|WRL Duo-Bander 80/40M SSB Transceiver|
The WRL Duo-Bander was an offering from World Radio Laboratories in the mid-1960's and was designed as an SSB only transceiver covering portions of the 80 and 40 Meter bands. On 80 Meters it worked in the range of 3.8 to 4.0 MHz and on 40 Meters from 7.1 to 7.3 MHz
There were several unique features to this radio in that it used a 5.55 MHz four pole Crystal Filter and a VFO that covered a singular 200 kHz range around 1.65MHz. The low VFO range added greatly to the stability and the Sum of the Filter Frequency + VFO produces an output on 40 Meters and the Difference of Filter Frequency - the VFO produces 80 Meters.
Band switching was a single pull knob that engaged a bank of slide switches on a common shaft that Shorted/Opened various capacitors to put in line a rather sophisticated grouping of Band Pass Filters. There are four coil sets in the radio that are tapped so that the full coil is used on 80 Meters and a portion of the coils use on 40 Meters. Some clever guys lived in Council Bluffs, Iowa. Then again Leo Myerson founder of WRL was nothing short of a genius.
In a strange twist of fate the venerable 12BY7 which is often used as a driver stage for a pair of sweep tubes (in this case 6HF5's capable of like maybe 150 watts output) is also employed as the receiver RF Amplifier stage. Now that is the 1st time I have seen that in tube gear. I use that same approach in some of my solid state rigs I have designed with relay steering.
Another feature was that it was a hybrid rig in that it employed a many NPN transistors for applications such as the VFO, Balanced Modulator, AVC control. Microphone Amplifier and even several were wired as diodes to provide clamping actions. Oh did I mention the transistors were all the same part number although I did see some paint marks on the transistor cases where they must have graded them based on the Beta gain figures
A minute (or longer) about the analog VFO in the Duo-Bander and in other WRL rigs such as the Galaxy III and the V series. It had linear dial markings across the band and used a variable capacitor not a PTO. Other manufacturers at that time such as National and Hallicrafters who also used variable capacitors had non-linear markings. For these rigs the bandspread was huge in the CW portions of the band; but you had to squint to see the 5 kHz tick marks in the phone band. The WRL radios had two speed vernier drives with additional 1 kHz tick marks on the outer drive hub. The mecahnical drive on the Duo-Bander variable capacitor used some stout gears including a spring load split gear so there was no backlash in the tuning. Some really smart designers in Council Bluffs!
The one that I bought off of eBay was a basket case and that is being kind. It had low receive but I did get it at a discount. The eBay ad mention you might have to get a better 12BY7. Well that was not the problem. There is an issue with the AVC which completely shut down the radio. I temporarily removed the AVC transistor which now lets it receive with lots of volume --but I still do not know why. Oh in case you are thinking bad transistor --well no. The transistor is good as I tetsed it in the VFO --works just FB. It is Q5 in the schematic below. I also pulled D2 --it too is OK.
Scratching my head on this one and in case of further wonderment --all those resistors and capacitors were replaced with NIB parts. I have literally replaced all of the resistors and caps in the radio. The only ones not fully replaced are in the VFO and Microphone Amp. I have a lot of parts in stock used in prior boat anchor restorations so any new hardware buying was very minimal. Finally I got to use that bag of .01 ufd 500 VDC ceramic caps. Luckily I also had a stock of 2 and 3 watt carbon resistor -- they are a tad big for a compact shirt pocket SSB transceiver.
I actually made a contact on 75 meters with this radio on 5/29. The report was readable but the audio is a bit distorted. Man that is not good news; but very positive as when I initially got the radio --- it did not transmit nor did it receive. Some of the audio distortion was traced to the Microphone amplifier where since that contact, I have replaced an electrolytic capacitor and added a cap to improve the low frequency roll off. I got these two hints about audio distortion from a ham up in the SF Bay Area who has two of these and an extensive blog on how he restored his WRL Duo-Bander(s).
My goal here is to get the rig working really well and have some fun poking fun on the West Coast Vintage SSB Nets that operate on 75 and 40 Meters. I can't wait to say the rig on this end is a WRL Duo-Bander. I am also thinking many of those hams who are on that net if they happen to be younger than 54 years old --WERE NOT EVEN BORN when this rig hit the market. That will add to the fun.
One immediate thing should jump out at you --cost cutting by WRL and safety! There is no protective cover over the plate caps on the 6HF5's -- there is 800 Volts waiting to jump out and kill you. Not a good design practice. But Leo wanted to save $1 in hardware and $2 to install it. The original price in 1966 was about $160 without a power supply! $3 was significant!
Check out the photos below --lots of NIB parts installed. I even removed several of the IF cans just to make sure there were no smoked coils --after awhile you get pretty good at removing and replacing parts --lots of them.
Stay tuned as I will update the information on this project.
5/30 ~ 5/31/2019
I am happy to report that the AVC is working now and the audio distortion first reported on my initial contact on 5/29 has been cured with the changes to the microphone amplifier circuit. BTW was runnig 600+ watts on todays's contact -- if there was audio distortion it would be very telling.
The AVC problem was traced to the triode section of the Audio Output tube. I made a bad assumption that since there was audio output the tube must be OK. But the triode setion of the tube is not used in the audio chain. The bad triode section was cutting off the receiver. Onward and upward.