The Panasonic RS-279US
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(The RS-279US has been repaired, but this report is currently a 'work in progress')
9/07/2025
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Playback Pre-amp Inadequate Bias Issues
It wasn't until much later did I realise that the new OnSemi KSC1845-FTA 'front-end' transistors that I used may have caused an issue, and that was - poor low frequency response, and low frequency distortion in playback mode only. Of course the problem could have been there prior to replacing the old transistors?
How can this happen?
Observing the 'shape' of the low frequency test signals suggested that non-linearity was responsible for high levels of distortion especially in the low frequency region between 30Hz and about 50Hz, allthough it was noted up to about 100Hz.
Since the playback pre-amp is configured as a de-emphasis, a post-emphasis, or 'PB EQ' circuit, there is effectively a large 'bass boost' present, and at such low frequencies the gain of the amplifier is at its highest.
This may have 'stressed' the PN or the Vbe junction of the first stage signal transistors Tr1 and Tr2 into a non-linear region of operation, even through this amplifier is in a voltage-series feedback configuration, this non-linearity could not be avoided.
The solution was to increase forward biasing of both Tr1 and Tr2, and this was achieved by (a) lowering emitter resistors R11 & R12 and, (b) lowering both biasing resistors R5 and R6.
Decreasing R11 & R12 to 3.3KΩ from 8.2KΩ would indeed also increase the open-loop gain of the amplifier, and increase Tr1/Tr2 collector/emitter bias currents.
The collector bias or quiescent current of Tr1 & Tr2 prior to this issue wasn't measured; indeed it wasn't initially suspected. However, post-modification raised the collector quiescent current Ic to 3.27v/33KΩ (3.27volts across RL of 33k) or just above 99µA.
Also at this point, stage 1 Vbe of Tr1 and Tr3 ~ 0.56v, and stage 2 Vbe Tr2 & Tr4 ~ 0.62v which is about right for these low current, high current gain transistors.
The issue of low frequency distortion, with severe non-linearity, and poor playback low frequency response was resolved.
Of note, a 'test tape' made on a Sony TC-177SD with a frequency-modulated back and forth sweep from 30Hz to 500Hz was later played back in the Panasonic RS-279US deck. Playback on the Panasonic was well within 2dB.
(Subject to alterations without notice)
