THIS INFORMATION IS NOT UP TO DATE - PLEASE CHECK THE POST OF JULY 1, 2010!! After my first experiences with this problem, I had recently proposed a simple preventive measure, consisting of adding a series gate resistance to the driver unit, which apparently suffers from instability with destructive results in many cases.
This update is the result of further study of the problem. It attacks the problem in a more efficient way, taking more measures to eliminate the suspected instability but also taking measures to possibly prevent the destruction of the driver unit even if the instability or other deteriorating phenomenon occurs.
The update consists of two parts, the first about improving the original simple modification, and the second about taking extra steps to ensure better stability and protection of the transistor in the driver unit. One can perform just the first part which is very simple. The second part is quite a bit more complicated and requires lifting the PA PCB and performing more alterations and additions to the circuits - but it offers considerably more safety. Please understand that although all the mods aim to improve the situation, perhaps a better understanding of the transient underlying phenomena is needed for a full cure. For example, there is a possibility that the failure is caused by frequency-dependent secondary breakdown of the LDMOS device, which would potentially require extensive redesign of the amplifier chain to eliminate, and may not be practically feasible in our case.
Click on the pictures to see a large version.
ADDITIONAL PREVENTIVE MEASURES
The second part attacks the problem from different angles. It requires considerably more effort and skill. This part is for the more experienced technicians, so detailed baby-step instructions (e.g. "lift the PA PCB by desoldering... and then..." etc., will not be given here. Please be extremely careful.
You can choose to do only the parts of the mod that don't require lifting the PA Unit PCB, omitting the modification of the drain-gate feedback network that follows.
You can choose to do only the parts of the mod that don't require lifting the PA Unit PCB, omitting the modification of the drain-gate feedback network that follows.
1) Modification of the drain - gate negative feedback network of the driver unit
Lift the PA UNIT PCB and remove the driver unit. The drain - gate negative feedback network flattens the gain vs frequency response of the broadband driver amplifier stage. It uses a capacitor (C101, 10nF) and a resistor (R102, 100 Ω). Change the capacitor to 0.1 μF, 50V (see the schematic at Part 1 above). This decreases the lower edge of the range of frequencies for which the network provides negative feedback, stabilising the amplifier there also. Apply a thin film of silicone grease to the heat sink's surface and tighten the driver unit's fastening screws well when reinstalling the driver unit.
2) Addition of an extra bypass capacitor
3) Addition of a driver unit protection fuse
IF YOU DON'T WANT TO LIFT THE PA UNIT PCB, USE THE ALTERNATIVE METHOD
DESCRIBED IN THE PINK DIAGRAM BELOW!
Adding a fuse to the 13.8V DC line feeding the driver transistor's drain improves the chances that even if instability occurs, the transistor will survive. (There won't be the usual fireworks and smoke, in any case!) Having lifted the PA UNIT, locate L302 on the PA UNIT (see the schematic diagram on this page and the next pages for diagrams and photos). Unsolder it and transfer it to the "bottom view" (the one you see after removing the rig's cover) of the PA UNIT (as shown in the service manual), soldering only the lead that connects it to the node with C305 and L301. Then solder an 1.5 A, fast-blow 20mm glass fuse (or equivalent) to the free lead of L302 in the way shown in the photo, soldering the other end of the fuse to the PCB trace that L302 used to connect to (it's the "b" line with 13.8V on it). Use a small piece of wire to solder the fuse to the trace. Don't locate the fuse elsewhere using long wires! Do it exactly as shown. Lay the fuse flat on the PCB. Use a small piece of thick paper or plastic sheet to insulate the end of the fuse soldered to the free lead of L302 from the PCB trace under it. After you have finished, cover the fuse with a piece of electric tape to prevent shorting the 13.8V line when replacing the rig's covers.
(Note: Originally, I thought about installing a current limiting circuit using two NPN transistors and some resistors in the DC line to the driver stage, instead of a fuse. This circuit would conceivably prevent a catastrophic secondary-breakdown scenario. Due to the severe lack of space and the relative complexity of this solution, I opted for the fuse. Anyway, I think this idea has merit, in the future I may try it.)
FINALLY,
4) Lowering the idle current Idq of the driver unit
In order to lower the gain of the class A driver stage, also reducing its thermal stress and the possibility of secondary breakdown of the LDMOS device, without seriously affecting its linearity at the RF drive level used, we can lower the idle current via the service menu.
The service manual procedure sets the idle current of the driver unit at 1A. Reducing it to 0.6 A produces no serious ill effects on linearity (as measured in a two-tone test in SSB).
Follow the procedure at page 4-3 of the service manual, "transmitter adjustment". Set the current at 0.6 A as per the instructions and exit the service routine.
This concludes the mods. Good luck! Enjoy using your IC7000!
Bibliography
1. John Pritiskutch - Brett Hanson, Understanding LDMOS Device Fundamentals, AN1226, SGS-Thompson Microelectronics, 7/20002. John Pritiskutch - Brett Hanson, Relating LDMOS Device Parameters to RF performance, AN1228, SGS-Thompson Microelectronics, 7/2000
3. S. Juhel - N. Hamelin, PowerSO-10RF: THE FIRST TRUE RF POWER SMD PACKAGE, AN1294, SGS-Thompson Microelectronics, 2/2001
4. Norman Dye, Helge Granberg, Radio Frequency Transistors, 2nd edition, Newness 2001
5. Prasanth Perugupalli, Larry Leighton, Jan Johansson and Qiang Chen, LDMOS RF Power Transistors and Their Applications, Ch. 14, Ericsson Inc., Microelectronics Division
6. Various constructional etc. articles in QEX, The ARRL handbook
7. http://www.mwrf.com/Article/ArticleID/5899/5899.html
