Paraset ~ Electrical Build

The Kd6VKF Mk-VII Paraset, Ser. №  001  is now essentially complete!  The wiring turned out to be the easiest part of this project; it was compete in about a week with only a couple of evenings this week , and Saturday afternoon/evening.

All the circuits are working and are shown here no particular order.  The plans for the next few months is to operate this radio on the 80M and then the 40M ham bands.  The goal is to have at LEAST one  confirmed QSO per band ~ but secretly I want to WAS and some DX too. 

KD6VKF Mk-VII Paraset - Top View

The box I built is obviously in need of a serious upgrade, it was simply made from a leftover sheet of plywood, just basically to keep from electrocuting my self while testing.  The three waterfaucet looking knobs will be replaced with some more close to the original, and  I may redo the wiring with some cotton covered wires and simulate the old paper capacitors and resistors, this radio has come a a long way since Part 1

POWER CIRCUITS:
The method to wiring this radio was done in stages.  I started with the power supply and voltage divider  (lower left of the photo)  I wanted to test the linear regulator for the 6.3V filaments as well.

The only real issue I had with these circuits was the linear regulator would not start with all three tubes!  The large current drain of the three cold filaments turning on caused the regulator not to work.  I had to wire the switch to supply constantly power to the regulator and switch the output.

KD6VKF Mk-VII Paraset - Bottom View

As you can see; for wires I started with  some YELLOW braided sleaving, in homage to the original, but it turned out to fray at the ends, I mostly used this on the RF circuits.  For the power circuits,  I used about 3 Ft. total of RED and BLACK Teflon tubing on both the component leads and to cover the enamel wires leftover from the TX coils I used for hookup.  Using the enamel wire was probably more trouble than it was really worth; I had to scrape both ends to make the solder connections. It took a lot of time to prepare the wires.

The 6V6 TANK circuit power input turned out to be a busy little junction.  I used plenty of RED Teflon tubing on the leads here. 



KD6VKF Mk-VII Paraset - Transmitter Power Close up

Note: the new solder tag I added in the lower left of the photo.  This two-tab tie point was added because the switch did not have lugs, but only two insulated wire leads.


TRANSMIT CIRCUITS:
The transmitter circuits were actually the last I wired in.  The first tests with the 3.5 color burst and 7040 QRP crystals were disappointing, no oscillations or output. Tuning of the original circuit was very high due the smaller inductance of the TANK coil.  Fortunately I had an 8MHZ crystal, or I would have thought the circuit was dead.  I plugged it in attached the dummy load and tuned it up to full brightness on the lamps.  I added a parallel capacitance to the low range 100pF and 3.5 MHz oscillates just fine now.  The 40M crystals actually do oscillate, it is just at the very end of the TANK cap and too weak to make the bulbs glow.  I will need to fix that later but I will only use 3.5 MHz crystals to start with.

KD6VKF Mk-VII Paraset - TX Power and Caps

I will probably need to  re-wind the TX coils to make a better tuning solution for dual band operation.

The internal key works fine. I wired the EXT key Jack in parallel and used the "RING" connection as a ground point.

KD6VKF Mk-VII Paraset - Transmit Side + Key

The transmitter layout was clean. The only issue I had here was mechanical.  You may note the tabs on the 6V6 socket are slightly longer than the ones I used on the 6SK7 tubes.  The longer tab interferes with the tip jacks for the A and E connections, not allowing me to put these jacks behind the panel as planned.
 

KD6VKF Mk-VII Paraset - Transmitter 6V6 Tube and Crystal Socket

 KEY:
The internal key was described in an earlier post.  Here it is wired into the transmitter's circuits.  The contacts were changed to semi-tubular rivets, set wit the Tandy snap tool. Also with some washer spacers were added for better continuity and contact spacing from the original contacts.  The copper braid  ground contact to the arm was made from some fine wire de-soldering braid:

KD6VKF Mk-VII Paraset - Internal Key

The crystal socket was fabricated from two RCA type jacks mounted on a small fiber board.  This allowed the sockets to seat behind the panel.  Since ground connection was compromised by the hole repair, a solder tag was used to make the ground connection more reliable. The solder lugs were swapped to above the socket and the EXT Key jack was a convenient ground tie point for some components since it was well grounded.

KD6VKF Mk-VII Paraset - External Key Jack

The gimmick capacitor  (BLUE  wires below) has a few more turns than the original. I thought the added feedback capacitance may help when using smaller crystals, and it is easier to remove turns than to add.  The two lamps were connected to the TX coil GREEN for AERIAL and RED for TANK Circuit.   I found that a 2V flashlight bulb works best for the Aerial indicator while the 6V bulb glows brightly in the Tank circuit.

KD6VKF Mk-VII Paraset - Transmit Coils and Gimmick

RECEIVER CIRCUITS:
The actual second circuit built was the receiver.  I decided since it was the most complex in terms of the number of components I would build and test it before the transmit circuits.

When a new telescope is made, the firs time it is used astronomers  call it "first light". The "first sound" for this set was pretty un-eventful. I hooked up the power, warmed up the 6SK7 tubes, plugged in the headphones and...nothing!  Tinkered with the Regen control a bit and then was just going to try a sweep with the signal gen when I noticed the problem;  the phones were connected to the wrong jack!  

When connected to the "Phones" jack  static came rushing in and a few twists of the dial and ...a SW Broadcast!  I think the first heard station was: Radio Havana Cuba International, but the tuning was just short of the 40M and 80M ham bands:

As you can see due to the "Q" of the tuned circuit with the smaller than normal coil inductance used in this circuit was making for a narrow bandwidth, which has restricted the tuning range.  I could not listen to CW that night, but I tuned around and had a great time listening to some international broadcasters, some utility SSB stations, and WWV on 5.0MHZ was almost centered at 50 on the scale.

I then made the tuning graph by setting the dial on the 10's (0-100) and tuning a signal generator, not coupled to the receiver, until it was heard on the set.

KD6VKF Mk-VII Paraset - Receiver Side View

FINE TUNING:
The receiver was "band spread" for 80M.  After I found the resonance frequency of the coil / tuning cap I thought it would be a strait forward tuning job,  I calculated the required capacitors; one for parallel to increase the overall capacitance, and one in series to reduce the tuning range ...simple?  



KD6VKF Mk-VII Paraset - Receiver - With Band spread Caps for 80Meters



Well, it turns out the  values of these ceramic discs were off by 10% - 20% or so caused the errors, not my calculator!

I had to use my MFJ -259 antenna analyser to measure some parallel and series stacks to finally get the required capacitance.  Once that was done I had a beautiful spread, the entire dial 0-100 was now only 80-Meters centered on CW QRP!

80M Band spread tuning chart

 

[Note-- the colorburst crystal frequency is marked w/ red diamond]

I came up with some interesting band options in the process, which prompted me to put a small metal disk that I could put a rotary switch for multi-band operation! This can be seen to the Left of the crystal socket and EXT Jack in this view:

KD6VKF Mk-VII Paraset - Transmitter Side Ext Key and Tank Circuits