AKAI CS-705D

The AKAI CS-705D

Original ebay Advert Images of the CS-705D.

 

Advertised for 'parts or not working", I got this little attractive looking deck up and running in less than 30mins.

However, there's much work to do on it - the record head was quite thick with dirt and oxide, and the pinch roller looked as if it hadn't been cleaned for decades. The CS-705D was a little corroded internally in places, but in fair condition. The VU meters were misbehaving on Play.

So far ...

• Removed internal debris
  
• All contacts and switches cleaned with Servisol Super 10
  
• Capstan shaft lubricated with Revox recommended capstan oil. 
• Pinch roller - removed all tape oxide with diluted acetone, later isopropyl alcohol, and finally water. Roughed-up surface by rolling pinch roller on fine grit paper, removed excess rubber (vacuumed), then applied some Rubber Renue.
• DC Motor (dated Nov 1976!) removed, applied Servisol Super 10 down the shaft. The fitting appeares to be standard, and so a replacement may not be difficult - looks like a 2mm spindle? 
• Cleaned associated motor pulley.
• All muting and similar switches cleaned.
  

Measurements -

• Erase head bias frequency/voltage: ~85Khz/~35v peak.
• Record Head bias frequency/volatge: ~85Khz~9v peak.

 Observation -

• Electrolytic capacitors seem good, may replace later?
  

The deck is now functioning, recording, and playing back almost as new.

Issues to be Addressed

1. Record/Replay Head replacement or lapped? So far - recording and playtback are fine, much better than expected, but the head wear is obvious. 
2. Motor speed seems to drift over time?

11/10/2021

A New Record/Playback Head Installed

Original Record/Playback head -

This doesn't appear to be the original P4-303 or
HN-424822 record/replay head?
Head wear is actually deeper than it looks in this photograph.

Although the old head worked well it had obviously done some miles, so I chanced a few potential replacements. Looking at the two 'new' heads I bought, and the original head fitted in the CS-705D, I decided to perform some measurements.

The results are listed here ...

(a) Original: '512 K6AB', R ~ 192Ω, L ~ 110mH (at 10Khz)

(b) New: 180-T1905, R~250Ω,  L~ 78mH (at 10Khz)

(c) NOS: 445-20 E S1X10, R~ 350Ω, L ~ 127mH (at 10Khz).

Again, I suspect the 'original head' was not the exact one that came with this machine back in 1976? (Edit: It may indeed be the original, as other images of the CS-705D on the internet show the same record head identity number.)

Head Capacitances: While it is true that the inductance value L should be independent of excitation frequency, we have to appreciate that a record head also has parasitic capacitances, which is why (I suspect) that the apparent measured inductance varies a with test frequency?

Before I decided to replace the head, I measured (under load) the peak head bias frequency voltages in record mode on the CS-705D. They were ...

Type I (Normal Tape): default 9v, (can be varied from 5v to 18v)

Type II (CrO2): 14v, (can be varied from 7.5v to 26v)

Type III: Fe-CrO2: 12v, (can be varied from 7v to 24v)

 

Calculating 'Normal' Bias Record Head Current

From Ohm's Law, we have I = V/Z, and for our existing record head the impedance at approximately 85Khz is going to be -

where Z = √{R²+(2πfL)²} ~ 59,000Ω

It should be noted that the impedance from the inductor L is frequency dependent and dominates the overall impedance Z.

From this 59KΩ impedance, both peak and RMS bias current can be estimated -

Original '512 K6AB' Ibias ~ 0.15mA peak, or 0.108mA RMS.

Ibias is the high frequency bias current that will flow through the head, even when there is no signal applied during recording. Any additional audio signal added to the bias is very small in comparison.

Which 'new' Head to Choose?

Good question - I don't have the defintive answer, but I decided to try the 180-T1905 first. At 85Khz, its impedance was going to be lower since its inductance was less.

Ignoring the internal DC resistance for this calculation, Z ~ (2πfL) ~ 41600Ω, and so the bias current from the 'new' 180-T1905 head can be calculated, and found to be 0.15mA - almost identical!

