The tape speed control adjustment for the WM FX443/445 is the same as for WM FX433/435 as shown on post #15.

It consists of a potential divider circuit of three resistors:

R605 - 5.6k
RV601 - 4.7k variable pot
R604 - 4.7k

The control voltage at the wiper of the pot is about 0.2V
We need to modulate this voltage from about 0.15V or 0.25V to 0.2V.
We have to determined what direction to move in order to lower the speed. This can be checked with a voltmeter as one adjusts the trimpot one way or another.

That's the easy part.

The next step is to find and purchase a suitable range finder. Most ultrasonic range finders will output a digital pulse whose width represents the time-of-flight for the sound to be transmitted and reflected back from the target, i.e. twice the distance to the target.

Next, we can convert this information to provide a voltage to modulate the tape speed.

 

Brilliant, thank you once again MrChips.

Tomorrow morning i will source a voltmeter - will any voltmeter do the job?

I will let you know as soon as i have the units open and i am able to test. I should be able to do this tomorrow afternoon, UK time hopefully.

 

Make sure you put a test tape into the unit before adjusting. Once you twiddle the adjustment it will be very difficult to get it set back to the original setting unless you have a copy of a speed alignment test tape.

If you do not have an alignment tape, create your own by recording a test tone of known frequency such as a musical instrument, piano, organ, flute, pitch pipe, tuning fork, telephone tone etc.

 

...
The next step is to find and purchase a suitable range finder. Most ultrasonic range finders will output a digital pulse whose width represents the time-of-flight for the sound to be transmitted and reflected back from the target, i.e. twice the distance to the target.

Next, we can convert this information to provide a voltage to modulate the tape speed.

The Maxbotics ultrasonic distance sensors from Pololu.com have a number of outputs, one is an analogue voltage output which is linear to sensed distance.

 

Make sure you put a test tape into the unit before adjusting. Once you twiddle the adjustment it will be very difficult to get it set back to the original setting unless you have a copy of a speed alignment test tape.

If you do not have an alignment tape, create your own by recording a test tone of known frequency such as a musical instrument, piano, organ, flute, pitch pipe, tuning fork, telephone tone etc.

A steady frequency, such as an organ note, will be a great substitute.

After all these years of using, repairing, cassettes and old reel to reel recording machines; I have never bought the Master head alignment tape.
Maybe I've been extremely lucky.

 

Thank you everyone for this advice.

I have to wait until tomorrow to use voltmeter, but i have made progress in disassembling the machine. The ribbon cables were a little tricky, and it relies mainly on clips instead of screws, but i've managed to get inside it and locate the potentiometer. Will also get on the case with that test tape MrChips.

MrChips, can i also ask about the voltmeter. I've never used one before, but i understand the principle. Also i will be working with someone who is very familiar with them. But, where in this case will i put the contacts?

Also, can i ask what the function of the two other resistors is? Just trying to get my head around everything!

The_RB - someone else recommended that model to me today. What is it about this model, over the one previously mentioned in the thread that would help do you think?

Hoping to get a feel for the different functionality of these sensors.

 

The speed adjustment requires about 0.2V at pin-11 of IC601 LB1674V. This voltage is derived from 0.5V reference at pin-8. Trim-pot RV601 is part of a resistor chain consisting of

R605 5.6k
RV601 4.7k
R604 4.7k
TH601 3.3k

TH601 is a thermistor and is used to provide temperature compensation.

The resistor chain is what we call a voltage divider. Resistors R604 and R605 are used to reduce the range of the signal at the wiper arm of the RV601 pot. Instead of swinging between 0V and 500mV, the wiper arm will produce a voltage from 150mV to 280mV. This allows for a finer adjustment.

Such an arrangement with fixed resistors on both arms of the pot is quite common when you want to reduce the adjustment range.

You will set the voltmeter to a 0-2VDC range.

Touch the RED lead to the centre connection of RV601 as shown.
Connect the BLACK lead to a convenient 0V reference point as shown. If the circuit board is coated with a solder mask you may have to scrape off a bit of the mask to make electrical contact to the trace.

If you post a photo of the circuit board we would be able provide better advice.

Without adjusting RV601 your voltmeter should read about +0.2V.

With a test tape playing, adjust RV601, write down the voltage reading and make a note of whether the tape slows down or speeds up.

Adjust RV601 in the opposite direction and test as before.

With this information we will be able to design the circuit to modulate the tape speed in the desired direction.

 

The Maxbotics ultrasonic sensor from Pololu.com as suggested by RB looks like just the thing you need.
Model MB1300, AE0 #1660 has a range of 0-7.65m (25ft). It outputs an analog voltage of 4.9mV/cm.

Let's hope it is in the right direction you want.

 

This is great. Thanks Mr Chips.

I should be able to do this this afternoon i hope. I do have one other question - how do i adjust the RV601? Would that be with a micro screwdriver?

Here's a scan of the board (scanner seemed to work better than my camera)

I put some arrows where it seems that the RV601 is, just to check. Am i looking in the right place?

 

I am surprised that if you insist on working with Walkmans, that you wouldn't also insist on Polaroids famous ultrasonic senors. Regardless, digging into the past, the most annoying thing I remember about the Walkman was it's frequency shifts when the user moved around. The most striking thing I remember, clearly to this day, was when I got out of the car and the music came with me. It was a life altering moment.

 

In order to perform the tape speed tests you will have to reassemble the WM back to working order.

You will have to do some soldering work. Get two lengths of fine insulated wire of two different colours. The finer the better. 30 AWG wire-wrap wire would be ideal.

Gently scrap away the green solder mask at the trace I have indicated in YELLOW using a razor blade or XACTO knife. Tin the copper trace revealed with a tiny bit of solder. Tin the end of your wire and solder the wire to the trace.

Solder the second wire to the point I have indicated in GREEN. This is your GROUND connection. Twist the two wires together and run the wires outside of the WM. Reassemble the WM.

Get some resistors from 1k to 10kΩ. We will use this to bridge across the two wires and listen to hear what happens to the tape speed.

 

Thanks MrChips. Soldering i can do, so this bit i should be able to manage i hope. Will need to pick up the resistors and wire, will try and do that tonight or tomorrow morning.

 

Have you done the initial testing of speed control range? If not, you should power it up and play some music or a fixed tone tape, and vary the trimpot through its full range and see what range of speed control you get.

That trimpot might only give you +/- a couple of percent speed variation.

 

Hi THE_RB,

Thanks for the suggestion - could you let me know how i would go about that? Sounds like a good idea.

I have a variety of resistors, and the other equipment needed now so i will start to experiment this evening.

 

All you need to do is play a tape and turn the trimpot and listen. You're the one who knows how slow it needs to be to get the right effect. You can turn the trimpot with a little flat-blade jewellers screwdriver.

 

Brilliant, thanks THE_RB, i'll give that a try and see what happens.