Technics RS-671

The Technics RS-671

A risky purchase?, possibly?, possibly not? So, as a precaution I studied the aluminium fascia for scratches and corrosion, and then estimated the potential corrosion or rust internally. After some time I decide to take a chance and conveyed my buying concerns to the seller, and offered to buy for 35% lower than his asking price.

There's little doubt that this machine had been left in 'storage' for 10, 20, or more years. There was even a small twig to be found inside the machine next to a smoothing capacitor - my only conclusion was that it had been left in a garage for a very long time.

 

Some Original Advert Images:

 

The paintwork on the top of the deck was so poor, I could easily scrap it off, but concerns were more directed towards the inside of the deck.

More original advert pictures ...

I have to confess, I did not take any photographs of the insides of the RS-671 - I just went to work on diagnosing the main problems and fixing many issues.

Internally it wasn't as bad as the external condition of the deck; just as I had calculated!, but much work needed to be done. Smoothing capacitors were probably leaking since during initial testing, only 10v was available to the motors, when it shoud have been around 14v to 14.5v.

The audio section PSU was cleared of debris, and the moldy areas were cleaned and allowed to dry. Later the main power supply capactors were replaced. Even the zener diode on the audio regulator circuit was replaced with a slightly higher voltage value; 20v instead of 18v. I don't have any 18v zener diodes in stock at this moment.

The usual cleaning and lubricating practices were applied to the mechanisms, including all rotating components of the cassette transport.

Since reporting on this Technics RS-671 repair is very time consuming, what follows below, is not generally in the order that these repair jobs were done. Indeed much of the tedious repair work is not written about.

Loose Take-up Spool Idler Tyre

This had worked loose, although the outer rubber was fine, the inner diameter had apparently stretched!

There was no obvious replacement available, so I bought a few idler tyres to try. The actual diameter of the aluminium rim on which the idler will sit, is 14mm. 

 

A 16.4mm x 11.8mm x 3mm was the best option, but I had to 'sand it down' to an outer working diameter of about 15.5mm. The reason being - the idler would interfere with the take-up spool/reel when the Technics was set to RW mode.

The effective idler diameter was reduced by employing a tight fitting drill bit, and putting the drill into a drill stand while I carefully applied a sanding block to the rotating tyre. Later the tyre was cleaned and Rubber Renue applied.

 

Rewind/Fast Forward DC Motor

 

Is a DC motor which appears to work on a centrifugal force principle to regulate it own speed. At a certain angular velocity enough centrifugal force is generated to throw out 'weighted contacts' and temporarily disengage the DC supply from the motor. An access port is provided if angular velocity settings are to altered by the user - in this case I didn't change anything.

 

The motor was thoroughly cleaned with the aid of a glass fibre pencil and switch cleaner.

 

Before Cleaning.

Showing it Access Port.

 The motor has to be isolated before returning its original casing.

 

The Capstan Motor

Based on the uPC1003C2 botor controller; a 'hard to find' IC that (if found) will be costly, so beware!

Note: the photographs that follow, are not in the order in which I worked on the motor.

Initially I just replaced the two electrlytic caps -
both indicated with a white mark.

Both motor brush (left) and commutator (right)
were cleaned with switch cleaner and a glass fibre pen.

The rotation of this motor is clockwise.

Motor current demands are dependent on load torque, and if we ignore the basic electronics of the motor controller circuit, Imotor ∝ Tmotor.

At a set speed (usually around 2400rpm) the total load torque is derived from the take-up slip friction action, the flywheel mechanism, and other mechanical frictional losses,

Initial DC current demands from PLAY action were around 125mA, which from my experience is considered too high. This will put a mild current strain on the brush-commutator interface. At that time I used a temporary flat belt from the motor to drive the flywheel. The belt was really too small - 122mm (diameter) x 5mm (width) x 0.6mm (thickness).

After fitting a similar belt with a slacker diameter of 135mm, the load current dropped to a mean of 100mA; much better! This figure was further improved, when the 'new' modified take-up spool/reel idler was fitted to replace a temporary 'O-ring'. Now the mean PLAY DC load current was hovering around 89mA to 90mA.

This figure could be improved still further if and when I decide to reduce the amount of take-up spool slip friction. This can be achieved by effectively reducing the force offered by the compression spring within the take-up hub.

Of interest the motor plus controller load currents with the motor running in PAUSE mode ~ 52mA, and in PLAY mode (without a cassette 'load') ~ 60mA.

With the take-up hub removed and disassembled, I attempted to reduce the amount of compression that the default spring offered. This was more difficult than I first anticipated; how do you reduce the size of a compression spring effectively?

However, with a little effort I did manage to get the PLAY load current into the motor controller down to around 79mA ... 85mA, but a mean value of about 75mA was achievable with another third-party compresson spring that I opted not to use, as it was slightly ill-fitting.

Since this is effectively a 'top loading' cassette deck, it is worth noting that total take-up spool friction during PLAY will statistically be a little higher than for a front-loading cassette machine. So having a good amount of take-up torque is essential, but of course being mindful that it must not be excessive.

 

Original Compression Spring for the Take-up Hub

DC PLAY loading current ': 79mA ... 85mA.

Although the motor now works well, I am not 101% confident that all is good; I've had issues with it, and yet have not been able to pinpoint minor problems. So far it's stable with little speed drift, typical figures are usually within ±3Hz (±0.1%) from a 3150Hz ABEX reference tape over 10-15 minutes.

Wow/Flutter

 

Better than expected considering the state of the deck when I bought it.

After careful 'sanding' of the pinch roller, and a slightly reduced load on the motor, wow & flutter now hovers between 0.04% to 0.07% (JIS WRMS), with an occasional excursion beyond those figures.

All these fixes are provisional, since the RS-671 has so many pending issues to address. Two such issues that are on the list are: (1) Pinch roller replacement, (2) Record-Play multiswitch, which does need to be removed, and internally cleaned due to occasional 'pegging' of the right VU meter.

03/12/2024

 

Record-Play Multi-switch Fix 05/12/2024

 

The said switch was carefully de-soldered, and very carefully dissassembled. Then the internal contacts cleaned with a glass-fibre pen and switch cleaner. This procedure is indeed risky, but with patience and perserverence it can be achieved.

Solder side view with switches removed.

Retrieving the switches - note the different sliding switches

After cleaning with contact cleaner and a glass fibre pencil.

Preparing to return the switches.

All good, no sporadic oscillations in either channel.

System Control

This version of the RS-671 has a set of 4 x 20mm fuses post-transformer, but pre-rectifier.

Attention was brought when the 2000mA 'slow-blow' fuse open-circuited after many testing cycles of the RW/FF circuit. Of interest to the reader - RW/FF DC motor loading current was typically anywhere between 125mA and 225mA, and would rise to approximately 300mA when the RW/FF motor reached the end of the RW/FF operation. This is effectively a stalling condition, where we can expect armature current to be in that 0.3A region.

The circuit below illustrates some circuit loading ac current conditions under either RW or FF, and indeed the large rise of secondary current when the autostop (the final of two phases) is initiated.

Original Schematic from the RS-671 Service Manual
Note: No fuses are suggested?   

 
18/12/2024

(Article is subject to corrections, and additions without notice.)

Technics RS-673 Cassette Deck

Technics RS-673 Cassette Deck

 

I saw this on ebay very recently, and I was hooked. This is a quite a rare machine, but before I bought it, I just wanted to be sure it was repairable as I mulled over all the pictures, and the description of this 1978 machine.

 

Original Advert

 

 
Before continuing, the reader might wish to know that there is no obvious or available service manual to the RS-673 as far as I am aware. If you know where I can find a service manual to this machine, then please do help, thank you.

Another Technics RS-671 has an available service manual, and much of what is in there applies to this RS-673 model, except for the fluorescent metering.

Initial Testing

I could see that the deck was 'basically' operational, but it would not Play, nor rewind (RW), nor fast forward (FF).

The transport mechanism has seized up - decades of 'dry' lubricants, a fine layer of dust, and enough oxidation to grind things to a halt stopped the deck from making any sound.

Before any other work was done, the transport had to be working, and so the top side of the transport was cleaned carefully.

The tape supply hub/spindle has seized completely, so much so that I could hardly move it with my hand! This was the reason why the auto-stop kicked into action once Play was pressed.

However, after injecting some OB41 freeing-oil down the spindle of the hubs and 'working' the spool a little with my hand, free movement was restored to this and the take up hub. The remainder of this side of the transport was taken apart and cleaned with a mixture of detergent/water, switch cleaner, IPA, and acetone. Note - take care when using acetone on plastics, there will be a chemical reaction with regard to some plastics and this will ruin the 'look' of the said plastic. Please check online for more details.

Later, all idler tyres were removed, cleaned, carefully 'roughed up', and with Rubber Renue applied, returned to their original postions.

Other concealed but essential sliding mechanisms had to be cleaned and lubricated too - 'renewing' the idler tyres was simply not enough to cure the problem of zero RW and FF!

 

 

Later, the pinch roller was carefully 'sanded', cleaned, and Rubber Renue applied, and then allowed to dry. Associated mechanisms were removed, and cleaned to allow free movement of the pinch roller.

Earlier on, both heads were cleaned with acetone, with special attention applied to the record/play head. There is obviously some wear, and as a result I am aware that this could impact on performance, possibly more in the right-channel (innermost track) more than the left-channel  (outer track)? The capstan and heads were also de-magnetised.

