Wednesday, November 25, 2020
The GAIN knob is just that, you are trying to find a level that just matches the noise so that the result is zeroish. It is not to find the best listening level. Use the radio knobs to do that.
The two PHASE knobs jointly adjust the phase of the noise signal. You should adjust each for a minimum. Unfortunately they also mess a little with the level, so you have to go back and forth a bit between all three.
The NOISE Attenuator control is rarely used and is on the rear panel. When the LNA is installed it is used to get the noise signal in a range that can be matched by the GAIN control. Without the LNA it will probably just be set to max. It will depend on the size, type, and location of the noise antennas, as well as the strength of the noise source. If you find that it works best with the attenuator below half way, try reducing the gain by plugging the LNA into the 9V power source, or consider removing it completely.
The DELAY ADJ is inside on the pcb. This screwdriver adjustable trimpot is used to adjust the delay time of the relays after transmit. It is not critical if you just use AM or SSB, the time can be fairly long without being annoying. CW will require a shorter time and a little more fine tuning.
ASSEMBLY of the PCBs:
This kit may not be as challenging as building a HeathKit color TV, but it will still require a bit of time and skill. If you have not built an electronic kit for a while, you should probably practice with a simple DIY kit of some sort from eBay. Before you start the ‘180 assembly, thoroughly study the schematic and understand the function of each part of the circuit, it will be a help in construction and operation. Review the instructions and be certain that you can identify all of the components. Begin by sorting out the parts so as not to loose anything, get some good light, and a place to work comfortably for a couple of hours at a time. Complete assembly is probably a two evening job; there are a lot of parts.
You will need some basic tools: soldering iron (about 40W) with a smallish tip, your favorite solder, sharp flush cutters to trim leads, screwdrivers, needle nose pliers, and if you are not 13 years old, a good magnifying lens. A volt-ohmmeter, even a cheap one is pretty much a necessity (Harbor Freight has several for $10 or less, as does eBay).
You will notice that when soldering, that the ground plane is quick to dissipate the heat on grounded pads so you will have to stay on those pads a bit longer to get good flow to the top of the board. 3-4 seconds is usually about right.
It is generally best and easiest to install the smallest parts first, and gradually work up in size. Don’t stress about getting the components close to the board. It may look nice if they are all flat and neat, but keeping them a little above the board would make any rework easier and generally improve air flow. This is especially true of all the small transistors. Refer to the board layout drawing and install 6-7 parts at a time, then solder them. Then trim the leads and put in the next group. Checking them off of the BOM as you go will save time in the long run. Don’t worry if you have parts left over, there are often a few extra parts in the kit (I don’t count so good, but I try hard to never be short).
When the board is complete, you can clean the backside with a little rubbing alcohol and an old toothbrush to remove the flux from soldering. Check to see that all leads are trimmed enough so that they will not short to the case. Check especially those along the edge of the board.
MAIN PCB ASSEMBLY:
[_] 1) It’s a good idea to install the Zener Diode D7 first, this will keep it from being confused with the 1N4148 diodes which look almost identical. Then install the 1N4005 black rectifier diodes (D1 & D18). Now you can put in the three 1N4148 switching diodes (D2, D13, D19). As with all the diodes, watch the polarity and match the band with the silkscreen image.
[_] 2) Install the two full wave bridges, they are in the 4 pin dip packages. Note the + and – location on the board layout drawing.
[_] 3) Install the inductors, be careful, there are two different values and they can be difficult to tell apart. They look like resistors, but will measure nearly zero resistance on your ohmmeter.
[_] 4) The resistors are next. R9 and R7 will run warm in operation so mount them about 1/8” (3mm) or so above the board. Save the potentiometers until later since they are large and get in the way. Some resistors have 4 value bands (1%) and some have 3 bands. It didn’t matter but the 1% types were supplied when available.
[_] 5) Install the capacitors, just save C3 until later. Only C2 and C3 are polarized, the rest can go in either way. Each is marked with a very small 3 digit value code.
[_] 6) Now is actually a good time to install the board connectors. The pre-wired cables should be oriented correctly: The 4 pin cable should wind up with the RED wire in the square pad. The two-pin cables should also be inserted so that the RED wire goes to the square pad. The green connector and the SMA connector may require a little more heat.
