Tuesday, February 21, 2017

Checkout of HW-100 transceiver and preparation to use DDS VFO in place of the heathkit supplied VFO


I did some initial cleanup on the apparent rust on the HP-23 Power Supply and discovered that it is mostly a brown gunk adhered on top of the paint on the main transformer and on top of the varnish on the choke. I got about half of it removed with a very mild cleaner (Formula 409) and the areas that were exposed show the paint or varnish, not bare metal. 

I went through all the initial resistance checks using my V7 VTVM, with its 11M Ohm input resistance and everything checked out spot on. I flipped the unit on its side to give me access to the underside of the circuit boards where I can make all the voltage measurements specified in the checkout section of the manual.

I flipped the power switch on, but nothing happened. HP-23 plugged into mains, cable connected between HP-23 and HW-100, but zero signs of life. This may be yet another problem in the cable, due to the flimsy small gauge wire and total lack of strain relief. I will pop the cap off the connector at the HP-23 where the earlier defects occurred and take a look. 

Nothing was visibly wrong, so I plugged it all back in and voila, the transceiver turned on. I began the alignment as outlined in the manual - using the built-in 100KHz oscillator selected by moving the slide switch to CAL. I haven't verified the correctness of the frequency of this oscillator, but it is something I can do using my frequency meter before I go too far through this process.

I decided to pull out the VFO (main tuning knob and its associated electronics) to see how I would modify it to work with my direct digital synthesizer (DDS) VFO. The existing box sits behind the main dial, contains a variable capacitor for the tuning and an oscillator, plus a 6AU6 tube which amplifies the oscillator before its signal exits the box and travels down to the rest of the transceiver circuitry. 

My DDS needs the tube amplifier,  thus I just need to disconnect the oscillator half of the circuit in the box, run in the DDS signal and connect it to the grid of the amplifier tube. I have found two components, a 10 uf capacitor and a 47 ohm resistor, which are the only coupling between the oscillator and the tube amplifier.

Plan to disable oscillator on left, add input jack for input from DDS VFO
I disconnected those components, mounted a signal input connector for the generated frequency from the DDS, and made sure that the oscillator was disabled from operation. I had to choose a good spot for the new connector, permitting the existing oscillator to be reconnected at some future point if desired. 

Upper half is the oscillator, bottom half is tube amplifer
Top of VFO box - need to find entry point for DDS signal
I decided to remove the trimmer capacitor labeled "SHIFT ADJUST" and use that hole to pass the DDS signal inside. I completed this and remounted the oscillator box.

Trimmer oscillator to be removed to route input cable to tube grid
While soldering on the B+, bias and filament wires to the feedthroughs, I had the small wire connecting B+ to the PCB below snap off. I had to remove the wire fragment and install a new wire to power the VFO box. 

I have to hope that the shielded cable that runs out of the VFO box and through the transceiver cabinet out to my digital signal synthesizer will not pick up spurious frequencies from other circuitry such as BFO, IF or final outputs. If it does, I shall need to install the DDS in a metal box just above the VFO unit. That complicates things because I will need to remote the rotary encoder, pushbuttons and LCD panel then.

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