The AM Forum

I have another post asking for information about my Transmitter labeled KW62. This transmitter has two 4-400s modulated by Two 810s.

I am about to fire it up for the first time.  I am going to use the ranger to excite the 4-400s and to modulate the 810s.  The ranger is up and running and ready to go.

IN all the documentation, I cant find any information on operating guidlines for loading the transmitter. My main question is on the two variacs, I must set.

One is for the 4-400 Screen volts. The other is for the plate voltage on the 4-400s.  IN the hand written notes it states 600 volts screen and 4000 volts Plate.  These are maximums I understand..

These are both set to ZERO now.  Can someone give me starting voltages to do the initial tuning on the 4-400s? I was thinking 2000 volts plate and 300 Screen.  I understand the importance of having both voltages up at the same time.  Should I start lower?

My plan:

1. load the ranger up to 10 watts AM carrier on 75 meters in to the dummy through the Transmitter.
2. Plug Transmitter in, Flip switch and wait the 50 seconds for the HV to come up.. (scary moment)
3. If all is well let the filiments cook for 20 minutes or so.
4. KEY transmitter with NO drive and increase Plate and screen voltages(both have meters)
5. Key Ranger and tune the plate cap for resonance.
6. Tune load for max power and retune plate cap for resonance.
7. Let run at 200 watts or so for 10 minutes.
8.  Unkey and make sure everything is ok.

I need starting Plate and screen voltages you suggest.

I need Plate current that you suggest for the two 4-400s in low power.

I need MAX plate current allowed at high power.

I need Typical Screen current and max screen current.

I need to know max Modulator Current for Two 810s running at 2700 volts.

The overall idea here is to get it up and running at around 300 to 400 watts so I can use it for a day or two to locate and repair any issues I might find. 

All it states in the manual, I have are test notes..  "4000 volts plate, 600 volts screen,  1200 watts AM output-100% mod for 4 hours" ect...  No notes about currents.  I wont run this at 1200 watts ever.. It will likely destroy my KW matchbox and 18Gauge antenna.. LOL  I intend to use this at 300 to 400 watts.

Thanks for your time!  Wish me luck!   The 50 sec time delay scares me.. I think I will have my hand on the plug at the wall ready to yank it out if something goes BANG!

Clark

Just make sure you don't apply screen voltage to the 4-400s with no plate voltage.  That's a recipe for a couple of melted toobs.

Also ensure that there's fixed bias on the 4-400s or a clamper circuit w/grid leak, if you key it with no drive and no bias it'll draw LOTS of current in a hurry!

Believe you're looking at a max mod current for the 810s of about 300 mA.

FYI, my GPT-750 runs about 3KV to a pair of 4-400s.  Loaded to the book values, Plate = 300 mA, screen =60 mA, Po= 750watts.  Grid drive is about 10 mA if I recall correctly; this will affect screen current.

Best way to tune up is look for a peak in screen current; the plate dip isn't as pronounced as the peak in screen current.

You might look at the manual for the Johnson Desk KW which uses pretty much the same toob set.

You should have that turret any day now.

Clark,

I would follow JN's advice and the tube lineup is the same as the Desk KW.  I usually run my Desk in the low power position loaded to around 300 watts carrier output.  In the low power position, voltages are as follows:

HV (both final and modulator) 1,500 volts
Screen  250 volts
Fixed bias (final) -60
Fixed bias (modulator) -33

If there is not already some interlock to prevent screen voltage without plate voltage I would suggest using a relay setup with the coil controlled by the plate voltage variac set up so that the screen supply does not receive power until the plate control is advanced to provide high voltage.  Although the 4-400 screens are pretty tough compared to a lot of more modern tubes applying screen voltage without plate voltage will quickly turn your 4-400's into low mu triodes.  A fast acting low current fuse (in the screen lead) may also provide additional protection.  The supplies should also be interlocked so that neither plate nor screen voltage can be applied unless the grid bias supply is also active.

I know your feeling about waiting for the "big bang".  My Johnson Desk KW was in pieces when I bought it and it took about a month to get everything hooked up again.  I did the first testing with the screen and HV supplies disabled and all went well until I switched the mode switch.  The works were outside of the pedestal and when the VERY loud mod transformer shorting relay changed over I jumped high enough to nearly hit the barn roof.  All was well except for my nerves.  For the Desk, I created a temporary power cord with additional fuse holders and I used fairly low values fuses for the initial very low power testing to make sure that nothing major was wrong.

73, Rodger WQ9E

Your #3 should be first.   Cook the filaments at least 30 minutes.  Does it have mercury vapor rectifiers (866s or 872s, etc.) ?  Cook them at least 30 minutes (60 is better) with NO anode voltage.

I would disconnect the all HV and LV from the PA and modulator and bring the HV supplies up by themselves, slowly as long as you have variacs.  Another trick is to wire a 100W bulb in series with the AC supply so even if there is a short on the DC side, the AC side current will be limited to 100W or so.

Be careful!

73, Bill  N2BC

Thanks for all the replies!!

