I rebuilt my Heathkit SB-200 that I obtain from e-bay. This one was in bad shape but the price was right. It was dropped, bad tubes, and definitely been used. I slowly rebuilt the amp and got it fully functional. Initially I rebuilt the power supply using Harbach Electronics PM-200 Replacement Power Supply Module. Refer to their website.
I came across a web site, http://www.xs4all.nl/~pa0fri/Linairs/SB200/sb200eng.htm, which PA0FRI explains in detail on how to modify the sb200. I went ahead and modified my SB-200 according to his website and followed most of his recommendations. The following items are what I wanted to accomplish with these modifications:
- Tube Protection. Fuse the high voltage to reduce catastrophic failures.
- Soft Start to limit the primary in-rush of current and slow the rise of anode voltage. This protects the filaments and rectifier diodes and permits the use of a lower current primary fuse.
- Change amplifier key to support modern radios.
- Reduced filament voltage. On my SB-200, I had almost 7 Volts on the Filaments. I ended up using a loop of 18 gauge Teflon wire to drop the voltage until the filament voltage was in an acceptable range.
- ALC Adjustments is now easily adjusted. This is nice when I use it with my Drake transmitter which I can easily overdrive. The amplifier is very stable and is well protected.
- Faster QSK Operation. I wanted a faster transmit/receive transition, by updating the input and output relays did the trick.
These modifications are based upon PA0FRI’s modifications. I give him all the credit. I am impressed with the stability of the amp and have not had any issues. Please refer to his website: http://www.xs4all.nl/~pa0fri/Linairs/SB200/sb200eng.htm,
The modified SB-200 schematic diagram is shown below. This uses a “Key-All” circuit board for keying the amplifier. This board may still be available from Jackson Press. I also incorporated a “soft-start” from Harbach Electronics.
Input/antenna relay uses a reed relay for the input and a VAC relay for the output side. I had to had a 24V power supply for these relays since the original used 117V.
SB-200 Modified Amplifier Gallery: