AKAI GX-F31 Cassette Deck

 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.  

 

 

 

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. 

Line Out Not MUTED

This is hand written approximation, no simulation software used, although later the figures were verified by measurements. 

 

 23-06-2026

 

The content of this article

may be modified or corrected  without notice  

 

 

ITT 8025 Cassette Deck

 ITT 8025 Cassette Deck

Some pictures and commentary while I've been working on it for a future sale.

Cassette Transport

Rightmost is the back tension brake and 'new' felt pad.

Drive Belt: Out of sight is a 79mm x 5mm flat belt for the motor and two flywheels.

Lapped Record Head

Original Playback Head Lapped

Some New E-clips

 

There is a small issue with this transport, namely head assembly penetration, and a slight leftward misalignment of the record head. 

Observe that the record head is aligned slightly to the left, although it does not catch any cassette shell to date.

 

 
With record head removed.

With record, playback head, and head assembly plate removed.

Head plate, was later partially reamed to allow a slightly further drop or effective reduction in head penetration during PLAY.

Three bolts hold the returned head plate. The threads for the bolts are located underneath the head plate. The effective drop in head penetration during PLAY is now approximately 0.5mm less than previous.

All heads returned and re-aligned using a M-300 gauge (for tilt), a Hans Peter Roth 'narrow track' 1Khz head height tape, and an ABEX 10Khz full track reference were used for playback head alignment.

However, there was another yet-to-be-fixed issue. And that was - the record head was still too far left-of-centre, so the whole process was repeated above, but this time extra reaming to allow the plate to be shift a little more to the left. 

 

The result was to ensure that the small record head could penetrate (almost) dead-center into the cassette shell aperture.

 

Recording Failure

 

Much later a new issue surfaced - the ITT 8025 was only recording in the left channel.

 

Checking the bias signal at pin 1 of the HA-1457 pre-emphasis voltage amplifier revealed that one of the two HA-1457 amps had probably failed. 

 

During a recording session, pin 1 normally comprises of the (bias-trap attenuated) 85Khz bias signal and the audio. The overall display resembles Amplitude Modulation with the carrier 'amplitude modulated' with a test signal, although technically it is not. 

 

A high frequency test tone is best employed to observe the two on an oscilloscope since pre-emphasis ('record EQ') exaggerates the tones above 400Hz. However, an audio 10Khz test tone can only be seen if sufficiently large, but was found only in the left channel; the right channel was dead.

My assumption of a faulty HA1457 later turned out to be incorrect, however that didn't stop me from removing the old HA1457 ICs and replacing them with a TL071 OP Amp based solution with step-down voltage 1N4148 diodes in series.

 

Original HA 1457 Pre-emphasis Amplifier

HA 1457 Replaced with a TL071 Circuit Solution

 
It wasn't until I had ran the unit with the new pre-emphasis pre-amps that I realized the fault may have been caused by a faulty Dolby integrated circuit, namely the LM-1011 in the recording chain.

After interrogating input pins 5 and output pins 7 on each LM-1011, the right channel had clearly failed at the output on pin 7. Both ICs were carefully removed, and with new 16 pin DIL sockets in exchange, I was tempted to try the  Signetics NE545B Dolby chip.

After consulting the datasheets on the NE545B series, I was partially confident these would work. 

The supply voltage to the previous LM-1011 was 20v (exact), and the maximum rating of the NE545B is '24v' or is it 20v? So, just be to be 'on the safe side', I wired into the supply rail a single 1N4148 signal diode to drop the voltage by about 0.7v. This was done by cutting the track on the solder side of the board and positioning just one 1N4148 signal diode.

Two Dual In Line ('DIL') Sockets host the NE545B Dolby ICs.

 

Both left and right channels are now recording, and playing back well.

{More on this may follow} 

Tape Transport and Tape-Slowing 

Playing some cassette shells still resulted in the tape slowing down, which I initially thought was due to the record head being too left-sided, and overall head penetration too excessive?

Well, the above were indeed issues to be corrected if possible, but the nagging issue of some cassette tapes and their shells causing mischief for unrestricted transportation still proved a stumbling point.

That is, until I notice something in an earlier image I took of the deck. It became apparent that there was a conflict between the supply tape guide and the cassette shell - the two sides were making contact on some cassette shells, most noticeably on an old 1970s AGFA, and 1990s TDK D46.