I got the Turrett!! Its the exact one I need!  Thank you so much for sending that!  If you need money for postage or anything.. Just let me know.. What a guy!!!

I am going to download the DESK KW manual if I can find it.  This is pretty close to that except the power supplys are way larger.

Ok... I think I have what I need for ratings on the Plate and screen.. 1500 and 250 to start.  This transmitter has interlocks and time delays built in.  I understand about the screens burning up..  I will apply some Drive when I first key it. 

Once its up and running and I know the iron is all good.. We will start the upgrade process.. On the list are new diodes to get rid of the old Metal ones.  Different bleeders in the power supply as there are 330 watts being disipated now.  I want to install the tuned input back with the new turrett that was sent to me. I am also going to drive the modulator with hi fi outboard gear and audio amp. I am also going to install a third variac for the huge blowers. At the low cycle of ham use, I just dont need the 100% output of the fans going in this room.

Thanks for all the help... Now, I just need to get better so I can move this thing in the room and get to work!

I use a small fuse in series with the screens, say 200 mA or so.  That should protect the screens in case the plate HV goes away,

Hi Clark,

Here is some data based upon my fairly long experience with both a single and a pair of class C plate modulated 4-400As:

My first homebrew plate modulated rig back in 1978 ran a single 4-400A, plate modulated by a pair of 833As. I ran the 4-400A at 2750 VDC at around 275 Ma Ip, for about 750 watts plate input on 75M. I got close to 600 watts output at this plate voltage, which represents an efficiency of around 78%; quite good in my opinion, considering the plate voltage I was running. 4-400As like to see a minimum of 3000 VDC for really good efficiency.

In Class C plate modulated service, when you run the 4-400A at around 2700, 2500 VDC or less Ep, the Esg must be reduced from the published Eimac specification at the normal level of output loading, or else the Isg will run too high. With 2750 VDC on the plates of the pair 4-400As in my present rig, and at the level of Ip I normally run, I pull about 80 ma of Isg, and the Esg must be held down to 400 VDC at this set of operating conditions. If you run the Eimac spec of 500 VDC Esg, the Isg will come close to exceeding the dissipation rating of the screen.

This is true not only with my rig, but similar type transmitters including the Collins KW-1 and the Johnson Desktop Kilowatt when run at their normal plate input level and an Ep of 2500 VDC. Both rigs ran the 4-250As or 4-400As at an Esg of 400 VDC for the reason I have described above.

When I run my rig in the low-power mode; I reduce the Ep to 1800 VDC, and the Ip to 350 Ma. The Esg is reduced to 300 VDC, and the Isg drops accordingly to 60 Ma. The grid drive remains the same at 25 Ma.

John/W3JN: 10 Ma of grid drive for a pair of plate-modulated class C 4-400As is way too low; the Eimac spec is 12 Ma per tube. I also agree with your comment about watching the 4-400As Isg when tuning the HPA; always tune the final for a peak in screen current.

As others have pointed out in this thread, you really want to watch that screen disippation, both during the initial fire-up/testing phase as well as during normal operation. As a minimum, use a 125 Ma fast-blow fuse in the screen circuit, or some kind of screen overload relay protection. You don't want to roast a pair of expensive 4-400As! I also use a series of interlocked time-delay relays to control the proper turn-on and turn-off sequencing of the various power supplies in my transmitter when going from transmit to receive and vice-versa. Very important!

My experience with BC rigs employing 4-400As is that the Ep is usually around 3100 VDC; this permits maximum efficiency, and the Esg can be run at the normal Eimac spec of 500 VDC for plate modulated class C operation, without exceeding the screen dissipation rating of the tube.

Good luck with what will probably be a fun and rewarding transmitter project!

73,

Bruce

Clark, that turret does a LOT more good in your xmitter than it does in my garage.  All I ask is that you pass the radio karma around to someone who needs it someday.

Bruce, that figure was from memory... I rarely look at the grid current in the GPT because if you have the right screen current it follows you have the right grid current.

Ok.. Good advice.. I understand..  The Rig has meters for Plate volts, screen volts, Grid, plate and screen current.

So no more then 80MA of Screen current?   

I can see this relationship as I have loaded my Globe 500 many times while watching the Screen current. They suggest less then 40 MA of screen current and that rig has one tube.

Thanks again for the turrett. I will try to help someone down the road!

CLark

Clark,

Sounds like you are on the way.  By the way, the corresponding voltages for the high power position of the Desk KW are:

HV (both final and modulator) 2,800 volts (the manual spec is a bit lower but that was for lower line voltage)
Screen  400 volts
Fixed bias (final) -90
Fixed bias (modulator) -65

As built, the Desk KW doesn't monitor screen current so I added a small external meter in its own little case to monitor the screen.  The Desk is set up to meter total cathode current so the known amounts were total cathode and control grid current leaving you to guess at how the remainder divided between the screen and plate.  The assumption was if you followed the tuning instructions in the manual it would be correct-adding a meter proves that was a bad assumption and with it tuned as Johnson indicated the screen current was usually high (although not dangerously so for 4-400's).  The desired level for the Desk is 75 mils of screen current loaded for 1KW input on AM.

