Nakamichi 600
This is the original 'Mark I' which I bought off ebay. Purchasing a 'spares or repair' machine is always a risk, which is why I never pay 'top money' for anything.
Many ebay sellers just want your money, and care not if you're unable to fix the deck - so be aware readers. With that in mind, I especially enjoy and make a point to inform them that I've repaired the deck.
Some of the Repairs
Repairs are many, and what follows was not necessarily the order in which several repairs were made to this deck.
Initial problems -
- Seized DC motor
- Pinch roller had slightly hardened surface
- Heavily worn record/playback head
- Heavily worn erase head
Strangely, for a deck of this age, the drive belt was fine, although it will be changed later.
The initial motor seizure was relatively easy to deal with as I just freed the frictional neck by applying switch cleaner on to the pulley-side bearing, and 'worked' the pulley until the motor was free. Then after cleaning the neck, ISOFLEX PDP65 oil was liberally applied to the said bearing, and the motor was up and running.
Many hours later in the repair cycle, the DC (FG) motor was opened up and serviced.
The Rotor: Be aware of the spacer or washer left-side which is hidden from view. It is very easy for this to get trapped inside the permanent magnet chamber in the second picture.
The Rotor: Be aware of the spacer or washer left-side which is hidden from view. It is very easy for this to get trapped inside the permanent magnet chamber in the second picture.
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The bearing on the brushes side of the assembly was very dry, and so again ISOFLEX PDP65 oil was injected into the bearing cavity. At this point a spacer was lodged (out of shot) into the stator chamber where sits the permanent magnet.
For a 50 year cassette deck that has seen many hours of use, the brushes appeared not to be heavily worn.
For a 50 year cassette deck that has seen many hours of use, the brushes appeared not to be heavily worn.
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Carefully returning the motor to its casing.
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It is important to carefully re-assemble any such motor - we want the least possible internal friction for the servo controller to work the motor. So my advice is to: re-assemble, and before tightening the three screws that secure the left-side cap, spin gently the motor shaft and 'feel' for the least torque load on your fingers and thumb. Then incrementally tighten each one in turn until the whole unit is done.
Worn Record/PB Head
It was obvious that the head was worn from the playback level, but moreover - the lack of treble in the audio. As a quick check I demagnetised the heads and later adjusted the azimuth just to see that this wasn't purely an azimuth issue, and so it proved to be so.
Disassembled head with wear grooves.
Disassembled head with wear grooves.
There were two options - lap this original head or find some replacement.
However, a 'straight forward' replacement isn't so easy to find, there are probably few or zero amount of NOS Nakamichi heads out there on the internet so I was stuck.
In my stocks of various electronic artefacts, I had a Sony head that would fit, it had a short penetration depth unlike most other possible options. From memory its inductance was comparable, and so this would disturb too much the bias oscillator resonant frequency.
The Sony head proved very successful, although it lacked playback sensitivity. I could easily achieved 30Hz ... 19Khz within +- 3dB (Ref: -20dB 160 nWb/m DIN standard) after a record/playback session. Truthfully, I was astonished by this amazing response, especially from the Sony head, which was originally from a Sony TC-K44 and lapped,
However, just now and then the pre-amp would become unstable with very low frequency oscillations of the order of about 5Hz, yes five cycles per second. This instability could begin after 50 seconds or 50 minutes!
At first I thought this issue may have some instability within the pre-amplifier? After all, there had been some re-capping done within the pre-amp before I bought it. Since I had not played the deck for long with the original head fitted, I could not determine for certain if this was a pre-amplifier issue or not?
I was at a loss to find what the exact cause was, and after lapping the original head, I returned the Nak head to its original owner - this Nakamichi 600. Finally, no instability!
You may notice the two small 'pits' close to the gap on the right channel. They could have been there before lapping?
Provisional record/playback frequency response measurements also returned surprisingly great results - 30Hz to 19Khz +-3dB at the theoretical 160nWb/m DIN level.
Pinch Roller (13mm x 8mm x 2mm ): The reader will have noticed too that the pinch roller had already been replaced. Incidentally, there is apparently only one compression setting available between pinch roller and capstan.
Erase Head Problems
At the time of writing this has not yet been resolved.
Although full erase for Type I tapes is excellent, Type II is more problematic. I first noticed this since the left channel would not fully erase - there was, and still is a periodic resurfacing of sound in the left channel. Studying the erase head reveals that the erasure track fully covers both L/R channel record tracks, so probably no erase head height issue there.
Closer inspection revealed another story however - severe wear on the erase head!
Both ferrite and the plastic surfaces are badly worn, and since Type II tapes require a substantially higher erasure B-field strength, then perhaps it is not that surprising that full persistent erasure is not happening? The effective cross sectional area is being reduced and with it the relative permeability of the ferrite.
