Experiment only, not Recommended.

Aiwa AD-F770 Capstan Motor

Regulator Replacement
(Just an Experiment)

Quite recently I decided to return the original troublesome '12v 2400rpm CCW' capstan motor back to the Aiwa F770 cassette deck for further investigation. Previously, I had replaced it with a probable Chinese copy of a Mabuchi DC motor. 

I chanced my luck and replaced two internal electrolyic capacitors hoping that the onboard regulator would work correctly. It wasn't long before the transport (playback) speed of the deck began to show the same subtle variability as before - the very reason I replaced it!

Backplate of flywheel - note the regulator circuit inside the motor circuit.

Front view

I again examined the original regulator circuit and ordered some 2SA684 (PNP) transistors. There were also zener diodes in this old regulator circuit, and I soon realised that I may need to replace all the components to be sure this would work again!? 

While I waited for the transistors to arrive, I mulled over the idea of designing a simple, but highly stable voltage regulator, centered around the popular National LM317.    

The original voltage regulator sits on the back of the dc motor.

Desoldering the old regulator circuit from the original motor I needed to find out what voltage the motor operates at the 'correct speed'? - the result was approximately 5.5v. With this in mind, I built and tested a circuit to deliver trimmable motor voltages between 4.7 and 6.0 volts, approximately.

The LM317 Based Voltage Regulator Circuit:

Note: A tantalum capacitor was initially added across effective-R2 as recommended by National Semiconductor (to reduce ripple effect), but this made the output slightly unstable - approximate deviation of about ± 2% when stable, so I removed it. I also added a small 10uF tantalum capacitor at the output with the addition of a protection diode across the LM317, and another across the motor. Probably not needed, but I put them there.

 
Note that as Vout = 1.25(1+R2/R1), the effective value of R2 is given by ....

R2= 1200(1200+Rv)/[2400+Rv]

Where trimmer Rv is approximately 2600Ω (by measurement), at maximum setting.

Simulating the 'new' regulator circuit.

Using stripboard ("veroboard"?), the said circuit was then assembled, and tested again. Later the circuit was encased, and secured into a simple plastic insulating case. All connections to the F770 were resumed.

The motor needed to be returned to its casing - all previous regulator components (except an 8-pin chip) were removed, and a new lead was soldered in.

Correct Playback Speed Alignment: Using a full track width reference tape, and an external digital source I aligned the motor speed by listening to the two sine waves 'beating' together until there is zero, or a very slow drift between the two.  (17/04/2020)

 *************

18/4/2020 ... This morning I ran some more tapes through the F770, and again the speed began to wobble slightly - it's very subtle, but noticeable over a period of about 15 minutes.  It's so subtle that you can be forgiven for thinking "did I hear that pitch wobble?...".

Conclusion: Now that the regulator issue may have been eliminated, the motor is probably partially worn? I suspect brush-commutator contact is partially failing?

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

New Mabuchi Motor Fitted EG-510ED-2B2 (18/4/2020)
After realising that the old motor was showing its age, I decided to discard with the idea of a new stable regulator - it worked well, but the motor wasn't behaving itself! 

I've now fitted a genuine Mabuchi motor into the Aiwa AD-F770, after putting the parts back together, the deck is now working very well.

The Aiwa supplies 12.5v to the dc motor, and the motor draws an average of about 58mA when in play mode, and a little less when just running free.  Of interest too, when the old motor was in circuit, it drew only around 40mA - perhaps there was an issue within the old motor were brush/commutator contacts were more resistive due to dirty or worn contacts?

The old motor dissasembled is shown below.

From close-up inspection, I could see why the motor was varying its speed - the commutator brushes had partially broken. There was also a small build up of carbon deposites on the commutator.

General Comment about the AIWA AD-F770

This is a very good performing machine, but personally I wouldn't recommend buying a F770 due to the complexity of the internal circuit layout. They are awkward to fix or service, so lot of patience required!

7/05/2020: Judging the feedback from another website, I think it is important for the reader to understand that the use of the LM317 voltage regulator (as stated above) is an *experiment* - it is not recommended. 

