Kenwood KT-7500 Tuner

 The Kenwood KT-7500
Stereo FM/AM Tuner

I recently bought this to add to my interest in HiFi receivers and tuners.

The Kenwood KT-7500 is a beautifully engineered and finished piece of HiFi with a gorgeous fascia and controls, and of course high-end performance to match.

This particular version of the KT-7500 is slightly unusual in that there are two switchable de-emphasis configurations to choose from: 25µs and 75µs time constants; symbolically represented by 𝛕. If a tuner is going to give you a choice, then I would have expected both 50µs and 75µs time constants, not 25µs and 75µs!?

The time constant parameter 𝛕, is an accepted convention when describing 'corner frequencies' in the playback of a first-order, low-pass filter frequency response. Each time constant 𝛕 is directly linked to a corner frequency fc.

Here in the UK, the playback de-emphasis curve adheres to a time constant of 50µs, that is 𝛕 = 50/1000000 seconds, whereas, in the United States and Canada, they employ the 75µs time constant.

Therefore, it is quite possible that this KT-7500 was purchased in the US or Canada back in the late 1970s? I say this since 25µs was intended to be used for Dolby encoded broadcasts in the United States, possibly Canada too? For this to work correctly, a Dolby outboard would be required?

An explanation of de-emphasis, and the associated time constant will be explained at, or near the end of this article.

The KT-7500 in its Wooden Housing:

Opening up the KT-7500 reveals a beautifully laid out circuit.

Highlighted above in white is the final low-pass de-emphasis circuit that needs to be modified so that both 50µs and 75µs time constants are easily accessed, at present I've only 25µs (for a Dolby outboard?) or 75µs (United States) to choose from.

Observe below, a picture taken for the original advert by the seller, and the limited choice: 75µs (US or Canada) or 25µs (Dolby encoding) de-emphasis setting.

Identifying De-emphasis Stages

Studying the schematic for the KT-7500, we are able to track down the output stages, and identify the de-emphasis low-pass circuit.

Both left and right channels have been highlighted for convenience: right in red, and left in blue.

Studying the circuit diagram reveals that a NJM4558 dual OP Amp has been used in the final stages of audio post-processing; that is FM de-emphasis.

The OP Amp appears to be configured in a non-inverting, first-order low-pass state. Similar to that shown below -

The derivation for the voltage gain for this non-inverting amplifier can be shown to be: 

 Av(ω) = 1 + Zf(ω)/R

Where Zf(ω) is the impedance of the parallel network containing the 33kΩ resistor and the 750pF (and, or the 1500pF) capacitance. The R is the 2kΩ resistance as shown above.

This voltage amplifier's gain is frequency dependent, but can easily act as a voltage follower if Zf→0Ω; in that case the gain is simply unity, ie 1.

However, returning to our low pass de-emphasis amplifier and filter - in terms of the resistive and capacitive components, the complex form of the voltage gain of the OP Amp in this non-inverting arrangement is:

 

Where Rf = 33000Ω, C = 750pF or 2250pF, and R1 = 2kΩ.

Note: In the KT-7500, C is actually switchable between 750pF and (750+1500)pF. 

Also, be mindful that periodic frequency f (Hz) is related to angular frequency ω=2𝝿f (radians/second), or f=2𝝿/ω.

In the above 'complex' form, both voltage gain and input vs output phase shift can be analyzed.

In an attempt to keep this write up as simple as possible, the mathematical issues of derivation of formulae are going to be by-passed.

Computing OP AMP Gain Av(f) vs Frequency (Hz)

Applying a complex conjugate operator to the above expression, and we have ...

And so, the modulus or absolute value of the gain is ...

Mapped out, it will look like this ...

Frequency Response Curves
for CR value of: C x R = 50µs

If my calculations are correct, the plot in red would be the theoretical curve if the OP Amp was in an inverting configuration, and in blue is the theoretical plot for our non-inverting KT-7500.

Corner Frequency and Time Constant

The corner frequency is that frequency f, for which 

2𝝿fCR = 1, 

or

f = 1/{2𝝿CR} Hz.  

