"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ----------------------------------------------------------------------
I decided to bench my TR7 - a week before Field Day - to make some PA checks. I had perviously noticed a power drop-off when keyed down - most apparent on 10 meters, but affecting all bands. This sounded exactly like the symptom of predriver thermal runaway on late model driver boards as described by VE3EFJ in his excellent Drakemods paper ( http://www.zerobeat.net/drakelist/drakemod/drmod122.html ). Initial tests showed close to 150 watts CW into a dummy load on 75 meters, and the predriver did not seem to get unusually warm. 10 Meter output was 40 watts CW, dropping to 20. The predriver was somewhat warmer, but not what I would describe as hot. I implemented the emitter resistor mod to the predriver as described by VE3EFJ, but did not notice much difference. At this point, I noticed that the CW output power had become somewhat erratic and unstable on some bands. I did a full power keydown test at this point to see what was getting hot and what was not. The predriver was not, but the PA heatsink got extremely hot - especially, it seemed, in the area of the drivers. A voltage check on the base of the driver transistors showed about .6 volts in RX - not good - and .7 in TX. I then lifted the base terminal of each driver transistor and repeated the measurement. The top driver transistor (measured with a Fluke 8050A) measured .6 V at first, but then quickly jumped to 13.5 V on the base! The bottom driver measured slightly over .6 V. Leaky drivers. Restoring the base connections to do further testing, I decided to measure the bias current of the drivers. Things started to really go south at this point. Power output had become very erratic on some bands, in some cases pinning the needle on the 200 watt scale of my wattmeter - a sign of PA oscillation. I removed the predriver mod, and the results were the same. A cool-down period with the FA-7 energized did not seem to help any. Oddly enough, driver idle current was only around 800 mA at this point, which would seem to meet the Drake spec. I then removed power and did the customary ohmmeter check of each transistor, from base to collector. The bottom transistor measured as would be expected for a good transistor, slightly over .6 forward and full scale (open) reverse biased. The top transistor measured between .2 to .4 in both directions. I had no doubt that both transistors were leaky, but it was definitely confirmed for the top transistor. It was time for a call to RF Parts to see if they could get me the parts I needed before Field Day. But it would have to wait until Monday (this all happened on a beautiful Saturday afternoon). I decided, as long as I was in there, to go with the full PA restoration as described by Floyd Sense K8AC. Terry at RF Parts was very helpful with what turned out to be a difficult order. Difficult in that I don't do credit cards, and I couldn't wait for a Paypal bank transfer to take 5 days to clear. Once we found a mutually agreeable way to exchange money and parts rapidly, I ordered a matched pair of 2SC1969, a matched pair of 2SC2879A - the RoHS complaint version of the 2SC2879, with an Aluminum Nitrite insulator instead of the (politically incorrect) Beryllium Oxide - and a 2SC2166 (specified by both RF Parts and VE3EFJ as a replacement for the MRF476). My parts arrived on Thursday. As PA rebuilds go, this one didn't suck. Drake left room to work around the PA transistors, and they came out easily after I loosened the hex cap screws to break the tight thermal contact with the heatsink. I completely removed the heatsink, cleaned it and regreased all the devices with that nasty white goop. All the resistors that I checked were well within tolerance. Replacement of all 5 devices that I ordered went relatively quickly and easily. I also reinstalled the emitter resistor mod in the predriver circuit. After reassembly and checks for shorts, power output was solid and stable. I replaced driver bias resistor R2203 with a 100 Ohm 1/2 watt, which brought the driver idle current down to about 240 mA. I had created a set of "loop terminals" in place of the original R2203 when I had the heatsink off, so I could swap in different values if needed. I didn't need to. The PA heatsink ran much cooler, even without the FA7 running, than before. Power output after ALC adjustment is as follows: 160 - 110W 80 - 132W 40 - 135W 20 - 130W 17 - 110W 15 - 105W 10 - 93W This was measured with the wattmeter in my MN2700, into a Mil surplus Bird gigahertz dummy load. My HyGain 421B wattmeter agrees closely. Power on 10 Meters was slightly lower than what Floyd achieved, but still better than 3 dB higher than before. I'm not complaining. On thing I did not do was reverse the flow of the FA7, as has been suggested in several places. This just seems wrong to me when you consider the design as a cooling *system*. If the fan just cooled the PA heatsink, it would probably be OK - but the PA compartment is not sealed and there are gaps that allow airflow to/from the PA compartment and the rest of the radio. With the fan pulling air out, it draws a small amount of air through the whole radio, which helps keep the whole system at a cooler and more even temperature. If the fan were to blow in, some of the warm air would find it's way into the rest of the radio. This incursion of warmer air, which would vary in temperature with the transmit duty cycle, would make the entire radio run slightly hotter and potentially affect stability. All of this is unverified, but I chose to err on the side of caution. The test - Field Day. The TR7 performed as expected. Power output was solid and reliable. Case temperature remained cool to the touch, and exhaust air from the FA7 never seemed more than lukewarm - even during long runs on 20 Meters with the SP7 on. Audio reports were good, except when I had the Yagi pointed west and was plagued with RF feedback. Bench tests after Field Day showed that measured power output was exactly as before. I would like to publicly thank Floyd K8AC and Wayne VE3EFJ for their excellent work on the PA power output question. They turned what would have been a "rebuild to as it was before" into a "rebuild it better". Also, thanks to Thom K3HRN and Ron WB4HFN for providing homes on the internet for this valuable information. This is what's it's all about, folks! 73 -Jim NU0C ---------------------------------------------------------------------- Submissions: [email protected] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
