Friday, February 24, 2017

DDS working in enclosure, now debugging the transmit issues of the HW-100

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

This morning I tested the wiring of the DDS enclosure and for correct operation. The unit tunes up and down, the 'up' and 'down' bandswitching buttons work, and the RIT (Receive Incremental Tuning) button works properly. RIT allows me to hold the button and tune an offset so that transmit and receive occur at different frequencies, using the button to control which frequency is active.

The enclosure is just about done. I still need to mount the Heath style feet on the bottom and to put in the sheet metal screws to hold the box closed. It is operational, however.


DDS VFO wired up in its enclosure and passed all tests
I hooked the DDS and the microphone up and attempted to begin the transmitter alignment. My first try didn't seem to work, but I need to work methodically through the steps since the very first act is to set the bias; that could keep the final amplifier tubes in cutoff if not adjusted properly. 

After adjusting the bias potentiometer, the transmit tubes do draw 50ma cathode current when energized with no signal. The next step in the alignment is to switch to Tune mode and advance the Mic/CW gain until relative power is seen. It wasn't. Time to start tracing the signal through the various stages and learn where something has gone awry. 

After some work this afternoon, I had a few conclusions. First, the carrier crystals are quite close but not exactly on their 'ideal' frequency. Second, the output of the isolation amplifier is essentially zero. 

I looked at the output of the carrier oscillator, which varies based on the mode of operation. That shifts the carrier to the left or right for SSB operation, or directly on the tuned frequency for CW or Tune modes. 

For CW and Tune, the oscillator should be at 3395.4 KHz but was about 200 Hz high, an error of ,006%.  USB should be at 3396.4 KHz but was off by 20 Hz or less than .0006%. LSB should run at 3393.6 KHz but was off  by 30 Hz, also vanishingly small. I could cut a trace on the PCB and put in a trimmer capacitor for the CW crystal, but the error is too low to care about.

The output of the isolation amplifier was varying randomly from about .85 MHz to about 1.6Mhz for receive modes, then in the Tune mode it sometimes ran at about 2.3MHz and other times was off. This is not good. 

I think I should have the carrier oscillator frequency amplified here for Tune or CW, and the carrier balance control should eliminate the carrier entirely for USB or LSB. That means that output for LSB or USB will only be on the sideband away from the carrier when there is voice on the microphone during transmission. CW only passes the carrier if the code key is pressed, biasing the IF amplifier on. 

I need to back up earlier in the chain, before the isolation amplifier, where I should see the carrier oscillator signal. Next spot to look was the output of the cathode follower that drives speech signals to modulate the carrier.  

I have to admit that I performed a local modification, swapping the coupling capacitor between the speech amplifier and cathode follower/mixer to a larger capacitance to allow through a bit more bass from voice input. It shouldn't affect the carrier oscillator at all, but if I messed up the change it could be an explanation.

If the carrier is not getting through then the isolation amplifier will be boosting random coupled signals. I need to probe further back but it is slightly challenging to find scoping points. With a PCB and tightly fitted components, it takes time to find a place to hook a scope probe anywhere near the desired signal. 

Also, the high voltages require extra caution. With 800V, 350V and -130V present, I could get quite a shock. Only the final section uses 800V  but across the rest of the unit it is possible to bridge 350 and -130, almost 500V of relative potential. 
Section of schematic I am observing

If I have the signal on the cathode of the cathode follower but didn't see it on the other side of the capacitor that couples it to the next circuit, That could be because one of the diodes is shorted, since the voltage drop across working diodes would leave the AC signal observable. 

I had to redo the tests on the grid of the cathode follower, because I should see carrier on all four settings, even if the follower isn't letting that signal through. The signal that is missing comes up into the diode bridge, so that any coupling of it back to the cathode follower is coincidental. The cathode follower exists to drive the audio signal into the bridge.

The routing of the carrier oscillator signal to the diode bridge is somewhat indirect. While I know that the carrier is produced correctly for all four mode settings, I am less sure about how it gets up to the bridge and the schematic is a bit circuitous ( pardon the pun ).

