[Boatanchors] TCS Receiver Resurrection Update, Part 3 (last)

[David Stinson]

TCS Receiver Resurrection, Part 3 (last)

RF, Converter and LO stages.
The phenolic board behind these stages mounts
most of their suspect parts. Remove the screws
and lift the board to get to an oil-filled cap. 
If this cap is bad, you can nip the wires to 
it and mount replacements above. One will 
go at the tube base of the LO and another will
mount on the top of the board. Just follow 
the wires.  Replace any bad resistors etc.
https://goo.gl/photos/3B6UfSK2dNKTink86

On the "postage stamp" caps in the tuned
circuits: These are all in low-voltage grid circuits
and I've never had trouble with them. They are
mounted in places that would require removing 
coils to reach them and I worry about that old
ceramic cracking. Unless you have reason to 
believe one of them is bad- like a stage that won't 
align- I recommend leaving them alone.
Open a can of green-pea soup and pour
it over the tuning capacitor.

The IF/Detector/Audio Modules.
Remove the screws holding the phenolic boards
in the modules to reach the suspect parts.
https://goo.gl/photos/JVekpq39V1zv6TsC7
https://goo.gl/photos/9t89ZyFCuXvBL7nr6
Replace all the "postage stamp" caps in these
modules, even if they check "good."  Consider
it "preventive maintenance" so you don't have
to take this apart again.  The big "transmitting" 
caps in the Audio module are likely just fine.
Remember to solder a "pig tail" to C235 to 
reconnect that "too tight for an iron" connection
from the wiring harness.

There are two kinds of TCS IF transformers:
1.  A hollow cylinder with slots on opposite sides
that pistons an iron core.
2.  Tuned with two variable capacitors.
https://goo.gl/photos/u6AMKd4zqjY5f1MH8
If you have these, we will be covering an issue
with them below.
Note that all the BFO adjustments I've seen
were one of these variable caps.

I've heard that some early IF transformers have 
a "Philco" adjustment- a 1/4 nut soldered to the 
top of an iron-core adjustment shaft, but I've
never seen one like this.

The phenolic in the IF transformers and 
Detector/BFO cans is hygroscopic- it sucks-up 
moisture and will leak high voltage to the chassis. 
If you have a working TCS receiver and hear 
what sounds like "thunder storm static" 
with no antenna, it's likely to be this problem 
and you need to fix it right away because it will
quickly blow-open IF transformer windings and 
finding replacements is tough.

Two 1/4 inch nuts hold the IF and BFO shields
in place.  Mark the shield so it goes back on
the same way.  Be careful- the wire inside is
fragile.  The phenolic board at the bottom is
held to the chassis by two screws.  Remove 
the screws and carefully lift the transformer
to provide some clearance.

Coat the top and bottom phenolic plates
with Corona Dope.  Coat the chassis where
the IF transformer mounts, especially the insides
of the screw holes- a favorite place for the
high-voltage leakage tracks.  Allow to dry
completely before reassembly.  
The IF transformers have fixed capacitors
across some windings.  I've never found 
one bad.

If you have the "two variable caps" IF transformers, 
you likely have a tricky problem with some of them.
The rotor shaft has and adjustment nut that was
"press-fit" on the shaft, holding the rotor at the 
correct position.  Many of these nuts have cracked
and the rotor falls down, shorting the cap. 
This type cap is also in all the BFO cans I've seen
and they had all cracked.
There are different opinions on how to address
this issue.  I put a small dab of JB Weld epoxy in 
the top of the nut (not to much; don't want it to squeeze
out the bottom), covered it with the wax paper to 
prevent sticking and used a small "C-clamp" to hold
the rotor plates centered while the epoxie set over night.
There may be better ideas.  Would like to hear them.
 
Time to reassemble. Be sure to put all the screws back 
in the plate over the tuned circuits. 
Those wires that feed through the rubber grommets
on the 1st and 2nd IF modules: 
It is very easy to get those caught between the 
module and the chassis when re-installing 
the modules. Tuck them out of the way before 
reassembly. I got caught with that one and 
nearly pinched a wire off.

I recommend connecting the B+ and bringing it up slowly
without applying filament voltage to check for any shorts.  
If all seems well, cross your fingers, say a prayer 
and power up.

Align the radio per the manual.
Note: If you plug a speaker into the headphone 
jack, your audio is being fed through an 1800
Ohm resistor. Check the diagram. For best
audio, use the audio feed from the power plug
and a properly-matched speaker. 
You'll get all the audio you can stand.
The rig should be quite sensitive and, after a 
warm-up, very stable.

If you use the rig on CW according to the design,
you'll get "ear pops" in the speaker and phones.
You can fix this with a couple of zener diodes.
We've already exhaustively covered how 
"this won't work." Actually, it works just fine.
Two 1N4734A Zener diodes connected in parallel, 
but in opposing directions (anode to cathode and 
cathode to anode) soldered across the earphone 
jack terminals fixes the "pop" in headphones.
https://goo.gl/photos/oZJYbzutX4Dw8Bww5
In the original circuit, there is an 1800 Ohm resistor from
the audio output transformer secondary to the headphone
jack. Assuming this resistor hasn't been removed, this
should make earphones on CW work well.

If using an external speaker via the connector,
use "steering diodes" before the zeners.
https://goo.gl/photos/VsdamG7NsHuEzjYZ6
With these diodes in place, CW with the TCS is
"pop-free" and as comfortable as any modern rig.

I hope this has been helpful and I will be glad to answer
any questions I can.  If there is any interest, I could cover
the restoration of the TCS transmitter, which isn't nearly
as much work ;-)/  I don't know everything and 
there are lots of people here smarter than me.
I welcome any comments, contributions etc.
Between us, we can get you going.

GL OM ES 73 DE Dave AB5S.