The gain then becomes .....

Under these conditions, and without the '1' term, the gain is (Rf/R1)×(1/√2)⁢ or -3dB below its maximum value. However, because the gain for this OP Amp configuraton is Av = 1 + Zf/R, the '1' offsets this calculation a little.

The time constant 𝛕 mentioned earlier is generally derived from RC networks and their ability to charge or discharge, and in particular - the initial rate at which charging/discharging occurs on response to a voltage step function.

𝛕 definition: If this initial rate of charge were to be maintained, then the time taken for the capacitor to become fully charged or discharged is equal to: C x R seconds.

That is ...

𝛕=CR

The reader may have already deduced that frequency f and 𝛕 are related, that is, at the corner frequency fc: fc = 1/{2𝝿𝛕}.

UK FM De-emphasis

Here in the UK, the 50µs de-emphasis time constant 𝛕 is employed, it is directly related to a de-emphasis corner frequency.

fc = 1/{2𝝿ᐧ50µs} = 1000000/{2𝝿ᐧ50} = 3183Hz.

In the context of the KT-7500, and to obtain 𝛕=50µs, C needs to be 1500pF, neglecting component tolerances, the actual values of CR used above gives ...

 𝛕=1500pF*33KΩ = 49.5µs

or 

fc = 1000000/{2𝝿ᐧ49.5} = 3215Hz.

Now examine the above plot to confirm the -3dB drop at approximately 3183Hz. This equates to (1/√2)*17.5 at 3183Hz.

United States/Canadian De-emphasis

In the US and Canada, 𝛕=75µs, that is ...

fc = 1000000/{2𝝿ᐧ75} = 2122Hz.

Turning our attention to the Kenwood KT-7500, the slide switch is turned to '75us' and so we obtain C=(750pF+1500pF)=2250pF, which leads to  𝛕=2250pF*33KΩ = 74.25µs. Which in practical terms yields: fc = 2143Hz. Again, we neglect small variations in component values.

Concerning Dolby de-coding de-emphasis, it is left to the reader to compute fc from 𝛕=25µs.

Re-configuring the Kenwood KT-7500 De-emphasis

Returning to the problem of incorrect de-emphasis for this KT-7500, and inspecting the values of the polystyrene (close tolerance) capacitors suggests that swapping both sets of capacitors will provide the correct playback equalization that I require.

Note: JRC4558 dual OP Amp used in this Kenwood.

Previously, playback from FM transmissions sounded as if the treble control had been turned down by a few dB. All good now!

14/09/2023: This article may be subject to minor corrections, and additions.

AKAI AA-1030 Receiver

 The AKAI AA-1030 Stereo Receiver

Back in the late 1970s I drooled over many HiFI seperates, knowing that I could never afford many of them. However, sometime in 1978 I bought a Rotel RX-402 receiver for £128 from COMET.

More recently I've been looking into buying and reviving some old classic receivers, and in particular the AKAI AA-1030.

This is a heavy 13Kg, 30 watts per channel stereo amplifier, and tuner. Although 'HiFi Choice Receivers' review back in the late 1970s found that this AKAI was able to deliver around 47 watts per channel into an 8Ω load.

There were other AKAI receivers for sale on ebay, other similar models were: AA-1050, AA-1135, and the AA-1150; eventually I took the risk and chose the AA-1030.

As always with purchasing second hand gear, there are risks, so one can be certain that both repairs and modifications will have to be made.

Original Advert

Opening Up the AA-1030

Dust had settle mostly on the power amplifier side where the venting was situated, there was also signs of rusting.

AKAI AA-1030 Class A-B Amplifier Board,
this shot was taken after some initial cleaning.

Main Amplifier - at top left,
Main 6800uF electrolytic capacitors (for amplifier) - bottom left,
Power Supply & Regulator Unit: right side.
After initial dusting.

After cleaing the face plate, and repairing the broken antenna switch - top right.

