{Collins} KWM-2 No Grid Drive



I have a very clean KWM-2 that I purchased several years ago. About 2 years ago, I finally got around to re-capping the KWM-2, and following that the unit was working fine. I was out of town for a couple of weeks, and when I returned and attempted to tune the rig, there was no grid drive. Given other priorities, the unit went back on the shelf for a couple of years, and I am just getting back to it. When tested, the unit still has no grid current at all in tune on any band. Following previous advice on this reflector, I pulled all the tubes and checked them on a high quality Hickock tube tester, all showed good. I also exercised the band switch and tightened all the chassis screws as instructed with no affect. I then proceeded to take resistance measurements per the manual. Resistance values in general were high although most were still inside the 20% tolerance. About a dozen were outside that range, with 4 readings that were nearly double the spec. However, none appeared to be so wildly off that they would cause the unit to have no grid current at all. 

As such, I proceeded to take voltage measurements. For these measurements, the KWM-2 was running off a variac that was set to be sure that the unit was getting the spec 275 volts in from the 516F-2. power supply. As I guess would be expected, many of the voltages were lower than the spec, although again, most were in the ballpark with respect to the specified value in the manual. Of the tubes that appear to be in play when the unit is in the tune mode, the most suspect readings were from pin 3 on the 6146W finals - only 80 volts vs. the 240 volt spec. I traced the voltage level back in the schematic, looking for where it dropped off. The voltage was at the correct level at the jumpered J5 - J6 jacks, but prior to the DR7 diode, the voltage was down to the 80 volt level. The only thing in between these points is the S9A mode switch wafer, so I applied deoxit to that wafer switch and exercised it multiple times hoping that oxidation on the switch wafer was the cause. Unfortunately, this had no impact. I then checked the value of R70 and it was well within spec at 21.8K. The resistance value for pin 3 of V9 and V10 was also in spec at 14.7K vs 15K, which would indicate that R69 is also in spec. So, those two resistors do not seem to be the cause of the low voltage level at pin 3 of the finals. At this point, I am at a loss to explain what might be the cause of the low voltage level on the finals. 

The only other transmit voltage levels that appear to be seriously off are on: 
- pin 7 of V6 (2nd mixer), where the voltage is 5.3 volts vs. the spec level of 0.5 volts 
- pin 2 of V13 (xtal osc) where the voltage is-0.5 vs the expected 2.5 volts. 
- pin 7 of V14 (vox) where the voltage is 15.5 volts vs the expected 82 volts. 

So, any help on where to go from here would be greatly appreciated. All of the resistance and voltage measurements are available in an Excel spreadsheet if someone wished to look them over. 

Randy, K0FA 



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