SB-303/SB-301

[Kechkaylo Dave (ATLINKS)]

To All:

I really enjoyed reading about Glen Zook's experiences with the Collins &
Heathkit receivers. He did mention that the order of performance preferences
listed the SB-301 over that of the SB-300. I wonder: with the expection of a
few added nice conveniences such as front panel selection of VHF outboard
downconverters, WWV, an ANL, and RTTY receive on the SB-301; otherwise, the
circuitry of the SB-300 is almost identicle to that of the SB-301 (RF amps,
1st & 2nd converters, IF strip, BFO, etc.). So why would the performance of
the SB-301 be any better than the SB-300?

The first production run of the SB-300 had some front end instability on a
few of the lower bands and preselector tracking problems. This was solved
(quickly) in the later runs by the addition of a parallel resistor in the RF
amp grid & RF plate coils to lower the resonant circuit "Q", and a
modification of the screen voltage in the RF amp. These engineering
modifications first incorporated in the SB-300 were carried over to the
SB-301.

The SB-303 used those Heath No. 417-240 (40673 variety) dual-gate MOSFETS:
one as an RF amplifier, and two more as a first and second mixer. These
MOSFETs overloaded easily, especially in the RF amplifier as it doesn't see
much front end filtering (mainly a single-tuned resonant circuit in the gate
and drain circuits. This is why Heath added an RF input attenuator, but the
SB-303 had great sensitivity. I remember the SB-303 IF strip being very
"hot" and would oscillate if aligned just so.

Naturally having used and serviced both receivers while employed at Heath, I
really like my SB-300 over the SB-303 in terms of its lower noise floor and
better strong signal handling capability (without having to use the input RF
attenuator).

Dave Kechkaylo, W8QIZ

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