Initial Repairs

• Clean circuit boards throughout - partially completed.
• Switch clean all potentiometers, switches, and contacts.
• Broken antenna attentuator switch (fixed immediately) 

Electrolytic Capacitors:

• Replace all power supply (PSU) electrolytic  capacitors.*
• Replace all power amplifier electrolytic capacitors.*

Faceplate:

• Brushed aluminium face plate requires some very careful local 'sanding' and cleaning with warm soapy water.  

Audio:

• Loud output at headphone socket, with excessive noise and hum. (fixed, see below)
  
• Random low-level burst noise in right channel, sounds like discharging. (fixed, see below)
  
• Inspect main Class A-B amplifier and set biasing currents. (done, followed service manual advice, but the SM is a little abiguous!?)
  
• New lamps needed for tuner display. (replaced with DC powered LEDs, may alter the arrangement later?)

Radio Section:

• Inspect FM Tuner section, and the Front End, may recalibrate at a later date?

Loudspeaker Protection Circuit

During the process of switching on, and off, both headphones and the loudspeakers can experience a transient surge of charge. In most cases this doesn't present any problems but can be unpleasant to listen to. In time this could cause damage to a loudspeaker or connected headphones, which is why 'modern' amplifiers incorporate time-delay relay and circuit which disengage the Class A-B 'push-pull' output stages from both loudspeaker and headphones when the amplifier is switched on or off.

Unfortunately, the AKAI AA-1030 does not facilitate this kind of protection. This was not uncommon back in the early to mid 1970s.

Capacitors*

There was no obvious call to do this, but as the amplifier is very old, why not?

For now, I've replaced the 6800uF units
with KEMET 10,000uF capacitors
that serve the main Class A-B amplifier.
I will return new 6800µF units very soon to
the circuit.

 

I actually replace them again later with 2 x 6800uF SamWha. Just look at how small the new designs are!

The electrolytic capacitors for the PSU were also replaced. I don't believe there is any need to populate non-audio paths with expensive 'audio capacitors', so I didn't.

Random Burst Noises in Right Channel

A random burst-like noise was picked up in the right channel. I suspected that this may be coming from the tone control board which serves the volume, bass, and treble controls?

There are four 2SC1222 NPN transistors here, and all were replaced with KSC1845FTA types. This completely resolved the problem.

A warning here: the pin out configuration of the 2SC1222 and the KSC1845FTA were identical, and not different as suggested in a data sheet for the 2SC1222. Since I followed the advice of the C1222 datasheet my wiring had put the base and collector the wrong way around - all volume/bass/treble controls did not work. It was only then I suspected that the old 2SC1222 datasheet was incorrect?

On-semi data for the KSC1845-xxx

I was initially tempted to employ KSC1815-GR NPN transistors, but noting the circuit diagram above, there is a suggestion that the base-emitter voltages should be around 0.6v, and not higher at around 0.7v. 

The KSC1845 probably biases slightly 'earlier' than the KSC1815. 

Regarding transistor current handling capacity, there is no problem with excessively high quiescent collector or emitter currents during standby.

Observing the circuit diagram above suggests that the quiescent current through R5 will be approximately (12.9v-6.3v)/1500Ω, or 4.4mA.

In theory, if during high level audio stimulation, TR1 is in its 'saturation' region (unlikely), then VCE ~ 0.2v, the current in R5 will be approximately (30v-0.2v)/(3900+1500) or 5.5mA. This transient case is well within the limitations of the KSC1845 at 50mA.

Headphones Issue

Since 1976/1977, it appears that one or more individuals have been inside this unit as it had both a name and date written in; it was dated as '29-12-89'.

At some point in time, the output of the headphone amplifier circuit had been modified, I became aware of this to my cost, the output was far too high. Infact I blew the right-channel internal diaphragm of the headphones I was using during either switch on, or switch off, although I suspect it was during the 'off' state?

So, what was happening?

Examining the output of the Class A-B push-pull amplifier, and how the headphones are integrated into the circuit will explain.

