Repairing the IC-735 optical encoder

Repairing the IC-735 optical encoder
Dr D. White MEng EngD AMIMechE MW3UZO © Jan 2009
V1.0 11th January 2009
Warning!
The optical encoder unit is very sensitive and only works properly when everything is aligned and in
order. Simply by undoing a few screws on the encoder itself, you can put it out of alignment and even
warp the internal masks, making it useless. If it still works and if the problem is intermittent but
bearable – leave it alone! Only attempt the repairs if you are prepared to accept the fact that the
problems could get worse and that you may have to find a replacement encoder that is quite rare and
expensive.
Introduction
This document covers the repair of the optical encoder for the Icom 735. It may be relevant to other
radios that use the same encoder assembly. Symptoms, reasons for failure and solutions are covered for
each case. Figure 1 shows my radio that has suffered from many problems, the latest being that the
optical encoder refused to work until the radio warmed up. This didn’t really become an issue until it
needed 5 to 10 minutes to start working.
Figure 1 - My long suffering IC-735
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I spent all of the evenings of a week and the weekend to repair the radio, it was very frustrating, fiddly
and it took 8 or 9 attempts at various solutions before it worked enough to be usable. So be prepared to
get annoyed. When you get tired and frustrated, put it down and start again the next day, otherwise you
may make a mistake and risk damaging the radio further.
The problem with my radio was tracked down to a phototransistor that had failed. I thought it wouldn’t
be too much of a problem replacing it and started to take it all to pieces. This is where I fell into a
number of traps waiting for the uninformed. I probably fell into all of them, which is why this document
is fairly comprehensive! My pain is your gain – don’t make the mistakes I did and you should get your
encoder working 100%. A good rule to follow here is DO NOT DISMANTLE ANYTHING UNLESS YOU ARE
SURE WHAT YOU ARE REMOVING IS THE PROBLEM. This will save you much frustration in realigning
everything in the encoder.
Symptoms
The symptoms of encoder failure are quite varied, but obviously involve some problem with tuning the
radio:
Radio will not tune when VFO knob is turned
Radio will only tune in one direction
Radio only tunes after warming up
Radio stops tuning after warming up
Radio will tune up and down randomly when you turn the VFO knob in one direction
A certain portion of the encoders range stops tuning or only tunes in one direction
Tuning quickly reverses the direction of tuning
Tuning slowly tunes up and down randomly instead of in the desired direction
One or more of these symptoms may be present.
Removing the rotary encoder unit
Only the top cover needs to be removed to perform the inspection and repairs. There are 12 screws to
be removed, two on either side of the radio, 4 on top and 4 around the speaker grille. When this cover is
removed, you can see the speaker and the heat-sink part of the final unit as shown in Figure 2. Before
removing the final unit, remember to disconnect the small phono patch lead at the rear otherwise when
the final unit is removed the phono connectors will be tugged and you may already know what sort of
problems this can cause. The final unit is fixed by two countersunk screws at the front of the radio and
two screws at the back. The servicing location for the final unit is as shown in Figure 3.
The optical encoder flying lead is connected to the main circuit board at the bottom left of the radio, as
shown in Figure 4. Disconnect the lead from the board.
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Figure 2 - Final and speaker unit
Figure 3 - Servicing position of the final unit
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Figure 4 - The optical encoder connector
Figure 5 - Removing the VFO knob with an Allen key
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Figure 6 - Removing the encoder unit
Figure 7 - The rotary encoder unit
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The VFO knob can be removed with an Allen key inserted into the hole in the knob edge that contains a
grub screw as shown in Figure 5.
The encoder unit itself can be removed from the radio by unscrewing the 4 screws on the encoder unit
and by carefully threading the flying lead and plug through the access hole inside the radio.
Figure 7 shows the rotary encoder unit as removed from the radio. Mine as already been heavily
modified and parts of the lead cut – yours will be nice and pristine.
Reasons for failure
Case 1 : Alignment of PCB
The PCB that contains the IR LEDs, phototransistors and switching transistors is mounted on top of the
encoder as shown in Figure 8. It is attached via two small screws. The alignment of this PCB is important,
as it has some play around the fixing holes which will shift the location of the phototransistors inside the
unit.
A typical problem caused by this is losing the ability to tune, or only tuning up or down. These problems
occur when one of the optoelectronic pairs stops working properly for some reason.
Figure 8 - PCB containing optoelectronics on top of the encoder
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Try this first – attach the encoder to the main board, loosen the screws and move the board around to
see if proper operation can be obtained. DO NOT REMOVE THE SCREWS AND PCB YET, as you could
damage something vital! Read the next case to find out.
