Hohentwiel Kit Manual.   Contains schematics, parts lists, board component outlines.

The kit has 4 boards, a coil kit (you wind them!) and 4 bags of parts, one for each board.  The crystal filters are not shown.

This is the Control module.  Contains the AF amp, Voltage regulators, CW & VOX, T-R switching, and side tone generator.

The 10.7MHz  IF module. 

The coil kit.   Can, forms, magnet wire, and slugs,   You wind 'em!

The RF module contains a 5W PA.

The 133MHz VCXO module.  Switchable in 2 bands.

This is the bottom (left photo) top side (right photo) of the nearly completed assembly.  The Control board is on the right behind the front panel.  The front panel pots side just on top of the base board and fit against the control board.  No extra room there.   The IF board to the left is still unfinished.  I have several more transformers yet to wind and the crystal filters to install.   Both boards mount on standoffs I have soldered to the base PCB.   Temporary wiring is in place to test the control, VCXO/LO, and RF board receive sections using another radio at the 10.7 IF.

The pictures above are views of the bottom side of the Chassis.  I chose to use leftover PCB for RF and IF board shielding and for the base to mount all boards to.   The base is sandwiched between the 2 case halves.  The case (chassis) is a surplus box I found.  It just happened to be an tad longer than the longest board (RF board) so I figured it would work.  After extensive mechanical drawings accurate to 1mm, I figured out how to position everything strategically to fit.  It does not get much tighter.  I print out the 1:1 drawing and tape it to the panels and use it as a drill template.  The same drawing becomes a front panel label later.

Most pots, knobs, and switches used were from a surplus store or my junk box.  Unfortunately things were so tight I had no room to mount a nice edge mounted S/RF meter I have had in my junk box for years just waiting for this opportunity!   It even had a S9/5W scale on it.

This is the almost finished case work.   Labeling and a decent set of knobs yet to be come.  The rear panel has TX level and Mic gain pots, key, power, PTT out  for transverter control (next mod) and 2 BNC jacks for split RF.   I am just using common RF for now, but my home station uses split.  I want to try this rig out on EME.

I changed the tuning voltage resistor ratios to favor tuning across a very small segment with fine resolution.   It is dedicated to 144.195 to 144.223 right now.    I might widen this a bit later once I get a feel for the resolution feel.  There is a capability to select bands but I found for some reason difficult to place in the same range.  I will likely revisit it later. The RIT bypass was the band select.

The front panel controls are left to right:

• Audio Level with Power Switch

• Future Squelch mod

• VFO, a 1.8" 10 turn counter knob, found surplus.

• RIT knob

• RIT bypass and CW/USB switches

• Headphone/speaker 1/8" stereo jack

• Mic jack. Wired to be the same as my K2 MH-2 Heil mic.  The stock mic will be an old Radio Shack dynamic mic.  My Heil Pro-Set Plus with switchable HC4&HC5 elements work great.

6/6/2004 - Finished Packaging and alignments, on the air testing show very low noise making weak signal copy a pleasure.    Found a bad TL071 op amp for the audio/AGC  buffer and low pass filter.  Bypassed the stage until a spare arrives.   With the low pass filter bypassed and good quality headphones and the supplied 2.4KHz  crystal filter, the fidelity is excellent.   I found adding another preamp was very helpful with the weak signals.  The weak signals were weak, but above the very low noise floor so adding an intermediate preamp (I already had split RF so this was painless) helped bring up these weak signals to make them the easiest copy ever.  I compared this with my K2/XV144 setup on the same antenna and the noise level on the K2 was higher.  This is very nice.  The K2 is significantly better than most of the other rigs I have tried.  I have a FT-847, FT-100, IC-706mkIIG, and had compared these against the FT-817, FT-736R, and IC-730 (as a IF  rig).  An interesting observation is the effect of variable AGC settings when available.   The FT-736R allowed for easier copy at times since it was more adjustable that the other rigs.   The K2 was judged by me as the best, and was a hands down winner in contest conditions when there are nearby KW and rover stations on the band.  All the other rigs front end folded making them useless, but the K2 rejected them very well and I could operate 10KHz away no problem.   I have not tried this withteh Hohentwiel yet.

I  made these modifications

• Squelch - I did not have space for a S/RF meter, so I used the S meter driver op amp as a comparator to activate the mute in parallel with the TX mute.  Just a diode wire-OR setup.  This required swapping the + and - op amp inputs to invert the signal.  Added a 25uF tantalum in place of R64 feedback resistor.  R64 was reused to connect the + input top ground.   These mods reduce the thump the Squelch muting was giving.   R67 was replaced with a 1N4148 diode and Pin 5 was connected to the junction of R10/R14 on the control board.  R68 is not install and the TX meter buffer amp is unused for now.    I removed pot P3 (10K) and extended the terminals to a front panel pot.  It seems to work fairly well.  It now is triggered by S-level.  This is similar to my K2.

• TX meter amp unused - Considering repurposing this to drive a LED to indicate signal strength by varying the brightness of a LED.  Possibly change the color of a tricolor LED for3 strength ranges.

• Added split TX/RX operation by inserting a short piece of RG-316 from the RF board L1 1/4T tap solder pad to a Rear panel BNC chassis connector.  TX is still common RF and is connected to the TX cable up to the tower T/R relay, but the RX line is driven by a tower mounted GaAsFET preamp and the receive performance does not seem to suffer much.  There is maybe a 3db receive attenuation.   I plan to add a rear panel switch to force the T/R relay to TX position during split operation.  Waiting to acquire a suitable small pushbutton switch.

• USB/CW switching - I reduced the switches due to limited front panel space.  I do not use LSB voice, and I also only use 144.0 to 144.250.  I wanted finer tuning control.  So I hard wired the UB/OB circuit to UB.  A 3PDT toggle switch now selects USB or CW (which is LSB).

