[Heath] SB-200 problems...again...

[Kenneth G. Gordon]

On 6 May 2016 at 7:46, Bill Bear wrote:

> Ken,
> 
> One of the first things I did after an initial visual inspection was replace C19
> ( 2uF) and C3 (20 uF).  They just looked too old and my experience with old
> electrolytics is that they are no longer up to specs.  So, I must have something
> else to investigate.

OK. Let me see if I can remember exactly what I ran into: for one thing, I 
began having that problem after I had installed a Harbach Soft-Key interface 
to help my more modern transceivers key the amp. 

It turned out that the high 60 Hz AC ripple in the DC to the grids was causing 
the Soft-Key to partially key on, increasing the bias voltage to below ("above" 
i.e. more negative)  -2 volts.

In order for the 572Bs to draw 90 mA of idle current, the grid bias must be  
-2VDC. If the voltage increases "above" that (becomes more negative), the 
idle current will drop, and the tubes will possibly be operating nearer a 
non-linear region of their operating curve.

Secondly, there is also the fact that in order to read that 90 mA, the metering 
circuits have to be working properly. That 90 mA shows at the very bottom of 
the scale and in fact is very hard to read there, and is not very accurate. 

What one really should do is to add another section or rewire a seldom-used 
section, of the meter switch to read idle current at a point at least half-scale 
on the meter.

Thirdly, idle current will also fall if plate voltage is not close to where it should 
be. I get 2150 VDC under load.

> After you changed C19, did you get a nominal idle plate current reading near 90
> ma?

Yes.

>  I am also wondering what resistance check results you had after the mods: 
> plate to gnd and tube pin 3 to gnd ?

I have not checked those, but will as soon as I can. 

My most-used SB-200 has recently come up with a problem very similar to 
yours in that both idle current has pretty much disappeared, and it is requiring 
a lot more drive to get the grid current and power output to be where it should 
be. 

Tubes are brand-new RF Parts jobs too. And this problem suddenly occurred. 
This indicates to me that some component in the grid circuit has suddenly 
failed.

Due to our youngest daughter's upcoming wedding, I have not been able to 
do much radio operating for the past several months, but hope to be able to 
get back to that after May 21. :-)

Therefore, I have not been able to dig into the SB-200

vy 73 for now,

Ken W7EKB