Dedicated DC motor controllers typically sample the change in motor shaft speed due to a change in armature current, or terminal voltage.

If the load torque 'T' increases during an interval of time 𝚫t, the motor will slow down, the back EMFwill decrease (since EMF∝ speed) , and so armature current increases. 

(Note: T ∝ armature current). 

The controller then (typically an AN6651 in circuit) increases the terminal voltage by a proportion so that constant speed is maintained.  The reverse is true if the load decreases.

8/05/2020: I have just ordered some AN6651 controller chips after managing to decipher the AN6651 (and other similar) datasheets. I hope to devise a general solution to the problem of ageing DC motors in cassette decks by using and modifying third-party DC motors that can run at 2400rpm.

I have begun a new article here ... https://cassettedeckman.blogspot.com/2020/05/external-motor-controller-experiment.html

Over the next few weeks I hope to establish a reliable working modification that can be used for other third party DC motors. 

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(Note: www.blogger.com has editing/software bugs, and so I may need to revise or edit the articles without notice. It's very irritating for me to do, and for you to read - so apologies.)

Line Signal Muting

Line Signal Muting

While studying my Sony TC-K61 cassette deck, I was intrigued how the muting transistor action works? After sketching out a similar circuit of my own to analyse and assemble, I think I may now understand how this muting is achieved.

Here is Sony's muting circuit for the TC-K61 cassette deck for both recording and at Line Out - marked in green.

After sketching out my own generalised circuit, I had some components easy-to-hand, and so put this simple circuit together.

Here we have - an external signal generator shown to the left with its own internal resistance of around 600 ohms. Then I just added a simple, non-specific RC network to simulate general external circuitry. After that there is a load resistance of 20,000ohms. An oscilloscope was connected across the load RL.

The muting transistor here is simply a S9014 NPN which is about to be switched 'on'. A sine wave signal voltage of 1Khz across RL was set to 2 volts peak-to-peak with no interference from the S9014. Once I had switch the S9014 'on', the output completely muted. 

The results ..

VL (before muting) = 2v.
VL (during muting) = 5mv.

This gave me an effective reduction of 20Log(0.005/2) or -52dB.

So why does this work?
Firstly, note that - the transmission line is effectively dc decoupled. Secondly, the S9014 is not experiencing any external one-directional electric field (hence voltage) to motivate charges across the collector-base-emitter junctions. 

So how does forward biasing the base-emitter junction result in an efficient -52dB muting reduction?

My theory: Both base-emitter, and base-collector junctions are forced into forward biasing modes.

There is no, or very little dc current running through RL, but since both forward biased junctions are now offering a conduction path, and in particular to an ac signal - it is this ac signal voltage component that gets shunted through both junctions as both junction slope resistances 𝚫V/𝚫I (thanks to biasing) are very small.

Effectively, the source ac signal (component only) voltage drop, almost entirely occurs across the 600Ω, and 670Ω+j/𝛚C impedances since the slope resistances (collector-base, base-emitter) are so low in comparison. It's basically potential divider law, the ac signal voltage at the two junctions is 'lost' earlier across the said impedances. *

(To understand slope resistance 𝚫V/𝚫I or slope conductance 𝚫I/𝚫V in this context, you'll need to study transistor NPN junction characteristics, and in particular the Ic vs Vbe (or even Ie vs Vbe) curve which are not always illustrated in datasheets) *

Shunting of the ac signal works in both directions, only one direction shown above.

Can we expect a forward biased base-emitter junction to offer a better conduction path than that of a forward biased base-collector junction?, probably, but I haven't investigated this.

I noted that during this electrical state, there was a very small dc potential across RL (collector-emitter) of 6.9mV, and again 6.9mV when I completely removed the circuit to the left of the S9014. This suggests that while one junction (probably base-emitter) was approximately 0.7v (forward biased), the other PN junction was at around 0.7v-0.0069v; effectively the same!

One final note - efficient muting was also realised when I reversed both emitter and collector.

23/03/2020.

