SB-200/1 & HL-2200 Replacement Capacitor Bank v02
First… This amplifier has potentially lethal voltages present. Regardless of your background, if you are not afraid of high voltage you should not be doing this. Get help from someone who knows what they are doing and is afraid.
Heath engineers originally designed the amplifier’s capacitor bank with what was available at the time. It was a good design, and the voltage doubler approach has been the standard in high voltage supplies for years. However the engineers were faced with using 8, metal cased electrolytics that posed a significant shock hazard, as well as being difficult to put in kit form. Their solution to put them in the plastic sandwich enclosure worked but also restricted ventilation. (electrolytic capacitors maintain their rating and last longer if kept cooler). Those old can electrolytic caps also varied widely in value, especially over time.
I received the new boards for the capacitor bank, and finished the design for the sheetmetal that holds it in place. I had to spend nearly a thousand bucks at a sheet metal fab house but I have lots now. Will hopefully get that back by selling them through ebay. It will put some new life into a bunch of aging amps. Wish I could install one in myself.
The board came out nice. It sits sideways from the old cap bank. (that is why the new sheet metal) Today we have capacitors available that are not only more stable, they are smaller, and at least to some degree, individually insulated so the plastic insulators were a waste. The original caps were 200uF so 8 in series created a filter capacitor with a value of 25uF. This new board can support caps in the 200-600uF range giving a filter value up to 75uF. This doesn’t increase the total power available, but does significantly increase the instantaneous power available, and of course improves filtering.
The value of the ‘bleeder resistors’ selected by HeathKit was low (30K) to quickly bleed off the charge in the interest of safety. However it did result in a great deal of unwanted heat which was waste of power and shortens the life of the capacitors. Most upgrade projects today change the resistors to around 100K and rely on good judgment to stay away from the high voltage until it has bled down. Using larger value caps makes the selection of bleeder resistors more interesting. The larger caps mean that the high voltage will be present for as much as three times as long! The included resistors were selected to optimize the power supply and reduce heat. (they also serve to equalize the voltages across the capacitors but a higher value still serves this purpose on the newer caps) If you are concerned that a more rapid bleed down is necessary for safety, feel free to add lower value resistors (or put similar values in parallel). In any event, wait until the meter reads zero, and then another 5 minutes before accessing the chassis (meters are known to fail). Even then, ground the HV before touching anything!