[Heathkit] Question on SB-200

[Dennis Berry]

I have a few questions on the SB-200.  Perhaps some of you have run into these problems and can point me in the right direction?

I purchased this amp about a year ago, and had it on the 'to do' list.  I really didn't have the antenna system to handle the amp at the time, so it was put on back burner.

I've recently opened it up and dug into it a bit more.  I still have not applied power, as I wanted to go thru it electrically first.  It had additional modules in it, so I assumed they were the Harbach modules I've heard about.  A little web snooping and I discovered it has the Soft Key, Soft Start and power module installed.  The soft key was wired, except the 'blue' wire to the antenna jack was not installed, and the 33 ohm resistor was still on the antenna jack?  All of the other wires had been added for the softkey, so I finished the mod for that installation.

I checked the wiring on the soft start, and it appears to be wired correctly.  That all I could tell at this point.

When I got to the power supply board, it appears it had been toasted at one time.  The caps have been replaced, some obvious light burn or smoke marks, and the diodes were replaced with 1n4006, which I felt were too light and replaced with 1000 piv, 2 amp for the time being.  Also, the series of 1m resistors on the board were missing, and the old (3) 4.7 meg resistors were soldered to the board (for the metering circuit I think).

So my questions are, has anyone had a Harbach power supply board give them problems before?  Any ideas what would cause one to self distruct as it looks like this one did?

Any suggested checkout from this point on.  My intentions are to pull the tubes and check the HV reading and the voltage going to the soft key module from the filament supply.  Then I want to 'close' the relay with the soft key module using my multimeter and read the current rating going through it.    Lastly , check the filament voltage with the tubes out.  Will it make a difference if the tubes are in socket for this reading?  I understand it is good not to go over voltage on the high side with filament voltage in that it shortens the life of the tube?

Has anyone checked the input tuned circuits with a MFJ antenna tuner?  I was wondering if this was a good method to see if they were tuned for 50 ohms at the frequency in use or not?  Or, does the tubes need to be installed in order to get a correct reading?  (If they are, I assume one would not want to apply their MFJ to a live circuit?).

Any ideas on checkout procedure or why the board may have failed will be great information.  This is not a 'hot' project and I'm wanting to take my time and make sure the amp is in decent shape to put back on the air.  Overall it looks pretty clean, tuning caps do not appear to have arced, nothing looks fried.  So I'm cautiously hopeful it will not take much at all to get this old Heath ready to put back on the air when I need that extra 'punch'.

Thanks to anyone on the list that responds.  If you prefer, you can send directly to nu8s at qsl.net.

    Dennis Berry, NU8S