Looking solely at the headphone circuit (marked in red), we see at the headphone socket a series 330Ω, 2 watt rated resistor. With or without the louspeakers switched in, the 330Ω resistance is there in circuit to lower the audio voltage across the headphone load, all thanks to potential divider action. 

Typically, back in the 1970s, 8Ω to 16Ω 'impedance' headphones were not uncommon. The headphones I used for monitoring the audio here were the AKG K-55, which I think are approximately 33Ω in impedance. However, this was not the reason why the headphones were ruined. (Note: the AKG K-55 phones were later repaired with new drivers).

During the last 'service', a very low valued resistor was soldered in parallel with AKAI's own 330Ω resistance. The resistor markings are difficult to read, but an ohmmeter suggsted that the overall resistance of this parallel combination was less than 3Ω, yes, that's 'three' ohms!?

This explains why the volume and noise were so high, and why a headphone driver blew when the amplifier was 'powered down' after a few times.

During 'power down', the large electrolytic capacitors will discharge through the amplifier and a momentary imbalance will occur between the complementary push-pull NPN/PNP pair giving rise to a voltage 'blip' at the output. As there's no effective potential divider in the output stage, the headphones absorbed the effects of a high voltage discharge surge.

I later removed this idiotic 'modification' and inserted a 680Ω 3W resistor on its own. All is now good, no noise, no hum, and no damage to any headphones!

The original 330Ω specification is electrically more suited to lower impedance (8Ω-16Ω) headphones.

Facia and Lighting

As can be seen the unit is now clean, but not devoid of scratches. Careful fine grit sanding has been applied in some areas, but care must be taken not to 'ruin' the facia completely by incorrect application or any application.

New LED Lighting (22/08/2023)

Three white LEDs are now affixed into the original filament lamp holders, and now powered by a DC source. The DC source being tapped off the original 15v AC (rms) primary transformer windings, then fed to a simple full wave rectifier, and a 680Ω 2W resistor is placed in series to act as a voltage dropper.

The 15v AC primary winding terminal point.
This was the original distribution point where
all three lamps received their 15v AC voltage supply.

DC LED Display Circuit
Each LED is running at about 11mA,
which is sufficiently bright.

 

Shown above is a fused (200mA) AC to DC full-wave rectifier for the excitation of the three display LEDs. Although provisional, it is likely I will keep this circuit, and later make the arrangement more electrically secure.

The AA-1030 is working well - both AM/FM tuner, and amplifier.

Re-capping:

Tuner: AM/FM section .... completed.

Phono Pre-amplifier: not completed.

Tone Controller Board: not completed.

Main Power Amplifier: partially done, incomplete.

It is worth mentioning that there are no component reference marks on the circuit boards, so precautions must be taken when re-capping or replacing transistors.

***********************************

To be updated and probably corrected without notice.

29/08/2023.

The NEAL 102 Cassette Deck

The NEAL 102 Cassette Deck

I bought this on 26th December 2022 and received it before the New Year. It was advertised as ‘faulty’ and indeed upon receipt of it, it was!

Original Advert

 

On inspection the deck was wired to the 13 amp plug without the earth connected, and what appeared also to be a blown fuse.

The first task was to establish why the fuse had blown which lead me to initially de-coupled the audio board from the power supply. All that now remained was to investigate the integrity of the transformer, fuses, and the 115v induction motor.

Power Supply & Induction Motor

Disassembling the motor it became obvious that the motor rotor had seized, probably as a result of dried grease and oil? Freeing the rotor wasn’t difficult and so this and the rest of the motor was taken apart.

Neither the 500mA fuse from the primary winding circuit, or the audio board fuse had blown, so I could assume no short-circuit had taken place. Not sure why the mains fuse had blown though!?

Later, with the audio board isolated from the power supply, the motor and transformer were 'checked over' to confirm all was well.

Understand too, that at this time I removed the large flywheel, cleaned and lubricated it. The idler gear (in the middle) was also cleaned with denatured alcohol and later with Rubber Renue. All other moving parts were cleaned with either denatured alcohol or Servisol Super 10 (or equivalent) switch cleaner.

Motor field windings, and Rotor.