2nd revision, 24/3/2020.
** 3rd revision: 20/03/2024

 

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

I also later simulated the circuit in https://www.falstad.com/circuit/circuitjs.html

The results were different concerning muting ability - this simulation was less efficient for some reason?, but the remaining analysis was reasonably accurate. You can see that both PN junctions were effectively forward biased and the slope resistance very low.

 
25/03/2020.

Recording/Playback Level Instability

Sony TC-K61 Recording/Playback Level Instability

For some time now, I have noticed that the deck's ability to record and playback at stable levels is questionable. Quite often I would have to recalibrate the playback levels (via 400Hz Dolby Level ref tape), and re-configure the record levels for my Maxell UR tapes. These random deviations from the target values were not small, but in the order of 2dB-4dB.

Frequently, it was the left channel that began to vary with time - was an hfe or hFE (ac and dc current gain) transistor parameter drifting as the deck warms up in the playback circuit? - if so, which one? Often small variations in hfe in a circuit with negative feedback have little effect. However, with reference to the playback circuit below - are the signal inverting transistors Q104/Q204 exhibiting problems?

New Components: So far I have replaced every capacitor in the audio and control boards, including all bias frequency generator capacitors. The bias frequency is stable at 100Khz, and generates around 30v (peak-to-peak) for Type I tapes at the record head terminals. Type II setting yields 34v, and about 65v for Type IV tapes.

Signal Path Relay: I even changed the Record/Replay Head signal relay to eliminate any possibility of a potentially and slowly oxidizing contact?

Old G2V-2 relay replaced with OMRON G5V-2. Note: old caps were still in circuit at the time of this image.

However, this full capacitor and relay replacement did not cure the problem, so what was it?

Playback Circuit: It was only recently that I began to study the playback circuit which consists of three transistors in an apparent Current Shunt negative feedback configuration - not Voltage Shunt as I originally thought? They are: 2SA836 (PNP), and the 2SC1345 (NPN). These are low power, low noise-figure (NF) transistors, which are no longer made.

There may be several schematic errors in the service manual?
Example: above Q103 and Q203 there are a biasing resistors in circuit, not capacitors.

2SC1345 Replacement: After searching the  internet for suitable replacements, I decided to 'chance' a KSC1845 Fairchild transistor which I had in stock. And which offered similar specifications - in particular dc hFE characteristics. As mentioned above, the basic purpose of Q104/Q204 is to act as a signal inverter, current-shunt negative feedback is later fed from Q105/205 to Q103/203.

From voltage measurements, the default 2SC1345 dc quiescent collector current for Q104/Q204 was approximately 310uA. Replacing both Left/Right channel 2SC1345 transistors with KSC1845s worked well - producing the precisely the same quiescent collector currents!!! Wow!

Board Modifications: Note - I have reversed the record and playback level potentiometers, re-soldering them so they are easily accessible. The 'new' KSC1845 NPN replacements are marked in red. (long pins)

Assessment: Currently, the replay amp is working well. After playback and record calibrations, the TC-K61 seems to have stabilized for both left and right channels.

Time will tell if I have found the source of the problem, fingers crossed!

Article subject to revisions, and correction of mistakes. 18/03/2020.
+10th minor revision, 31/03/2020.


26/03/2020: I have now replaced the Q103/203 and Q105/205 (2SA836 PNP) with Fairchild KSA922FBU PNP transistors, and recalibrated the TC-K61 - the deck's playback circuit is very stable! The dc quiescent collector currents Ic, and Vce quiescent voltages differed from the original 2SA836 configuration by between 2% to less than 5%. Virtually identical - good!

Finally, the buffer amplifier at the input line-in stage appears to be a simple emitter-follower configuration. The old (but not faulty) 2SC1345E was replaced with a Fairchild KSA1845FCK NPN transistor.

 The TC-K61 is working superbly well for Type I tapes!  

Marantz SD35 Cassette Deck
Servicing and Alignment

Received this 1989 Marantz SD35 cassette deck off a relative - it had been in his loft for a number of years, and initially we didn't know if it was working. It was thankfully, and so now I thought it was time to check this thing out in more detail.
 

