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Author Topic: sb200  (Read 3722 times)
AE4WG
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Posts: 1




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« on: February 06, 2014, 02:03:23 PM »

 I recently bought a sb200 amplifier. And while I was setting it up I think I fried the tubes. The softstart board came loose and shorted to the inside of the cabinet.
 I have the softstart board removed and wired as a normal sb200 also I have the tubes removed and I get the following readings. The resistance from each anode clip to ground is abiut 180k, pin 3 of each tube socket is around 12k. The plate voltage on the front meter is about 2.4kv. The bias voltage on each tube socket is about -135v and when keyed goes to about -1.8v and the plate voltage still shows about 2.4kv. The only thing I see out if spec is the 2k relay resistor is reading about 1kohm..
 Is there anything I should check before I replace the tubes.
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W9GB
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Posts: 3380




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« Reply #1 on: February 06, 2014, 05:55:30 PM »

Heathkit SB-200 Assembly and Operation Manual
http://www.crompton.com/hamradio/heath/sb200/heathkit_sb200_manual.pdf

Page 47
===
SB-200 Schematic
http://www.w6ze.org/Heathkit/Heathkit_033_SB200.pdf

There are NUMEROUS Internet pages devoted to Heathkit SB-200 restorations
http://blog.kotarak.net/2008/03/sb-200-part-1.html

(and some bad hacks)
« Last Edit: February 06, 2014, 06:01:48 PM by W9GB » Logged
WA0SSV
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Posts: 12




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« Reply #2 on: February 06, 2014, 06:55:30 PM »

Your voltages look good, assuming the respective measuring resistors are not far off. If the plate current measuring resistor is ok, plate current in xmit with no rf drive from your transceiver, should give value of 80 to 90 milliamps, this is your idle current. At this same point in time, there should be 0 milliamps of grid current. If you start to give the amplifier some rf power, say 10 watts when in transmit mode, your grid current will start to increase and if so, tune your plate and load controls for maximum power output. Typical output, say on 40 meters, with roughly having 50 watts of drive, should give around 450 to 500 watts out. If you see numbers such as these, tubes are fine. Best to do this with a dummy load so you know  your load is good to do testing with.
73's
Larry
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WB1FFI
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Posts: 38




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« Reply #3 on: February 07, 2014, 05:57:26 AM »

Just a head's up. I installed the Harbach soft start on my SB 200 and was using a line voltage of 120 volts. The resistors in the kit were not the proper resistance and designed for 240 volts. So the resistors overheated. This was a few years ago and Harbach modified their website to notify hams that if line voltage is 120v to use different resistors. Not sure if that is what fried in your softstart. Good luck in your troubleshooting,
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N8CBX
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Posts: 565




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« Reply #4 on: February 07, 2014, 03:35:44 PM »

The softstart board came loose and shorted to the inside of the cabinet.
A silicone glue job, I guess? I mounted mine with metal standoffs & screws. Of course, one has to modify the board to do this. Poor PCB layout that forces the use of glue.
Jan N8CBX
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