Rotor in Isolation

 

The motor current on driving the idler gear and large flywheel was measured at 138mA AC (rms). This is a figure I would expect; anywhere between 75mA to 150mA would not be unreasonable.
 

The NEAL 102 came with the standard original user manual, various other literature and a ‘Service Instruction’ manual. The latter appears to have been a temporary document until the official Service Manual was printed? I can only assume that this was an early Neal 102 MKI; serial numbered at 2291.

The power supply circuit diagram is shown with some measured voltages; these were later done 'under load' conditions.

There are no voltage markings on this Service Instruction circuit diagram.

At this moment, ac supply and motor circuit integrity was the main focus of attention.

Much later in the restoration project the transformer was tested ‘offline’;

I initially found it difficult to believe that the audio board dc voltage was designed to be about 40v!?

Measurements ‘offline’ confirmed the above SI circuit diagram was correct.

I subjected the transformer to a simple signal generator where I set the frequency to 50Hz (UK mains frequency), and the input to the primary to 2.40v (two point four) ac rms. Within this ‘safe’ environment, the results were:

Primary Windings: Brown = 2.40v, Red = 1.2v, and Magenta = 0.18.

Secondary Windings: Yellow-Yellow = 0.48v, and the other seconary winding was 0.11v AC (rms).

With this, I knew the winding ratios were correct, the transformer was then returned to the deck, the motor re-connected and tested. This side of the circuit was stable.

Motor to Idler Alignment

With the power supply switch on, and observing the motor pulley, idler, and flywheel in action, the line taken by the idler and the pulley (set for 50Hz operation) was questionable.

A very small metric (1.7mm) allen key was needed to loosen the pulley from the motor spindle. Moving the pulley up and down the motor shaft wasn’t easy; I’m not sure that is by design? Although there is no thread on the motor shaft, a screwing action seemed to be required to move the pulley up and down the motor axial.

Later, idler to motor pulley alignment was improved; here observe that the idler was possibly too low?

Circuit Board Initial Checks

The audio board was removed and checked for any obvious short circuit. The procedure was to connect the (unknown at this time) 40v dc rail and earth point to a variable DC, current limiting power supply.

With a regulated variable power supply the initial current limit was set to a low figure of about 50mA, and then the dc supply voltage gently increased observing the current demanded by the audio board. 

Here also, I gently opened up the current limiter and saw all was good, no obvious short circuit!

The audio board was then returned to the system, and the deck could be generally tested to see if the NEAL 102 was ‘basically working’.

The answer to that question was – yes it was.

The deck was indeed playing back tapes, but the original pinch roller has disintegrated, which rendered it useless from a fidelity point of view.

Pinch Roller & Mechanism

With the deck disassembled again we can see the ‘floating’ pinch roller and its assembly. A most unusual design which seems to have been replaced in other and later versions involving this Wollensak based machine?

The pinch roller was clearly beyond repair and so a complete replacement was necessary; that is both the pinch roller and the mechanism that guides it.

At first, a replacement seemed challenging until a makeshift idea of utilising an old Sony mech became possible, complemented with a 2mm shank steel rivet and temporary pinch roller.

However, this wasn’t as easy as it first appears. Underneath the roller axis was a ‘resting’ post used by the previous assembly which would have to be removed if the ‘new’ pinch roller was to be centred in line with the cassette tape. The complete head assembly had to be taken out to access the rivet-like fitting for this post or boss.

The rivet-like finish that secured this boss was ground down using a Dremel, and the boss was slowly taken out with pliers. Care was taken not to distort the head assembly platform.

A strong, 'hold down' spring underneath had to be temporarily disengaged from the sliding mechanism, and was held ‘suspended’ with string as shown; a ‘neat’ little trick worth remembering!

There were still two outstanding issues to be resolved here: the base of the record head ‘catches’ the swinging mechanism if the roller is withdrawn far enough in, and a ‘stopper’ needs to be added to stop the pinch roller from springing out beyond the capstan.

