2E26 Transmitter

[Carl WA1KPD]

Picked up the following TX from eBay based on an article in the January 57 issue of QST. Very nicely built and very true to the circuit except the builder added a VFO

Several problems. I get very limited power out- like less the one watt and the loading cap does not seem to have any impact on the output. I have checked all circuits, components and everything looks good. The only thing that seems off to me is that the screen voltage on the 2E26 is about 70V. I think it should be closer to 180 volts.
I have replaced the 2E26 with a NOS one with no difference. I checed out output ahead of the relay to make sure that it was not the problem with no change in reading

Secondly it looks to me as if the osc will always run? Can I lift the cathode when in rcv mode to get rid of the sig?

I appreciate any thoughts

Thanks

Carl
/KPD

[WA1LGQ]

        Good find Carl, that would be fun to play with. Is the 2e26 plate getting HV? Is the loading cap any good? Is it making contact with the output coil and gnd connections? How about the screen bypass cap on the 2e26, is it shorted, does it get hot and bubbly? I guess you checked the screen dropping resistor already? How is the grid drive current?

.........Larry

[W3SLK]

Carl, can you peak the grid and dip the plate? Being able to tell what's on what will tell you where the problem lies. If you aren't getting a grid current, chances are you won't get anything out the other end. I think you will find it but it will be one of those "Doh!" moments. Gud luck.

[Carl WA1KPD]

Is the 2e26 plate getting HV?

482 VDC measured on the cap itself

Is the loading cap any good?

4-137 pf- on the money

Is it making contact with the output coil and gnd connections?

All the way from the cap to the center of the SO239

Is the 2e26 plate getting HV? How about the screen bypass cap on the 2e26, is it shorted, does it get hot and bubbly?

No problem but I replaced it anyway

I guess you checked the screen dropping resistor already?

On the money but the voltage seems low to me when looking at the tube chart

How is the grid drive current?

Looks fine assuming the shunts are still good. If anything a bit high

Carl, can you peak the grid and dip the plate?

Yep both of those work fine. The problem is the dip on PC is about 10 MA and the loading adjustment does not let me bring it up any. It acts like there is no cap there.

I think you will find it but it will be one of those "Doh!" moments. Gud luck.

I agree- Thanks I found it on ebay listed under CB one day.

Thanks for the help guys. I will keep you posted.

73

Carl

/KPD

[AB2EZ]

Carl

The rig is behaving as if it has too much loading capacitance or too little inductance on the output winding (e.g. the output winding is shorted to ground somewhere in the middle or at the top)

Either of these would lead to too high an RF load impedance on the 2E26.

[I.e. as the output impedance goes down, the load impedance seen by the tube goes up]

Stu

[Carl WA1KPD]

Carl

The rig is behaving as if it has too much loading capacitance or too little inductance on the output winding (e.g. the output winding is shorted to ground somewhere in the middle or at the top)

Either of these would lead to too high an RF load impedance on the 2E26.

[I.e. as the output impedance goes down, the load impedance seen by the tube goes up]

Stu

I will pull that output winding apart. There is no indication of a problem but there is just general crud in the area. Maybe something is shorted as it is hard to see. I'll also measure the actual inductance of it as compared to my calculated inductance (.4 uh) 3 turns @ 1 inch dia and 3/8 in long
As an experiment I put a .001 cap in series with the center of my coax and taped the plate coil by sliding the other end of the cap leadup and down was able to find a point where I showed 40 ma of plate current and about 12 watts into the dummy load. A little low but not bad

[WD5JKO]

Glad were finally discussing real technical issues from old long expired ARRL QST Copyright material.

Good discussion..

Jim
WD5JKO

[KC4VWU]

Carl,
        I'd make sure the output link and load cap is very well grounded as well as double check all the solder connections after the tube plate. Inspect the blocking cap and plate choke also. If you are getting good grid drive, it has to be in the output circuitry somewhere. Even with low screen voltage, you should see more output than a watt unless the supply is severely sagging.

        I would say it is pretty compareable to an ARC-5 tx. and you probably should expect  no more than about 20w. out to the antenna.

Phil

[w1vtp]

Carl

That screen voltage is too low at 70 volts.  Is the plate dip rather shallow?  Or does it peg off resonance?

Nice find

Al

[Gito]

Hi

The transmitter looks like it's loaded lightly or not loaded at all,when A transmitter gets it screen voltage with a dropping resistance from it,s B+ ,with the right grid drive/grid current,then the Screen voltage will soar up/get high when the transmitter is loaded lightly ,so the screen voltage will drop to a low value/If the R/dropping resistance is the right value

So maybe the output coupling Capacitor is at fault /defect,or the output coupling circuit is not right value/not the right load/antenna ,have You tried it with a dummy load?

Gito

[WA1LGQ]

Kinda sounds like the output circuit is not resonant at the correct frequency.......

[Gito]

Hi

when experimenting that You have done,You managed to load it to 40 ma.Than it is obvious ,than the problem is in the "loading Part" in the transmitter,

 Is the transmitter design for a certain load like 50 Ohm load.
Can it be that Your Antenna has a higher/different  input impedance as seen by the transmitter?

So the transmitter "sees" a higher load than it was design for,and makes  the transmitter harder to load to it's speck with it's present circuit.


Gito

[W3RSW]

Mentioned before, but I think the screen bypass cap is lossy and is sucking down the screen voltage.  Since this is a low current application, it doesn't take much loss to suck the screen down.  The cap may not get hot or bleed, etc. Many screen bypass caps were under spec'd. for real world screen voltages over time.