[_] 7) Take a break from soldering and wind the transformer. Wind 10 turns of the enameled wire, and then three turns of the insulated wire. The wires can exit the ends or sides of the core, it doesn’t much matter. Just make sure that when it goes on the board the enameled wires terminate on the C7 side. Trim the wires to about the right length to fit the board (not too short). The insulated wire can be stripped with a good pair of strippers. The enameled wire must be prepped by scraping the ends with a razor blade or sandpaper, then tin the ends to make sure that all of the enamel has been removed before soldering. Double stick tape will hold the core in place on the board.
[_] 8) Install Q2,Q3, Q5, Q6. Leave out Q1 and U2 until after testing. Leave all the transistors well above the board, it will make removing them possible if that ever becomes necessary. On Q5, install the provided heat-sink before you solder it in.
[_] 9) Install Q4, the large PNP power tab transistor. The metal side of the transistor goes toward the inside of the board.
[_] 10) Add the bit of plastic shaft to the shaft of RV1. Use the adhesive lined heat shrink provided. This will allow this control to be adjusted from the back panel. Use a heat gun or very small flame to shrink the tubing.
[_] 11) Install the ‘delay adj’ trim-pot, RV5.
[_] 12) All of the larger parts can go in now. Don’t forget to install C3, and the 2 fuses (lamps). Solder in the controls and the relays. Note that RV1 is a 1K pot. The others are 5K. Be sure that the pots are firmly seated against the board so that the front panel will align with the enclosure correctly. Some of the pots have a metal anti-rotation mounting tab that will interfere with mounting in the front panel. Cut or break that tab off of the 5K pots before soldering.
[_] 13) You can now install the front panel. Do a test fit then carefully bend the LED to line up the the hole. Attach the panel with the washers and nuts on the three controls, just finger tight for now. Make sure that the LED is firmly snapped into its bezel and solder it in place.
[_] 14) This is not critical but it is a good idea to electrically connect the ground plane of the main PCB to the back of the front panel. Just solder a bit of wire (a piece of resistor lead) from one to the other. Doing this will allow operation of the unit outside of the case for testing. (the only other ground for the front panel is through the case and the paint can insulate connection)
[_] 15) Cut one of the two pin cables (red/black wires) to about 2 inches and solder it to the power switch. Polarity doesn’t matter. Install the switch into the front panel and tighten the nut. Plug the cable into the PWR SW connector on the board.
REAR PANEL ASSEMBLY:
[_] 1) The rear panel is really a PCB. It will provide a location for the fuse and protection diodes for the noise input.
[_] 2) You will need to surface-solder a full wave bridge pack to the back of the rear panel. Note the orientation and match the + & -. This is as close as we get to doing surface mount.
[_] 3) You should install the remaining 4 connectors as indicated. Tighten well, and if you have a dab of Locktite TM ,glue, or nail polish, this would be the place to use it.
[_] 4) Solder a small lamp (fuse) from the center pin of the BNC connector to the L-shaped pad next to the four diodes.
[_] 5) A 5.6uH inductor is also soldered between the ‘L’ shaped pad and ground. It is easiest to cut one leg to about 3/8“ (8mm) and prep it with a small bend. Then you can hold the other long leg to solder it in place. After the short end is soldered cut the long end and bend it into place and solder. Depending on your noise source you may get better results without this inductor. For now, a good plan is to leave the ground end floating so you can easily try it later.
[_] 6) On the four pin connector with wires, cut the white and red wires (don’t cut the yellow and black) to about 3.5 inches long. Then wind 3 turns of both the yellow and black wires through the green ferrite ring. This choke helps to block RF from getting in or out through the power wire. It also a good idea to solder a 0.1uf cap across the power connector on the rear panel. The yellow and black wires can now be soldered to the power connector on the rear panel. The yellow wire is connected to the center conductor, positive terminal. The ground lead should also be jumpered to the pad on the rear panel.