I would definitely get used to your transmitter (and find any "bugs") running low power for awhile before you crank up the variacs.  Do be careful to match the audio amount so you don't create excessive voltages and blow a mod transformer.  Have fun!

Rodger WQ9E

Under some loading conditions you can get reverse screen current. The screen supply should have a low enough resistance bleeder to absorb it. If screen current is too high, it could be the loading is too light for the applied drive. It's more of a concern in smaller tubes but worth watching out for.

This transmitter also has GRID Voltage knob and Grid tune knob.  The Grid Voltage has a Meter.

Going to light off the Filiments in a few moments with NO HV and NO drive.  Just in Standby. 


Clark

Ok.... The good and the bad..

We plugged it in to the 240 vac line. No problems.

Turned the mains on.. Power light lit up.. Great.

Turned the Filiment switch.  810s and 4-400s lit up nicely. 

Waited about 20 minutes.. No smells no noises.. 

Hooked up external Keying relay 110v to a Foot pedal.  Stepped on it. Got volts!

1000V Plate.  100 Screen. Bump in plate current back to zero. 

Turned up variacs to 1500 plate and 250 screen.  No problems. Unkeyed it.

no smells no smoke..  Looking good!

Keyed it again and watched the volt meters move up to 1500 and 250 again.  Perfect.

Keyed it up again and plate Current meter PEGGED OUT MAX.... Unkeyed..

Slight smell of burning. Hmmm.. powered down and inspected. Found nothing. 

Turned back on and warmed up.. Keyed Relay for half a sec... Saw voltages come up and plate current PEG.. Unkeyed..


Thats where I am at..  So for a moment.. The thing looked normal..

Plate volts: 1500
Screen volts 250
Mod current. 40 to 50 mills.
Mod volts 2600.
Plate current bumped up to 100 then fell to zero. Later Pegged the meter.

Short in RF deck somewhere... Should I pull 4-400s and try again with no tubes in line to see if Short is gone?
Check 4-400s for a short? Which pins?

NO RF DRIVE the entire time. My wiring for the plug on the ranger is not correct.  I hooked up the 600 ohm out of the ranger to the mod deck as the manual states. Now the ranger has no RF out.  I need to look into that acc plug further. Hence.. No drive tonight.  Just checking the voltages and the plate meter pegged.

We ended up stopping for the night... To much stress and not enough know how to go further. I guess good and bad news.. The volts and iron are good. The mod deck fired right up.  Just need to figure out why the plate is shorted.

Clark

Hallo Clarck


Reading your problems ,may be some of my comment/replay can help you,looking at your grid voltage of the 4-400 it's to low the spec of grid 1 of the 4-400  is -220 volt in class C continuous service,but most of transmitter used a compromise voltage,just to cutoff the plate current(zero current) ,so we need not to drive it with high  R.F voltage,that you must exceed the -220 v off the bias to make grid current flow.So Rodger said the final bias is -90 v,usually there's a Resistance that connect between this - bias to the grid 1 of  the 4-400.so when we drive this tube there's a -220v dc on the grid (-90 + (R X grid current))

It is normal when you keyed the transmitter the Plate current bump up to 100 ma,and back to zero,it's because the - bias is not settled  ,so when the - bias gets to the operating conditions , The plate current is back to zero."with no RF drive".


After sometimes when you keyed the transmitter ,the plate current pegged out,Yes it can be that the 4-400 is shorted ,just try to put in one tube at a time,if the problem is still there .
I suspect the there is something wrong with the - bias circuit,maybe the resistor that is connected to the Grids of the 4-400 is burnout, so there's no bias in the grid.Or the - bias supply is  defect .(seeing that the - bias is so low/-60 volt?)

When I tuned a transmitter  ,I put the screen at low  voltage , as the screen voltage can control the plate current of the tube,than I drive the final gradually, first with small drive ,looking at the Grid Current,tuning the grid input to resonance,peaking the grid current, and looking of the plate current ,loading and tuning the output of the tube ,get a dip of the plate current/ so it's resonance.
The plate current will be low because there "no screen voltage", then slowly  we raise the screen voltage,the plate current will get higher ,when the screen voltage get higher.
when you get the right screen voltage voltage ,and the plate current is lower than we expect ,increase the drive,by doing this we can get the right operating condition. 
used low drive as possible ,so the screen current is low,the data sheet tells that the screen voltage is 500 v maximal,so you can adjust  the screen voltage to make the plate current lower or higher


Regards


Gito

Thanks for taking the time to help me Gito!

I think that one of the 4-400 is shorted.  I am going to pull one out at a time and try again.  It worked perfectly until something caused the short. If its still shorted, I will remove RF deck and check the Grid like you suggest.

I wonder if its possible for the 4-400 to Oscilate out of control with no RF drive?  The plate cape and load cap are probably not set anywhere near where they should be to start. Is this possible?

The good news is that the plate, filiment, screen and modulator HV are all there.  This means the iron and related components are good. 

