[Heath] Heath SB200 Input

[Kenneth G. Gordon]

On 16 May 2011 at 11:08, James A. Sutherin wrote:

>     I´m finally getting my SB200 on the air. I noticed when driving
>     it with a TS570D I had very little grid current,

There can be any of a number of reasons for this: among them 1) worn out 
finals, 2) low drive, 3) biasing off, 4) voltages low....and others.

> I suspect a
>     metering problem and will address this later, however I had an SWR
>     meter inserted between the 570 and the SB200 and it indicated a
>     high swr >4:1 on 80 meters and also on 40.

Yes. That is not too unusual. There have been several different methods of 
fixing that problem.

Look at this website first:

http://www.xs4all.nl/~pa0fri/Lineairs/SB200/sb200eng.htm

> I also noticed I could
>     not get much of a dip with the plate and loading controls. My HV
>     was about 1800v which I also suspect as the metering voltage
>     divider which I am addressing with a new rectifier / cap board
>     replacement.

I don't think the rectifier/cap board will address the issue of incorrect B+ 
voltage showing on the meter. For a proper solution, again see the link I 
provided above.

The resistors used to meter the HV are quite simply outside their VOLTAGE 
specification, and go bad in relatively quick order.

Your HV at no load SHOULD be right at 2400 VDC, dropping to around 2100 
VDC under load of about 500 mA.
 
>     My question is there any way to check the tuning of the input for
>     better drive?

An SWR meter between your driver and the amp is one way. My SB-200 
shows an SWR between the driver and the amp of no higher than about 
1.8:1 across any band. I even use it on 17 meters, with the bandswitch set to 
15 meters, and both drive and power output are within spec. But my input 
tuning circuits have been "tweaked" to achieve this.

The link I provided above will give you ONE way to correct the "high input 
SWR" problem. Electric Radio magazine has published a second, and a bit 
easier, way to do it.

The problem is that the component values used for the input Pi Nets are not 
quite right, or have drifted way out of speck, although they CAN sometimes 
be tweaked for better SWR between the driver and amp. However, in this 
case, the length and type of coax can effect the SWR too.

Using the method from the link above, the length of coax will no longer effect 
the SWR.

However, with an SWR of >4:1, your driver has probably folded back its 
output power.

If everything is correct, the SB-200 only needs between 60 and 80 watts of 
drive for full power output.

Also, be aware that when your 572Bs get worn out, and their emission drops 
way off, the final impedance changes enough so that it is difficult to load to 
full power output on the lower end of 80 meters. The loading cap will be all 
the way meshed, and power output will still be below spec.

If you look at the schematic, you will see that the bandswitch automatically 
adds a large mica cap in parallel with the loading cap on 80, at least, and 
maybe even on 40 (I have forgotten, for sure).

In my case, I had to add a bit more to that to enable me to load to full power 
output on the LOW end of 80.

Even with those added caps, when my finals wear out, the caps don't help 
much.

However, when your SB-200 has been restored and brought up to spec, with 
new finals, it is a very, very reliable and dependable amp.

I have two.

Ken W7EKB