Case 2 : Alignment of phototransistors
Before removing the PCB board from the encoder, be informed that the optoelectronics inside are
positioned around a set of very delicate and sensitive foil masks that can easily be bent and ruined by
the PCB board being twisted when inserted inside the unit. This is Trap 1 that I fell into. DO NOT DO THIS
IT WILL CAUSE YOU NO END OF WORK AND PROBLEMS. Place the encoder in a secure place for testing,
if it falls while the board is inserted but unattached, you will bend the foils particularly if you are holding
the flying lead at the time. With this in mind, carefully remove the PCB.
Typical problems caused by this is losing the ability to tune, or only tuning up or down. Again, these
problems occur when one of the optoelectronic pairs stops working properly for some reason.
Figure 9 shows the IR LEDs on the right, marked DS1 and DS2, and the phototransistors on the left
marked Q1 and Q2. For the encoder to work, the lenses on the sides of the devices must be pointing
directly at each other. Also, the devices must respond identically to changing IR light conditions so that
the quadrature phase (i.e. the square waves outputted by the encoder for each of the two optical pairs)
is correct. You may find that slightly offsetting the phototransistors or IR leds may improve performance,
but it is best to have the lenses pointing at each other when aligning other parts of the encoder.
,
Figure 9 - Position of opto-electronics on rotary encoder PCB
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Case 3 : Alignment of static mask or rotary mask
Figure 10 shows the rotary mask inside the encoder unit. It is fixed by three screws and a metal plate
that holds the mask flat against the plastic part of the rotor behind it. At the bottom of the image, you
will see two screws and washers either side that fix the static mask to the body of the encoder. The
static mask has two sets of five slits cut into it and the rotary mask has a circular set of slits cut around
its circumference.
Typical symptoms are tuning in the opposite direction to rotation, tuning up and down randomly or poor
performance when tuning slowly.
Do not loosen the screws attaching the static or rotary mask unless you are sure they are the problem
and they have to be removed. Do NOT lubricate the masks – this is Trap 2 that I fell into. The masks on
my unit were warped and lubricating them caused the masks to lie perfectly flat on each other due to
the surface tension of the oil. This appeared to improve performance, but in the end proved to be the
cause of tuning up and down randomly due to fluctuations in how the IR light passed through the masks.
They can be cleaned by removing them carefully and washing with switch cleaner and lint free cloth.
Read case 4 if you are going to do this. Remember that any loosening of the screws (Argh! - Trap 3!) will
require time consuming alignment of the masks.
Figure 10 - Foil masks that generate the optical pulses
To align the masks, first align the rotary mask. Rotate the shaft and look closely to see if the disk edge
wobbles even slightly at the edge of the disk. Loosen the screws and tighten them until it looks perfect.
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Do not attempt to touch or move the disk. Use only the action of loosening and tightening the screws at
the centre to align the disk.
What really affects the behaviour of the encoder is the positioning of the static mask. This is quite fiddly
to align. Attach the PCB to the encoder in the most central position you can and attach the encoder to
the radio. Loosen the static mask fixing screws and with a mix of loosening and tightening and moving
the mask with a fine pick or screwdriver find the best location for the mask. In some locations, you will
find the encoder tunes up when it should tune down and vice versa. When it is in the right place, you
will find that it tunes in the right direction with accuracy when tuning slowly or quickly in both
directions. Good luck!
Case 4 : Warping of static mask or rotary mask
Figure 11 shows the masks sitting on top of each other from the edge. The optoelectronics fit around
the disk. The masks must lie against each other and not be warped in any way. Even the slightest
amount of warp will cause problems. In the image, it looks like they are pretty flat, but they are not flat
enough, or in contact enough to work! From what people are complaining about on the internet it looks
like this is a common problem.
If there is a warping problem with the masks, then a symptom is that only a certain portion of a VFO
rotation will work, or the radio tunes up and down randomly when the encoder is rotated. However if
they are badly warped then the symptoms are as for cases 1 and 2 as one of the phototransistors will be
permanently switched on due to IR light getting through the masks due to the gap in between them.
Figure 11 - The masks must lie perfectly flat on top of each other
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It is possible to repair the warp providing it is not too severe. If the masks are only very slightly warped
then it is possible to close the tiny gap between the masks by folding a small piece of A4 paper in two
and placing it in the gap between the case and the static mask. This pushes the static mask against the
rotary mask. Figure 12 shows such an arrangement which solved many of the problems I had with my
encoder. Make sure that the piece of paper does not cover any of the slits in the mask. Do not use glue
to affix the paper. The spring from holding the masks together is sufficient to keep the paper where it is
located.
Figure 12 - Possible solution for slightly warped masks
If the masks are badly warped, then they will have to be removed. Note that which side of the mask
faces forward is important for both masks. The masks will not work correctly the wrong way around.
Using two strong, perfectly flat surfaces and some monitor screen wipes, sandwich the mask inside the
wipe and compress with the two perfectly flat surfaces. Striking with a hammer may aid flattening the
masks. Once this is done, then the masks can be refitted, a paper spacer inserted as described earlier
and the masks aligned.