I hope its just a tube! I have spares!  I have searched the RF deck over and over with my meter tonight and as you can see I am up at 1am.   I have not yanked the tubes out because I am alone and do not want to work on this without someone with me in case of accident or fire.  I will wait until a friend is here to assist me so I can be safe and power it up with one tube at a time.

P.S  I asked about the ranger hook up again.. I downloaded the Desk KW manual and realized my problem. I did not put a jumper on the plug for the Final HV on the ranger. I will fix that tomorrow and will have RF drive to the unit for testing. If its a tube or a simple resistor/cap failure, The unit will be up and running on the dummy load for testing.

Going to bed. Thanks alot for all the help! I feel I am close on this project!

Clark

youre a hero to me Clark, you rescued old homebrew and that means to me you love radio more than money. there's some groovy AM gangstas out that way..... you should get in touch and see if any of em can lend a hand in person. real radio people will jump at the chance to help.  8)

Clark


of course it's possible to oscilate,but if it's a factory build Tx ,I believed there are neutralizing circuit  in the transmitter that prevents it from oscilating,or the neutralizing cap is un tuned,

what did not make sense to me is the bias voltage,usually the -bias voltage is close to the tube spec,the - bias decide the operating class of the tube,since the - bias data s spec is -220vdc ,and your transmitter has -60 vdc,it again did not make sense to me .
Clarck i'm building small MW transmitter ,600 to 1500 watt transmitter used as gospel radio,so to my experiences ,I always used the - bias close to the data spec of the transmitting tubes,so when there's no drive the plate still get no current(zero current)

Are you not mistaken  the bias voltage of the 810 as the bias of the 4-400?

So I think there is something wrong with the bias supply ,maybe shorted turns in the - bias transformer.

Don't use the data sheet as a must, the goal is to tuned the transmitter for the power output that You want,but of course don't exceed the max rating of the tube.

So to begin just put enough drive,so there's a small grid current showing in the Grid meter,tuned the grid so there's a peak grid current showing,tuned the output of the transmitter for max power,with a low screen voltage,slowly raising the screen voltage,repeat the tuning procedure so there's more power output,get a more /higher drive ,repeat the tuning procedure,and if all is going well,the power output will be the power that you want.
You don't have to drive the grid with 10 watt driver (for example),you don't have to get a 12 ma grid current as the data sheet says,if you can drive it with a 6 watt driver and get only 8 ma grid current but can still load the amplifier to the power output that you want it's okay.
lower drive means lower grid current and lower screen current

Regards

Gito.

ps.Sorry for my English

I've seen many a tube-type AM broadcast transmitter oscillate with no drive.

And plate current always goes up as drive drops, in my experience.

I would NEVER intentionally turn on an AM transmitter with no drive.


I'm new here so don't flame me too bad, but I've worked a lot in broadcasting.

And a question--I thought the legal limit for AM on the ham bands was 375W carrier?

The legal limit for amateur operation is 1500 watts.  PEP or continuous (as in, RTTY, etc).

If you modulate your carrier 100 percent, 375 watts would be the theoretical maximum carrier.  HOWEVER, lots of people here are running broadcast equipment, and run in excess of 100 percent modulation.

--Shane

Thanks for the non-flame response...

I have given away two broadcast transmitters here at work and I hope they are enjoying a good life on 160 meters.

We have a couple more old RCA 1 kW transmitters not currently being used. I have considered taking one home, but if I do, I intend to run it legally.

Gito.. Bias is fixed. I studied the schematic.  Its just like the Johnson Desk KW.

I pulled the tubes one at a time. The short is still there.  If I key for half a sec, I see the plate meter peg.. I let off it.  Its not oscilating.

Now, With no 4-400s, The short is still there.

Screen: 200 (adjustable variac)
Plate: 1500 (adjustable Variac)

I pulled the HV off the RF deck.  NO short from connector to Ground on RF deck. Even Manualy closed relay and checked.  No short from Plate CAPS to Ground..  Screen and filiment bypass caps are perfect.

Mod deck seems fine.. Mod volts and current are at normal.

HV power supply seems fine as does LV power supply.

The only part that looks like its ever been hot is the Small Choke from Output to Ground at the RF output conector.  It is a bit brown and has some coloration on the wall where its mounted. I cant seem to tell if its damaged or not. I need to pull it out of line and test it. Its a 2.5 choke as per the schematic. It looks like its been this way for years.  I can smell a Rotten and sweet smell of what ever is shorted.. Just cant locate it.

I tried 10,15,20,40 and 80 meters on the BW 805 Tank.  All short when keyed. I dont want to keep keying it like this so I stopped. I have gone through everything I can think of with my meter trying to find this dang short!

I am also studiing the INTERLOCK system.  I do NOT see the interlock actuator anywhere. I think its bypassed. I can see that it is showing the interlock from Screen to plate voltage. If you dont get plate volts, the screen drops to protect the tubes. This seems to be working as I have both plate and screen volts.

Thanks for all the help!  This is like a soap opera!

I am on a committee against automotive companys so it takes alot of my time. We are trying to get the max speed of any car sold in the US to be no higher then the speed limit.  THis way, NOBODY can have the power to break the law.

Clark

Clark,

Given your description it sounds like the coupling/DC blocking cap is suspect.

Rodger WQ9E

Thanks for the tip.. I was going to pull the RF deck and check the 4 door knobs.  I have two at the top of the choke, One at the bottom and one at the HV input.

Clark

Well, I've read through this thread and wonder if a schematic is available.  As far as RF drive is concerned: if the control grid has cutoff bias applied there should be no problem with no RF drive.  In fact, with my 2 X 813 rig. it was SOP to start with no RF drive and advance it slowly while dipping the plate current.  This would be done until the proper grid drive (ma) was achieved.  At this point, the plate current might be a little low so the coupling would now be advanced until proper plate current, control grid and screen current is achieved.

Do you know if there is a combination of fixed bias and grid leak bias in in place here?  That would be important in my mind.  So, after you check the 4 x 4's for shorts and get a tube in the final that you have some sort of confidence it's back to square 1. 

I really think you need to know what you have by way of design.  Again, a schematic would REALLY be nice.  Fixed bias should be such that the 4 x 4's are just in cutoff with grid leak providing the rest of the bias for class C operation.

After reading WQ9E's post.  I am more convinced that a tracing of the RF deck and control circuitry is in order so you know where things are design-wise.  If this a home brew rig, it's a mistake to assume it was designed after the Johnson desk KW or for that matter, the Collins KW1.

Duplicate comments.  Please delete

Happy to hear that its not the tubes. I bet it will be something simple once you found it. May be that little choke on the output failed and shorted.

Paul, is that 5 kw Gates still over there? Got any pics of it?

I agree with Rodger.  One, or more, of the doorknob blocking caps is either leaking or is shorted (or breaking down under application of HV) putting HV on the tank coil which then travels down to the little RF choke across the output.

As you can see there's a VERY good reason for having this RF choke.  If it weren't there, someone could get killed dead in an instant by toucing the antenna.

Suggest you disconnect one end of the doorknobs then key it up again... if you get no heavy current draw, that's it.

Its a motorola rig. The Schematic is very close to the Johnson Desk. I am not assuming anything. I compared the schematics.  The bias is fixed as is on the Desk.

If I cant get it going in the next few hours, I will go down and scan the schematics and post them.  The Door knobs all check out. No shorts in RF deck.  I will disconect the one at the bottom of the plate choke and try again. Maybe its breaking down under load. 

If this does not fix it.. I will have to call the RF deck good.  Meaning that something else is shorting the HV out on purpose like an interlock of some kind.  I can see two terminals on the back labeled (24 Volt INTERLOCK) and nothing is hooked to them. I looked over the schematic and it clearly shows an interlock relay with the coil that goes to these terminals.  Maybe this relay needs to be energized... I am not sure yet. 

The other issue is that there is an RCA jack going out the rig with the PTT RCA line.  This is not labled. I traced it to the INPUT relay on the RF deck.  On the schematic it shows TWO RF inputs.  I am going to have to pull the RF deck out again and flip it over and trace this relay down.. I think someone decided they wanted this rig up and running with any one of two exciters. So they added this relay to switch the inputs.  If this is the case, I will rip all that out and wire it direct like it shows on the schematic.

For now.. The 24 volt INTERLOCK is my main culpret in my mind.. I am about to throw 24 volts on that coil to see if the short drops out.

If I unplug the HV from the RF Deck it shows Short from Center pin of cable to Ground. RF deck side of connetor has no short.  Maybe its a Shorted 872? They really had a design flaw here as the 872 are at the front of the power supply deck. We will need a Cherry picker crane to get at them for testing.

Anyways.. I am moving from an RF deck problem to an HV power supply short..  I think the plate meter pegging is causing all of us to look in the wrong area...

thoughts?

Clark

Yep, depends on where the plate current meter is located electrically (ie in the RF deck, or before the RF deck, etc).  Could be the cable is shorted, something shorted in the modulator deck if the HV transits there, or in the PS.  THose little Millen HV connectors sometimes will short, too.

Take those 872s and throw 'em as far as you can.  Replace 'em with solid states or at least 4B32 xenons.  They're pretty, but mercury vapor toobs have fried countless transformers over the years due to their tendency to short and flash over.

Isolated the Short..

Unhooked the B+ from the RF deck.
Short stil there.. 500MA+
Unhooked the HV from the Modulator deck..
Short GONE.

The Short is from the HV line TO the Modulator deck.  Must be a shorted Mod transformer.. Oh well. We all tried..

Thanks for the help guys! 

Clark

Had a similar incident once.  Turned out there was a short in the plate choke between the bottom of the choke and inside the mounting screw hole. Won't hurt to check.  Good luck  On another note, check the HV connection where the HV connects to the mod chassis.  If it's the mod iron, it may be still useable if you slide a piece of plexiglas underneath the mod iron so as to isolate it from the chassis.  Good luck

could be, but even if so, they can be repaired. or another one subbed in for testing.

You sound deflated way too soon - that rig is worth getting back on the air. why dont you just ditch the modulator for now and see how the RF part of the rig works? 

you could locate a new mod xfmr in the meantime. it may be something super simple in the mod deck, you're chasing it down, why stop now?

