Hello from WA0OPW in Omaha (ex-WA1OPW in Boston.)
My Viking 2 has developed a new problem. There is no grid drive on 10 meters, but plenty below 18 mc. Drive drops off on 21 mc, but is still enough to tune up the rig.
I checked and cleaned the band switch; it appears OK. The wafer for OSC is ok and the coil checked out ok. The band switch wafer for the 6AQ5 BUF is very hard to access, but voltage is ok across coils L5A & B.
The VFO and OSC tubes were both a bit weak, so I replaced them. The 6AQ5 BUF was running hot, so I swapped it with a 6AQ5 from my receiver. It worked in the receiver, but still ran hot. So I ordered a new one. The 6AQ5 from my receiver produced much stronger grid drive in the Viking, but still none on 10 meters.
The Buffer is reading fine on 10 meters and all other bands, but zero grid drive on 10M. I've checked R15 and R19. The RF coupling caps, C14, C20 & C25 seem ok. I jumped across each one with another cap, but no difference.
The rig has the Johnson grid block keying modification but is otherwise stock, except for a solid state LV rectifier. I also converted 11M in the VFO to cover 17M. (Sorry Good Buddy.)
Note- I need to tune up the OSC and BUF using my S-meter because the rig meter barely shows a dip/peak.
Another pair of 6146s made no difference.
Any thoughts would by appreciated. Thanks.
Update- The 25k drive control is giving me strange resistance readings. So I ordered a new one. Firing the parts cannon usually works after a while...
PROBLEM SOLVED - The Viking 2 was obviously designed by WWII vets who also designed Jeeps capable of driving with one wheel missing. The 25K drive pot (R25) was fried, but there was still plenty of grid drive on all bands but 10M.
I ordered a new 6W 25K pot from
https://www.etsy.com/shop/UsefulComponents for $15 plus $4.88 shipping by Royal Mail. The item arrived from UK in less than a week.
Installing the pot did not require removing the front panel. I managed to squeeze it in from the back. The key was to pre-wire the pot, jam the new wires through the grommet, then reconnect the wires. I put a 7.2K 6W resistor in series with the pot's B+ lead to protect it.
This has been a long wild goose chase. The band switch turned out to be fine and the interstage rf tank coils were working. The coupling caps were ok. The resistors were all good except the 56 ohm grid resistor feeding the 6146s. It had drifted to 79 ohms. This resistor is apparently to suppress TVI so I just left it for future replacement.
I also discovered a problem with the Viking II documents. In Figure 10c, R35 is actually a choke, L23. On page 45 of my pdf, a choke is labeled R20. It should be L20.
I haven't worked on an old rig in years. I am in dire need of a schematic overlaying tube pin numbers, voltages and resistances. Maybe Sam's makes one? Well, I suppose the market is pretty small.
An autopsy photo of the 25k 5W drive pot is attached, next to the modern 6W version.
Now to reinstall the 4,278 screws in the Viking II case..
Thanks.
