This is Heathkit's one and only kitted solid state sideband transceiver. (Except for one very short lived disastrous experiment, the later transceivers were fully assembled from a well known manufacturer and badged as Heath.) I love this rig for its simplicity and straightforward design. I used mine for years, with no bad incident. I have repaired dozens as "rescue dogs" to be rehabilitated or for friends to prevent their untimely destruction. Do be aware that the SWR protection features are not the same as modern standards. Keep the antenna tuner adjusted right, at lowest power before going to full power, and it will give you great service for a long time. Before transmitting, be absolutely certain to understand the transmitting time duty cycle discussion in the manual. Long tune times, RTTY, speech processing, and other misconduct are severely punished by failure of the finals. I also have a very rare HW-104, which is the version without the digital display. This is similar to the relationship between the SB100 and the HW100. Due to the lack of the display current, the HW104 in the low power position is an excellent low drain full feature QRP radio. As a bonus, you can switch on the finals quickly if you need them. The HW104 receive is a bit cleaner from lack of very weak birdies generated by the counter board for the display. Installing a retrofit shield between the high voltage inverter for the display and the output filter board (which is also in line on receive) helps. Heathkit added this as an update. But you can make one easily in your shop - just put insulating film on the side facing the alc filter board to prevent shorts. Note that it is not needed in the HW104. The differences between the HW and SB flavors of the '104 are as follows: HW has no digital display but has a crystal calibrator with outputs at 100 and 25 Kc. HW uses epoxy paper boards which do not resist heat from soldering as well as the G10 boards used in the SB. The CW filter is always IN on CW in the HW; it is selectable, at least in the SB104A. HW is not available in the A version as it did not survive more than 2 years in production, although many of the mods are applicable. Not a lot of difference in performance or price, which is why most people bought the SB version, and the HW faded away and became a collector's prize. NOTE: the receiver in the HW uses FET amplifier common to the non-"A" version, and is more sensitive. The lack of a counter and its spurs makes for a cleaner receiver. HW has low current drain and in the low power mode, makes a nifty full feature QRP radio with minimal battery drain, since it does not have to run the high voltage inverter for the digital display and associated TTL and 5 Volt power supply. SERVICE NOTE: The B board, which does not have the 5V regulator parts, is not interchangeable to an SB104. The SB104 B board WILL operate a HW104. The FET and direct mixer input receiver front ends are NOT interchangeable without chassis mods. All other boards will swap for troubleshooting purposes.
I offer this one stop SB104 lover's site for repairs, adjustments, and modifications. He absolutely nails it, all good stuff here.
I have the following ideas to highlight important points he makes, and to add to his fine work.
The noise blanker works very well, If you make the vox adjustment knob into a blanker level control. Put a small screwdriver adjust pot in a convenient location for vox adjustment. Just splice the old front panel vox control in where the adjustment pot on the board is located. It will still overload on close in signals, which is a problem on 40 meters at night when the foreign broadcasters are on. But if you want to cut the noise from power line arcing, ignition, and other pulse noise, the level control will allow you to set it just right. Also, there is no adjustment for pulse length in the stock version. Since you now have a level control (from the VOX control), you do not need the NB on/off switch. It is in line all the time anyway. Just use the level adjust to turn it off. The NB switch can now be used to switch a slightly fatter capacitor into the circuit across the existing capacitor that controls the length of the blanking. When dealing with short spikes, switch the bigger capacitor out. Or you can ignore the front panel switch and pick the best value for your interference. You then have a free switch that some people use for a reed relay network to switch the signal line into and out of the circuit for an actual signal bypass, not just a disable as is done in the original design. This fixes the overload problem completely.
Some people also add a second receive SSB filter, maybe even a fox tango filter, in the spot where the noise blanker could be installed. If you have a fox tango filter, leave it alone and appreciate the sharp skirt selectivity and lower blow-by. All the SB104s had a problem, due to shielding and switching diodes used, which limited the ultimate stop band rejection - this extra filter completely cured that problem. Using better diodes for switching also helps the blow by problem in a stock unit. By the way, putting the fox tango filter on the "E" carrier generator filter board does not do as well as you might think. The problem is not the filter to begin with. Any filter, even the original filter, in cascade, at the noise blanker position works just as well. Note: you need a buffer amp with mild gain to overcome the filter insertion loss.
The power supply was re-issued in an "A" version, which was more rugged, and did not use the (failure prone) unobtainium integrated circuit in the earlier unit. It also added a well designed crowbar circuit. I added a fan on the heatsink. You can use a 24 volt DC fan on the unregulated B+ before the regulator with the right dropping resistor. It really helps longevity. Absolutely under no circumstances mount the power supply inside the speaker accessory. It gets too hot. They acknowledge this when they tell you to discard the top cover if you do that. If you use the old non-"A" version, install an aftermarket crowbar circuit. A fast trip dc circuit breaker (similar to the ten tec current protector kit) instead of a fuse is also good. Also, the 110 VAC wires for the front panel power switch on the SB104 are bundled with the dc power; I like to disconnect them completely and mount a switch on the HP1144 power supply. If you choose to ignore this, the 110 VAC can get connected to the 12 VDC and turn a minor repair into a basket case. A much better choice is to use an Astron RM-20A or RM-35A, which is a more modern design based on the venerable 723 chip, which is commonly available. They also come with meters. The actual transmit current with correct load is less than 20 amps, key down. There is little difference in price to get a used 35 amp unit. A switch mode power supply is noisy and not recommended. See comments below.
