Tuesday, February 11, 2014

Robyn 240D

These are pretty cool little radios. All tube-type, except for the PLL circuitry for frequency synthesis. A friend picked this rig up, and it needed some help. Lots of little problems needed attention, but nothing major. There's practically nothing published on this rig for mods (except for one bogus mod that is all over the web), so I set out to comb through it and fix a number of problems and upgrade / improve a few things. You'll need a schematic or Sams Photofact to locate some of these items, but the work is pretty straightforward. 

Power supply & modulation issues : 

Once working correctly, I noticed this rig (and another my friend brought to me) lacked low frequency response in the transmit audio chain, and didn't have enough headroom in the power supply to allow for 100% positive modulation peaks. Negative peaks were 100% easy, while positive were clipping at about 80%. Here's the list of changes...

1- Install a .5uF 25vdc cap from pin #3 on V6, to ground. Keep leads short. + terminal of cap to pin #3. 

2- Bypass R27, 100 ohm resistor in the 278vdc source, this raises it to about 290 vdc.

3- Lift one end of R28 1K 5W and install a 150 ohm 5W resistor in series, or replace it with a 1150 ohm 5 watt resistor. This restores the 211vdc source to the correct voltage, after the mod in step #2 above. Actually, it raises it from 211 to about 218, and improves the rx sensitivity. More on that, later...

4- Relocate C905 1uF 160v cap from across R906 1K, to across R904 6800 ohm. (-) of cap to the L907 end of R904.

5- Bypass R906 1K. 

6- Change C601 to .1uF 500vdc disk

Receiver sensitivity: 

In step #3 above, I stated to lift one end of R28 1K 5W and install a 150 ohm 5W resistor in series, or replace it with a 1150 ohm 5 watt resistor. This restores the 211vdc source to the correct voltage, after the mod in step #2 above raises it to about 225vdc. This restores the primary receiver strip voltage (normally about 211vdc) to about 218. I have many ham band receivers that place 230vdc or so on those same tubes, with no long term effects. This simple mod improved the receiver sens from .5uV @ 10dB SINAD, to about .3uV @ 10dB SINAD, a very noticeable improvement.

The following changes brought the receiver sensitivity up to better than .1uV @ 10dB SINAD.

Change R106 from 47K to 220K

Change R201 from 680 ohms to 1200 ohms...

Install a 56k resistor in parallel with R303 (100K)

Delta Tune issue: 

I've seen many comments on the web about users having to operate the receiver with the delta tune at about the 3 o'clock position for best reception. I found a 10k ohm 1/2 W resistor installed from the wiper of the delta tune control, to the high side of the pot, will move the varacter voltage to about 4.5vdc and bring the rx center frequency spot on with the delta tune control at 12 o'clock. You could also install a small 15k pot in the same place as the 10k resistor I mentioned, and dial it in that way. The issue is caused by drift in the 15mhz crystal, and padding it results in osc instability...this seems to be the best and most stable approach to correct the problem. 

HEAT!! : 

 These radios run HOT!! I installed a small 12vdc fan inside of the radio, to pull heat out (never blow cool air in, simply pull the hot air out slowly to cool the rig). I picked up the low voltage DC supply for the fan at the high side of R191 10 ohm, and ground directly to the chassis. I installed my fan on the inside, it is nice and quiet, and works great. 

  Instant On: 

Another problem with heat, is where Robyn decided to keep the radio mostly powered up all the time, to give the illusion of it being "instant on". I really don't like this, and want the rig off when the switch is set to "off". This also keeps the rig cool when you're not using it. 

1- Locate the red and brown wires that connect to the power switch. Trace those around the center of the chassis and back to their connection points near the main power socket on the back of the rig. Clip them from their connection points on the terminal strips, and connect a jumper across where they were connected. 

2- Pull the ends of the red & brown wires up and towards the rear power socket on the back of the rig. Locate the (usually yellow) wire coming from the power socket pin #3, and clip it free from the terminal  strip. Connect the brown wire to the terminal strip where you clipped the wire from, and connect the red wire back to the free end of the wired coming from the power socket pin #3

Now, the rig will truly shut down, when you set the power switch to "off", and help with keeping the rig cool.


- Swap the 6BQ5 modulator tube out with a 7189. I use a SOVTEC EL84M for even better modulator performance.  

- Replace the 6GH8A tube out with a 6EA8A tube. It's much more stable over time than those flakey 6GH8A's, easier to locate, and has a little more output for the reference oscillator.