>I opened up my SB-220 the other day for the first time (to replace the
>diode board so I could see my real HV) and discovered something odd. It
>appears to have the Measures mods to the parasitic suppressors in the
>anode circuitry (2-turn coils, paralleled resistors, and the 2
>unequal-length wires from the plate choke to the blocking cap) but there
>are also what seem to be high-voltage resistors in series with the Measures
>suppressors, and between them and the plate RF choke. They are pretty
>discolored, but the color code seems to be 10 ohms.
We do not use color-coded resistors in the anode circuit, Pete. We use
paralleled 1 ohm MF resistors as an anode fuse (Ra). We also use 2,
paralled 100 ohm MOF resistors for the anode suppressor resistor (Rs)..
These resistors are numerically marked with black ink on a blue ceramic
paint background. In the presence of RF, the blue paint normally changes
to mottled brown and light blue. The electrical condition of these
resistors should not be evaluated visually because they can look
terrible, and yet be within tolerance.
>Anyone have any idea what this mod is, or what it's for?
The purpose of the resistance-wire suppressor modifications is to reduce
the VHF-Rp at the SB-220's anode resonance (110MHz). This reduces the
VHF V-gain of the 3-500Zs.
(QST, 10/88, 9/90, 10/90). However, there are naysayers who say quite
the opposite.
- Low VHF-Rp suppressors reduce VHF gain, so they are bound to have loss
at 28MHz. Typically, the added loss is roughly 1%.
>Could it be
>responsible for my low output on 10m.(ca 750 watts)?
>
Normal full-throttle output from a SB-220 on CW@10m should be 850w to
900w.
- A quick way to evaluate your problem on 10m is to first see if there is
a input swr problem. Switch the SB-220 to CW. Apply full drive and
tune/load for max. Pout. The plate/anode I should be at least 0.7A. If
the anode I is less than 0.7A, check the input swr. If the swr is more
than 1.2 to 1, a solid-state exciter may be having trouble delivering
full drive to the 3-500Z cathodes. If this is the case, the Q of the10m
tuned input probably needs to be increased. (QST, 11/90, 12/90) If the
swr is ok and the output is still low, check the tune C to see if there
are two peaks on 10m. The first peak should be at approx. 2:30 o'clock
and the second peak should be around 3:30 o'clock. If only one peak is
observed at 3:00 o'clock, odds are that intermittent VHF parasitic arcing
has evaporated the 10m contact on the output bandswitch wafer. However,
if everything seems ok except the low wattmeter reading, check the
resistance of the dummy load with a DMM. If the load is other than 50
ohms, wattmeter readings will be affected. I once saw an amplifier that
delivered an apparent efficiency of115% due to wattmeter error.
. . Rich---
R. L. Measures, 805-386-3734, AG6K
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