The five volt logic power supply will fail to 13 volts out if the ground is lost on the regulator board "B" in the front of the card cage. This damages a lot of logic chips in the counter and display driver. Before it fails, replace it with an LM309 or LM323 regulator. It is current limited, and will not allow the damage to occur. K5BCQ also suggests a fix for the 11 volt regulator, but I did not find that necessary; none of the circuits on the 11 volts will fail if it goes to 13 volts. Further, there is a big issue of headroom between the 13 volt and 11 volt supply. Designing a regulator with only 2 volts of headroom is beyond the scope of this article and the capability of most radio hackers. If you make these mods, study the circuit carefully and remove the card edge contacts involved; if you inadvertently plug a modified board into an unmodified transceiver to test it, it isnt pretty. If you plug an unmodified board into a modified transceiver with the right pins removed, it just doesnt power up at worst - no harm done. Mine is done in such a way that I can debug the VOX and audio circuits on an unmodified board and ignore the regulator circuits.
WARNING WARNING WARNING!!!!!!! Never, ever, power up an untested transceiver without going through the assembly manual initial check out procedures of the power supply regulators without all the other boards unplugged. I also recommend disconnecting the 5 V output to the display driver board or unplugging the socketed driver chips. The procedure in the manual also tells you what to expect on the display as you plug each board in, very useful for troubleshooting counter board problems. Don't forget to disconnect the high B+ inverter which runs off 5 volts as well or you will have to open the can and replace the inverter transistors. If your display is completely dead, dont blame the digital drive chips. First check the +5, then the +200 (approx) before you tear out your hair with a scope probe in your hands.
The VFO drive uses ball bearing reduction "Jackson" drives. Replacements are commonly available. You will have to saw off one shaft a bit to make it all fit. If the VFO tuning is jumpy or binds, it can be washed out and greased with new bicycle grease as is discussed with Ten Tec classic Omni and Corsair rigs, if it hasnt gotten too bad yet. Fix everything else before you address this problem, if you can get by during debug.
The S meter does not move at all on weak signals. Then it quickly advances. The site listed has an easy fix for the "F" board. Just change some diodes. It also is backward compatible with unmodified transceivers. This is quick and works really nice.
The instructions for VFO assembly do not adequately warn you of shorts from B+ to ground on the power connector. If you have a short, try unplugging the VFO. Getting inside the VFO is difficult, but hey, you won't quit easily if you got this far. This might be a good time to do the Jackson drives while you got it out.
Many complain that the receive sensitivity of 0.5 microvolt is not comparable to modern transceivers. Yes, it does not compare to a contest rated station. But it is as good as a Ten Tec Jupiter, or any other low end rig. Keep in mind the price point at the time. In any event, there are mods which use hot carrier diodes or mini circuits mixers for the receiver front end. If you carefully align the bandpass filters on the "G" board, especially if you have access to a sweep generator that will do 9 megacycles, it will be hard to hear the difference. Not unless you are one of those people who can hear the difference made by gold plated cables in your stereo. In which case, I probably cant help you. You can put a broad band amplifier between the input RF bandpass filters and the first receive mixer, with a switch to bypass it; many modern rigs have this feature. Having a decent antenna seems like a good idea too. But in many locations, the noise generated by power line insulator arcs and switchmode power supplies in consumer electronics renders the extra gain unuseable. You didnt really buy a switchmode power supply for your radio, did you ? Aaaaaauuuuuuuggghhhh! Use an Astron linear supply. The ignorant are dumping them cheap at the swapmeets now to replace them with a new switchmode power supply. QST recently did a comparison review and found only one out of the four to be as good as a linear supply on noise specs.
There are modifications that eliminate a transmit pulse at initial key down. They are Heathkit updates well worth doing, mostly strategic locations of added bypass capacitors. Do them all or they dont work right. A true "A" version generally has them. An older SB104 no "A" may or may not have it depending if the owner did it. The way to know if it is a true "A" version is not the narrow strip along the bottom of the front panel, provided in the upgrade kit. If the front panel has a marking for the right hand most push button that says 100Hz (to switch off the right hand digit in the display), it's an older SB104. If it says 400hz for the CW accessory filter, its a true SB104A. It's a lot of work for a small improvement, and many did not do them. But mine has the mod, because I am fussy. To test for the mod, set your wattmeter for a low full scale reading and key the mike without speaking; if you get a kick of high energy on the meter, the problem has not been fixed. Before you dispair, many modern rigs have this problem and have not attempted to fix it. It can cause problems by blasting the grids of your linear amp with a pulse. If you dont use a linear, maybe no problem. Pay attention to the other signals on the air and see how many of them start with a loud click or thump, even from modern rigs. It isn't just vibration in the mike. It's in the radio. If you dont believe me, substitute a resistor for the mike element in a connector and ground the PTT line.
