SB-220/221 CONTROL BOARD V03
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 It was later superseded by the functionally identical but prettier, HW-2200.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.
24VDC Power
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.
120/240 Selector
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.
Soft Key
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”).
Antenna Relay
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.
ASSEMBLY
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.
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:
Soft Start
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.
Soft Key
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.
Temperature Control
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.
Capacitor Selection
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
