Firing up the KW Transmitter.. Need help with operating guidlines.

[kc6mcw]

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?

[w3jn]

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.

[ke7trp]

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

[w3jn]

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.

[ke7trp]

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

[KB5MD]

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

[N3DRB The Derb]

Must be a shorted Mod transformer..

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?

[ke7trp]

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

[kc6mcw]

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

[WQ9E]

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

[ke7trp]

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

[Gito]

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

[Gito]

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.

[ke7trp]

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

[flintstone mop]

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

[ke7trp]

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

[Gito]

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.

[ke7trp]

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

[KD6VXI]

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

[ke7trp]

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

[w3jn]

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.

[ke7trp]

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

[WD8BIL]

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!

[KD6VXI]

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

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

[ke7trp]

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