After the usual dusting or wiping off of debris, I opened the SD35 up and found that all was good internally. There was naturally some dust and corrosion present as expected, but was only minor.
 

I needed to investigate whether the drive belt needed replacing. The cassette tape transport and motor mechanism are cheaply made, and a bit delicate, so I had to handle with care. After extracting the mechanism from the main unit, I realised that to replace the belt meant more careful disassembly. I checked the elasticity of the drive belt, and everything seemed to be fine - so no need to fix that issue!
 

Cassette transport and motor - top right.

On reassembly, the machine worked but then a little later decided not to work!? The next day, I opened up the deck and carefully disassembled, and reassembled again the cassette transport unit. All was working back to normal!

Later, all external and internal potentiometers were switch-cleaned with Servisol, and then the heads and capstan were demagnetised.
 

Next on the to-do list was the record/replay head alignment - using an ABEX 10Khz reference tape I aligned the head azimuth to the tape. The original setting was out by a fair margin, ~ 180° at 10Khz.
 

So now I could use my ant-audio.co.uk 400Hz full track width Dolby Level reference tape to calibrate the output at Dolby Level, making sure both left and light channels were equal - I eventually settled for an output of slightly under 500mV (RMS) at Dolby Level.
 

At this point the peak reading LED meters were only a fraction out, but I re-calibrated these regardless. Unfortunately, in order to set the peak-level meters, I had to take the LED display and control board off the chassis and adjust in situ - thankfully it was not difficult.

I later calibrated the internal record levels to match that with the sensitivity of the current batch of Maxell UR C90 tapes. That is: record at 315/333/400Hz at 0VU or Dolby Level (Dolby Level = +2.7dB above 0VU), and the tape now plays back at calibrated level.

The deck is now playing well - very stable sound, very pleased.

Back in 1989, this retailed for around £149.99 in the UK.


Article subject to alterations and corrections without notice. 21/02/2020

Sansui SC-1330

Cassette Deck

Maintenance/Servicing

Bought off ebay for little money, this was advertised as ´For Spares or Repair´. It was in good physical condition, and the heads had very little wear. The pinch roller looked aged, but after some cleaning with isopropyl alcohol, and later filtered water on a cotton bud, it is nearly as good as new!

So far I have replaced the old drive belt - a 72mm or less diameter is fine, 3.5mm width, although I have just ordered a 64mm diameter 6mm width belt, let us see how that works later when it arrives!?  

12/2/2020: The 64mm diameter, 0.7mm thickness, and 6mm wide belt was more difficult to fit. Overall, the tension was too high, due mainly to the thickness (0.7mm) of the belt. Also, this belt wouldn't always run well - not sure why!? So I reverted back to the 72mm belt.

I think a 68mm-70mm diameter, 3.5mm-5mm width belt would be optimal?

Replaced most electrolytic capacitors, and cleaned all potentiometers with Servisol. 

Rubber Renew

Bought 125ml of this toxic chemical, and so decided to use on my machines. It smells like rubber solution, only stronger! I decided to buy this stuff hoping that it would return the power and hence internal 'grip' for the tape rewind, fast forward, and the take-up spool during recording. It did the job, but I had already cleaned much of the idler gears with isopropyl alcohol, and diluted acetone anyway - I'm not sure if Rubber Renew is worth the money?

If you use Rubber Renew, be sure to follow the safety instructions.

Noisy ON/OFF switch 
The Sansui SC-1330 makes a small electrical arc when switching on or off. This interference can be clearly heard through both headphones and the loudspeakers. On investigate I saw that the internal mains switch does not have any form of arcing/noise suppression.

After studying the circuit, I decided to bridge the switching contacts with high voltage rating, ceramic capacitors - marked below in blue.

The diagram above was taken from the service manual. I also noted that only Japanese versions of the SC-1300 series had suppression support!?

I carefully soldered in some 3Kv (not 3.3Kv as suggested in diagram) rated capacitors - yes these are probably over-rated, but best to be safe than sorry?! These completely eliminated any sound blips due to on-off switch arcing issues. I didn't bother to add a small resistance in series with the capacitors - these limit the transient currents which only last for some micro-seconds anyway. This type of suppression circuit is know as an RC Snubber Circuit.