I realized much earlier that manually lifting the cassette shell during PLAY resolved the issue, but I was still certain that the left-leaning record head and general head penetration were solely to blame. 

As a final attempt, I decided to check out the supply tape guide, its positioning and the baseline on which the shell sits. After loosening the baseline guides, I realized that these had probably slipped downwards over the years through casual insertions of the tapes. And so it proved to be, by resetting the baseline height the whole transport became fully cooperative!

No more tape dragging! 

   

Tape Creasing:

How much the tape dragging problem concealed tape creasing I'll never know, but the machine does crease tape on some cassette shells - it seems to begin on the supply side. 

The old supply pinch roller measured 9mm x 8mm x 1.5mm, but I only had a single 9mm x 7mm x 1.5mm in stock. This new roller worked, but 'end play' had to be reduced. End-play was reduced with suitable nylon washers. 

However the new roller did not eliminate any tape creasing. 

Tape creasing is sometimes caused by ...

(1) the tape being fed into the supply capstan/pinch side which is slightly uneven - either due to ineffective back tension, a poor cassette shell, or an out-of-parallel capstan/pinch roller line.

(2) on some occasions concurrently, it can also be invoked by insufficient supply capstan/pinch pressure, as the take-up side is perpetually pulling the tape along - the tape also 'micro' slips here?

(3) Head tilt which is not 90° to the plane of the traveling tape. There is obviously a margin of error here which would be acceptable. 

For this ITT 8025, increasing back tension will stop tape creasing, but on this machine back tension was (I believe) sufficient. Increasing capstan to pinch roller pressure also cured this problem, and to achieve this I had to configure the tension springs into a slightly more tensile state.

I first swapped the springs - I may have made a mistake earlier by swapping the, I cannot remember?

Secondly, the tension of each tension spring was increased a little more by placing a plastic sheath over the anchor point in each case as illustrated below.  

Finally, the playback head tilt was checked again and altered very slightly to favour an even tape wrap across the head.  An M-300 gauge is essential.

 

Tape creasing tests were performed on cassettes without a tape pad.

 

Reverting to Original Rollers

After some thinking, I later decided to return the original rollers - both supply and take-up rollers were in very good condition. Of course - no tape creasing, and tape flow is very stable.

 

 

Wow & Flutter:

Just a quick test: 0.07% ... 0.08% WRMS so far. 

Frequency Response: 

At -20dB, casual white noise 60 second recorded test - better than 30Hz to 17,000Hz using a TDK D46.

More may follow.

(20/04/2026)   

 

 

 

NEAL 102: Take-up Spool Torque

 NEAL 102: Take-up Spool Torque

 

 

The idler that drives the take-up spool's torque needed attention since the acting torque was either weak or intermittemt. 

 

Initially, the edge of the rubber idler was 'roughed up' and cleaned, and treated with Rubber Renue, but a few hours into PLAY, the same problem arose - torque was intermittent, and this could be heard on playback.

 

The solution was to either replace the idler, or remove it completely and re-work its surface or edge.

 

Flywheel

Audio Board & Removed Flywheel

Flywheel Removed, Idler Exposed

Idler Mechanism Removed

Seperated Components

Note the idler's form, which is unlike any idler normally made.

The rubber idler was first carefully sanded on its perimeter, cleaned, then soaked in Rubber Renue for about 5 minutes. After wiping down and allowing to dry, the idler was returned to its mechanism.

 

 

Out of sight and on the other side, there is a tension spring which needs to be returned, this provides the necessary force so that the idler presses between flywheel and take-up spool gear.

 

Idler Spring Returned.

JFET Measurements

JFET Measurements

 

As measured by the PEAK DCA75 Pro: 12mA current limitation. 

Note: JFETs do vary from their specification from sample to sample. On offer here is just a guide, which may be cross-referenced with official datasheets. 

