W0CCA Corsair II Modifications

Ten Tec Corsair II Modifications by Cap Allen W0CCA

This list is not intended to be a do it yourself, if you don’t have at least a few electronic skills at ferreting things out. Mostly this requires boldness in violating a commercial circuit board. But we ARE hams aren’t we?

1. DDS Conversion- Number ONE improvement. this is documented elsewhere under N4YGs very excellent website. In there you will find a narrative, pictures for converting Corsair II (and now Corsair I as well) to DDS from PTO VFO source. Works great, new radio, read the info., no further comment needed. This is the best mod you can do, you will never rebuild a PTO again, and you will have a seemingly complicated conversion, though not really, under your belt. The primary addition I would recommend is adding a full PC Board box shield around the DDS. After all the PTO was in a bomb proof little aluminum box so that its output only went where intended. I have built this box on all my DDS conversions and birdies are virtually nonexistent.

2. QSK conversion- relay- The early Ten Tecs had a amplifier driver relay (not the little reed T/R relay, that stays) that was noisy and annoying. Plus it was too slow for effective QSK. I have always disconnected this relay, the Omni VI has a software command to disable it. Phil, AD5X, documented a very good technique for small relay QSK switching of amps such as Ameritrons or SB220, etc. I have used this relay scheme with the small DIP relays successfully for 7 years at 500-800 watts with my SB220 and not a peep of malfunction. Look it up on Phils site. In all my Ten Tec radios, I disable this relay and use the NPN driver transistor in the Corsair or other that pulls down the relay for 12 volts as the driver to my AD5X relay setup. It is a relay system that is getting its signal from the T/R voltages of the Ten Tec, the timing is good and there is no hot keying. There is even a delay pot. Simply disconnect collector of driver to relay and directly connect that to the relay jack on the radio. The transistor is Q10 on both control boards. My CII has a “QSK” out jack as well. I have modified it to go low for the QSK switching, but I like the relay out better because it gives me an adjustment with R35 (R10 CI).

3. AGC- folks talk about the Ten Tec flawless QSK. I love these radios, but the QSK is far from flawless. The AGC is too fast on QSK and contributes to the “popping” and thumping that annoy ops. On the CII, I played with C32 and C33 on the If/Af board. I ended up with a 2.2 uf electrolytic in C32 and a 3.3 in C33. This basically gives me two CW AGC positions. I am not a SSB op, and don’t need long hang time. I don’t have quite the speed of recovery that a 1 uf C32 gives, but much more comfortable turnover. Folks still can break me with a single dit. Easy to play with these to your liking. In fact the whole R50/C32 or R51/C33 time constant could be played with. It is a purely empirical solution. On the CI it is C64 and C67.

4. QSK- If you look on IF/AF board again you will see that slow QSK grounds both C55 and C54 for 11 uf. Fast QSK grounds only C54 for 1 uf. (C40 and C41 in CI). The logic here is start playing with C54 until you like it. I found that a simple change to either a 2.2 or 3.3 uf at C54 did a world of good.

5. Sidetone- probably the most common complaint for the CII is the sidetone. No idea what Ten Tec was thinking here. Sidetones have always been below my expectations on ANY rig. The CII may be a low in this regard. Steve N4LQ led the way with the CI. For unknown reasons, Ten Tec uses a nice Twin T oscillator in the CI for the “spot” function, and uses a lousy oscillator for the sidetone. In that case just steal the input to the spot function, which most of us never use, and hook it instead to the sidetone key line and you have the good sounding oscillator. You can use the CI twin T but I recommend the keying solution below for both. The CII needs a whole new sidetone. The CI needs mods. The Twin T oscillator is a widely available circuit. Unfortunately the method of keying the osc. for the CI spot function is not a good one for fast CW. So this solution applies to both CII and CI. Free running the Twin T oscillator and using a switch such as a 2N7000 that grounds the output to the Twin T is the preferred method. Sounds great and you have a stable sounding oscillator. My Twin T board in the Corsair II is about ¾” x ¾” using every perf board hole !!! For the CI, you only need to implement the free running oscillator by grounding R2 instead of hooking it to Q11s collector and putting in the 2N7000 keying circuit. In the CII I cut into the line at R90, on the CI cut in at R93. You need to use a capacitor, just a 0.01 or 0.1 between that and the audio amp (C56 on U5 on CII, C44 on U4 CI) to keep from grounding your audio from the rx also !!! This keeps the sidetone volume constant while you change audio gain in the rx.

6. Joe, N4YG also makes an excellent sidetone generator for the Corsair using DDS technology. It sounds very good as well. I would defer to his directions for his unit for a description but it retrofits quite well in the Corsair I and II. (not sure this is still available)

7. LED readout. I have seen complaints about the LED readouts on the Corsairs going, missing segments, etc. The LED system of each radio is totally different. Each has its own solution. I started by simply taking out all the LED boards and carefully resoldering everything. For the CII that did it, for a while. I did a rerun on those boards and have been good for a long time now. An hour of careful work and I have all digits and all segments, no flutter. For the CI it was a little more involved. I did the resolder and thought I had it but stuff kept cropping up, getting faint “reverse C’s” on all digits etc. No evidence of reading the LO (all 9.000 or 91.000 at turn on) until warmup. I finally found two of the pullup resistors on the 7490 drivers had gone way out of value and replaced them. You just have to be persistent.

8. LCD replacement- replacing of the LED unit completely with a simple AADE or equivalent digital readout is quite simple. At finish you have smaller digits if using a 16x1 display but very reliable. You take ALL of that stuff out (unfortunately including the keyer in the case of the Corsair II. I have never used that keyer) You have to switch the 9 mhz offset of the readout in and out depending on low bands or high bands because of the conversion scheme. In cases where you have replaced the PTO with a DDS, you now have at least two bandswitch wafers that can send that ground information to the AADE or equal readout and it is a simple step to have total automatic frequency readout reading the output of the premixer from the VFO string just like the LED readout did previously.

When all mods are done, we do not have a K3 or an Orion. But I had a lot of fun tweaking, felt like a real ham and still have money in my pocket. This is the last of the analog rigs and it is the one to grab if you feel you understand stuff. From this rig on out, it is digital and its out of my league. With discrete transistors I am willing to jump in with both feet. It is lots of fun and you develop a kinship with the radio that is hard to duplicate when bought off the shelf.