I ran 5 watts from a Heathkit SB1400 HF rig on 3885 into the RF deck on AM. Got  300 watts out at 2000 volts and 300 screen right at the dip. Plate current was a super low 100MA or there abouts. I stopped there.. At least I got power out.

Its cool.. I will Roll it out of the room. Put the station back together.

Most likely, the rig needs to be taken all the way down and built back up again. I am not interested in doing all that work to it at this time.  I have plenty of other rigs like my 500C to talk on!

Plus.. The turrett needs to be installed on the input. Right now, I have a string of resistors for 100 ohms on the inputs of the 4-400s..Thats gotta go.

Thanks again for all your guys help.. I hope I was at the very least.. Entertaining.. hahah.

Clark

Ya, if it is the mod iron, Ive heard of guys repairing their own. Shoooot, Ive rewound a couple of 12" subs and made them 2 ohms with good luck. That was BEFORE they were even available like that! I wouldnt do it again BTW...HA

Clark,

Before you write off the mod transformer check all of the feed through insulators, modulator shorting relay, etc.  The odds of one of the ancillary devices being defective is fairly high so hopefully you are lucky and it isn't the expensive piece.  Not knowing how your metering is setup, it could even be an 810 problem.  I would start by disconnecting the mod transformer and checking to see whether the leakage is it or somewhere else.

Rodger WQ9E

I looked over the schematic and then poked my head in there.. Its transformer.. one .01 and out.. Not much to go wrong there!!   Cant be 810s as the HV is shorted so that would mean the trans right? 


The Amphenol screw on HV connectors look new and are perfect. I even jumped them with a clip lead for a test.

Man, I hope the band is decent for AMI tonight on 3870.  Its been just $Y(U last couple of days. Fire up your BIG rigs and give me a call!

Clark

Hallo


Clark,it's true what W1VTP wrote if the bias is right (cut off bias) There's no problem with no drive,when you keyed your transmitter.

Try to find where the plate current meter is placed,directkly  after the B+ power suplly.Usually  it's in order B+ --- Meter---Modulation Transformer---RFC choke ---,RF Bypass C at the junction  of the RFC and B+ to ground ----the parasitic coil then to the plate cap of the 4-400,coupling capacitor  from the junction from The RFC and parasitic choke to the output tuning parts of the tube ,at the output is an RFC conekted to ground to proteckt if the coupling capasistor is shorted ,and the B + apear at the output conecctor so .the fuse/protection circuit will put the transmitter down.

If the order is true, there can be :

1.Shorted modulation trafo to ground
2.Shorted By-pass capacitor to ground.
3,Shorted output Coupling capacitor so that the B+ apear at the output tuning parts and shorted by the RFC (near the output connector)


Regards


Gito.n

P.S just wake up it's 4 o clock in the morning in my country

Clark


I think you have found the problem in your Transmitter ,it's a little to late to post my reply,it's because when I woke up this morning,I directly  wrote ,without reading first the reply of the others friend.
anyway good luck and may your transmitter works well.

It's true what K4PDM wrote don't turn on AM transmitter with no drive
 
Yes if it has no fixed bias(cut of bias)and using automatic bias.with no drive there's no bias,so the plate current soars up.
If it has it screen voltage drop with resistor from the B+ of the plate voltage.
the screen voltage will soar up to the B+ of the plate voltage with no drive
 
But if the transmitter is neutralize it .it won't oscillate.and the plate current drops to zero if the grid  fix bias is enough, with no drive



Regards

Gito.

It has Seperate Plate and screen with an interlock system. Bias is fixed.  I ran the RF deck with no drive. No plate current to speak of.  Added Drive up to 10 watts and tuned the output to 200. THen I stopped. The RF deck works. The mod deck is shorted out of the B+

Thanks for all the help guys.

Clark

Hi Paul.
Thanks for being a good broadcast engineer who sees the value of Ham radio to GIVE those old transmitters away. It's sad that engineers and or stations think that olde tube transmitters are still assets.
It's too bad you live too far from PencilTuckey/ Pennsylvania. I wouldn't mind a second B'cast TX.
Yea, that PEP ruling is a joke. And I'm sure there are a lot of SSB folks running 3-4 KW. People using tubes with handles.

Phred

More work done on this rig.  The short remains on the Modulator deck.  I have checked the trans from each standoff to ground and there is no short. However, When the B+ lead is plugged in and then leaves this deck to go to the RF deck, The HV shorts and blows the fuse. I cant seem to figure out where this short is.. Its not the tubes and like I said, Nothing is measuring short.

W8QBG is going to come over tomorrow mid morning with more test gear. He offered to help me locate the problem. We hope its an oil cap shorting on HV. That would be an easy fix.  I will report back what we find..

So far,

Screen Volts are 300
Mod current is 60ma static.
Mod volts are 2600+
HV is 3000 but then shorts.

Clark

Hi Clark

When you managed to get a power output from the RF deck ,is the modulator transformer connected in the circuit?