My alteration to the mains board is shown below.

Sansui SC-1330 Calibration
I have also calibrated the machine so that -

• Test Tape Input: 400Hz Dolby Level RefTape, gives Vout = 500mV (RMS)  or ∼1400mV p2p.
• Calibrated the peak level meters to read Dolby Level at RefTape Dolby Level input.
• Calibrated the internal Record Levels (for Maxell UR), this means .. record at 0VU, and achieve 0VU on playback etc.
• Bias adjustment: 1000Hz to 10,000Hz flat for Maxell UR tape. 
• Rec/Replay Head azimuth check with my ABEX 10Khz ref tape - the original setting was spot on!!!

21/02/2020

29/04/2020: I decided to give the shorter and stronger 64mm x 6mm belt another go. This time I had cleaned the pully wheel carefully, "roughing up" the surface for a better grip. I also allowed the flywheel to guide the line of the belt around the pulley - this was done by losening the pulley screws and turning the flywheel with my finger so that the belt would find a natural line, the pulley would then slide outward a little more. Finished this alignment by tightening the pulley grub screws.

Results: Both fast-forward (FF), and rewind (RW) are much better than before - no stalling!

So far, so good!
 

Common Emitter, Voltage Shunt Feedback

Common Emitter Transistor Configuration

NAIM NAC32.5 and NAIM NAP110 Servicing.

This intended 'servicing' of the above old NAIM products, is going to be in 'slow motion', in other words - a slow, but on going job, and one that will reveal some discoveries, and pending problems - no doubt!

Not used '32.5' and the '110' for several years, I decided to fix a broken stereo/mono toggle switch, which from start to finish took nearly three hours.

In order to get the board out, I decided to desolder the rear DIN and phono connections. The replacement mini toggle switch was a Dual-Throw, Double-Pole type 'DTDP' with no 'latch' effect.

Top Toggle Swich was broken - no easy direct replacement available.

Having got the NAC 32.5 working again, I was puzzled that the pre-amp in mute mode didn't null-out the sound completely - something that I'd not noticed before? 

Working the NAP110 again
Switching on the NAP110 Power Amp soon revealed that something wasn't right - the NAC32.5 was not receiving 24v from the NAP110. The only indication that some power was getting through was a faint light from the LED on the 32.5. Re-connecting the NAP110, the NAC32.5, and a SNAPS unit (to run the 32.5) lead me to believe that the power supply from within the NAP110 was faulty? Both pre-amp (32.5) and amp (110) played well once the SNAPS unit was in use - although on occasion, a sort of disconnecting noise could be hear randomly. Further investigation later revealed that the regulators were not functioning properly.

A quick look into the NAP110 revealed that the amplifier's in built low-current LM317 voltage regulator (and/or 3300uF cap?) was faulty - only showing +2.9v dc, as opposed to +24v dc. This commonly used regulator circuit appears to supply any associated NAC controller of that era - I'm thinking mainly of the NAC 42, and NAC 32/32..5 series? 

Obviously, a few or all of the components need to be replaced, then some re-testing......

NAIM NAP 110 Power Amp with regulator for external provision.

With LM317 based regulator removed.

NAIM 36v-to-24v Voltage Regulator
 
The standard NAIM regulator for 36v-to-24v conversion is a common circuit configuration, but with additional ripple effect reduction via C2. 

 

This is able to reduce ripple to around -80dB, according to Art of Electronics (Paul Horowitz/Winfield Hill). Their research suggests a 1v ripple reduced to a mere 0.1mV!, at 60/120Hz I assume?

Below, I've drawn out the circuit.

The Components of the LM317 Design

Flywheel BYF406 diode: 1.0A Iout, 800V Vrrm, Fast Recovery Rectifier Diode. Operational temperature range from 0^°C to 175^°C. If a replacement is required I'll use a 1N4006 diode, although I only have the 1N4004 (1.0A Iout, 400v Vrrm) in stock, so maybe I'll use this instead? 