[_] 7) Solder the white wire to the center pin of the RCA jack. Then, solder the red wire to the L-shaped pad. You can also solder a 0.1uf cap across from the center pin of the RCA jack to ground if you like. Although not necessary, it can keep some RF out of the power.
[_] 8) Strip and prep the thin coax cable with the gold SMA connector, and using the pad next to the BNC connector, carefully solder the shield to ground and use the connectors ground lug as a strain relief for the cable. The center conductor should be soldered to the L-shaped pad. Use minimal heat to avoid melting the center conductor.
[_] 9) Using three pieces of about 2.5 inches of 18-14 awg wire, connect the SO-259 connectors to the three pin Phoenix connector. Note, the center pin is ground (GND on the PCB silkscreen)
[_] 10) You can solder the remaining 2pin red and black wire connector to the LNA. Run the wires through one of the mounting holes as a strain relief and connect the red wire to + and black to -. Don’t install the LNA until you do some basic testing.
[_] 1) It’s not a bad idea to check to make sure that the board slides nicely into the case and that the front panel holes line up. You may have to loosen the controls or re-solder them to get a perfect fit, but don’t install the case just yet. Double check the rest of your work and verify that all of the transistors are in correctly and that all of the diodes are in the correct direction. The LNA can be left out for now. Steps 2 thru 4 can be done with the boards out of the case.
[_] 2) Before the 4 pin power cable is plugged into the board, plug in the power supply to the rear panel and verify that 12V is present on the yellow and black wires and that yellow is positive.
[_] 3) Now plug in the 4 pin cable and the green 3pin connector. When pressing the power switch, you should hear the relays click and the the LED should light in a yellowish color. If it is green, the LED is reversed. If nothing happens check for 12V power at the power switch, the orientation of Q4, 5V across D7, and the location and orientation of Q2. If C3 gets hot and explodes violently, it was in backwards.
[_] 4) If everything works so far, and with power on, try grounding the control input (the RCA jack). The relays should click and the LED should change to red. If this fails, check D7, and the other diodes and components around Q2. When you are satisfied that the power and relay control is working, then install Q1 and U2.
[_] 5) You can put everything in the case at this point but leave the top off. You can tighten the controls and install the knobs. (It’s good time to put the rubber feet on the bottom as well)
CAUTION: When the transceiver is connected, there is a possibility of high voltage and RF exposure.
BE CAREFUL! Turn RF power down on your radio and take care not to transmit.
[_] 6) Continue testing with the transceiver, the power supply, your main antenna, and a noise antenna, connected. PTT is optional since you will not be transmitting. Hook up a noise antenna to the BNC connector on the rear panel. Maybe a 2m antenna or just 5-10 feet of wire in the shack for now.
[_] 7) The LNA board can be installed now, it simply sits above the main board held in place by a 90 degree SMA coupler to the LNA’s output. Power is provided by one of the two, two-pin connectors. Use the 12V connector for testing. The signal input to the LNA comes from the rear panel cable (the one with the SMA male connector).
[_] 8) When everything is in the case and connected, turn the QRM-180 power on and off. Listen for the relays to click and observe the power LED. When the power is on the LED should be yellow-ish. Set the gain, and the rear attenuator to max.
[_] 9) Turn on the transceiver. With the QRM-180 off, the receiver should work normally.
[_] 10) When you turn on the QRM-180, there should be a lot of noise in the receiver. Play with the controls to get a feel for them and try it with and without the noise antenna connected.
[_] 11) At this point you should be able to adjust the two phase controls to get a sharp null point. If not, recheck your work and use an ohmmeter to verify that the rear panel cables are connected and not shorted. You can lift one end of L1 on the rear panel to get a check of the resistance of the SMA cable.
[_] 12) If everything works the unit is now complete!! Go to the section on operation and begin testing your options for a good noise antenna. If you still have issues, review the “list of common problems” and continue testing.
You can operate with or without the LNA as needed, depending on the noise level and size of noise antenna. If you remove it just plug the SMA cable from the rear panel directly into the main PCB. For less gain, you can also try plugging the LNA into the 9v power source. While normal is 12v, the LNA is a bit cleaner on 9V. Try both.