Thursday, February 23, 2017

C-3 and HW-100 work, construction of DDS box

HEATHKIT C-3 CONDENSOR CHECKER RESTORATION

While removing the old capacitors that will be replaced, I found a funky modification made to the underside of the chassis. Someone had soldered 20 ohms of high wattage resistors across a new tie strip that was added. It is a poor quality change, just as I found quite a few solder joints which were amateurish at best.


Resistors inserted in series with magic eye tube filament
Upon investigation, I found these resistors were added in series with the filament on the magic eye tube. Looking at the tube, I discovered it was a 6E5 magic eye, not the 1629 tube supplied with the kit. Aha - the filament on the 1629 is 12V but the 6E5 requires a lower 6V supply. The series resistors were intended to drop the voltage.

Until I replace the capacitors and power this up, I really don't know if the 6E5 tube is working. If it works well, I will dress up and leave this little mod in place but if I need a new tube, I will just go back to a 1629 and drop the extraneous resistors. The wiring of the socket for the 1629 is different from that of the 6E5, thus I have to rewire that also if I switch back.

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

I am working on the layout for the external box that will hold my direct digital synthesizer that replaces the VFO inside the HW-100. I need to lock in the orientation I want and the placement of the LCD, rotary encoder and three pushbuttons. Further, I need to decide on the power supply and RF output connector types and placement.

Aluminum box for DDS VFO

DDS and LCD to mount inside
Rotary encoder and RF output jack


Buttons and knob to complete the front panel
Perhaps I should create a small board to deliver 12V from inside the HW-100 for use in the DDS VFO. I would likely take the filament power, rectify it and regulate it, putting some small board inside the HW-100 to accomplish all of this.

Meanwhile, the microphone arrived - a historically accurate choice, an Astatic D-1604, and I hooked it up to the transceiver. Now, I could perform the transmitter alignment steps from the Heathkit manual. That is, I could if I hadn't disassembled the DDS VFO. First, I have to complete the construction of the DDS in its box.

I punched the holes for the three pushbuttons and mounted them. The hole for the LCD is cut out, although a bit uneven because of the way I had to make the rectangular slot. My hole for the rotary encoder is too small, thus I have to step up one size and punch again.

I will need to carefully measure, mark and drill the small holes for the mounting hardware that will hold the LCD in place behind the panel. At the same time, I should create the holes on the bottom to mount the main DDS PCB.

I still have not finalized the RF output and power input jacks, thus can't punch the holes in the rear yet. I have a number of heathkit style rubber feet and will place four of them on the bottom of this enclosure to allow it to stand a bit above the HW-100 cabinet top.

I chose to run over to Anchor Electronics to pick up the RF and power jacks, which would let me complete the project tonight. Sadly when I mounted the LCD panel, the backlight broke from flexing of the board. It works but need external light.

After dinner, I had both boards mounted and was wiring up the controls. I finished at night but wasn't ready to test it out until the morning.

This and that

Worked on one of the 1401 systems at CHM today, replacing a front panel switch we thought was bad but the problem persists. The Start Reset button does not reset the machine properly. This means that once an error condition arises, the system must be power cycled to run anything else. 

Participated in an oral history session with Bob Feretich who, along with Grant Saviers and others, created the Tape Emulator box that helped restore the first 1401's Tape Adapter Unit (TAU) about a year faster than would otherwise have been possible, and provided virtual tape drives and educational program support. 

HEATHKIT HW-100 SSB RESTORATION

I picked up a nice aluminum box to mount my DDS VFO into, beginning to cut the openings and mount up parts. I removed the rotary encoder and jacks from the circuit board and will place them on the face of the metal box. The LCD is also to be installed on the face of the box. 

HEATHKIT C-3 CONDENSOR CHECKER

I picked up this unit, which uses a 'magic eye' tube to indicate when a resistance or capacitance bridge is balanced, in order to determine the component value. It also tests for leakage at a variety of voltages up to about 450V, which is a great way to test and reform old electrolytic capacitors. 

