AKAI GX-F31
Distortion of Output at Line Out and Headphone.
After playing the deck for a short while, distortion began to 'creep' into the left channel and also began to fade in volume.
The fault was difficult to pin point so as a basic starting base I re-capped the PSU - the Power Supply Unit, and later re-capped most of the area surrounding the Dolby IC. However, this made no difference.
On injecting a test tone of 1Khz at Line In, the muting JFET 2SK177 was good at the JFET's source side, but not at the JFET's drain side for the left channel - there was signal here for audio line out.
It was only 'by chance' I checked various DC voltages around the Dolby circuit and the sound mute area that I discovered that two electrolytic capacitors were in opposite polarity to that suggested by the voltages my meter was reading!? Not only opposite in polarity, but also the marking on the circuit board had been printed opposite to my readings!
These voltages across 'C114' (according to the service manual) where in fact marked as C97 on the actual board were approximately -6v. This is the correct voltage, but as stated above the original electrolytic capacitors were placed incorrectly.
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Over a period of time 'C114' must have begun to break down and the internal electrolyte deteriorated to such an extent that the capacitor was partially conducting?
In the mean time, prior to knowing what the fault was, both JFETs Tr36 (2SK117) were removed and the source and drain connections were shorted. This allowed the AKAI GX-F31's audio to work, but at the cost of no muting.
Once the fault was rationalized, and new caps were put into circuit at opposite polarity to that printed on the PCB at C97 and C97b, a search for a suitable JFET had begun.
In my stocks I have some 2SK30A, 2N5457, J113, and other N-channel JFETs.
For the muting to work, the Gate to Source Vgs(OFF) voltage will have to negatively high enough to invoke n-channel depletion, or a 'pinch off' state of the n-channel semiconductor.
A temporary three-pin platform or 3-pin 'mount' was soldered in so various JFETs could be tested without resorting to soldering and de-soldering on the main board. The 2N5457 tested fine, but so too did the J113. The latter is not normally used for audio applications, but it's job in this context would be to -
(a) act as a small resistance path during PLAY, REC/PLAY, or
(b) to act to impede signals (very high resistance) to line out during the depleted n-channel or Vgs(OFF) state.
Note: the original pin-out configuration from this view is: drain, gate, source, but the J113 pinout is drain, source, gate, so the leads had to be carefully bent to fit the configuration.
The J113 JFET solution works flawlessly.
Muting Circuit Analysis
This line out muting circuit is unusual, and rather complicated. I wondered why AKAI adopted this design?, perhaps it was 'just to be different' and so they (like others) could claim this to be another new design?
There are two switching voltage lines -
(a) At '-7.3v' or actually measured at -7.44v permanently, and
(b) The other switches between +26.2v (Mute=ON) and -3.2v (Mute=OFF).
These are represented by a single equivalent voltage source for simplicity. It is assumed their Thevenin equivalent impedance is low.
Line out MUTED
This occurs when the GX-F31 is not in PLAY or REC+PLAY modes.
The RL resistance is just there for completeness, it plays little part in the muting analysis.
The transistor's base-emitter junction when forward biased exhibits an approximate dc resistance of 1K ohm, perhaps more typically up to 1.5 K ohms.
Confusion must not be made with dynamic (small signal) resistance 1/hoe.
This is hand written approximation, no simulation software used, although later the figures were verified by measurements.
23-06-2026
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