[Heathkit] SB-200 Problem

[Donald Rowland]

Recently acquired SB-200 was refurbished and some mods added.  Worked FB on 40 meters for about an hour and then would not load any more.  Readings when working:

Tune Control:  Top of 40 meter range
Load Control:  2.8
Drive:  90 watts
Output:  600 watts

Readings now (40 meters):

Tune Control:  Top of 40 meter range
Load Control:  Almost full CCW
Plate voltage, no load:  2390
Plate voltage, 250 ma. (as high as it will load, regardless of amount of drive applied)
Plate voltage, loaded to 250 ma.:  2280

Tune control produces barely discernable dip in plate current at resonance.  Grid current remains well within the white arc at all times.  Grid and plate current both respond to drive power up to mid-scale for the grid current and up to 250 ma. for plate current.

On 20 meters, results are similar, except that resonance occurs with the Tune Control well to the right of the 10 meter arc (off scale).

The only parts that have NOT been replaced are the entire power supply, which checked OK, all the RF chokes, the plate tune and load variable caps, and the doorknob and other HV caps in the final tank circuit, and the relay.

The Harbach softkey mod, the Richard Measures parasitic suppresssors, a 20-ohm, 25-watt glitch resistor in the plate voltage line, a 10-ohm, 3-watt resistor between lug 8 of bandswitch wafer and C16 (to reduce parasitics and excess drive), plus two diodes for meter protection, have all been installed.  Also, the filament voltage, which was excessive, has been dropped to the recommended 6.09 volts range.  Finally, the silver mica caps on the 10, 15 and 20 meter tuned input coils (one of which was the wrong value and the others missing entirely) were replaced.

Since the malfunction occurred, all the parts that can be checked have been checked and nothing can be found wrong.  The bandswitches have been checked for arcs and whatnot and are OK.

I am out of ideas.  Can anyone help???

Keith