Sunday, December 1, 2019

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!

Notes on the SB-220 HV board by AG6YJ
The combination of being retired and being a ham leads to a lot of tinkering. And as the owner of a Heathkit SB-220 which for some reason was aging more rapidly than myself, I set out to do a rebuild. The internet is full of advice, some good, some bad, and some absurd, so I selected a reasonable subset of information and did a respectable job of rebuilding it and getting it back on the air.

But along the way the tinkering spirit led me to lay out a couple of PCBs to clean up some of my work. So when the boards eventually came in, the chassis went back on the bench and went through another revision. And like so often happens, I realized that the boards could be a little better and set out to design another set.

Well, when you order printed circuit boards, you don’t get just one. Generally I get several dozen so if I am lucky enough to have a useful design, I can have spares to give a few to friends.

In any case, this time I elected to sell a few on eBay to further support my otherwise frivolous tinkering. There is no intent here to compete with the other boards (W7RY and others), they do a good job and provide a reliable source for much needed replacement parts.

I did use this board to upgrade 3 different amps.  Two SB-220s and one HL-2200.  Of course along the way I did a lot of other upgrading.
This is the dis-assembled version of the HL-2200.  One of the 220s is behind it.  I decided to re-do the capacitor bank as well.  Boards are on the way.  I also added a fan speed control to the 220.  Used a controller from ebay.  It works fine but is a little kludgey...

The HV diode board:  (this is from the kit that I put on Ebay)
For the most part, the diode rectifier board is a drop in replacement for the original board in the HeathKit SB-220/221.
However, it has a number of additional features not included on the original.

  • 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 1N4007 diodes to replace the original Zener, including capacitors to filter the voltage.
  • 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.
To use the connectors to allow the front panel to be removed, you must add a few new wires to the board. The Relative Power wire must first go to the board so that it can go through the connector to the front panel. Similarly, you also have to route the lamp voltage through the board. Being able to remove and protect the front panel is worth the extra effort.

If you elect to replace the original #47 lamps in the meters with the provided Led arrays you will have to come up with the appropriate dc voltage. The arrays are a string of Leds totaling nearly 12v, so to operate from the original filament voltage of 5vac you will need some sort of voltage multiplier.

One method is to build a voltage quadrupler (about 28vdc). A 150-200 ohm resistor on the board is about right in this case if the two arrays are placed in series.  Use 200uf to 500uf 16V caps and 1N4004 diodes or better.
It is fairly easy to put this little circuit on the front panel using a terminal strip.