[SignalOne] Back On the Air

[EDWARD SCHUMACHER]

Hello All and a Happy Fourth!

Now that my CX-7 has been back up and running for a few weeks, it seems time to firstly thank all who have offered helpful suggestions and helped locate parts. I would also like to share a few of the things I have learned along the way since that is what this group is about after all. I also have to tyhank my wife who lost the use of half of her laundry table for the last couple of years while I sorted out what had happened to the rig, HI!

My problems began with a mundane symptom of blurring of the display which told me I had yet another driver chip failure caused by whatever had been spilled on the board before I got the radio. This would have been easy but I had decided to install a NOS 8072 the next time the rig came out of the cabinet. I replaced the old chip and got the corroded off leads out of the socket ok and the display came back. Then I installed the new final and started blowing fuses. At this point I noticed the surgistor had finally given up the ghost, cracked and was glowing along the fracture line. The rest was downhill from there as I tried to figure out what was going on. A couple of other problems cropped up from working on the display board and the end result was a mostly dead receiver and ditto for the transmit side with only about half power out.

The new tube came out since that seemed to be when the trouble started. Eventually I did learn about the nasty habit the 8072 has for going gassy while sitting on the shelf. I am in the process now of building a bench setup for evaluating and possibly degassing my stock of new ones. In the meantime, the original tube is still producing upwards of 130 watts or so output on most bands except 15M. Think I may have a tired xtal there but that can wait a bit now.

Finally got a big window of time back in January and just started back on the display board which was now dark but I found that the receiver was not completely dead as I had first thought. I knew from previous experience that the display was locked in the readin mode for some reason. I did find a couple of devices that looked like they were blown from spikes when the final was flashing. They were replaced but the real problem turned out to be yet another chip in the divider chain in the counter. This was found by simple signal tracing until signal was lost right at the chip. exercising the chip brought back the display andfull normal receive! When locked in readin, the receiver is muted constantly.

On to the transmit side now, again basic checking was begun with voltage off the Driver Board since the bias resistors for the output transistor showed signs of heating. The output transistor was confirmed bad but fortunately the resistors still checked in spec. It also appeared that the input MOSfet had taken a hit and was shorted again probably from the final flashing over. A call for help produced the Motorola transistors needed and one was easily installed at the output.

The  40468A MOSfet is another story. The current replacement from Mouser is a NTE222. It can be used but it is nota direct replacement! Looking at the part numbers it looks like NTE took over the old Sylvania ECG series. That series did have a direct replacement for the MOSfet anda device for the dual-gate MOSfets used in other circuits in the S1s. NTE only has the dual-gate device. Simply use the old trick of tying the gates together and then wiring to the correct points on the circuit board. You will end up with one lead crossing under the device. The good news is that now you have a diode protected device with somewhat better performance specs than the original 40468A. Be advised that the NTE222 is getting pricey now even from Mouser and has to be ordered. They do not have them in stock and it took a good two weeks for mine to come in. This all points to this device also possibly being dropped from production.

After reviewing the options for replacing the surgistor I finally settled on two NTC thermistors I found listed by Mouser. They seemed to be the closest thing to the original device which in fact appears to be a user mod that became a factory mod. I ordered the 10 ohm/7.5 A device P/N B57364S100M and the 2.5 ohm/11 A P/N B57364S259M. These offered cold resistances similar to what I was seeing in various step-start circuits plus being able to handle the current draw of the radio. I don't think I ever did track down anything giving a clue to the original surgistor cold value. The 10 ohm/7.5 A device was installed and is holding up ok with none of the signs of heating of the original surgistors. I have not tried the other one since this one seems to be good as is.

Well, that's my story and I'm sticking to it. Glad to have the rig back in place in my shack and I actually enjoyed the learning experience I gained! Will be on Saturday hoping for decent band condx and a few more check ins to the net.

73 ... Ed, WA9GQK