[N2DTS]

Its screen modulated right? So what is the screen voltage supposed to be with screen modulation?
maybe it was designed to work into something like a whip antenna?

Brett

[KC4VWU]

Looks like plate modulation to me. Figuring where the screen supply has went awry should be fairly simple; not much there. I still believe it's not matching to the load properly. Since it was designed as a mobile tx. may be set up to be worked into a shortened ant. Give him a little time to smoke it over good and we'll see what he comes up with. Should be a fun little tx. to play with.

Phil   

[w1vtp]

I agree: the schematic indicates that it is plate modulated.  

I'm having a problem getting my mind wrapped around the idea that the screen voltage is so low because of a loading condition.  An easy way to find out if the low voltage is because of excessive screen current (the implied cause of the low 70 volts) would be to measure the drop across the screen resistor (after first checking that resistor's value). I'd hate to think that poor 2E26 is drawing that much current!  I'd sooner suspect suspect another cause. Perhaps the screen resistor has changed value or the bypass cap (as someone has suggested) has changed into a resistor.

The reason why I asked about the plate current indication while going through resonance (dipping) is that typically when the screen voltage is forced low, the off-resonance current does not go very high.  Whereas if the screen voltage is near where it is supposed to be off-resonance will be very high with a sharp dip assuming a light load.  If the loading is too tight, plate current should be high and it should be hard to get a dip where you would expect it to be.

Good luck, Al

[N2DTS]

Oh, yes, plate and screen modulation!

70 volts sounds very low to me.

With a lightly loaded final, screen current goes way up, voltage goes down, maybe the rig was designed to run into an odd antenna.

The output circuit looks more like link coupled output stuff, where the cap tunes out reactance, is not used to load the final, the link position does that.
Its not pie net, so it wont tune like a pie net....

What does that output coil and link look like?

Brett

[N2DTS]

Also, it looks like the hv was switched on to tx, powering p the osc section along with everything else.

75 pf plate loading cap for 80 to 10 meters?
No band switching on any stage, and its supposed to work 80 to 10, or is it to be built for one of those bands?
I don't see how its going to work on 80 to 10 without switching coils.

Can you switch the coils or is it built for one band?

Brett

[Pete, WA2CWA]

Also, it looks like the hv was switched on to tx, powering p the osc section along with everything else.

75 pf plate loading cap for 80 to 10 meters?
No band switching on any stage, and its supposed to work 80 to 10, or is it to be built for one of those bands?
I don't see how its going to work on 80 to 10 without switching coils.

Can you switch the coils or is it built for one band?

Brett

The circuit diagram clearly states "single band mobile transmitter". Also, if you look at the parts list, it indicates that L1, L2, and L3 have to fabricated per a "coil table". Different coils for different bands but only one band at any one time.

[KC4VWU]

Yes, 70v is low; specs give 200v max. Something is going on there. 3.5 or 7 MC crystals? Could this be a 20M tx? Doubling in the oscillator and straight through or doubling in the PA would get it there.

Phil

[KC4VWU]

Oops!, just saw that. Will do 80-10. So I guess it's fundimental, doubler, tripler on the osc. and straight through and doubler in the final depending on band choice.
Phil

[WU2D]

Everything Carl said and check the relay contacts...

Is the output tank coil/s wound correctly? The thing may never have worked!

The 1635 push pull is interesting and it makes a compact modulator. That tube is basically a military version of the 6N7 which is a nice low impedance driver. I think the Super Pro uses those as the limiter.

Mike WU2D

[KM1H]

How many paper caps are in there?  Get rid of any first and then do a voltage check at all the pins. You might find a few resistors well out of tolerance or even the wrong value. Ive picked up several old HB projects that looked darn near professional but the builder never got them working due to wiring or component mistakes.

Carl
KM1H

[Pete, WA2CWA]

You might want to check the 33K 5 watt screen resistor to see if its changed value. And check the coil table information to see if the coils match the frequency of operation.

[AB2EZ]

Given Carl's posted measurement results:

A. Rig tunes up properly if he creates a temporary "pi" network at the output (by placing a capacitor between the antenna connector and a tap on the primary of the link coupled circuit.

B. Screen voltage is too low: indicating excess screen current... probably caused by too little plate voltage when the output circuit is tuned to resonance... which, in turn, is probably caused by too high an rf load on the 2E26 at resonance. [Check to see if the screen voltage comes back up when the rig is tuned off resonance... to produce about 50mA of plate current].

Since this rig is using a link coupled design (as Brett pointed out), the load on the 2E26 will be much too high if the turns ratio of the link coupled circuit is not correct. Check to see if the correct coils are installed.

Also, as Gito pointed out: if this rig was designed for mobile operation, in the 1950's , then it is possible that in the expected application, the resistance of the antenna (after adjusting the output capacitor) would be much lower than 50 ohms
. If that is the case, then with a 50 ohm load (and with a link coupled output circuit), the rf load on the 2E26 will be much higher.

If that is the case, you will have to use a lower turns ratio in the link coupled circuit... or consider changing the tank circuit to a pi network.

As an alternative, to use the existing output circuit:  you can convert a 50 ohm load to a 12.5 ohm load (for example) by using a ferrite core to build a 2:1 transformer. The 1 turn winding would face toward the rig, and the 2 turn winding would face toward the antenna. If that helps, you can experiment with the turns ratio to get the proper load on the rig.



Best regards
Stu