The flywheel was taken out for inspection, lubricated then returned and cleaned again as the oils ran into the sintered bearings and would probably transfer on to the pinch roller if I didn't clean this again!

Drive Belt

 

Estimating for now, the main drive belt is about 130mm in diameter, and about 5mm wide - this seems to have been replaced in 1996 and/or 1999? The belt will be replaced later, but at this moment it's fully operational and stable.

 
Basic Calibration

With already some 6 hours into this restoration project some basic calibration work was performed.

Before that could be done, all potentiometers, and switches were 'switch cleaned'.

Also, the bias traps were re-set to maximize rejection of the bias frequency 'carrier' - there are calibration points marked to do this.

After basic calibrations: Bias Trap, Azimuth, PB Levels, PB EQ, Record Levels etc, the machine at this moment is returning the following figures ...

Wow/Flutter: < 0.11% WRMS (more often under 0.10% wrms).

Speed Stability: WRT a 3150Hz ABEX reference tape, it is good at approximately ±2Hz after 10 minutes.

Frequency response: 35Hz ... 14,500Hz -3dB for Type I 'Normal' tapes.

THD: At +2dB or slightly greater above Dolby Level, it is approximately 1.5% at 1Khz.

Recording Level Instability

 

A small, almost un-trackable variation in recording level in one channel was sometimes experienced. This was seemingly fixed when associated electrolytic capacitors, and 2SC828 NPN transistors were removed and replaced. Possibly pre-emphasis was affected in a small way?
 

 

Observing the schematic above, it is quite possible that either one or both of the switching transistors were not functioning properly? When in record mode, they should switch into their saturation region, and offer a near short circuit for ac signals to ground.


So far, the Technics RS-673 is running very well.

This Blogger article is unfinished, and will be subject to corrections, additions etc without notice. [15th November 2024]

Technics RS-630TUS Cassette Deck

Technics RS-630TUS Cassette Deck

 

Lots of little issues with this 'faulty' deck that I bought off ebay. Cosmetically, it is in good shape.

Problems -

 

• Stuck sliding door.
• Stuck Front Panel Switch
  
• No mains plug.
• Worn pinch roller.
• Slightly slippy belt. (Measures approx 86mm diameter x 3.5mm width)
• Playback amplifier sporadic oscillations, mainly in the left channel.
• Dust in cassette mechanism.

Most of the above were easy to correct, however the main concerns were - reliable transport functioning and the sporadic oscillations from the playback pre-amplifier.

Pinch Roller & Transport

At first the thought of replacing the pinch roller looked daunting, but later realised that the 2mm internal pin that holds and guides the roller could be tapped through from the bottom side.

Here's how - very carefully break the plastic cap at the dead-centre point with a very fine drill, screw driver, or punch (~ 1mm diameter or less), this produces a small opening, and then very gently 'tap-out' the steel pin.

A 13mm or 13.5mm diameter pinch roller will work, but a 13.5mm diameter roller is recommended as this will give a slightly higher pinch-to-capstan pressure.

My initial attempt cracked the underside
of the support for the pinch roller.
The strength of the assembly was not
compromised, but still I re-enforced
areas of importance.

The torsion spring that provides the pinch-to-capstan pressure is slightly puzzling. According to the Service Manual, the torsion spring is fed with its longer tail to the right as we look down on to the transport mechanism.

However, observing some online pictures, when in situe, the torsion spring needs to be tensioned like that indicated in red.

 

I have attempted to set up the tension like so, but found it extremely difficult, even with tools made for this kind of work.

So another solution was found.

The torsion spring is first centred underneath on the plastic pinch roller mechanism, and selecting the highest torque by 'pinning' the leftmost (shorter) spring arm into the relevant guide (hidden in this picture) should be done before returning the mechanism to the main board guide. The rightmost part can be secured into a groove provided by the Pause sliding parts.

 

Upper Left Side: This frontside view shows the torsion spring fully located - the short side of the torsion spring is tucked on to the underside of the roller swivel mechanism (as normal), and the longer tail is tucked into a Pause mechanism slot.

To increase working torque exerted by the torsion spring - first, the spring has to be carefull bent to increase effective torque.

 

Factory Torsion Spring Positioning

I may at a later date try again with the spring in its factory default set up, but the spring will have to be bent again allowing me to tuck the shorter side under the roller assembly arm.

 

Auto Stop Problem

This would not activate, and the reason was quite simple - the spinning rubber idler tyre was not turning with sufficient force to activate the autostop.

This idler works off the brass knurled section of the capstan/flywheel, and was currently providing too much slip. The idler was 'roughed' with emery paper, and later cleaned with Rubber Renue.

