t) Uneven Response Across the Band / Bandpass Filter
It has been observed that in some rigs the power output is uneven across the band, or
drops off at the high or low end of the band. This is not specific to the band in use and
assuming a proper alignment has been performed, this may be due to one or more of
the following:
1. The two Pre-selector/Driver caps are not tracking together. This can occur as a result of the
rubber “O” rings slipping. Ensure that they are both fully meshed and fully open at the same
time.
2. Confirm that the LMO output is constant across the band. The LMO output can be measured
using either an oscilloscope or a VTVM with an RF probe connected to the RCA jack on the
rear of the LMO. It is normal to see some difference between the high and low ends of the
range.
3. The frequency response of the 8395 KHz to 8895 KHz bandpass circuit is uneven.
t.1. 8395 to 8895 KHz Bandpass Filter
The frequency response of the 8395 KHz to 8895 KHz bandpass circuit and filter can
result in a variation in drive from the low end to the high end on all bands. The
frequency response of this circuitry is determined by the Bandpass Filter (T202). T202
is factory aligned and sealed and there is no procedure defined for adjustment should
the response change due to component variation or aging.
T202 is a traditional LC bandpass filter configured as a pi network. L2, C2 and C5 form
a series-resonant circuit that is tuned to the center of the pass band (8645 KHz) and the
two end sections form parallel resonant circuits to boost the low and high endpoints to
provide a broader response.
If alignment is found to be necessary, it can be done without removing T202 from the
radio by removing the top screw and unsoldering the two tabs that hold the case to the
circuit board. It is not necessary to unsolder the five signal leads. Removing the case
provides access to the three trimmer caps.
When reinstalling the case ensure that the inside with the tape is positioned over the
trimmer cap adjustment screws to prevent shorting the adjustment screws to the case.
NOTE: Heathkit Service Bulletin HW-101-56 discusses this problem and describes a
method to perform a sweep alignment of the Bandpass Filter T202. I have not found a
copy of this service bulletin that contains the figures it references.
Alignment Page 40
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