(1) OnSemi: J112

Symmetrical Source/Drain

Vgs(off)=-3.66V at Id=5.2µA

Vgs(on)=-2.73V at Id=5.01mA

gfs=9.8mA/V at Id=3.0mA to 5.0mA

Idss>12.00mA at Vds=0.47V

Rds(on)=24.4Ω at Id=5.0mA and Vgs=0.0V

(2) OnSemi: J113

Symmetrical Source/Drain

Vgs(off)=-1.82V at Id=5.3µA

Vgs(on)=-1.00V at Id=5.00mA

gfs=11.0mA/V at Id=3.0mA to 5.0mA

Idss>12.00mA at Vds=0.82V

Rds(on)=34.3Ω at Id=5.0mA and Vgs=0.0V

(3) N-Channel: 2N5457 JFET

Symmetrical Source/Drain

Vgs(off)=-0.35V at Id=4.8µA

Vgs(on)=0.17V at Id=5.00mA

gfs=21.9mA/V at Id=3.0mA to 5.0mA

Idss=1.74mA at Vds=3.01V

Rds(on)=27.6Ω at Id=5.0mA and Vgs=0.0V

(4) N-Channel JFET J109

Symmetrical Source/Drain

Vgs(off)=-3.59V at Id=4.7µA

Vgs(on)=-3.01V at Id=5.00mA

gfs=26.9mA/V at Id=3.0mA to 5.0mA

Idss>12.00mA at Vds=0.09V

Rds(on)=4.4Ω at Id=5.0mA and Vgs=0.0V

 NEAL 103 Cassette Deck

Currently working on this machine, just posting images for now ...

Main Circuit Diagram:

This is the schematic for the NEAL MKII, I suspect it is very close or identical to the NEAL 103?

 

Transformer: 

Secondary voltages (under load) at ..

(1) Vyy ~ 41.2v rms,

(2) Vxx ~ 10.3v rms.

Induction Motor Armature Current: 

Ia ~ 133mA (ac), and unloaded motor (primary tapped) voltage is approximately 116.2v, RMS. 

Main Board:

Identifying where each circuit is not going to be easy - there are no markings at all on either side of the board.

All electrolytic capacitors were replaced, this solved a line in distortion problem. 

The Power Supply Unit (PSU) on this NEAL 103 appears to be near identical to schematic of the NEAL 102 MKII. The original 'axial' 1000uF main power supply capacitor has been temporarily replaced with a radial type, in this case I've used a 2200uF.

I have no formal service manual for this NEAL 103.

New Old Stock Pinch Roller Mechanism:

On both the NEAL 102, and 103 models they employ their own unique pinch roller mechanism and pinch roller. The tyre had worn and was in a poor state, although tapes could still be played.

The old arrangement was removed, and a protruding 'boss' was ground down to make room for a mech that was originally in a TEAC A-109.

So far, so good.

Induction Motor Inpection Images

 

The motor was about 2.5% to 3% slow which was surprising, so I opened the motor up for cleaning and lubrication. 

 

To increase the motor speed, theoretically another pulley will have to be made with a 2% to 3% percent increase in diameter. 

Alternatively, increasing the supply voltage, or even lessening (if possible) the air gap between the stator and the actual rotating rotor would bring about the increase in speed. All of which are difficult to realise to get the desired, but small increase in motor speed. 

Internal labelling states this machine can be switched to either 230v or 115v, I initially suspected that this machine was wound or configured for 240v? I now suspect that back in 1975/1976 this machine was configured for the 240v UK mains voltage, and so this deck's pulley was made to accommodate for that small (default) increase in supply voltage? The UK now runs on a main voltage of 230v/50Hz rms. 

Returning the motor back to the deck after cleaning, and the speed was now hovering around 1.5% slow. I may look into this later, but thoughts also do surface that each transformer manufactured will not be identical. Small differences in primary voltage tappings from one transformer to the next will statistically occur. 

Motor Circuit Now Fused

 

Motor is now fused; there's no requirement to do this other than to easily monitor ac induction motor current, which is steady at around 133mA.

 

 

 

This NEAL 103 report may be updated without notices. (30/12/2025)

Tandberg TCD-310 Cassette Deck

 TANDBERG TCD-310

I'm currently working on this circa 1975/1976 cassette deck. There so many minor issues with this deck - too many to write about.

However of note, the original pinch rollers were removed - they were effectively useless as their surfaces had become shiny and lacking in good traction abilities.

General areas on the component side of the audio boards have been cleaned, and most/all electrolytic capacitors will be replaced, and later most/all signal transistors.

Some images for now ...

 

 

Dust now removed from the motor

 

Motor pulley and cooling fan.

Record/Playback, & Erase Head Assembly: