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

[Kenneth G. Gordon]

OK. Today I removed both the plate current metering resistors, R-12, and R-14, and checked 
them with a calibrated ohm-meter. Both are reading within a very few percent of 
specifications.

I then carefully reinstalled them, but left the grounded end of R-12 un-connected, and 
connected a milliammeter between the free lead of R-12 and ground.

Activating the amp, and keying it on (with no drive) results in the milliammeter reading of 68.8 
mA, which while not the 90 mA that the manual says should appear, at least is closer than 
what I have been seeing. The meter, undoubtedly due to the added internal-resistance of the 
external milliammeter and resistance of the leads (0.01 ohms), read 500 mA, but I expected 
that and was not surprised.

Grounding the ungrounded end of R-12 through the clip lead (0.05 ohms) results in a reading 
at the meter of around 70 mA, which, given the imprecision of the meter scale at that low 
level, is certainly acceptably equivalent.

At this point, it is looking as though I have determined the source of several problems, have 
discovered the real problems, and have solutions.

1) In order to bring the panel-meter reading to equivalence with the external milliameter, I am 
going to shunt R14 (3600 ohms 1%) with a high-value trim-pot, and will adjust this until the 
panel meter reading corresponds with that of the external milliammeter.

2) I have been told that the "modern" Chinese-made 572Bs require LESS than -2 VDC grid 
bias in order to make them achieve 90 mA (for the pair) of idle current. At this point, I am not 
sure exactly what to do about it. I am thinking that even at ~70 mA idle current, instead of 90 
mA, the difference is not worth worrying about, but only a test with a proper spectrum 
analyzer would tell for sure.

3) The Harbach soft-switch I am using is clearly defective, and I am going to have to either fix 
that, or work with Harbach to replace it.

At this point, I am simply going to adjust the plate current meter reading to correct it, and will 
leave the rest alone until I complete the conversion to full-QSK which I am planning. At that 
time, the grid biasing circuit will be completely different from the Heathkit original, being set, 
instead, by Zener diodes in the cathode circuit.

Thanks for all your help and vy 73,

Ken W7EKB