Lapping an erase head isn't that easy since we are dealing with two different materials - both of which will have different wear coefficients.
Initial inductance of the worn erase had was estimated at 78uH, and post lapping approximately 73uH.
Post lapping improved erasure, but it has not eliminated it.
Alternative Erase Head
A direct replacement would be desirable, but it is highly unlikely. So another is sought.
Neglecting fitting issues for the moment, most cassette deck erase heads I have measured possess an inductance between approximately 800uH and 1200uH.
As these figures stand, they are useless as a replacement for the Nakamichi 600 for at least one good reason - oscillator resonance.
At the heart of the oscillator is the L302 ferrite cored transformer which was removed and examined 'offline'.
Unloaded, the primary or transistor side had an inductance of approximately 272uH, and the secondary or 'tank circuit' side, 585uH. Both sets are variable by turning in, or turning out the ferrite 'slug'.
This oscillator circuit has a single erase head bias current setting for both Type I and Type II tapes - which means the field strength is the same regardless of tape setting.
Resonance: A good estimate of the resonant frequency of the tank circuit can be got from the equation ... 1/{ 2*pi*Sqrt(LC) }
Where C315 = 39nF, and L is the effective inductance of the secondary windings and the erase head. The record head inductance has been ignored (approx 70uH) since that 'tapping' and circuit is also highly resistive.
Without proof, the effective (non-mutually coupled) inductance of two coils is given by
1/Lt = 1/L1 + 1/L2.
Here we have 1/Lt = 1/585uH + 1/73uH or Lt = 64.9uH.
So then, we can expect the secondary side to oscillate at approximately ...
f(osc) = 1/ {2*pi*Sqrt(64.9uHx39nF) or f(osc) = 100Khz.
If a 'new' erase head is going to be fitted, it will need an effective inductance in the region of 60uH ... 85uH. The final oscillator frequency can be 'fine tuned' using the ferrite 'slug' in the transformer.
From measurement, the 'old' erase had an inductance of 73uH which at a measured frequency of 106,000Hz offers a calculated impedance of around 2*pi*106,000*73/10^6 or 48.6 ohms.
The high frequency bias voltage was measured under the original erase head 'load' and was found to be 18.6v rms.
Translated, we obtain the working bias current Ibias.
Ibias=18.6/48.6 amps or 383mA.
This is the rms erasure current working to erase the tape, it is quite a high value, much higher than found in other cassette decks.
There is some heat generated by this 'large' current, part of it will generated from I^2R losses, and swirling eddy currents in the ferrite.
Based an an estimate of head resistance to be about 2 ohms, I^2R losses alone will generate (0.382)^2*2 or 0.3 watts. And that doesn't include eddy current and skin effect generated thermal activity.
The 300mW doesn't look much on paper, but consider the small size of the erase head. Surely over time, this will have had an impact on the softness of the plastic and ferrite area sections? Hence the excessive wear?
Potential Erase Head Replacement
The original erase head is still in circuit, operating normally and successfully for 'Normal' or Type I tapes.
For a replacement to work, it will need an inductance of about 60uH to 85uH as stated above, and is able to erase the tape with its generated magnetic B-field.
Alternatively, another head could be employed with higher inductance, providing of course that the tank circuit still works on the original 39nF and an approximate and effective 60uH to 80uH inductive load.
I have ordered an erase head with L ~ 390uH and DC resistance of about 2 ohms.
Alone this won't work, but if a dummy load in parallel can be found to make 60Uh to 80uH, then there is a chance of success.
Again, we use 1/Lt = 1/L1 + 1/L2
Here, our Lt = 75uH (say), and the dummy load (in parallel) L1 = 390uH.
So then ... 1/L2 = 1/Lt - 1/L1, gives 1/L2 = 1/75 - 1/390 or L2 ~ 93uH.
So, if indeed this 390uH head can be sourced, and fitted correctly (inc modifications), then the bias erasure current that the new head will 'sink' will be given by the current divider rule ...
Ibias = 0.383 x 93/(390+93) or 74mA.
Question: If and when this head is fitted, will it generate enough field strength to erase Type I and Type II tapes with 74mA?
Many high impedance erase heads erase their tapes with much lower currents than the 383mA calculated above.
For example, the erase head on the Sony TC-K44 has an inductance of 796uH, and for Type II erasure, the quote rms voltage is 32v.
Ibias = 32/{ 2*pi*105,000*796 x 10^(-6) } = 61mA.
Alternative Tank Circuit Capacitance
Another option is to fit a new erase head in circuit, but lower the value of C315? However, the potential problem still remains - can the transformer's output voltage of 18.6v rms drive enough Ibias current, hence field strength?
Also, I suspect under an effective higher impedance load, the high frequency bias voltage will be a little higher than 18.6v, perhaps close to 20v?
More to follow ...
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