After fitting the 180-T1905 head, all that was left to do was to - demagnetise the head, re-set the azimuth, re-calibrate PB levels (from a Dolby Level reference tape), adjust the internal record levels so that 0VU on record, gave back 0 VU on playback. And then finally alter the bias (actually bias amplitude!) from the potentiometers on the circuit board.

Success? ....  yes, excellent!

The AKAI CS-705D records and plays back very well indeed. It's 333Hz and above frequency response is almost flat to 13,000Hz, and about -3dB down at 15,000Hz. This is better than the original specification.

I haven't yet arrived at the point where I will undertake a full frequency response test, and weighted signal to noise ratio assessment.

Track Height Adjustment?

Possibly later I will add a fine shim and test for true track height alignment using one of my Nakamichi 3Khz track height test tapes.

 

14/10/2021: Both with, and without a shim added, I measured the output peak-to-peak from a 3Khz Nakamichi-DR10-made track height test tape. The difference was small, but with the added shim the playback amplitude was slightly greater. The head tape guides are now truly equidistant from the tape. So the shim stays in!

Motor Speed Variatons.

As mentioned above, the motor speed controller appears to let the motor drift off speed. It's not serious, so I will look into subject at another time. 11/10/2021.

14/10/2021: Old Motor Refurbish Attempt

I took the old motor out, and stripped it down to the bare motor assembly, then fired some more Servisol Super 10 down into the armature/stator chamber, just to be sure its truly contact cleaned! 

Also, as a precaution I desoldered two electrolytic capacitors on the motor speed conroller board, then replaced them with new 3.3uF and 4.7uF capacitors.

All re-assembled, and (so far) working well. Again, I speed-calibrated the motor, so only time will tell if I have found the source of the problem?

 

Motor Casing.

Note: There is a removed sheath that electrically
isolates the casing - not shown.

DC Motor Controller Board - can be removed by desoldering at X-points.
Note: Polarity is important, otherwise the motor will run clockwise - currently, it runs CCW (counter clockwise).

The best way to de-solder is to use a copper braid which 'soaks up' the molden solder during de-soldering.

14/10/2021: The motor was again starting to drift in speed, in a similar manner to the motor that was originally in the AIWA AD-F770 - the change was slow but noticeable.

15/10/2021: New Motor Fitted

 

Mabuchi (a copy?) CCW 2400rpm motor now fitted.
Runs off 12.6v DC supply, 12.6v under load conditions.

 

Played the CS-705D for more than an hour - all working well, stable sound. 

 

Motor Speed Drift 17/10/2021: The speed seems to drift again - it's very subtle, but noticeable. Confirmed this when I had to recalibrate the speed using a 3Khz calibration tape - twice. So I am not imaging it! 

 

Power supply is stable, so my attention is on (a) the new motor, (b) pinch roller and possible slow motion slippage? (c) Excessive belt tension? - motor load current is higher than expected Play=ON ~150mA, FF=ON ~260mA, RW=ON ~250mA! 

Comparing DC Motor Load Currents

 

Here I've compiled a small table of results comparing DC motor load currents with a TDK D46 tape inserted. 

 

Bare in mind, the torque (Torque = tangental force x radius) developed by a motor is proportional to current, ie T ∝ I. Example: A doubling of armature current is required to drive a doubling of load torque.

Cassette Deck Model	Play Current	FF Current	RW Current
AKAI GX-M10	~ 57mA	65mA … 74mA	59mA … 106mA
NAD 6050C	~ 68mA	72mA … 90mA	71mA … 120mA
AIWA AD-F770	69mA ... 75mA	NA	NA
SANSUI SC 1330	~ 54mA	98mA …135mA	103mA … 170mA
AKAI CS-705D (before belt stretch)	~ 150mA	200mA ± 50mA	250mA ± 50mA
AKAI CS-705D (after belt stretch)	120mA ... 125mA	200mA ± 50mA	250mA ± 50mA

We can immediately see that the current required for the CS-705D is more then twice that for other machines. Indeed, after deliberately and uniformly stretching the flat drive belt in the CS-705D, I was able to bring down the average Play current - but this is still too high.

I can only conclude for now, that the loading on the small DC (internally speed regulated) motor is beyond the specification of the motor!? Hence the reason why its speed 'wobbles' or drifts as it gets warm. Or is it more likely that the motor and internal speed regulator circuits are failing?