I inventoried the parts that likely need replacement and ordered them from Digikey. Essentially it is all but one of the capacitors inside, since they were types that are notorious for becoming leaky with age. When the parts arrive, I will replace the old components and power this puppy up. 

Tuesday, February 21, 2017

Aligned receiver section, preparing to do transmitter alignment of HW-100

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

I wrapped up the conversion to using the DDS VFO, powered up and confirmed that the unit is tuning properly. I haven't confirmed that the output is free of coupled frequencies through the cable that joins the DDS to the VFO, but at some point before operating on the air I will hook this to a spectrum analyzer and look at the output of the VFO. 

I proceeded through the receiver alignment, using the 100KHz built in calibration oscillator. All this assumes that the oscillator itself is right on frequency. There is a procedure to follow to validate this, which will be my next move.

I had a plastic project box that I initially though I would use to mount the DDS, but it is too small and not shielded, both are a problem. I am looking for a larger metal project box which I can install my DDS VFO and its LCD screen. It will sit atop or alongside the HW-100 rig.

I notice that there are minor differences in the frequency that the 100KHz signal peaks as I switch through the bands - this indicates that the crystal oscillators for the final frequencies that are mixed with the IF are a bit off their nominal values, each differently. There are trimmer capacitors on those crystals, I believe, which would allow me to get this spot on to the desired frequency. All that REQUIRES that I can trust the 100KHz calibration frequency, making that tuneup a priority.

I have about ten feet of wire stuck into the antenna jack and sprawling across the floor. Not much of an antenna but I was able to find some CW (Morse Code) going on in the 15 meter band. I need a really strong, i.e. local, signal to hear anything without a proper antenna, but at least I have reception, tuning and so forth. 

My dummy load arrived by early evening, however I still couldn't move on to aligning the transmitter section of the transceiver. I didn't have a morse key or a microphone with PTT yet, thus no way to trigger the transmitter. 

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

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION

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.

Monday, February 20, 2017

powered up HW-100 and fixed first problem

ALTO DISK TOOL

Marc will rerun the boot attempts with the pack I wrote and with a known good booting pack, adjusting the capture and formatting rules to give me traces I can compare more successfully.

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION

The dummy load and cable are at the local post office but with the postal holiday tomorrow, they won't arrive until Tuesday. I can't switch on the transmitter until the load is applied, but I can check out the unit under power, make initial adjustments and see how it receives signals with an inadequate short wire antenna.

I set up the power supply, joined the cable between it and the HW-100, and switched it on. Nothing. The power supply light didn't turn on at all. I pulled off the cable and tested the power switch pins on the connector that will complete the power supply primary circuit. They worked fine.

At this point, I decided to check continuity in the cable. Several pins had no connectivity at all! Bad cable. I opened the caps on the two plugs and the problem was immediately obvious. The wires had failed due to being yanked, separating several leads from the remnant down in the pins.

Wires snapped off on the power supply end of the cable
I took the opportunity to remove the phone jack cable that had been piggybacked on the connector to fit some local modification by the original owner. I removed all the wire remnants and prepared the cable ends to be soldered back in the connector. The design of the cable is very odd - no strain relief at all for the wiring on either end.

Receiver end, with spurious phone jack connected to unused pins

I soldered up the connector, checked connectivity with the meter and then attempted power up of the transceiver once again. It now powers up and I can hear hiss from the audio section when I turn the audio gain up. No antenna at all and not surprisingly, no reception.

I did the initial tests from the instruction manual, which showed that I have correct voltages delivered to the board and that all the tube filaments glow. I set the function to Calibrate, which will produce signals every 100 KHz to allow calibration of the receiver. I can hear the tones at various points on the tuning dial, but the S meter does not deflect from its zero setting.

