[Milsurplus] Powering the TBW-4

Thu Jun 13 17:49:12 EDT 2024

Hi Ray

My RMU had two of the three 800 Hz transformers missing.    Since I will be
running my TBW off of 120 VAC I decided to get rid of all remaining 800 Hz
related parts and wiring.    This resulted in the bottom deck being
completely devoid of parts and the middle deck almost free of parts.   The
middle deck still has R-219 which is still needed but can easily be
relocated.   I've attached two photos.

I plan to mount all new transformers (except the 2 KV supply) onto the
bottom two decks of the RMU.    This will be the 500V supply as well as the
2.5V, 10V, and 12.5V Filament transformers.

At present, I plan to have all primaries go through the Filament Rheostat
control.

The two 837's require 12.6V for the filament but they are fed by an 18.6
VCT secondary off of  the non-existant T-202.  I think this increased
voltage was needed due to the voltage drop in L-302 and L-303 in the
Oscillator filament circuit.    I think this then required dropping
resistor R-213 in the filament circuit of the Intermediate Amplifier.   I
plan on bypassing (shorting) L-302, L-303, and R-213 so that I can use a
standard 12.6V transformer for the two 837's.

The 2 KV supply will be external to the RMU and will be fed in via a new
plug on the bottom of the RMU, then to S-208C.     The new 500V supply will
feed S-208B.

What I still haven't figured out yet is what voltage the 803 suppressor
needs on Phone.   This voltage is developed from the non-existant T-203 HV
secondary CT via the bias adjustment pot R-205.  There is nothing in the
manual about adjusting R-205 (or maybe I'm not seeing it....).  Looking at
the 803 data sheet,  for 2 KV plate voltage in Suppressor Modulated Class C
phone operation, I'd need about -110V on the suppressor.     Still not sure
of this area yet,  but I should have room for a bias supply in the RMU.
The bias supply would feed into the ungrounded end of R-205 then to the 803
via S-209A in Phone operation.

The front panel AC ON/OFF and DC ON/OFF switches will be used as intended.
   Both required a good bath in DeOxit for them to work  (Well, they are 85
years old.....)

I plan to repurpose some of the 4 front panel AC Compensation SPST
switches.

One will be for a  “TX/RX” switch for lack of a better term.   In TX, relay
K-301 in the HF transmitter will close as it's supposed to do.
 Except.....   I plan to ground the Oscillator with a new modern relay
instead of K-301B.    That new relay will be controlled by the key or mic
switch from the RMU.

When receiving, the 2KV and 500V power supplies will still be providing
voltages to the 803, but the oscillator will not be enabled.   I was
discussing this with Howie and we feel this is OK as long as plate
dissipation is not exceeded, and that concern may require me to run less
than 2KV on the plates.

I ran a TBW-5 for a few years with 1200V on the plate and that gave me more
than 100W output on CW which is adequate for my needs.     Running phone
with the TBW will be a new experience for me.

Any and all comments welcomed.

73 Mark K3MSB

On Thu, Jun 13, 2024 at 10:23 AM Ray Fantini via Milsurplus <
milsurplus at mailman.qth.net> wrote:

> Don’t know, maybe smarter people can tell me how but think the wire that’s
> used is something like the stuff they use on heater elements so it can not
> be soldered, if somehow you were able to get a couple turns un-wound maybe
> you can just tie them together but looks to me as far as the power pot is
> concerned, its game over!
>
> How are you doing the entire T-202 mess? The original circuit for 800
> cycles had that pot (R-201) and all those weird power compensation
> capacitors switches and all that stuff. I did separate filament
> transformers for all the tubes except for the weird 2.5 volts for the 843
> and a separate huge 10 amp transformer for the ten volts for the 803, used
> the power stat or whatever you call those huge wire wound pots just on the
> AC primary of the ten volt 803 supply, the good thing if you got to replace
> it you don’t have to get an exact match, just something that fits in the
> same hole.
>
>
>
> Ray F/KA3EKH
>
>
>
>
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