ShumaTech Construction Notes

please send me any of your own tips or experiences to include on this page

Just a list of general tips that can help you with construction

Soldering Transistor Q1

When you take a close look at the PCB, you will notice that the pads for transistor Q1 are very close together. Additionally you may notice that the solder mask does not cover the space between the three transistor pads.

I find this problematic to solder.  Often I will find myself bridging one of these pads.  Take care here, it is the only difficult part of soldering in the whole project, so take your time, examine closely.

The best approach is to minimise the amount of solder feed into the joint which should lower the chance of bridging these pads.

Resistor Arrays

The 3 resistor arrays (R12, R13, R14) are sometimes difficult to obtain.  When this happens, I use plain 1/4 watt carbon film 150 ohm resistors.

You will need to bend the legs very close to the body of the resistors to enable them to fit the DIL spacing.

Due to the heat generated by the resistors, you will also need to keep them up of the board by a millimetre or two.  I use a small piece of plastic cut to size so it will fit under the row of 8 resistors.  Once the resistors are soldered in place, this piece of plastic can be slid from underneath the resistors.

Soldering Switches onto the PCB

You must make sure that the switches are fully seated and flush with the board.  If you have only one switch that is not fully seated then you will compromise the performance of the whole key pad.

This is important - spend some time here to get it right.

Mini-Din Sockets

The mini-din sockets are really meant to be mounted on a 1/16 thick inch metal chassis. The case specified is thicker than 1/16 inch. This extra thickness will not allow most (all) mini-din plugs to mate correctly.

To overcome this, use your 90 degree counter sink tool and counter sink the OUTSIDE of each of the mini-din holes. This operation will allow the mini-din plugs to mate correctly and they will no longer work themselves loose.

Power Socket

The power socket mounted to the case is not very secure and will likely fall out with a bit of vibration. This power socket suffers the same issues as the mini-din sockets - it is made to snap into a thinner material.

The solution is the same as the mini-din sockets. Use your 90 degree countersink bit on the INSIDE of the case to remove some of the material. You will now find that the socket will be much more securely mounted.

Machining holes for Switches

Make sure you use either a 1/2 inch or 13mm bit for machining the holes for the switches. The drawing states 12mm but that assumes that everything is perfect.

The extra space will allow for some of the switches to be a little off-centre and still operate freely.

Making the holes a little larger than 12mm makes no difference to the operation or appearance of the final product.

Soldering

The component holes on the PCB are not always plated through from one side to the other. Therefore the construction is dependant on the solder flow up the leg of each component to connect the two layers. Your soldering technique is relied upon to achieve this flow.

PCB construction like this is quite common and in no way does it reflect on the quality of the PCB - you just need to be aware of it. Therefore, you must make sure that solder flows up every component leg to join with the other side of the PCB.

My technique is: Soldering iron set to 280 degrees C. Heat leg and PCB for 2 seconds, feed solder for 1 second, retain soldering iron on joint for additional 2 seconds (this creates the flow to the other side).

My technique makes a 5 second joint at 280 degrees C which is well within the component limitations.

I am sure there are other techniques out there that are just as good or better, but I am in this habit now and you know the story about "old dogs and new tricks".

Scales, Batteries and Capacitors

Lots of discussion on the list about this. Let me tell you what works for me, and it is simple to do.

A 100uF capacitor is way too big for the job of filtering noise from the cable and/or internal operation of the scale. A more appropriate value would be around 1 - 10uf. You also want to make sure you catch all the little spikes and rubbish that may be induced into the circuit.

I solder a 4.7uF (or 10uF - whatever is at hand) tantalum capacitor in place of the battery along with a 0.1uF (100nF) mono ceramic in parallel. The mono ceramics are very fast which makes them ideal for catching those little spikes that may appear on the circuit.

Remember that the tantalum is polarised. The positive leg goes to where the outer case of the battery was connected.

Plenty of room for this in the battery compartment, it works a treat, and you can just order a few more of the components used to construct the DRO PCB.

Power Supply Reversed

Generally this is not good news! I have done it twice!

The first thing you will notice is that C15 is toast - replace it.

The LM7805 is most likely to have fried as well - check the +5V output if it is not correct - replace U16.

Now the fun part, if you are lucky, your DRO-350 will be functioning, if not follow along here.

The display is likely to be screwed up - don't worry about that now.

Do all the keys on the keypad work? If not, it is likely that you have fried the most vulnerable part(s) the 74HC573's. I recommend you replace the 3 of them - U7, U10, U13. It is quite difficult to determine which one is faulty as they all hang on a common bus.

Now have a look at the display. You may have some missing segments, segments always on and/or segments that are half-on.

The likely candidates are the UDN2983A's: X axis = U8, Y axis = U11 and Z axis = U14.

From my experience - this should fix it. If not you are on your own with the help and support you can get from the ShumaTech Group.

How to Remove a Chip from the PCB

Unless you are completely set up with all the right tools and equipment you may want to follow this guide.

It is highly unlikely that you can suck the solder from all the legs and just lift the chip from the board without doing some damage to the PCB in the process. Hence I recommend that you first cut the chip from the board.

Cut the chip out so that you leave as much leg in the board as possible. I hold my cutters vertically and cut the leg just as it leaves the chip. This will allow you to get hold of the leg easily.

Keep the soldering iron at 280 degrees C - you don't want to lift any tracks from the board.

Heat the pad with the iron for about 2 seconds, now grab the leg with small pliers (or fingers if you are a real man!) and quickly remove the leg from the board.

Once you have done all the legs, you can now use a solder sucker to remove the solder from the hole in the PCB. Be patient at this stage, if you can't get all the solder out, put more in and have another go - it should come out in one shot.

Before you install the new chip - CLEAN UP VERY CAREFULLY. This is important. If a bit of solder gets under a chip it is unlikely you will ever find it. Give the board a good shake and tap and make sure it is clean. Use some IPA isopropyl alcohol and a tooth brush to clean it up - give it a good scrub.

Dry it off, have a last inspection - now you are ready to instal the replacement chip.