Some mikes short the element or short the output to ground in receive. This can cause the radio's input capacitor to charge when you switch to transmit. The fix is to leave the element active all the time; this allows VOX operation also. Unfortunately, if you have a classic "Dee Ten Four" lollypop mike with the amplifier in the base, the power is switched on and off with the squeeze bar, and there is no fix for this, other than eliminating the amplifier. Same deal with electret condenser or any other amplified mikes. A lot of people just don't give a crap about details.
The transmit bandpass filters are very tight on later SB104A units. On 15 and 20 meters, a fixed mica capacitor of 9 to 25 pF to ground is replaced by a trimmer. This generally corrects low output on those bands. If problems are encountered on one band, swap the "C" board transmitter IF pre driver. If this corrects the problem, check the output filter board "K" for an incorrect component. If the PA generates 100W or close on one band, it likely is not the problem. If nothing is generated on any band, it also might be the "J" driver board. If that generates 1 or 2 watts on all bands, the "K" filter board is probably OK. But remember the "D" and "C" boards must generate a good signal first for any of the power amplifier or driver board can work. If you get more than 2 watts out of the low power position, fix the ALC circuit, or the driver board will fry. Problems could be in the sampling circuit on the driver board, ALC hi/lo switching network, or the "C" board transmit mixer/hfo power control ALC circuit.
IMPORTANT IMPORTANT IMPORTANT! The troubleshooting procedures tell you to measure the bias on the final PA. If it is not correct, they propose the final transistors are shot. Not necessarily so. One of mine had burned relay contacts that drive the fat round bias resistor on the back panel. It just wasn't getting the juice to generate the proper bias. If you replace the relay, use a socket too. Do you really want to ever do this thing again? I do this in all SB100 and HW100 style rigs that cross my bench. Replacement relays are commonly available.
The CW filter is a worthwhile addition, if you like CW. Check for the components on the board "E" if you do not have a filter. If they are there, but the filter is not, the filter has likely been removed and sold on eBay for about $100. But you can find them cheaper, and the components are already on the board for you. If the board is bare, sometimes you can find a parts SB104 that has not only the CW filter but some nice spare parts you may need in the future. I buy junkers cheap every time I can. About 40% have been completely rehabilitated and on the air again.
The display units may get dim or fail after time. Buying up junkers usually will score you some replacements. If people pull all the boards to sell them on eBay, they often forget the valuable displays. Also, if you are very determined (as determined as me), the output from the counter board will nicely drive modern chips and modern LED displays which are brighter, more durable, and come in any color you like. You can also use the space for the callsign on the front panel for a cool ammeter! Even room for a digital clock if you use smaller LEDs for frequency.
The original VFO accessory SB644 on the surface seems like a good idea. True, if you work split on 40 meters or need to split up 5 for DX work. My Ten Tec has the external VFO, has legendary CW breakin, and I do not use my SB104A for split frequency DX. WARNING WARNING WARNING! The stock remote VFO will generate a background signal on the unused VFO frequency in both receive and transmit! This got me my one and only OO report. Thanks for the heads up OM! There is a Heathkit mod series that makes this much better. Also, there is a mod out there to switch the actual B+ on the VFO in use to prevent this, but it no longer is good for full breakin and causes frequency drift. I have actually done a different mod with reed relays, two in series, with shorting the unused output to ground in between the two in series switching the signal path. With only picofarads in the open position, and the short in the center, the bleed through is much smaller. You also have to do mods to the 11 volt regulator power. And a bunch of other stuff. You can make it work on phone, but I do not plan to document it or recommend it to anyone but the most persistent detail freaks. If you are capable of pulling it off, I have given you enough information here. Some people have added rit, and there are mods out there to do that; they are much easier than making the SB644 work right.
UPDATE FOR FM OR VFO DRIFT WHEN CHANGING TO TRANSMIT 12/27/2013
This is an added thought addressing VFO changing frequency or frequency modulating in transmit. I encountered this for the first time not long ago. I got reports of being on the wrong sideband (really off frequency and FM). It was hard to diagnose since it only happened when running full power output.
The problem is caused by the 11 Volt regulator. The voltage drain increases in transmit, lowering its voltage or causing the voltage to change as the TX power goes up and down with modulation, causing FM to occur.
You MUST adjust the main power supply for 13.8 volts. The 11 Volt regulator in the SB104 must be set a bit low to allow enough headroom. Low gain transistors or IC regulator may need to be swapped out to see if they are the cause. This is a tricky trade off. If you set the 11 volt supply too low, you may not hit spec on the power output.
Good luck with getting some of those magic green radios back on the air!
UPDATE 2/22/2014: Some Additional links
|
