2019 Fun With Vintage Radios

Tinkering with Boat Anchors.

The plain old simple relay is an amazing device that frequently finds use in our rigs. It is not unusual for me to have more than six relays in one of my homebrew rigs. I know of one rig without any relays and that is the cousin to the SBE-33 (which has one 4PDT) and that is the SBE-34 with none.

Another one I just investigated is the Heathkit HW-18 which is a 200 watt SSB transceiver that was designed for use by the Civil Air Patrol. Not sure we even have a CAP today. The 1st thing you don't spot when you look at its schematic is that indeed there are no relays, yet it works as a transceiver. How can this be?

I have often said that in the days of old some of the best radio designers aside from those at Collins worked at Heathkit, R.L. Drake, WRL, National, Ten Tec and Swan. So it is no surprise that Heathkit figured a way to TR without any relays.

The first thing I also spotted was that the microphone on the HW-18 was hardwired to the rig. That seemed strange as in most other rigs in the Heathkit line you had a panel mounted mic connector. After much study it became clear as to the why. In the Microphone is a DPDT slide switch operated by the PTT button. You have to have the microphone connected otherwise the HW-18 will not receive. 

Now here is the clever part. One set of contacts on the microphone DPDT switch in Receive is grounded to the shield. That wire connects to a grid on one of the tubes (triode half)  and also grounds the Receiver cutoff line, which is nothing more than about - (negative) 125 VDC. Because the grid of one of the triodes is grounded, it is conducting to ground through the cathode and in the plate lead is a 220K resistor whose other end is connected to 250 VDC. Thus there is a drop of about 250 VDC across 220K. Roughly 1 milliamp of current. There is a connection to that triode plate that is routed elsewhere.

So when you hit the PTT, the cut-off bias is ungrounded and tubes in the Rx section are now cut off including the receiver mixer stage. 

The input section is different on the HW-18 from the other HW rigs. A capacitive tap is made on the Final Transmit Tank Coil which BTW is always connected to the antenna port. The tap is fed to the anode of a TR diode and the diode is forward biased from the 250 VDC screen supply. Thus the diode passes the received signal to a link coupling on the driver plate coil which also is connected to the Receiver RF amplifier. Thus diode steering provides the received RF. 

Earlier I mentioned that there was a connection to the Plate of the TR Triode. This connection ends up on the cathode of the TR diode. So when you hit the PTT the voltage on the plate goes to 250 VDC and now this reverse biases the TR diode so no signal is being passed from the Final Tank Coil to the receive circuits.

So some lessons here. The microphone must be connected for the receiver to work. I would change that out to a small DPDT 12 VDC coil relay. So when you key the PTT, the relay would take over from the internal microphone switch. That TR diode is critical! If it fails you will smoke the Rx input circuits. In the other Heathkit HW series there is a set of relay contacts that physically disconnect the receive circuits from any transmitted RF.

Oh there is a panel mounted slide switch labeled LOCAL and DX. Essentially the slide switch connects or disconnects a 220 Ohm resistor to the cathode of the Receiver RF amp tube. Man without that cathode resistor the receiver must sound "dead". The Heathkit engineers must have value engineered this rig to eliminate the TR relay and a RF gain control and other frills. No VOX in this rig --strictly PTT. After all the HW-18 is a two channel crystal controlled rig so it wouldn't be used for contesting or chasing DX, thus no accessories or extra functionality!

But think about it … some clever engineers had to think long and hard how to make a SSB Transceiver without any relays, and to find clever ways to cut costs (or charge the same and make more profit). 

Pete N6QW

Why are we Hams? 

There are many answers to that question ranging from I like to tinker with stuff to the competitive side such as last weekends WPX contest.

I tend to be more of the tinkerer type and have undertaken some bizarre projects with the old boat anchors one of which is the rebuild of the hallicrafters FPM300 (above) and making it all solid state. You can listen to that rig as it was operating last Tuesday evening on the West Coast Vintage Sideband Net. A minute or so in the W6DQ is speaking and when he said Pete's signal … That was this rig!

The vintage rigs can be found inexpensively on eBay and other equipment for sale sites. Recently I got a SBE-33 for $52.30 and a National NCX3 and Power Supply for $69. Now these old rigs do have some shortcomings like a lot of heat, dried out capacitors, resistors that have changed value, weak or in-op tubes or plain butchering from some not so well thought out modification(s). 

But they can be had cheap and in some fashion fixed and/or made better. The FPM300 was a basket case, basket case (it was that bad). I kept the main IF Board and S Meter and designed a whole new rig around those two elements. Everything else was sold as spare parts --so my going in costs after the sale was $20. How would you like a $20 rig like this?

But we are hams because we have FUN whether fixing these rigs or being the high score in the class double sideband 40M QRP in the Northern Virginia section. But in all cases we like the challenge that tests our skills. 

But in the end we all take away a huge learning experience whether it is learning the search and pounce tail ending of the adroit contester or hand making parts for a boat anchor long out of production. It gives us an opportunity to expand our creativity and improve our skill set.

Here is an example of hand making parts. The original Plate Meter for the SBE-33 was blown. I posted and note here asking if anyone had a SBE-33 plate meter. With the passage of time a kind ham provided me a meter from an SBE-34 which was the same type of meter only a different style. So I adapted the SBE-34 to the SBE-33. Don't ask how... This is where a big junk box and lots of tribal knowledge comes in handy. The SBE-34 Meter works FB!

The rig as is with the blown meter is shown below.

Here are some sleeper boat anchors that could be "rejuvenated" with a bit of digital trickery. First on the list is the Ten Tec Triton IV Analog version known as the Model #540. I reworked one of those that I literally got for free and for those who still subscribe to QRP Quarterly an article documented my adventure with that rig. You can see it on my QRZ page. Right along with that is the HW-100/HW-101. You can also add digital capability to that rig.

Also on the list are commercial crystal controlled rigs like the Ten Tec Model 150A (8 channels 2 to 12 MHz). Also look for a cheap Heathkit HW-18 which was a rig configured for the Civil Air Patrol. It had two channels in the 4 MHz range and opens up possibilities for 80, 60 or 40M. 

While you are at it the HW-12, HW-22. and HW-32 can be easily "digitized" to give full band coverage and rock solid frequency stability! While you are at --the earlier models can be made USB/LSB select as well as having two VFO's. The USB function on the HW-12 and HW-22 opens up possibilities for FT-8. The later A models of course did have the sideband select; but the non "A" models can be found cheaper and made to have selectable sideband functionality.

BTW there was bonus to the Ten Tec Model 150A adventure and that was I learned how to use LT Spice to simulate various band pass circuits that needed to be adjusted. Armed with the circuit values from the Ten Tec Manual I set up a circuit and looked at the response. Then using the TT circuit as the baseline I was able to reverse engineer the new filter constants. Yes the rig is operated from a key pad to change frequency tuning bands and an encoder is used to tune frequencies within that band. The final control head including the keypad, encoder and display is in a small Mini-box and the LCD was changed to an 8X2 --read small. 

We do it because we can!

Pete N6QW

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