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Author Topic: Heathkit SB-201 question  (Read 6000 times)
AE6RF
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« on: March 20, 2007, 02:55:37 PM »

Greetings,

I've got the new (to me) SB-201 all cabled up and everything seems to be working, but with one hitch.

The instructions say that the plate current should be roughly 90 mA when idling. The meter doesn't show _anything_ in plate setting when idling.

Is this something I should be worried about?

73 es thanks de Donald
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K0BG
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« Reply #1 on: March 20, 2007, 03:18:08 PM »

That should not be at idling, but keyed with no drive.

Alan, KØBG
www.k0bg.com
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K6AER
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« Reply #2 on: March 21, 2007, 08:03:45 PM »

As Alan mentioned above, the key line has to be grounded to bring the tube bias from -125 VDC to-4 VDC while engaging the T/R relay. When the tube bias is brought to the condition of AB1, the tubes are biased about 20 % on and in the case of a SB201 the 572B’s will draw about 90 mA. The RF drive completes the biasing of the tubes.

Depending on the age and condition of the tubes your plate current at 600 CW watts out (20 meters)  will be from 425-500 mills. This is of course dependent that the metering on the amplifier is calibrated.
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AE6RF
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« Reply #3 on: March 25, 2007, 08:17:07 AM »

OK,

Thanks guys. The amp is up and running. Works well.

Now the hitch is that the HV is only showing 1800V rather than 2400V.

The guy who had it before me noted that the meter was wrong and replaced the 4.7M series resistors. We're going to open it up on Monday and see what the real scoop is.

Thanks!

-Donald
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W9GB
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« Reply #4 on: March 25, 2007, 08:34:35 AM »

You may want to eventually change out the HV electrolytic capacitors.

Harbach Electronics
http://www.harbachelectronics.com/

The Heathkit Shop
http://www.theheathkitshop.com/sb200r.html

w9gb
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AE6RF
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« Reply #5 on: March 25, 2007, 09:16:07 AM »

"In theory" the entire amp was refurbished just prior to the previous owner.

I'm a newbie to amps (and any voltage greater than 120V) so took my friend's word at it.

We're going to open it up to check things out on Monday.

I can learn more about the amp, and appropriate HV safety precautions at the same time.

-Donald
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AE6RF
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« Reply #6 on: March 25, 2007, 09:18:19 AM »

P.S. I'm looking forward to my Amp Keyer...
hi hi hi hi

-Donald
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KA5N
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« Reply #7 on: March 25, 2007, 12:59:05 PM »

The 4.7 meg 2 watt carbon resistors will change value.  It is better to use 14 1 meg 1% resistors in series to give a total value of 14 megs(which can be mounted on a small piece of perf board (the actual total value calculates to be 13,714,286 Ohms the error
is less than what you get with the original resistors).  Harbach Electronics makes a replacement board that changes the electrolytics, bleeder/equalizing resistors meter resistors etc.  You can also get individual electrolytics and replace them.  I recently got a set of caps from Allelectronics (330 ufd 450 CDE) for less than $3 each (you need six).  The only problem with replacing the caps is that modern caps don't have the same type connections and snap-in types require small holes to be drilled in the board.  Another good move is to replace the 16 silicon rectifiers with 8 1N5408's (the 1n5408's are 1000 v prv @3 amps instead of the  600 v originals).  The Harbach board has all these things and all new resistors.
Good Luck Allen  
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AE6RF
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« Reply #8 on: March 29, 2007, 01:03:08 PM »

OK,

HV measures out someplace between 2200 and 2450 VDC depending on the probe/meter used.

That says the resistors are wrong. We're going to be replacing them with 5% carbon film resistors. We'll tweak the value so that the meter agrees more closely with the external instruments.

That board does look attractive. Next time something needs to be fixed in the amp I think I'll put one in.

73 de Donald
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