It is fairly robust circuit so you should not experience any problems if you managed to get all the parts in the right holes and in the proper orientation. But don’t panic if you do have an issue. Should something not function, check the schematic for that area and recheck the parts related to that function. It is surprisingly easy to miss-read a component’s value. Start with getting the relays to click when the PTT input is grounded. When that works, you know that the power circuitry is right.
If you still have signal problems, try connecting to the receiver but without the antennas. Use a small probe or bit of wire, and following the schematic, touch various points along the signal path. Just your body as a signal source will introduce plenty of static to locate a fault. If you can mess with the knobs and get a null in background noise, it is working properly. You just need to get the best noise antenna and get good at fussing with the knobs.
Common Problems: Power issue or relays don’t click:
• 12V red/black wires switched on the rear power connector. Check the power at the board.
• The rear panel gets its ground through the green connector. Be sure that it is plugged in when testing the PTT function through the RCA jack.
• A 1N4148 diode switched for the Zener? There should be approx 5V across the Zener.
• Is the power tab transistor backwards? Tab toward C3.
• Check pads that have top traces, sometimes solder doesn’t flow to the top and the plated-thru hole can break leaving the top trace unconnected.
• A few pads are very close to the edge of the board and can short to the case if the leads are not trimmed flush on the back of the board.
• Check the resistance of the SMA coax cable to the rear panel, it is easy to overheat the shield when soldering and short it to the center conductor.
• Is the LNA in backwards? RF in goes to the rear panel, RF out goes to the board. Is the power correct to the LNA? Plus voltage to VCC on the LNA?
• Are the enameled wires of T1 sanded and soldered?
• Check all the fuses with an ohmmeter. It is difficult to tell when they are open. Spare fuses are included but if you need more, they can often be found in hobby shops or on eBay by looking for ‘model railroad lamps’. 6V 40-100ma lamps are best but small 12V lamps will work.
• Check the grounds between antennas and the radio. A loose connector will cause lots of grief.
All else fails, email me and I will try to help. firstname.lastname@example.org
As most of you know, I have been selling kits on Ebay. The QRM-180 kit, the HV Diode board and parts for the SB-220, and the upgraded capacitor bank, also for the Heathkit SB-220. I like the kits, and it feels good to promote some of the old hands-on experience that used to be the heart of ham radio.
Ebay is fine, but I get a lot of inquiries from folks (not hams) that have no idea what to do with the kits but want a lot of information. Ebay is also expensive. They make more on each kit than I do. They are also pushing me to set up a business account which is even more expensive.
So... I am not selling on Ebay, at least for a while. If you are one of those that want one of those kits you can always contact me...
I still have the kits and can ship anywhere in the US by USPS priority mail ($8).
International is more difficult (and expensive) but if you give me an address I will try to get a shipping quote.
Payment will be by PayPal only.
Thanks for the interest.
Saturday, August 29, 2020
Thursday, July 16, 2020
Heath started selling kits in 1947 and the HeathKits were a wonderful way to get equipment that would otherwise have been too expensive to own. And the kits were easy and fun to build. The compromise was that HeathKit often had to omit some less-than-vital features to keep it simple enough to build and to keep the costs down. For those products that have are still useful today, there is a long list of upgrades, improvements and suggestions in magazines and on the internet.
In 1972 Heath was already building a very popular array of amateur radio equipment. The SB-220 was perhaps their best, and most enduring piece of ham radio gear. It was later superseded by the functionally identical but prettier, HW-2200. The AG6YJ Control Board combines a number of the popular SB-220 upgrades on a single pc board and does a bit to clean up the underside of the amp as well. I like to think that this is the way that the Heath engineers would have done it, given the budget.
Adds Soft Start
The board adds a “soft start” to the power circuit to gradually power up the amp. Sure, the 3-500 tubes are instant on (and the filament transformer does its bit to limit starting current) but the other components still take a hit on power up. The later, HL-2200 version of the SB-220 is known to regularly burn up the (now unobtainable) primary power switch. And the capacitors take a pretty solid hit on power up; the series string sometimes exposing one or two to a momentary over-voltage. The HV transformer also experiences an effective short circuit until the capacitor bank begins to charge up (and an upgraded Cap bank adds to the load). The fix consists of a simple power resistor in series with the input power with a timer relay to bypass it after a second or two.
