Technical information

LO Card Band Pass Filter Adjustment
My bad past experience tells me never mess with band pass filters to avoid damaging unobtanium coils
etc. Since I was having output problems on 15 and 10, I ventured on. The alignment manual references
a desired waveform figure but this does not exist. I tried both of the following with good results:
1. Manual described alignment procedures - I also played with my HP sweep generator and
spectrum analyzer as well to achieve a nice flat response. Sweep 15.338 MHz to 15.838 MHz
(from N4PLs most excellent “Tempo 2020 Mixing and Display” spreadsheet. The spectrum
analyzer made it much easier to perform the final tweaks for flatness at the ends.
2. Easy street REVISED – I discovered that my spectrum analyzer probe must have loaded down
the LO output in 15 and 10. The new procedure is to monitor the level on the TRANSVERTER
OUT RCA jack with the PA slide SW off with a scope or RF voltmeter. Place dial midpoint at 50
KHz. Sequence though all the SEGMENT buttons, peaking PRESELECT each time and adjust
the 2 coils for each band for as close to possible to the same output levels on all SEGMENTS.
This is just as satisfactory as 1. above and much, much simpler. The LO out is high impedance
and easily loaded down. The driver output is well isolated and results in optimum tweakage on
15 and 10 – I now have >100 watts on 15 and 10, all SEGMENTS.
VFO
No changes were required in my external VFO, end points were right on and no changes were made
other than case cleanup. It had a CB WARNING sticker that left what looked like permanent paint
damage. I used many applications of soap and water to fix. DO NOT USE alcohol or any chemicals on
elderly paint, ever. I learned this the hard way on other griefkit projects.
I had to reset the internal VFO endpoints after fixing the 6 VDC regulation problems on the AVR card
since VFO 5.3 VDC is now where it should be at 6.0 VDC. I suggest warming up for an hour before
adjusting so your dial indicator will be centered when warmed up. The addition of a NTE962 (below) to
regulate the 6VDC VFO supply substantially reduced the long term negative drift after 30 minutes. It
was within specification but it’s mo better now.
Does Not Transmit and Receive on the Same Frequency
I measured a 50Hz annoying delta. The first problem was that the 6VDC VFO regulated supply wasn’t. I
was seeing 5.380VDC on receive and 5.430VDC on transmit – not well regulated for sure. Even after
the Q658 change to NTE85, both 6 volt and 8.2 are not perfectly regulated and still have a small
XMT/REC delta of 20Hz.
Adventuring further into the abyss, I added a 3 legged modern regulator NTE962 for a more precision
6VDC VFO voltage. Cut the trace near PIN 5 (6 VDC output) on the AVR board. Double side tape the
NTE962 to the bottom of the AVR. Solder NTE962 output to PIN 5. Connect NTE962 center pin to the
ground plane. Solder the NET962 input to PIN 2 (12VDC). I still see a 20Hz delta and less long term
drift so this is not worth the effort.
I recommend just replacing Q658 (above) for the best bang for the buck if your AVR VFO 6VDC is less
than 5.5 volts (spec is 5.5-6.5) replace Q658.
Now on to Valhalla – 0Hz delta. It turns out diodes D751-D754 on the Operations Board (085) had
substantial forward voltage drop differences. I replaced them with 4 matched HP diodes and perfection
is achieved. You have to remove the Operations Board (front panel drop) to replace them given they
are hidden under the rotary switch. See the photo below of the board removed where the diodes live