BYF406 Purpose: probably to shunt any charge from C2 should either C3 or the load 'short'. And additionally to protect the LM317T from any back emf if the load become immediately disconnected.

Capacitors: both 10uF capacitors are tantalum (a transition metal) bead types.

Resistors: Are all 5% tolerances.

Regulated Voltage Output: The Formula for output voltage Vo of the regulator is: Vo=1.25(1+R2/R1). And here (without error tolerances taken into consideration) we obtain..

Vo=1.25(1+3980/220) = 23.86volts.

So I can conclude that the layout 'checks out' nicely with the theory, and with parts currently in the post, I can look forward to rebuilding the regulator.

NAIM SNAPS for the NAP 110 System

Coincidentally, the NAIM SNAPS (power supply) unit also employs the same LM317 based regulator circuit, but here there are two of them.

To my complete disbelief, one of the regulators in this unit was also faulty, delivering zero output, that is 0V! While the other was working nicely at 23.7v dc. 

 

Seems, that this unit also requires a complete service too. I did note that the unit soon got warm after switch-on, an indication that the LM317 was shorting via C3?

11/12/2019: With reference to the NAP110 Power Amplifier
Replaced unit with new regulator: the LM317T, 2 x 10uF tantalum capacitors (where one had initially 'shorted'), and the flywheel/protection diode was replaced with 1N4004 type. 

Since then, I have been testing the NAP110 with my NAC32.5 - so far all working well!

The meter was flickering between 23.8v and 23.9v. The calculation above was 23.86v, the target is 24v.

Testing the NAC32.5 and NAP110 after regulator circuit fix.

12/12/2019: NAIM SNAPS unit now has had all LM317T and output 10uF 35v tantalum capacitors replaced in the voltage regulator circuit. Unit working well.

Large 10,000uF Electrolytics:
(18/12/2019) I will eventually replace all 10,000uF electrolytic capacitors in time - have just ordered KEMET 10,000uF 63V soldered-tag type (35mmx55mm). This is an over-specification, but I prefer to over-engineer. The original voltage limit of the old capacitors was 40volts. 

So far all units are working well!

Power-on Diode: (13/12/2019) The NAP110 power-on diode failed a long time ago, so I decided to replace it. Not sure of the exact specifications so I decided to put my own circuit in place - a simple red LED, in series with (effectively) 30K ohms. The supplied voltage to the circuit is approx 36v. I wanted to try to match the brightness of this with the brightness of my NAC 32.5 contoller's LED. After a few investigations, even 30K (10K+20K) ohms was not high enough resistance to create a good match, but it's good enough for now - I'll change it again later to perhaps 51K ohms?

 

The circuit is shown exposed (ie un-isolated) for the purpose of the photograph. I used an old small circuit board and adapted it, but I will re-configure it again with some veroboard I recently bought.

27/12/2019: Finally settled for 2 x 100K ohm in-series resistors for the LED, which gives a good match to that of the NAC 32.5 and SNAPS LED intensity - not quite the same, but close enough.

(The circuit has been modified slightly since I took this photograph)

The revised circuit board is isolated underneath with a rubber mat, cut from an old bicycle inner tube, super-glued and shaped to fit. The LED is temporarily centred with Blu Tack. There's no danger of the assembly falling out - with or without the Blu Tack. The hole in the fascia, is actually threaded, I didn't realise this until re-assembly.

Later on, I'll re-cap the NAP 110, and the SNAPS with new 10,000uF KEMET capacitors. These arrived this morning...

Also of note - a heavier duty full-wave diode rectifier for the power amp, which I may or may not fit later?

20/12/2019: The SNAPS unit is now fitted with a 10,000uF 63v KEMET electrolytic capacitor. The 'spades' on these are not as easy to work with as the original ITT capacitors. I applied plenty of solder and secured the initial soldered connection, and later went further by wrapping the soldered joints with wire, then applying more solder so that the original joint could never seperate. The downside of this additional procedure is that the joint 'looks' untidy from the outside - this I find a little annoying if I am honest!

(The servicing of the NAP 110, NAC 32.5, and SNAPS unit is unfinished. All working fine so far. 18/12/2019)