• RIT bypass switch - The former OB/UB DPDT switch was reused to bypass the RIT for easier zeroing.  I could not find a 220ohm potentiometer.

• I found a 10T 1.8" turns counter on auction, great timing.  A wirewound 10T 10K pot was used, but a new type continuous pot would be better at the fill 200-300KHz range.  There is a distinct step change due to the wirewound characteristics.   I settled on 150kHz coverage which just avoids this.

• CW offset - I wanted to be able to tune in a CW  in sideband and then switch to CW without retuning.  In VHF contests we do a lot of cross mode contacts.  I padded the LSB trim caps C47 and C48 with 10 to 47pf so that I can switch between USB/CW and there is no pitch change, and on TX, the carrier is pulled 800Hz lower for the correct transmitted tone.

• Audio pot/power switch.  I used a 5K combination audio taper/power switch to save front panel space.

• Front panels LEDs.  I used surplus 5V miniature LEDs.  I left in the stock value resistors.  The brightness is good.  The TX circuit seems to have a very small/fast pulse for some reason, and the TX LED detects this and pulses.   Different but OK.  I could likely drive the LED harder and avoid this but I chose to leave it, saves battery!

• PTT Out - To key an external amplifier and switch my mast mounted T/R relay, I added a phono jack with a 2N2222 type transistor on the terminals.  a 4.7K from base to ground and a 3.3K in series with  the base was connected to +10'S' to turn on the transistor and ground the phono jack.

• RIT Offset Adjustments and Center Detent Potentiometer - I found a surplus 1K center detent pot and used it for the RIT to get a reliable means of knowing where zero was.   This would also mean I could use the RIT bypass switch for band switching later.   I plan to change the crystal so that I can have a 144.060 to 144.140 range and a 144.180 to 144.260 range giving reasonably fine tuning rates and coverage for the vice and 144.100 IF bands.   I added a 100 ohm trim pot to the RIT pot, and replaced R22 with a 1K 10 turn trim pot since 500 ohms is half the pot value for TX.   The pots allow for matching the TX t the RX when the RIT pot is in the center detent position.

• Audio PA gain - I paralleled R5 with a resistor to lower the value and raise the gain a bit to drive my external speaker better with weak signals.

• Mute improvement - On transmit spoken audio would bleed through the PA stage very loudly.  I determined that the side tone circuit around pot P1 on the control board was picking up the audio energy despite the low resistance to ground offered by P1.   I resolved this by soldering a 2N7000 directly to the LM386 PA IC leads 2 (side tone input) and 4 (gnd).  The gate was wired to the IF board pad 16 which has +10V in USB mode (I do not use LSB except for CW.   This way when in USB the side tone input is grounded.  In CW the side tone is normal again.

• Noise filter input bias mod - The product detector tends to sit at 1.25VDC and the AC signals tend to drive the average voltage downward.   I initially observed low audio output and when signal tracing I noted that the TL-071 op amp did not like signals approaching too close to ground.  The output would attenuate on strong signals in the noise filter stage.  I cut the trace at R53 and inserted a series 2.2uF electrolytic cap and two 1 Megaohm resistors at the R53 and cap.  One goes to ground, the other to 8.3V at R51 IC3/Pin7.  This centers the incoming audio at 4VDC and enhances audio fidelity.

Here are the "final" pictures.  Nothing is ever final.   I may choose to repackage this later to allow adding more goodies like a frequency counter module, ovenized crystal oscillator and/or PLL for GPS locking and driving the microwave frequency reference.   A S/RF meter would be nice.  

These pictures are of the RF board and VCXO boards in their PCB cases and thin brass sheet is used for the cover to save space in the box - things were tight.   You can see this in the close ups of the front and rear panels.  The left photo above is behind the front panel and the large black pot is the 10 turn VFO.  This is a bottom view and the mic connector is on the upper right corner.  The right photo shows the bottom view from the front and you can see the feed-through capacitors on the VCXO.

Rear panel showing TX level, Mic Level, RX, Common RF (TX), key, PTT out, power (2.5mm coaxial DC 'M' jack).   There is maybe room for one more and there are some spots in the IF top half rear panel area to squeeze more in later if needed.   I mounted the VCXO box close to the rear panel to leave as much room as possible for future options.  I will likely use the 1"x2"x1/2" space for a preamp module.

The IF board (left side) and Control board - front panel is on the right side in the first photo.   I used surplus 24AWG and 22AWG teflon silver stranded wire for all hookup.  Most of the wiring is 24AWG, and I had a single 500ft spool so all are the same color!  Not much of a problem though.  After installation and wire harness formation and bundling with tie wraps, the teflon is slick enough that you can usually tug on a wire and trace it though the harness as it slips slightly.

All closed up and in service.   Now to finish the 10GHz setup.  This radio will be the first IF rig for it.   I had a few construction troubleshooting issues but that is part of the experience.  The bad low pass filter op amp was a stumper for a while, since signals would get through, just seemed weaker than it should have been.  I aligned the rig using RF generated by using another transmitter key down at it's lowest power setting though several attenuators and a 0-100dB variable attenuator.  With an oscilloscope I traced though each stage of the IF board and peaked the signals.  I found measuring the USB/LSB oscillator frequencies difficult on my frequency counter for some reason, but a easier way was to position an antenna lead (open end coax) from a calibrated receiver dial at 10.7MHz.  Be sure to account for BFO offsets.   I used CW mode and Spectran and shifted the Receiver between CW normal and reverse (LSB and USB) to get the difference to zero, and the absolute at 800Hz plus carrier on my dial.   This is also how I calibrate my other radios against WWV.