New Old Stock (NOS) Motor: Matsushita MMI-6S2LS

29/10/2021 

After trying several NOS (New Old Stock) 12V DC motors (a genuine Mabuchi, and a 'Mabuchi' Chinese copy), I finally installed a 1994 unused Matsushita MMI-6S2LS. All motors of course include their respective motor speed controller circuits. 

After 2 hours play, no problems to report, and no large current drainage!

  

The torque now demands currents of -

Play: 76mA ... 83mA, FF: 150mA ... 180mA , RW: 150mA ... 200mA

The above figures are dependent on which spools have the most tape on them. Any excessive tension applied by the belts will also increase Play, FF, and RW torque currents. 

One fact that struct my attention, was the internal 'slip' friction (and additional torque load) demanded by the take-up spool. The CS-705D has a strong take-up spool force, which when in Play mode, adds about 20mA. This means that when in Play + Pause mode, the dc current is only about 57mA/58mA.

Speed Calibration

Using an ABEX 3150Hz reference tape and frequency counter, I have the deck set up to playback somewhere between 3150Hz and 3160Hz - which is slightly fast.

Hopefully, this time the motor speed won't start to drift!

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

Note: Long exposure photograph.

31/10/2021

LED Illuminated VU Meters

I decided to change the old filament lamps for white LEDs. However, the original lamps worked off a transformer ac tapping, whereas any new LEDs in circuit will need DC to get them working without flickering.

 

The proposed modifications and circuit is - 

 

 

 

VU Meter LEDs ready to be used.
Note: all exposed wires were electrically isolated
after this photograph was taken.

The LEDs now in circuit are not working to their full brightness, they don't need to. Two LEDs in series for each VU Meter is sufficient, where each LED is only consuming between 6.0mA and 6.3mA. Some 12.4mA of dc current was observed which will split into approximately 6.2mA x 2, since this is a parallel circuit. 

The two original 1000uF (25v rated) electrolytic capacitors were replaced by larger 3300uF (actually ~2900uF) capacitors. The idea behind this was to increase the available charge for both the 'new' DC motor, and Q1 voltage regulator circuits.

As a matter of interest the AC fuse currents were measured at F2 and F3 - they were approximately 188mA (before the LED circuit was introduced), and about 107mA for the DC motor circuit in Play mode.

 

All working well - very nice bright and clear VU Meters!

 

 

Cassette transport speed is still very stable!

Other Images

 

 

Playback EQ (De-emphasis) Modification (12/11/2021)

A small change to the playback (PB) equalisation circuit and curve was made to help compensate for any new record/playback head that I fit.

 

All electrolytic capacitors in the area were also replaced.

The resistor R12 is now a 20kΩ variable trimmer
potentiometer set to about 10kΩ by default.

 

AKAI Record/Playback EQ

 

I do not have any Akai alignment tapes, so the earlier record and playback EQ specification adopted by Akai is unknown, but I can assume it probably approximates closely to a 1970s IEC standard, but with slight modifications to suit their designs and record head constraints? The same probably applies to Sony, Pioneer etc?

 

Using a Home Made IEC 1981 Reference Tape

 

Using a home-made (approximate) IEC 1981 specification alignment tape using the Nakamichi DR10, I set the playback response of the AKAI CS-705D to approximately -6dB at 10Khz in comparison to the output at 333Hz/400Hz.

 

Why -6dB at 10Khz?

 

Nearly all the (pre-Prague 1981 IEC standard) second hand decks that I've bought exhibit a -6dB (±2.0dB) output drop at 10Khz (reference to 315Hz/333Hz/400Hz levels) when playing back the home-made IEC 1981 reference tape, created with the Nakamichi DR10. 

 

Since my Nakamichi DR10 has not been used extensively, I feel reasonably confident with the general accuracy of this setup. To support this proposition, my (post 1981) Revox B215 and even a humble Marantz SD-35 follow the Nak DR10 playback curves reasonably accurately, to within ±1.5dB at 10Khz.

Just to reiterate, I simply use the Nakamichi-made 'IEC 1981' tape as a general playback guide at 333Hz and at 10Khz only, it is not a reference tape for older pre-1981 machines.

Article is subject to small changes, additions, and corrections without notice. 14/11/2021.

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