Both autostop, and take-up torque are now functioning correctly.

Shown here is the autostop at the point of being triggered.

It is worth noting that the autostop is only triggered for Play/RecPlay action, and not for rewind, or fast-forward operation.

If I have another criticism to make it is that the autostop will not be triggred if the take-up spool fails to turn, or lacks enough torque to turn the cassette tape reels.

Pinch Roller Tensioning - Again!

I finally decided to return the torsion spring back to its intended postion. I was always aware that its new postion could cause some small mechanical intereference.

The task was made easier by first bending (deforming) the torsion spring until it formed a new starting point angle. This means that the effective spring torque when setup would be less, but no less sufficient.

This made the return of the spring easier, but not easy! To keep the spring under control, I employed two pieces of tied string to keep the torsion spring stable in its position until I was able to release the long end into its target anchor point.

New Drive Belt

The previous drive belt measured approximately 86mm in diameter, and 3.5mm in width.

 

I fitted a few belts of different diameters and widths, but for better performance an 82mm diameter x 3mm width was best, and yielded the lowest (so far) wow and flutter figures. Wow & Flutter is averaging at 0.11% weighted rms (Japanese Standard JIS) figures.

The latest 0.11% wrms figure in green was achieved after the DC motor had been disassembled and cleaned with Servisol Super 10. The earlier figure in red was before the DC motor was cleaned, and also includes a larger 85mm x 5mm wide belt.

Initial Wow & Flutter Figures
with New Belts

 

Clearly the better figures are in green, but a noticeable trend of increasing wow and flutter was noted over this approximately 9 minute period. This will be investigated later.

Playback Sporadic Oscillation

There were sporadic oscillations of high frequency, mainly in the left channel on playback. The exact cause of this is uncertain, but applying switch cleaner to the Play/Record multiswitch, and 'working' the Record ON mechanism solved the problem. Since then, oscillations have only occasionally returned, but are soon stopped once the Record multiswitch is 'worked' again. 

 

Record-Play Switch

 

A serious attempt was made to stop these occasional bursts of instability, and so the record/play switch had to be completely disassembled as one line of enquiry.

This was de-soldered from the main audio board using a de-soldering station. Make no mistake, this is quite a time-consuming job.

 

In the end, the result was positive, so far - no more oscillations.

 

Switch removed from Main Board

Component Side with Switch Removed

After careful disassembly.

Contacts - All Cleaned.

Record-Play Multiswitch Ready!

Switch Returned to the Main Board

Bias Notes

On the front panel of the RS-630TUS deck are switches for bias and equalisation. It is quite easy to select a high bias for a Normal (Type I) tape, but of course using the default 120µS playback equalisation, or a 1326Hz 'Corner Frequency'. 

 

Currently, with the deck in its present state, a high bias record setting gives a frequency response from 30Hz to 14Khz at -3dB at either end, reference 0dB at 1Khz, from a TDK D46 Type I tape.

The selected high bias switch is insufficient for Chrome or Type II tapes in my opinion - there is too much distortion, any Chrome tape on this machine will be under-biased. I shall modify the circuit later and report back on this.

Front Panel Switches: Dolby/Bias/EQ/VU Metering

It was obvious that the action of the VU peak meter switching was inconsistent, and as a precaution since this deck is old, all four DPDT switches were removed. First desoldered, disassembled, and cleaned of long time oxidation and and accumulation of dirt.

Before de-soldering

Switch taken apart.

All four switches returned

 

Re-Capping

So far only the power supply unit and those electrolytic capacitors involved with the VU meters have been updated.

DC Motor & Controller

The DC motor and its controller circuit was opened up again, this time the brush and commutator design was investigated.

The commutator is a a little unconventional for this type of motor - in my opinion better than other simple DC motor designs. Here both commutator and brushes were cleaned, the former was cleaned first with a Q-Tip and switch cleaner, but later on with a little metal polish, then 'finished off' with switch cleaner again. Both ends of the motor shafts were lightly oiled.

A KSC1815-GR replaced the 2SC828,
for no ther reason that the 2SC828 was old.

The remaining 'power transistor' outside of
the motor with heatsink is a 2SB324 (PNP Germanium!)
The 2SB324 has not yet been replaced.

One electroltic capacitor was replaced.

The single blue tantalum bead cap of 100nF tested good.

Revised Wow & Flutter Figures

I was curious to see if any improvement could be made to the previous wow & flutter figures? To my pleasant surprise wow & flutter was reduced to a mean value of about 0.097% (JIS WRMS).

Speed Stability Revised

Casually, speed drift was examine over a period of about 14 minutes. Very pleasing results so far.

{Article unfinished, and also subject to changes and corrections.

16/10/2024}

Additional Pictures ...