You said when you unhooked  the B+ of the transmitter ....short still there.

If that's  so,it is possible that the primer winding(winding with is connected to the transmitter side)  of the M transformer is shorted to the secondary winding(winding connected to the modulator side) of the M transformer.(The only way the HV voltage can get to the RF deck).
If there's no Voltage on the RF deck how can the meter shows 500 ma?
So there must be a HV voltage there.....I think it comes from the HV voltage of the Modulator via the modulation trafo

Maybe these  winding is shorted to each other . there are times that you cannot measure it with an Ohm meter,because the high voltage "JUMPS" to each other(like a spark plug) because there are some damage in the isolation paper of the Modulation Transformer,it happens  only when you hook the HV or the B+.

regards

Gito.

PS.
when you unhook the B+,you mean the screen voltage and the - bias is off?Auto maticly
but the heater/filament is still on.
When it is like that ,maybe the HV voltage get to the plate of the RF final  from the short in the modulation transformer winding, since the bias is off the plate current (the meter) of the RF final rise to 500 ma ,


 
when you hook the B+ of the RF deck, 3000 v,it has a different voltage of the modulator 2600 v, because the primer and secondary winding are "shorted", so there is a high current flowing from 3000 v(B+) to 2600 v (HV),so the meter pegged.

I think The Primary winding(RF side) is shorted to the Secondary winding (Mod Site) and not to ground.

The B+ goes from bottom deck to to mod deck input. Then OUT of the mod deck and up to the RF deck. IN this config. Its shorted.. I key and the plate meter pegs and the fuse blows.

If I unscrew the B+ INput to the modulator and unscrew the B+ from OUT of the modulator and jump them together with a Clip lead. The RF deck will output power and the short is gone. 

So the Short only occures when the Modulator is "INLINE" with the B+.   When I test for shorts, There are none. I dont hear a POP or short when I key.  Today, We will muscle the Modulator deck out of the unit and test each component. Its very hard to find a short that is not shoing on the meter!

Clark

Bypass caps on the modulator deck are shorting under high voltage, or the mod xformer is shorted as Gito said.

Those are my two bets. 

If you test the mod deck HV input to ground, what is your resistance reading?  Mod deck output to ground?

--Shane

It cant be the input side. I have Mod voltage of 2600 on the 810s and mod current of 60 something at idle. The Caps look perfect. Have not tested them. Its 150+ lbs to get the deck out. The Mod trans does not show a short on the output side. 

Off to rat shack to get more fuses so we can test again.

Clark

Does this have those Millen HV connectors?  Seems those sometimes have problems, as do the ceramic feedthrus.  If they arc once, that's it, they're toast.

Are the 810s fed with the same HV?  I'd start removing stuff until the short goes away, ie disconnect the plates to the 810s, then disconnect the HV feeds to the mod xformer, then disconnect any caps or shorting relays, etc.

It uses the amphenol screw on type. Made of out bakelite.  They look shiny and new. No cracks ect..   I know the wiring and connectors are good. I ran the RF deck with a Clip lead from the input to output Connectors on the Mod deck. So the short is IN the mod deck.

The 810s have there own huge 2350-0-2350 transformer made by Chicago. 

Clark

While you're at it you best put limiters in the beast to make sure you don't have the power to break the 1500W PEP law!

Not a short from sec to ground, nor a short from either primary to ground.

A short from pri to sec.

Put one lead of your ohm meter to one side of the primary.  Put the other side to the secondary.

Read resistance ?  :)

--Shane

Pulled the mod transformer out.  Its shorted from OUTPUT to Ground. More on the left side then the right side. Its not a dead short but it shows resistance. The input side is fine. The mod deck is fine also. 

Looks like I need a 600 watt Mod transformer if anyone has any ideas on where to get one.

Clark

How about floating the mod transformer above ground and take great care to never, ever touch that critter when the HV is on?

73,

Bruce

we discussed this already.  It might work.  I was waitint to see if I could get a replacement before we try that.

Clark

Oh stopit!

Run down to the local grocery store.

Purchase teflon cutting board.

Mount mod iron on cutting board.

FLOAT mod iron from case to ground in said fashion.

Fire up new 4 kilowatt PEP transmitter errr....

375 watt AM transmitter :)

--Shane

OR........
Check with Gary about rewinding the transformer........
http://tubes_tubes_tubes.tripod.com/tubestubestubes/ (http://tubes_tubes_tubes.tripod.com/tubestubestubes/)

Yes! You must isolate the isotope. ..I agree with Shane.

I guess isolating the transformer will work but obviously its on its way out. Could last a long time or could fail completely soon.
I am of the opinion if it cant work as designed............replace it.
Band aids dont make reliable equipment.
Its nice to run equipment that u dont have to make excuses for.

Bill

I can try wood later tonight.. But thats not going to be a long term solution.  I found three options right now. One is a peter dahl.  Decent prices. Should fit.