The original HeathKit design cleverly avoided the need for any low voltage control circuitry. That was fine, at least until you want to add a feature or two. The new control board includes a 24VDC isolated power supply to provide power for new relays, meter LEDs, and the rest of the control circuitry.
Fan Speed Control
As many have pointed out, the fan in the SB-220 is actually not bad. With a little maintenance, the fan will do a good job and last forever. Don’t be tempted to put in a modern design ‘muffin’ fan. They will not work as well in this application. The stock fan is sometimes needlessly noisy however, so the new control board adds a temperature sensor and control circuitry to let the fan run at a slow speed until heavy use (and a rise in temperature) requires the high speed mode. When things cool down the fan slows to quiet mode.
The SB-220 is designed to operate from either 240V or 120V. It is a nice feature. 240V is generally preferable, but on a good 120V circuit the amp will run just fine as long as you don’t try to exceed the limits. An error in the original design however, could inadvertently burn both transformers (with impressive smoke and fanfare) if just one of them lost a primary winding (or the terminal just came loose). Heath issued a service notice to fix this by removing wires from the terminal strip and using a wire nut to attach them. It added considerable confusion when someone new went to switch voltages. The new control board isolates both transformers and provides a simple connector to switch between voltages. Simply align the plug to the left for 240 and right for 120.
The SB-220 was originally intended to work with the tube transmitters of the day. To connect the amp to a modern, solid state, transceiver the PTT input of the SB-220 had to be modified in some way to avoid exposing the radio to destructive high voltage and current. The new control board circuitry limits the input voltage to 5V and just a few milliamps to ground will engage the antenna relay ( “Soft Key”).
A subtle but significant improvement is the addition of a small relay to separately manage the bias for the transmit tubes. It allows the use of a simpler (and quieter) DPDT relay for antenna switching instead of the original high voltage 3PDT. The new relay operates on 24V since it is now available. If at some time you wish to change to vacuum relays (which are usually 26V but work fine on 24V), no other complex circuitry is needed.
The circuits for the soft start, the soft key, and the fan control, are nearly identical. This makes understanding/ servicing the circuitry simpler and keeps the component variety to a minimum. All of the components are standard thru-hole types; no surface mount devices to provide needless frustration to the assembly. There are bypass capacitors all along the power path, as well as capacitors across the relays to ensure that RF is not a problem.
Unfortunately I made a couple of errors in the board layout, requiring a few jumpers on the rear of the board. I will fix them if I do another layout but in all likely-hood I will add other, similar errors. The jumpers may already be installed on the board for you but if not, the rework instructions are on the last page.
Start by sorting the components and setting up a work area where the bits an pieces are not going to get lost. A good soldering station is a must, as well as good hand tools, a multimeter, and probably a magnifying glass of some sort. Install small parts first and gradually work up in size. It will leave the most room for fingers as you progress. As much as possible, leave components (resistors, diodes, and especially transistors) a little above the board. Things will be cooler and easier to repair if the need arises.
[ ] Before doing anything to the board, it is a good idea to inspect the chassis of your SB-220. You will want to drill a couple of mounting holes for the new board. Use the board as a template to mark the location of the holes. There are 5 mounting pads on the board, two of which should line up with two screws already under the HV transformer (since they are flat-heads you might have to insert a large dia washer to get them to tighten). You don’t need to provide all 5 mountings but try to get at least 3. Be careful not to drill into something valuable on the far side. The included spacers are just long enough that the board should be above the one interfering mounting bolt of the HV transformer. Reverse that bolt if it appears too close (although the PC board is clear in that area).
[ ] Then, following the layout diagram, install all of the small resistors.
[ ] Install the diodes. There are three black (1N4007) diodes and two 5v zeners. Make sure that you get the polarity bands correctly installed.
[ ] Install the capacitors. Only the 4.7uF and the filter cap are polarized. Leave out C4, it is not needed.
[ ] Install the three LEDs. DC pwr is green, Soft Start is yellow, Fan=Fast is blue, Xmit=on is red.
