the face here so this would be to monitor the plate current now we can also monitor the grid current so there's gonna be a grid shunt on here as well somewhere so the grid of the sixty one forty six and there's the grid shunt right there so we see a and B right there so this would be a and B and it says grid right there so there we go so grid and plate so this is gonna give us the ability to look at the grid current and this is going to give us the ability to look at the plate current that's that switch on the face and that's basically how we tune this thing up then I'll show you exactly how that's done here in a little bit and I think that's pretty much all the RF output or antenna output jack from the globe Scout is attached to the input of my Marconi 2955 B analyzer through this piece of coax here so we can monitor the RF output power and the modulation here in just a little bit I have the line cord for the globe Scout attached to my isolation transformer and very act supply which is fused and current limited so the globe Scout is really protected before I go about turning on the filament switch here I want to make sure that a bunch of switches and knobs are in the right position or very close to a very good start position let's put it that way I also want to make sure that the final tube has some form of Drive so I'm going to either plug in a crystal or I'm going to use a vfo for this now I've looked through my box of rocks and I found a crystal that may work so we'll try that first and if this works we'll just stick with the crystal the crystal has 36:40 carved into it so chances are some hammer you operator has moved this crystal around if you can see that right at the top there it's just 3640 above my finger you may or may not be able to see that at any rate we'll plug that into the crystal jack puts this into the 80 meter band very first thing I'm going to do is put this onto the 80 meter Bend now before I turn on the AC power or the filaments I want to make sure that the transmit and standby switch is in the standby position I want to make sure that the phone and CW switch is in the CW position I'll start with the oscillator tune at 12 o'clock I'll start with the final plate tune at 12 o'clock in the antenna load at 12 o'clock as well I also want to be monitoring the final grid current that'll help us tune the oscillator here so what I'm going to do now is turn on the filament switch and let the vacuum tubes warm up I think you can see the filaments glowing in that tube in the back there that's the rectifier tube that's a directly heated vacuum tube so that one will come into a mission first the other tubes will take about 10 to 20 seconds or so so hopefully this thing comes to life see what happens a bunch of things at play here hopefully the crystal works as well so we'll find out in just a moment alright so that should be about warm enough so what I'm going to do now is click this into the transmit position and I'm going to tune the oscillator tune for a maximum current here so I'm going to turn this until I see maximum current on the meter so here we go in the transmit I'll move this around and there we go so if I pass it I'll back up and right there is maximum current so far it's looking really good now what I want to do is monitor the plate current and I want to tune for a dip with a final plate tune here and when I do that we should get some substantial output power if the 61 46 is working right and the 6p 6 is working right and the crystal and all that kind of stuff is all working right so here we go it's in transmitted now watts of current I want to tune for a dip so I want to bring this down to the lowest position and there's the dip and I'm already making just about 11 watts of output power just by doing that already now you'll find that bringing the the final plate tune out of the dip a lot of the times will give you more RF output power but you have to be going the right way if you go the wrong way you'll bring the power down it'll come out of the dip but the power will be going down so you have to be tuning the final tuning capacitor that in the correct direction and with skill and time you kind of get used to doing all of this stuff when you're working with these old transmitters so I'll just show you on the analyzer here that we are getting some decent output power so those ten point nine watts so what I'll do is I'll bring this out of the dip in one direction and you can see it's going down so the plate current is rising so what I'll do is I'll just dip that again right there is the dip so that's the dip where we were at you can see when I was going in the other direction it was raising so no matter what way I tuned this unless you have an RF power meter this is really hard too so you'll see that if I tune this control this way you'll see that it comes out of the dip now if I tune it the other way you'll see it'll come to the dip and rise again you see that so anyway I tuned this it's going to come out of the dip but if you go in the wrong direction you'll actually detune the power to a point to where you'll damage the final so that's very important to watch so a really safe way to load these things up really is just to use a watt meter but again bringing it into the dip is always safe so zoom on in on the screen here so you can watch the wattage move up here or you can watch it here if you want this is modulation we're not doing that test yet so so in order to bring this up in the right direction with this particular tune it seems that I'm going to go clockwise and you can see that when I