Still digging through HW-100 transceiver and debugging

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

One of those mornings, I guess. I fired up the HW-100 and scope to continue checking signals when I discovered a complete lack of the carrier oscillator frequency where I expected it. I walked back through stages with no success. 

Back to basics, I guess. Stuck the scope probe on the carrier oscillator tube V16 grid where the carrier oscillation is developed, Good there. Followed it to the balanced modulator. Good there. Found it nulled out on LSB and USB, otherwise developed across transformer T1 on CW and Tune. All as it should be.

Hooked up to the cathode of the isolation amplifier V2 and found no signal. Transformer T1 output is an internal LC network with the coil tunable, coupled through C22 to the tube cathode. Zero signal on the far side of C22, so time to investigate that component and get myself hooked to the transformer output side.

Where carrier oscillator signal disappears

Right out of the transformer, the carrier signal is strong and clean, peaked by tuning the coil, thus it is even more puzzling why I see nothing on the other side of C22 that links the output over to the V2 cathode. The only thing on the far side of the capacitor is a 470 ohm resistor to ground and the tube cathode. 

I yanked the tube out just to confirm that this is not a problem within the tube. No change. I pulled the capacitor C22 and validated its value of 24 pf. I measured the resistor and it is almost exactly 470 ohm. Nothing makes sense here. 

I wonder if I am loading down the coil and circuit, changing the resonant frequency of T1 and blocking the signal?  Time to clip the probe near but not on the line, resulting in much less capacitive coupling. Unfortunately, not enough signal to detect this way. 

I did discover that the carrier null potentiometer was dirty and noisy, easily switching its resistance. Putting some Deoxit inside and working it a bit lead to a much more stable control, that allowed me to null the carrier and keep it that way. 

The output of the isolation amplifier V2 was then monitored, which should give me the same signal at higher amplitude, but it was in fact weaker. This circuit is a grounded grid amplifier (see pin 2 above, the suppressor grid), with the input signal on the cathode and the varying potentials on the control and screen grids determining the degree of amplification. 

The control grid is fed from the ALC (automatic level control) circuit and the screen grid is controlled by the CW/Mic gain circuit. The B+ to the screen grid is switched on only during transmission (including Tune mode), and read about 150V. The control grid potential was -15 to 0 depending on the MIC/CW gain control while running in CW or Tune mode. 

Something is still wrong, I get almost no carrier oscillator amplitude into and out of the isolation amplifier V2. Still haven't figured out the issue but will keep searching. Perhaps I should check the four diodes in the modulator, as these have a history of going bad in similar transceivers. Just in case I ordered some spare diodes from Digikey. 

Meanwhile, I decided to go through the 'voltage charts' provided by Heath, for the unit in both transmit and receive modes. I couldn't get into the one section because all the driver coils and crystals are covered by a RF shield plate, and it wasn't worth the problem to check there yet. 

Everything matched up except for one voltage, which turned out to be the crystal calibrator oscillator plate. That is not switched on unless the PTT-VOX-CAL switch is placed in the CAL mode, which it was not. If the chart had identified that dependency I wouldn't have spent the five minutes researching the 'missing' voltage in the schematics. 

I went over all the modifications I made to the audio, modulator, if and bandpass boards, carefully checking the work for errors, bridges or loss of continuity. Good to know that they were all correctly installed. A few were right at the carrier oscillator, but those changes lowered the plate resistors slightly to increase signal strength. Nothing should be causing the symptoms I face. 

The ALC system lowers amplification on several different tubes and is probably the reason why I have almost no signal strength. I will spend the remainder of the evening studying the ALC circuits and determining what I should be seeing at various tube grids, for when I test again tomorrow.

Zeroing in on issue with transmit operation of HW-100

A pretty busy day as I had forecast, so only a bit of time I could devote to the debugging. 

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

I set up the scope with probes on the first transmitter mixer V5A, cathode and control grid. The cathode is the VFO signal and the control grid has the carrier oscillator signal from the balanced modulator by way of the first IF amplifier V3 . I can see the VFO signal running about 700 mV peak to peak. The carrier oscillator with the gain all the way up runs at about 2V peak to peak. 

Assuming the two signals should be about the same amplitude for mixing, this means I need to add an amplifier stage inside my DDS box, to boost its output by about 3X before injection into the VFO cathode follower V20, so that its output is now 2V PP. This is a tiny design and construction task, with the resulting board mounted inside my DDS VFO box.

Missing signal

At this point, I should see a good output signal at the output of the bandpass filter between first transmitter mixer V5A and second transmitter mixer V6. However, all I see on that side is some 12395 KHz signal which is the output of the Heterodyne Oscillator for the 80 meter band setting. There should be a 8395 to 8895 KHz signal from the first transmitter mixer.

I set up a probe on the plate of the first mixer, V5A, to look for the sum and difference frequencies that are input to the bandpass filter. Nothing, not an oscillation to be seen. The tube is not amplifying at all. Time to look at the bias voltages and components surrounding it.

When I look into the circuitry, I see that the tube V5A is biased to cut off while the transceiver is in receive mode, but that should cut off when the unit switches to transmit. This occurs by relay contacts that ground out diode D301. The line to the control grids pass through that diode to a resistor to -130V bias voltage. The relay will ground the diode, which allows the tube to get to a small positive bias and begin conducting.

Grid bias to cut off V5A, V6 and V7 during receive operation

I need to use my VTVM to watch the voltage on the control grid of V5A, I should see it heavily negative when in receive and jump up to a small positive value in transmit mode. What I saw instead was the large negative voltage drop to a low negative voltage, not to a somewhat positive voltage. 

