[Heathkit] SB-200 Problem

[km1h]

Id start by removing the Measures mods as they are voodo science. Its not 
easy to find 33-47 Ohm 2W carbons these days but Id rewind original spec 
parasitics.

Next check the 33 ohm resistors at the grid pins as well as the 200pf micas 
to ground. Those caps have a habit of blowing when junk mods are done.

Just hope you havent fried a tube.

Carl
KM1H


----- Original Message ----- 
From: "Donald Rowland" <HASDRUBAL at peoplepc.com>
To: <heathkit at mailman.qth.net>
Sent: Sunday, May 22, 2005 7:34 AM
Subject: [Heathkit] SB-200 Problem


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
_______________________________________________
List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **
$$ See the vintage area on the HCI web site - http://www.w9wze.org $$
Heathkit mailing list
Heathkit at mailman.qth.net
http://mailman.qth.net/mailman/listinfo/heathkit