Wednesday, February 15, 2017

Got power supply for HW-100, replaced capacitors on VTVM


I finally completed the setup of the direct digital synthesizer (DDS) so that it implements the eight bands of the transceiver, generating the subtractive intermediate frequency of the VFO it will replace. When tuning any band, the low end of the band generates a 5.500 MHz signal, then as the dial is rotated upward through the band our output frequency declines down to 5.000 MHz. 

Each band is 500 KHz wide. The frequency counter verified the proper output frequencies for all eight bands. It was painful to set this up, since I had to reverse the earlier settings - e.g. take the band 8 start frequency from 29.5 MHz to -29.5 Mhz. 

That step alone took almost 120 rotations of the dial. The IF offset had to move even further, about 140 rotations. Multiply that by seven other bands, albeit with decreasing numbers of rotations particularly on the lower bands. 

The remaining tube arrived - a 6AU6 - which I verified on the tube tester since it was tossed into an envelope and mailed. It checked out fine so I installed it into the socket. 

Today I received the Heath SB-600 which is a speaker cabinet with the HP-23A power supply installed inside. This power supply will deliver the 800V, 300V, -110V and filament voltage needed by the HW-100. I have to check out all the filter capacitors and replace any that are still original.

The unit was packed foolishly, thus the outer carton was partially torn open, one foot of the speaker cabinet was lost and I had some minor shipping damage. Still, it is in good shape for the restoration I need.

I removed the power supply from the speaker enclosure and moved it over to my bench. All of the capacitors are original electrolytics. They all must go. I built up a list of the values I needed in order to go shopping. 

However, I came across a nice kit that replaces everything inside, ordered it and will wait for it to arrive. I found that my HP-23 has been modified - need to figure out what was changed and either transfer it over to the new kit or dump it.

The major modifications are filter inductors on the AC input line, plus bypass capacitors. These are intended to block any RF from leaking back into the AC mains, I believe. This was probably a change made by the prior owner as a concession to neighbors. 

The other modification I found was a phono jack attached to the power connector where it would plug into the prior ham rig it powered This jack makes use of a unused pin for ground and borrows the variable bias power pin for the center lead. On the power supply side, the cable is not soldered into the variable bias pin, thus that voltage is not carried on the cable at all. Fortunately, the HW-100 does not make use of the variable bias line. 

The Heathkit manual has a resistance chart, which I used to see that the resistances appeared close to the published values at all the points they listed. When I came to measuring tube V5 on the bandpass board, however, two points had open conditions but were listed to display relatively low resistance.

More mysteriously, when I looked at the PCB I could see that those two tube pins, 8 and 9, were on traces that had absolutely nothing else connected to them. The open circuit was what I should have, yet the PCB has holes missing components and the chart gave target resistances.

Hmmm. I took to the schematics where I found one half of the tube (V5A) but nowhere could I find the other half of this dual tube. It dawned on me that in fact the documentation was wrong about those resistances, because the V5B half of the tube is not implemented on the HW-100. 

The same circuit boards are used for the lower cost HW-100 and the higher priced SB100/SB101 transceivers. The HW series was created by dropping certain features from the SB version to lower costs. For example, the SB allowed use of an external VFO, but it was dropped in the HW machine. I believe that the second half of tube V5 hosted one of those dropped functions.

On the SB-100 schematic, V5B is used as a "crystal oscillator" for a fixed frequency, which the operator could use for transmit, while still receiving using the tuning dial, or could force both transmit and receive to the one fixed frequency. As an example, this was used with the MARS (Military Auxiliary Radio System) in which ham operators work with the military as a means of supporting and augmenting communications for the armed forces. 

Thus, I solved the mystery of the missing tube section and know for certain that the resistance chart included with the HW-100 manual is partially in error as it contains values that would be found on an SB-100 instead. 


My new capacitors arrived from Digikey, thus I put the VTVM on the bench and began to replace the originals. With everything in place, I fired up the meter. Unfortunately, the behavior was not good. I suspect I got further cracking in the circuit board from mounting it back together. 

I will put this aside and open it up again tomorrow, when I am ready to deal with the funky condition of the meter and repair as much as possible on the fragile PCB. It had been working nicely before the replacement of the capacitors and resulting reinstallation of the PCB on the meter and brackets. 

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