go clockwise we get about almost 21 watts out of this and if I move the other control and move this around a little bit we get about 21 watts or something like that and it looks like it's just below 150 milliamps and that's out of the dip again so this could be you know tuned down towards the dip and you know you can reduce the wattage if needed so yet to see if we're gonna get any type of modulation happening at that level so it you might have to actually turn the power down just a little bit in order to you know get achieve a hundred percent modulation with this particular transmitter that's yet to be found out so what I'm going to do is I'm going to click this into standby again so you'll see the power drop off so there's no high voltage flowing and what I'm going to do is flip the switch from CW to phone and there should be a little bit of a reduction in our folk put power so click this back on not too bad and I'll click that off again click that back to CW and as you can see it's up just a little bit coming up to about 20 watts as things are kind of warming up from drying current I have an audio signal generator attached to the microphone input jack and I have the mic gain turned up just a little bit so I'll use the gain on the signal generator itself to turn this up so what I'll do is I'll click this in to transmit mode and we'll take a look at the analyzer here so I'll see how much modulation I can get out of this thing with it set to this wattage here so here we go you can see the level right under the mod freak going up just a random frequency I chose with a generator and it goes to about 70 percent and it's topping out there and I'm pretty sure it's not a clean sign there at all so what I'll do is I'll just leave this up around 70 percent and what I'll do is I'll bring the plate more towards the dip so that it's drawing less current of course the wattage is going to drop down and we should see a rise in the modulation level as you can see yeah it's starting to rise quite a bit so it's actually it went over a hundred there really fast and that's why it's popping into another range so I'll just turn this down and then back up you can see it's heading right towards the hundred-percent no problems there I'll just turn that off so with the wattage no modulation or anything happening here so you can see the wattage is at about eleven point one watts in order for us to achieve close to a hundred percent modulation so yeah this thing could use a little bit of help in order to you know put a little bit more RF power and achieve some modulation there I have the positive lead of my volt meter attached to that exposed jumper on the rear of the globe scout transmitter and I have the negative lead of my volt meter attached directly to the chassis so any guesses as to what the high voltage level is going to be on that jac I'll flip the switch to transmit and let's take a look over 400 volts is present on that exposed lead and that 400 volts is present all over the inside of this transmitter so I'll just flip this back into standby so extreme caution needs to be taken whenever you work on a transmitter like this you never ever want to come across that high voltage so there's a lot of modifications that we can do to really increase the performance of this transmitter so if you're interested in seeing a modification video on this globe scout transmitter let me know in the comments below and we'll revisit this in the future and we'll hot-rod the globe scope thanks for stopping by the lab today I hope you enjoyed this video involving this globe scope transmitter if you did enjoy the video you can let me know by giving me a big thumbs up then hang around there'll be many more videos coming like this in the near future so I've got a lot of very interesting antique and modern electronic devices to share with you guys lots of very interesting repairs and restorations planned for the bench so if you haven't subscribed you might want to do that right now as well in the previous video I posed the question would you like to see me deep pot the main power transformer out of that previous video in that previous video involved a neat little tool from the late 30s the answer to that was an overwhelming yes so we are going to deport that transformer together right now I'm busy designing I guess I could say a tool to make that a little bit easier so when we depart that transformer basically what we're doing is we're melting the tar inside that transformer and we're gonna pull the insides out of the transformer and see what they've hidden inside there so it should be a very interesting little process that we're going to do together so thank you for your comments and suggestions in that video if you're interested in taking your electronics knowledge to the next level and if you're interested in learning electronics in a very different and effective way you might want to check out my ongoing electronics course on patreon I'll have the link just below this video in the description and I'll also pin the link at the top of the comments section as well so until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 166,337

Rating: 4.9331565 out of 5

Keywords: Globe Scout, Globe King, World Radio Laboratories, WRL Globe, AM Transmitters, radio transmitters, tube transmitters, WRL 40A transmitter, ham radio transmitters, repair transmitters, fix transmitters, am modulator, Heising modulation, high level modulation, plate modulation, Globe Scout Model 40A, WRL Globe Scout

Id: VtsyKXl-nwU

Length: 36min 37sec (2197 seconds)

Published: Mon May 28 2018