Hooked up a short bit of wire and tuned in some CW signals on several bands. I clipped a voltmeter across the meter connections and saw that current is flowing through the meter. I then noticed that as the current increases with stronger signals, the meter pins below zero. Hmmm. Something to debug.

I will be using a scope and some reference signal to do alignment, but still lack a microphone to drive the rig. It will arrive in about a week, after I have the dummy load to let me also align and tune the transmitter.

I found some tests to do for the meter circuits, but none of them moved the S-meter upwards. This is not good, seems like I have a continuity break in the meter wires or the meter itself is defective. I did check the two resistors hooked to the meter terminals, and found they both were high. That would subject the meter to higher voltages on each post.

With the meter removed from the panel, I hooked up a resistor in series with my ohmmeter to see if the needle will deflect. Worked perfectly, thus the problem is not the meter itself. The wiring seems good but all I can check is ALC, the first of three meter settings, because the other two require a dummy load since they switch on the transmitter (Relative Power and Plate Current).

The meter is connected to an IF amplifier tube circuit in two places and will measure the relative voltage of the two lines. When I have the RF Gain all the down, the meter should be at full deflection. As I advance the RF Gain, the meter deflects downwards, which is the correct direction.

Earlier I had picked up some CW stations but that wasn't working tonight. I did swap two 6AU6 tubes, perhaps one of them is marginal. I will swap them back later. I picked up carrier that appears to be SSB transmissions on 21 meters, but the receiver is too far out of alignment to capture audio.

Lots of noise and erratic contacts, which is to be expected. More cleaning will be required. 

Saturday, February 18, 2017

HW-100 power supply restored, disk tool archiving packs correctly

ALTO DISK TOOL

My copy of Contralto was corrupted, but when I reinstalled it, I was able to take the cartridge image I had uploaded using my disk tool and boot it up just fine. Whatever problem is keeping cartridges from booting, it does not impair the reading capability.

While we took traces from the logic analyzer yesterday, they are formatted oddly compared to the way we traced prior reading attempts. As a result, I am having difficulty following them to see what is different between booting one of the cartridges I wrote and others that work fine.

HEATHKIT HW-100 SSB RESTORATION

My update kit for the HP-23 power supply arrived today, I assembled it and installed it into the unit. I then tested the unit to check that it produced the +800, +300 and 130V DC as well as filament power. While it also generates an adjustable bias voltage, that is not used with the HW-100 receiver.

HP23 Power Supply as received
Original parts inside (plus big filter capacitor cans)
Parts stripped out and ready for new circuit board

Soldering the board itself was easy, but getting all the loose wires from the chassis to the proper points on the board was more devilish. One of the wires was just too short to reach the hole on the board, yet it was the 800V transformer output which I really didn't want to splice. 

I ended up with no choice in the matter, thus I had to put in a join on a spare terminal strip. There were issues with ground connections that required some improvisation. Finally, the original RF filters on the input were gone but the wiring had to be restored to stock connections between plug, power lamp, power switch, transformer and the AC switching pins on the connector that runs out to the transceiver. 

After this was all done, I double checked the wiring against the schematics to be certain it was right. Some careful comparisons, continuity, resistance checks and other tests were performed before I would allow power to flow in the supply. 

New PCB visible under holes from removed filter capacitors
Plug in, switch on and I measured everything on spec. A bit over 800V for HV, over 300V for LV, under -130V for fixed bias, about -80V for adjustable bias, 13+ for the 12V filament supply and 6,5V for the 6V supply line. The unit is ready to go.

Restored and updated HP-23 power supply

To do the testing, I used my recently restored V-7 Vacuum Tube Voltmeter. One reason I got it was to measure the relatively high voltages in old tube equipment.

V7 VTVM ready for use
When I received the power supply, it was mounted inside the Heathkit SB-600 speaker enclosure. It was cheaper to buy the power supply WITH the speaker enclosure than to buy one along - and I have a speaker to use with the transceiver as well. Needs a bit of cleaning on the paint job and some dings handled, but otherwise is fine.

Dusty, dirty and dinged speaker cabinet