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Author Topic: TBW K3MSB  (Read 1521 times)
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k3msb
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« on: July 19, 2023, 01:55:38 PM »

I did a TBW about fifteen years ago and have a video of it over on YouTube:

https://www.youtube.com/watch?v=sd4gPNGNYRU

The original design used a weird three pin ceramic plug, the filament, screen and +500 volts all come in on the big Bakelite connect next to it and you can use banana jacks that fit right over the pins to keep that all original. I had a 2-kV power supply that was under the rack that was connected by a chunk of RG-58 with crimp on connectors on the shield and center conductor that was attached to the screw terminals on the filter capacitor in the power supply and connected directly to C327 or C336 on the HV side of the plate tank and did not attempt to use the original HV plug on the bottom of the transmitter.
Are you going to use screen modulation like the original design or high-level modulation?
Do you have the center section?

Hi Ray

I have all 3 units (LF TX,  HF TX, and the Rectifier Modulator unit) as well as all the original cables.

I plan to mount the filament transformers in the RMU.   The AC Power switch on the RMU will control them as will the Filament rheostat.

I've already built the 2KV and 500V supplies in a standalone chassis and will bring those voltages into the RMU then onto the correct interconnecting cables.

I will use their modulation scheme as I want to keep the restoration as close as possible to original.

In the RMU I'm using the 800 Hz iron is missing.

Thank you for the video.   What is the "Power Control" on your unit controlling?    Normally it's controlling taps to an 800 Hz transformer that I don't have.  Actually, I do have all the 800 Hz iron in a second RMU -- just in case I ever find the generator....    Grin
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73 Mark K3MSB
York, PA
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« Reply #1 on: July 19, 2023, 03:18:00 PM »

The power control disabled the AC control to the plate supply, the 2 kV. Unbelievably they disabled the plate supply to the 803 tube and only provided screen in the tune mode. All my life was always told that you never run the screen without the plate on first but on that transmitter its designed that way.
Oddly it did not appear to have any bad effect on the 803 tube? years ago posed the question about the 803 tube thatís also used in the TDE and several other Navy Transmitters if anyone has ever seen one fail or get week? Itís a weird thing but no one was able to come up with a failed or bad 803 tube. You can abuse them to no end and they donít seem to care, and remember that now days your dealing with a tube thatís over seventy years old but they still keep on going.
I used one of the old sidetone contacts on the big keying relay , K301C so that when that was closed it controlled two relays with one for the 500-volt supply and a second that activated the plate supply, you donít want to leave the 500 volt supply on all the time because then you hear it in your receiver, also it would be scary to keep the 2 kV supply up when you donít need it. The original design used short time constant small filter capacitors in the power supply at 800 cycles and that worked good but with sixty cycles the filters will provide just enough power after removing the primary that the VFO and IA will produce a weird spurious signal that travels down the band so I also ended up providing a negative bias voltage to one of the terminals on M301, the grid bias meter that when K301B was open cut off the IA and VFO tube and eliminated that problem. Lot like what they did with the NAVY TDE transmitter, just speculation but think the TDE, GO and TBW are all most the same thing except the TDE was for shipboard use, the GO for aircraft like the PBY and the TBW for forward operating bases ops.
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k3msb
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« Reply #2 on: July 20, 2023, 08:17:25 PM »

Did you do any CW with your TBW?    I'm concerned about the waveform with the 500V and 2KV supplies with every key closure and opening going through K-301.   I'm considering just leaving the 500V and 2KV on all the time via the POWER CONTROL.  I had both supplies on all the time on my old TBW-5 and was fine.

I'm surprised you needed to add the negative bias to the IA under key up conditions.  I would have thought that with K-301B ungrounded you wouldn't hear anything.

Did you bypass L303 and L302 in the oscillator filaments?   I'm not sure if they are even needed when using a 60 Hz filament transformer.   If those can be bypassed, you can also delete R312 in the IA filament circuit and the filament secondary is now a standard 12.6 CT instead of 19.6 CT.
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73 Mark K3MSB
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« Reply #3 on: July 21, 2023, 11:01:07 AM »

I left all that junk in the filament for the oscillator, just used a 12.6 CT transformer and R312 being they were already there and did not appear to be any issue. The two tubes lit up just fine. Think it was like a five-amp transformer so its output under load was enough so there was not an issue.
You would think that K301B would kill the oscillator straight off but for some reason I had a carrier that was still there, low level and it did travel in frequency when the key was released without bias. Maybe it had something to do with the larger filters on the 500-volt supply or whatever. Think if I had relay switching on the DC side of the 500 volt and HV supply maybe that would have not been an issue but just found it easy to keep the original idea of switching the primary AC feed to the high voltage supplies.
Had full wave 500 volt and 2 kV supplies using solid state bridges without center taps on the transformers so maybe I could have lifted the negative side of the bridges or something like that but had lots of relays that were rated for 115/220 VAC and thought that would be better to do the switching on the primary side of the transformers instead of having to find high voltage relays.
In voice mode the keying relay is not that big a deal with its ďkerchunkĒ but in CW thing you will find that to be fun. Good thing is that no matter how much the relay shakes the transmitter, table and everything around it it has no effect on the stability of the VFO. I was able to lock down the VFO on 7290 and donít think I had to touch it for months because it was always on frequency.


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