I should see almost ground level, except for the voltage drop across D301. What I find interesting is that there is a user modification I made to this circuit, dropping out a 10K resistor between coil L101 and the control grid of V5A. I will replace the resistor and see what happens tomorrow.

Fixed microphone, working way through the transmit side of HW-100 debugging and tuning

HEATHKIT HW-100 SSB TRANSCEIVER RESTORATION AND MODIFICATION

I put the scope probes on the two ends of the transformer at the output of the balanced modulator, set up the the display to sum the two signals, then tuned it to minimum level while in LSB and USB modes. 

The balanced modulator is designed to block the carrier frequency, by driving equal and opposite voltages to the transformer. Any voice will unbalance the modulator, causing a frequency shift that produces a signal above or below, depending on sideband selected, and passes through because it is unbalanced by the degree of audio input signal.

In CW or Tune modes, the modulator is unbalanced by shorting on side of the transformer, producing the carrier oscillator signal on the output of the transformer. When balanced, the output of the transformer is only those frequencies that shift off the balance point, i.e. speech. 

I will debug the voice input circuit to be sure I am getting the voice signal into the balanced modulator and that it produces the shifted frequency I expect. Nothing, so moved upstream to mic amplifier. Nothing. Moved on to the Astatic mic itself which appeared to have the right connector on it.

The mic stand is mis-wired! The microphone head plugs into the stand with a three pin connector. Pin 3 is ground, hooked to the case and it should be hooked to the shield of the mic cable and thus to the radio ground. Pins 1 and 2 should be wired to two of the four pins in the mic connector at the HW-100, but in fact pins 2 and 3 at the mic stand are shorted together deliberately.

Time to rewire this as it should be, validating each of the four pins on the HW-100/mic connector and through connectivity to the microphone head itself. With that done, I get voice on the audio amplifier output. Further, I get imbalance on the balanced modulator when voice is present.

I then retuned the zero balance on the balanced modulator to absolutely minimize RF output in LSB or USB with no input from the microphone amplifier. In CW and Tune, I get solid RF. I moved on to the transformer T1 that couples the modulator with the isolation amplifier V2 and confirmed the signal gets to the cathode of that tube. I peaked coil T1 to produce the highest amplitude and nicest looking version of the fixed RF signal. 

I can't absolutely confirm that the balanced modulator produces sum and difference waveforms between the carrier oscillator and the voice frequencies because my scope gets confused in its frequency identification logic. Thus, I can't verify the frequency it displays but I do see varying RF signals as I speak. 

Next up, I watched the signal at the output of the isolation amplifier V2, before it enters the crystal filter. The crystal filter has a very narrow passband centered at 3395 Khz, thus allowing only the upper or lower sideband signal to pass, or the CW signal if in CW or Tune mode. The output of the filter will go to the 1st IF amplifier and further. 

I don't see any signal coming out of the crystal filter, but I might be loading it down with the scope probe I hooked up a jumper to use as a morse key substitute, triggering the amplifier when I hit the key. I can here a tone being produced. Also, the ALC goes up and down with the CW gain control. 

Moving on to the first IF amplifier V3, I see the frequencies present at the output, which would be coupled to the first transmitter mixer V5A where it has the VFO mixed with it to produce signals in the range of of 8395 to 8895 KHz as the sum of the VFO (5000 to 5500) and the carrier oscillator (3395 CW/Tune, with additional modulation from voice if in LSB or in LSB). 

The second transmitter mix will combine the 8395-8895 signal with the output of the heterodyne oscillator - the oscillator which is switched to different frequencies by the band switch - to yield a sum or difference that is passed through bandpass filters selected by the band switch. The sum or difference that was selected then is coupled to the driver V7 and ultimately to the final RF amplifiers V8 and V9. 

What appears odd, since the ALC level goes up or down with mic or CW mode transmission, based on the voice level or the CW gain control, is that the relative power reading is always zero. I will look at this next because it is pretty essential to tuning and alignment. It also serves to tell me if the meter circuit is bad or if the transmitter is not operating. 

The way the relative power is measured is a very simple circuit that rectifies and smooths signals from the final tank circuit, consisting of two resistors, a diode and a capacitor. I am pretty sure I am not energizing the transmitter with much power, since the dummy load would get hot if I were. Also, cathode current to the final tubes doesn't change noticeably. 

I have a dinner party to attend tonight, thus have to stop work right now. I also have a pretty busy day tomorrow, which means I won't make much more progress on this over the weekend. 

HEATHKIT C-3 CONDENSOR CHECKER RESTORATION

My replacement capacitors arrived late last night, ready for installation today. Fired up the unit and found it working plausibly, having tested an 8uf 450V electrolytic on the bridge and leakage functions. My 20K test resistor gave a slightly high answer on the Rx100 scale and a odd reading on Rx1. 

No interest in using it for resistance measurement, will not investigate further. The magic cye is a bit dim, but still usable. Back in case and ready to use testing/reforming electrolytics in the future.

ALTO DISK TOOL

We have a session planned for Friday where I will attempt to archive as many of the cartridges on hand as I can. After that, we will test the tool on the Alto's Diablo drive and also debug the booting problem we are facing with cartridges I wrote.

The session will begin early because we have a long-ish interruption at midday, where we are panelists at a special lunch at Xerox Parc, talking with the staff about our restoration experiences. Afterwards we hope to get a few more hours of work in.

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 ).

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. 

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.