At the time of writing, only the ‘stopper’ issue has been resolved; roller mech and record head base conflict doesn’t occur unless I exaggerate pinch roller withdrawal. The ‘stopper’ is somewhat crude, but it works well; a simple loop of insulated wire!

New pinch roller, NOS mech, and a new record/play head fitted.

Audio Circuit Board

A warning to anyone who wishes to restore a NEAL 102 cassette deck second hand: there are no component markings on the circuit board; none! Which translates that - a careful, and systematic approach is 'a must' to replacing all electrolytic capacitors. It is so very difficlt to follow a circuit diagram and an actual PCB without guidance.

It took several hours to complete the full electrolytic capacitor substitution.

An additional note to all prospective buyers of a second hand NEAL 102: the circuit board removal and re-assembly will involve de-soldering and re-soldering a Mono/Dolby/Cro2 selector switch. The whole thing is quite tricky to remove, re-insert, and align.

Later I intend to minimalize ‘wear’ on the main circuit board by attaching a simple home-made ‘8 line’ cable extension to the original design. This will enable me to remove the main board and the selector switch much easier without fatiguing the 8-line solder connection between the two. 

Correction: As I found out the other day, this is not the case - you can keep the Mono/Dolby/Cro2 switch soldered on to the main board and remove the whole main board quite easily by unscrewing the plate that 'mates' the Mono/Dolby/Cro2 switch to the main chassis. (9/02/2023)

Removed Selector Switch.

NEAL 102 Cassette Deck Alignment

Finally, after many hours of work, I arrive at the final stages of this restoration project. With a new record/play head in, a series of alignments need to be carried out.

De-magnetisation of all Heads: Easy.

Record Head Track Height: Required since a new and different head is to be used. I will use my home-made 3Khz test tone recording made on the Nakamichi DR10.

The procedure is to playback the 3Khz test tape, azimuth-align the tape (at 3Khz) and write down the amplitude for each amount of shims inserted. I had several spare shims, and here just 3 shims returned the highest output. The shims are very thin, perhaps around 20µm?, I'm not certain.

Record Head Azimuth: Easy, currently using an ABEX 10Khz alignment tape.

Playback ‘PB’ levels at 400Hz: Adjust to reflect Dolby Level to meet with Service Instructions requirements; SI quotes 100mV at line out on playback of a Dolby Level reference tape.

Record Level: Normally a permanent adjustment for a particular tape. However, in this case a temporary adjustment is made that facilitates both left and right record levels to be equal on record. Fine tuning for a specific tape will be undertaken later; perhaps for TDK D?

Playback EQ (de-emphasis): There is no NEAL (or NEAL-Ferrograph) factory alignment playback (PB EQ) tape that I can use, but from tests involving this new head, suggest that the level of de-emphasis (Playback EQ) is as strong as several Sony decks I have. (See note (C1) below)

With the new head in place, playback EQ levels between 333Hz and 10Khz from a Nakamichi DR10 home-made ‘reference’ suggest a reduction to just 35% at 10Khz compared to the level at 333Hz. 

This translates to about -9dB drop between the two said frequencies.
(See note (C1) below)

Note: This NAK DR10 reference tape was recorded at -10dB, ref Dolby Level. This tape is only used for comparison, or to compare other 1970s (pre-Prague 1981) machines with each other.

Allowing for small measurement errors and general ‘old’ deck ageing misalignments, differences of between -4dB and -9dB have been recorded from my Sony, Akai, and Sansui decks.

Bias Setting: Measurement of the bias ‘carrier’ at the record head when the old head was in place was approximately 29v peak. The same voltage was noted with the new head fitted, but on initial tests, this was insufficient as playback frequency response rose steadily after approximately 1Khz. Increasing the bias voltage to 35v peak brought down this high frequency drift, and playback was reasonably flat to 10Khz. Final tweaking will have to done later when all issues are resolved.

Bias Frequency: The period of the bias frequency was measured at be approximatey 2.7 x 5µS, ~ 1000000/(2.6 x 5)Hz, or ~ 77Khz. I will probably confirm this later.

Measuring Bias Voltage at the Record Head.