[ ] Install the 3 relays. Add the rework jumpers for K1 if you intend to use it to short out the 100K bias resistor.
[ ] Install the 6 transistors. The Q5 and Q6 power transistors are placed so that the metal side is down. Q3 is placed so that the metal side is facing the power supply. (plastic side nearest the Qnumber in all cases)
[ ] Install the connectors. Note that the Temp Sensor 2pin white connector is offset toward the edge of the board unless a screw type connector is supplied.
[ ] Install the trim pot and any remaining parts other than C10 or R9.
[ ] The power supply is held in place by 4 plastic, press-in rivets. First, inspect the Power supply carefully and trim all leads on the back as short as possible. Then put the rivets through the holes in the corners of the Power Supply, then put a white plastic spacer on each. Press the rivets (and the Power supply) in place onto the main board, 24V input to the left.
[ ] Wire the 24V connector from the power supply to the main board. Route the wires through the small hole, and solder the wires to the rear of the board. Red positive. Blk ground.
[ ] Stop assembly here for now. C10, R9, and the power to the 24DC supply will be installed after testing.
If you have a 24V bench supply you can use that to power the board for testing. Otherwise, you can make a temporary power cable to plug the board’s supply into 120v(or 240v). Use the Red and Yellow wires of the three pin connector temporarily connected (and insulated) to a power cord.
The three control functions of the board use nearly identical circuits. In all three circuits an NPN power transistor is used to drive a relay by pulling one side of the relay to ground when the base of the transistor sees current from the pullup resistor. A 2N7000 FET drives the power transistor by holding the transistor’s base to ground until the gate of the FET goes above a couple of volts. Note that the voltage at the base (1) of the power transistor doesn’t change much, what matters is the current through the emitter junction. A feedback resistor provides just a little positive reinforcement to give the circuit a little hysteresis (snap action) to avoid any relay chatter (not needed on soft key). If a function doesn’t seem to be working correctly, check all components in that area for value and placement. If you have trouble, checking the voltage at the gate (2) of the FET will tell you the most about how the circuitry is working.
With 24V connected, you should be able to check the function of each of the three circuits:
The green LED should be steady on. The soft start LED should also be on. Each time power is applied the soft start LED should come on a half-second or so after the DC Pwr LED. (The power supply also takes a second to produce power so it will add a second to the delay) You should also hear Relay K2 click each time. In case of problems, you can check Test Point 5 with a voltmeter. It should be high when voltage is first applied and then drop quickly. If not, a backwards cap (C3) or nearby component is likely your problem.
With power applied, you should be able to short PTT to ground and turn on the red (Xmit=On) LED. Relay K1 should also close. If you measure the PTT input with a voltmeter it should read about 5V.
Plug in the temperature sensor to J5. Apply power and adjust RV1 temp adj trim pot in both directions until the blue LED (Fan=Fast) goes on and off. Adjust the pot until the LED just goes off. Hold the sensor in your hand (or against your forehead), the LED should eventually go on. Relay K3 should also click on. Now adjust the pot until the LED just goes out. It is now “calibrated” so that the fan will speed up if the temperature goes above about 100 deg F (38C). It will be a bit higher if you have the flu. If you have trouble, TP3 should read around 5V and TP4 should go up and down between 0 and a couple of volts with the adjustment of RV1.
As a separate test from the control board, temporarily wire your cooling fan in series with one of the large capacitors provided and a power cord. Plug it into 120V (not 240v) and observe the speed. If you want faster you can try a larger value or temporarily put two capacitors in parallel. Be careful, the caps can hold a large charge after power is removed (short the plug when unplugged). When you have a satisfactory speed (quiet with decent air flow) install the capacitor(s) into the control board. Keep the overall height of the board below the highest point of the 24V supply. Don’t forget to install the AC power connector for the 24V supply.
INSTALLATION IN THE CHASSIS
If all is good so far, take this opportunity to clean up the power wiring to the circuitry around the breakers so that the board fits comfortably, then you can mount the board in the chassis using 3/8” (9mm) spacers. Use at least 3. And double check using a straight edge that nothing protrudes below the bottom of the chassis.