Case 5 : Failure of phototransistors or IR LEDs
This is what happened to my encoder. The symptoms were no tuning until warmed up. When warming
up, approximately ¾ of the way through the encoder would intermittently start working, sometimes
tuning randomly, sometime in only one direction. In my case, the reason the phototransistor failed was
because when it was being mounted, the device leg inside the case moved and stressed the internal
connections. This led to a fault that appeared much later on in service. Upon examining the device, it
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was noted that the central leg was ever so slightly loose due to the bending of the legs required for
soldering. It would not surprise me if many of the failures out there were due to this.
In figure 9, the transistors Q3 and Q4 (labels covered by the components) are 2SC3399 devices which
have their own internal 47k biasing resistors. This explains the lack of resistors on the board itself. The
image shows replacement transistors that I have put in, your PCB will have the original devices in a ZTX
series transistor style package.
Exact replacement phototransistors and IR LEDs are not available. The original phototransistors have
quite a high gain and generate quite a square waveform. Transistors Q3 and Q4 square up the waveform
into a nice square wave ready for the radio to process. I experimented with some phototransistors out
of a mouse, which did not generate a nice square switching waveform and were a pig to get the
collector current right so that they worked. It was while trying these that I managed to warp the masks. I
had already unaligned the masks by checking them.
After much experimentation, frustration and discovering of the problems in the other cases, soldering
and de-soldering, making small veroboards to fit in the flying lead to further condition the waveform I
finally found the solution.
If failure of the phototransistor is the problem then you will need to carry out all of these modifications.
You cannot just change one of the phototransistors as they need to be of exactly the same type to
maintain the timing of the quadrature output. Everything will need to be replaced apart from the series
resistor for the IR LEDs and the capacitor and the IR LEDs if they can be proved ok.
To check if the IR LEDs are working, measure the voltage across both of them and each of them. There
should be approximately 1.4V across each of them and around 3V across the pair. If they do not
measure this, then there is a faulty device or poor connection. If you have a camera with IR night vision
capability then this is ideal for checking that the devices are illuminated. If they cannot be directly
checked like this then just replace them.
Figure 13 shows the original circuit, and Figure 14 shows the replacement circuit.
Replacement IR LEDs are Sharp GL480E00000F devices and are available from Rapid Electronics in the
UK, order code 58-0962. Replacement phototransistors are Sharp PT480E00000F devices and again are
available from Rapid, order code 58-1062. The replacement transistors are of Darlington type so that the
processed waveform is as square as possible – I used MPSA13 devices. Order several of each device as it
is easy to make a mistake. The IR LEDs in particular need soldering with a low iron temperature or the
case will melt and the lead separate from the device! I was as surprised as you probably are reading this
when I removed an LED because it wasn’t working for some bizarre reason and the leg came off in my
hand!
The new circuit needs to be built on the little PCB – keep everything as short, tidy and low profile as
possible as there is not much room to work with when replacing the encoder back into the radio. Figures
8 and 9 should give some clues on how to fit the components into the space available. Note that the
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phototransistors are 2 leg devices which need to be attached to the left and centre pads on the PCB
when viewing from the foil side. The right pad is a ground connection, make sure the leg of the device
does not touch this pad.
5V
5V
5V
Original optical encoder circuit for
the Icom 735 transceiver.
Red
220R
Yellow
10nF
47k
Phototransistor
2SC3399
IR LED
Brown
47k
Blue
IR LED
Phototransistor
Orange
2SC3399
47k
Green
47k
Figure 13 - Circuit diagram of original encoder circuit
5V
5V
5V
Red
5V
Replacement optical encoder
circuit for the Icom 735
transceiver.
220R
10nF
IR LED
GL480E00000F
Sharp
Brown
Phototransistor
PT480E00000F
Sharp
Phototransistor
PT480E00000F
Sharp
Blue
Green
Yellow
IR LED
GL480E00000F
Sharp
4.7k
4.7k
Orange
MPSA13
4.7k
MPSA13
4.7k
Figure 14 - Replacement optical encoder circuit
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Conclusions
Hopefully this has explained some of the problems that can occur with the optical encoder on the Icom
735, and helped avoid further damage to the unit that can occur by accident. My encoder is now fully
functional which is a miracle in itself due to the amount of work, and unfortunately damage, that has
been done to it. However the journey has led me through all what is likely to go wrong with one of these
encoders.
The 735 is a favourite of mine due to its excellent audio and small size. It is typically used for portable
and alternative location operation. Good luck keeping yours alive.
Dan.
Dr. D. White (C) January 2009 Version 1.0
This document may be distributed freely, however when distributing via the internet, please link directly
to the file on Highfields ARC website where the document is kept and updated.
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