Clark

Clark,

its normal for one side to have more resistance than the other side. the ohmage will increase because you have more turns of wire as you approach the outside of the winding(s). that alone is not a sure fire indication of a bad transformer.

you need that thing to be hi - pot tested  to verify for sure. I have a Freed hi-pot tester but I dont know if it works or not. I need to look inside it first and repair/replace any weirdness. maybe after I inspect it and verify ok operation I can send it to you for a test.

 

Hi Clark

It must be a short between Primary and secondary winding(not a dead short) Called it R1,there's also a short between the output to Ground called R2.
The B + oil capacitor is charged by the HV  trough R 1 and then it bleeds the Capacitor trough R2 ,showing as 500 ma plate current,the oil capacistor is charged to R2 divided by R1+R2 ,

When you hooked the B+ The current flows directly through R2 and the meter is pegged

R1 and R2 is actually Carbon tracks  when the Mod. Transformer is damaged.

 maybe It shows high resistance or no resistance when you measured it  with an Ohm meter.
 But when you apply High voltage it is a different story.

Regards

Gito

Short lived.... 

We placed a wooden plate under the transformer and extended the wires.. Fired it up and the short was gone. HV was 2000, Screen 250.  Grid 15MA, Mod current 80MA Mod voltage 2600.

I applied Drive from a Johnson ranger and peaked the Plate tune and then plate load. The Plate current meter read just off the peg. I think the plate Current meter is faulty. It never read more then 50MA and I had lots of power out. Great.. Increased the drive and retuned power was up. Applied a small amount of audio with my voice (from ranger) and heard my voice inside the Transmitter. Left it keyed for a minute.. No trouble.. Stable voltages and power out on Bird meter.

Applied a bit more audio and BAM the Fuses blew. 

With new fuses the unit powers up fine. If you flip the HV on now the Fuses blow instantly. 

The mod transformer must have really shorted out this time. 

The good news is that I have located a peter W dahl 1000 watt unit that will fit and a Thermodor 600 watt unit that will fit. I need to decide which to buy now. I think with the new iron, The transmitter will work.  At least we saw power out!

I can smell the Mod trans burning inside..  I know now it is the problem.

Clark

yeh, if you can smell it, it's done. removes all doubt for me.

you should investigate the rf deck now to see if here are any surprises at full bore. get a handle on tuning quickly, etc. If there are any surprises there you wanna catch em now before you introduce the new mod iron.

Just curious, what was the old iron? Depending on frequency response power level and such it may very well be a good idea to have it rewound. Big mod iron is pricey. You'll never lose on such a deal. Demand for big mod iron is too high.

YEah.. I have been through the RF Deck.  Aside from no parasitic resistors being isntalled, It looks pretty good. It tuned smooth and I have it resonant on 3870 now.  I have no idea if the 4-400s are good or not. I have two spares so thats not a problem.

The MOd deck needs the new transformer and it also has 4 old Orange Sprague electrolytic caps that need replacing. Might as well do that when I install the new transformer.

I also want to pull the Lower deck out and inspect the underside.  The 872s are out and solid stated already as is the other Rectifier tubes.  I think I need to eventualy go through the bleeders. There are 330 watts of bleeder on the HV supply and 240 watts on the modulator deck. To me that seems like overkill. Just burning up with heat.

I am pretty sure that with a new mod transformer this will come to life. I had real stable voltages.  ROCK solid 2000 on the HV with the variac at 40%. Rock solid Screen at 250 to 300 with the variac down at 30%.

Just need to decide which iron to get for it. Prob just get the PWD unit and be done with it.  I just want to get it up and running for a week. Then, I can decide if its worth it to tear it down to upgrade it.

Clark

Here are some pics of the mod trans on the left. The Right one is the big Mod power transformer for the 810s. Its 2600V out.

thats some beefy iron. The mod iron looks sorta sad, with it's plate leads hanging down in defeat.  :(  from the tags looks like the power unit is a Chicago Standard. they're hard to hurt. good stuff.

there's formulas for figuring out how much bleeder you need for a given voltage, current, etc to ensure he best possible regulation for the supply. 330 watts does sound like a lot tho. Possible some of that is being used for screen dropping, or some other legit purpose not just bleeding.

It is a large Chicago trans.  2350-0-2350.  Probably big enough to run the entire box.. which has been suggested.

The Big bleeders dont run anything else. I have the full schematics. The Screen supply is 100% seperate.. Good and bad..

Why there are so much bleeder is anyones guess.  The previous owner realized this and purchased new bleeders and did the math and included that math on a sheet for me.

Clark

Clark.

The voltage of a power supply soars up to 1.4 of the RMS voltage of the transformer voltage ,without load
when you rectified a 2000 v RMS you got 2800 v DC,
so to make the output more stable ,it uses Resistance Bleeder as a constant load,
So when You load/tune the Transmitter lightly or heavily .The plate voltage is relatively constant,
They used high wattage bleeder to do it ,
but as I know they used 3 0r 4 times the wattage they actually   need,
if you need a 100 kilo ohm  75 watt resistor ( by calculating the wattage need) ,They used a 100 kilo ohm 300 watt rating. maybe for safety reason.

The power that is used by the Bleeder actually is only 100 watt.

Regards

Gito