[ ] The most tedious part of installation is probably connecting the power transformers to the green voltage-select connector. Take it slow, start with the easiest one and land one wire at a time. Use ferrules if you have them and the crimping tool, if not, tin the wires with a little solder before putting them into the screw connector J3. Only put one wire in each location. Refer to both the schematic and the wiring diagram. At least one of the transformer leads is destined to be short. Extend it with a similar color if possible and cover the connection with good heatshrink. Use at least 16awg wire for all of the power wiring. A good source of wire is to strip the jacket from old computer power cables. There are two places where 4 wires are spliced together. You can best accomplish this by removing about a 1/4” (6mm) of insulation in the middle length of two wires and then twisting one over the other and soldering. Cover the 4-wire junction with one or two layers of good heatshrink.
[ ] If you are replacing the transmit relay with a new 24V one (recommended) you should wire one side of the coil to +24V and the other to the ‘Xmit Relay’ terminal on J1. The rest of the relay is wired like the original except that only two poles are needed. The bias can be wired using the relay on the control board assuming the rework has been done. [To use the original relay, K1 can be used if ‘X’ is jumpered to ground and the original relay connected to ‘Bias’. The third set of contacts on the original relay then will switch a bias resistor.]
[ ] The Bias terminal on the control board (J1) goes to the center tap of the filament transformer. Pin 5 (X TRA) goes to the zener voltage on the Diode board (pad #E).
[ ] The power switch wiring simply goes from the two terminals on the power switch to J2 (AC power in) on the control board. This would be a good place to use 14awg wire if you have it.
[ ] Install R9 above the board and touching the aluminum chassis corner support. This will dissipate any excess heat and will do minimal damage if something fails and the resistor overheats.
[ ] The fan simply wires to the ‘Fan’ connector. The temperature sensor can be mounted where you like, but drilling a 7mm hole in the RF enclosure above the HV transformer and inserting the sensor through a grommet is a good location. (Keep the wires out of the RF area)
[ ] You can use the 24V to power LED lamps for the meters if desired. Probably putting the LEDs in series and inserting a suitable current limiting resistor.
[ ] As with any work on HV, be extra careful when applying power. Use a variac if available. It’s a good idea to disconnect the HV lead from the transformer (Red/Yel) to the Cap Bank and temporarily insulate it (well) while you test the power selection wiring.
Rework Instructions SB-220 Control Board V 2.1
This error was my mis-guided attempt to do something clever with the small relay K1. It didn’t work. The best use of the relay is to assist in setting the bias for the transmit tubes. If the filament center tap is connected to the 5v Zener reference through a 100K resistor, the tubes will find their own bias level when idle. In transmit one must simply short the 100K resistor (a method proposed by Richard AG6K and others). K1 does this nicely. Before you begin the rework install K1, R3, J1, and J6
[_] 1 On the back of the board, cut the trace going from one end of the 100K resistor to the upper left pin of K1.
[_] 2 Connect a jumper from pin 2 (Bias) of J1 to the lower right pin of K1.
[_] 3 Connect a jumper from pin 5 (X) of J1 to the right hand side of the 100K and on to the lower left pin of K1.
There was also an error (senior moment) in connecting the 120V for the fan to accomplish speed control:
[_] 4 On the component side of the board, cut the large trace going to pins 3&4 of J4. Cut this trace close to the pins of J4. Then install J4.
[_] 5 Add a jumper from pin 7 or 8 of J4 to pin 1 (the square one) of J6 to connect it where it should have gone.
SB-220 Control bd BOM
Sunday, December 1, 2019
Connectors to allow the front panel to be removed or extended during service. The original pads are still present and can be used if connectors to the front panel are not desired.
A fuse and a 5w resistor in series with the high voltage to minimize damage in the event of a tube failure.
A string of 1N5408 diodes for the voltage doubler (larger than the originals)
A filter cap across the high voltage output.
Diode protection for the plate meter.
A string of 2meg resistors to divide the high voltage for the voltmeter.
A limiting resistor to provide the current for LEDs on the panel meters if you decide to do that.