(Oscilloscope probe impedance ~ 10MΩ for accurate measurement)

Record EQ (pre-emphasis): It is not often you see a deck that allows the technician to ‘tweak’ the rise in frequency emphasis above mid-band (approx 300Hz .. 500Hz) frequencies, but the NEAL 102 allows just that!

This emphasis can clearly be seen on the VU meters; expect a standard rise of about 10dB/15dB from about 500Hz to 10Khz. Altering the relevant potentiometers, I was able to adjust this with ease to roughly ±3dB of the 10dB quoted above.

NEAL 102 VU Record Level Meter Current Readings

(Measurements taken from the VU meter circuit,
small errors in measurment are probably present)

Shown above is the variable audio signal voltage amplitude as driven into the record head. It is this curve that will be followed, or very close to it.

Record head impedance will naturally attenuate head current with frequency (basic electrical engineering theory), therefore for near-constant, or a controlled audio signal current (hence near constant magnetic flux), the signal needs to be emphasised in this second half of the graph, hence the term 'pre-emphasis'.

I have taken similar measurements from other cassette and reel to reel decks, but this time at the record head! However, with this approach it is difficult to obtain accurate results, as the audio signal is so small in comparison to the bias 'carrier' envelope. Oscilloscope triggering also becomes difficult.

Calibration 

For this NEAL 102, calibration is better done at the back; solder side of the circuit board. The Service Instructions are slightly confusing as they illustrate the potentiometers (and transistors) from the component side.

A permanent marker aided easy access to the calibration potentiometers.

Performance

If anyone has read the old sales brochures for the NEAL 102/103 series, you'll appreciate that this deck back in 1974 had a good specification; cassette players and decks were still in their infancy. However, let's not get carried away and expect 'Revox/Nakamichi performance'!

Below this paragraph, I hope to add some performance findings regarding this apparently early version of the NEAL 102.

Wow & Flutter: Firstly, I had to remove the supply reel belt to the counter, as sometimes the counter sticks creating unpredictable variable back tension.

So then, with the cheap, temporary pinch roller fitted, wow and flutter as measured from 200 samples from the WFGUI.EXE log file was:

Mean Value: 0.09% DIN,  (The standard deviation was 0.009)

Mean Speed Error: -0.62%

Test tape: ABEX 3.15Khz Full Track.

Frequency Response ('Normal Tape'/1980s TDK D): Not yet formally done, but observing playback of white noise at around -20dB (ref: Dolby Level), the original specification of 35Hz ... 12,000Hz is about right. Although it looks like the 12,000Hz figure will be extended based on observations to around 12,500Hz to 13,000Hz. The Fast Fourier Transform profile also shows mains hum and its harmonics, but that is to be expect when we record at only -20dB!

Other Images

VU Meter Lighting: 6.3v rated filament lamps in series,
running off the second 10.5v/11v ac secondary winding.






NEAL 102 under general testing with the original record head.

All working!

The fact that NEAL (North Easy Audio Ltd) provided such an array of performance tweaking options tells us that this was a serious machine for the serious audio enthusiast. Back in the 1970s, I remember NEAL, but only in adverts and in particular The HiFI Year Book 1976. Back in 1974/1975/1976, the NEAL 102 and 103 models were far too expensive for me to buy, I could only dream!


Small Fixes:

• 18/02/2023: I sanded-down the edge of the pinch roller mechanism so that the mech can swing back and forth without clashing with the base of the record head. Note - the head replacement is merely a DYNY62 we see on ebay and other places; probably Chinese made? I still have the original record head for this machine; should I get it lapped?
  

Note also: the mechanism 'stopper' is somewhat crude,
but it works and doesn't interfere with pinch roller pressure.

Outstanding Issues:

• DIN input distorted and overly sensitive, DIN output however, is fine.

Corrections: (C1) I quoted a '-3dB' or '-4dB' reduction earlier, that was not correct.

This blogger page will be updated allowing for corrections and additions without notice. 

19/01/2023

 
cassettedeckman@gmail.com