Troubleshooting a new PCB

Question

I designed and got a PCB manufactured for the first time, and have soldered the component and powered it up. However it is not behaving as expected. I think I may learn something by everyone here for advice instead of ripping it apart and trying it again before. The circuit is an 8-bit register for a CPU consisting of two 4-bit D type FFs (SN74LS173AN) and an 8-bit bi-directional bus buffer (SN74LS245N). There are 2N7000 N channel MOSFETs that are used just to show the bit status of the register so the TTL output load is reduced. Clock is rising edge and reset is asynchronous active low. Store signal has to be high to clock in data to the register and the enable signal for the buffer is active high.

The schematics, PCB layouts and PCB board are all shown at the bottom of the post.

This design was working on a breadboard previously so I know how it should behave.

So the upon powering the PCB with 5V from a current limited PSU set to 50mA, the current was reaching the limit, and even up to 100mA, the PSU voltage was only around 3.3V. Bit 0 LED was illuminated brightly and bits 1, 2 and 3 are lit. However the other bits are dimly lit. The resistance between Vcc and GND is around 5kOhm, but don't think that necessarily indicates a short somewhere.

I checked the board before soldering for trace continuity according to the schematic and it was good.

Where is a good place to start? Desoldering LED and MOSFET components leaving just the the ICs, then check for correct operation? If still faulty, some of the ICs must be causing the issue.

Schematic:

PCB Layout

PCB 3D Viewer (Top)

PCB 3D Viewer (Bottom)

PCB Photo (Top)

PCB Photo (Bottom)

EDIT

Image below shows the state of the board at most times regardless of what you do with inputs. They mainly flicker due to the power supply current limit being reached. Clock and reset do work but you cannot clear the register to zero and the LEDs will not go out.

Answer 1

The 2N7000 MOSFETs are not quite rated for logic level applications, especially for LSTTL. They may be getting into linear mode and dissipating enough power to cause excessive current. But the 1k pull-ups should limit current to about 3 mA each. You might try using 74HC logic components which have rail-to-rail outputs as well as lower static dissipation.

Another possibility is oscillation, so check voltages with a scope.

Answer 2

Here's one tip for troubleshooting old style PCBs with through hole components - put all the ICs in sockets. That way you can pull a leg out to see what happens etc.

Answer 3

With a multimeter, check for shorts to the very close ground trace as shown (just a hunch).

A few hundred Ω or more is okay, < 10Ω is not. It's really only two tests since they're connected togther at U3 and U2, but the solder under U2 looks suspicious. Remember that the solder mask over traces can leave copper exposed at the edges so it's not okay to have solder blobs that extend over edges that are not supposed to be connected.

In the future note that there is no reason to have the trace that close to the pads. Similarly, if you end up doing copper pours keep the spacing to pads generous since there are many more opportunities for shorts, either from soldering or from board manufacturing defects.

Answer 4

I have done this many times. The first thing I do is add the power to the board and test all the ICs etc to be sure power and polarity is on the correct pins. On your board I would add the LEDs and FETs resistors and test them. You can put +5 on each pin on J5 one at a time to be sure everything works. Then solder in sockets for each of the ICs. Install one and apply the appropriate signals at J1, J3, J4.At any point something does not work correctly solve that problem before continuing. Do this with each of the other ICs one at a time. When the last one is working add one of the others etc until they all work. While testing measure the voltage at each IC input and see if it has at least the minimum voltage +20% or more.You may have some families of ICs that do not work and others do. Also if this is headed for production be sure you have margin on all pins at both max and min voltage. See the probe described below.

I have a 3.3K resistor connected to a VOM probe. I connect the probe to the input, of the VOM, the ground to ground. If I am measuring grounds I connect the resistor to +5, if it is the positive side I connect the resistor to ground. I have also done this with a bidirectional LED powered by a 3V supply. The probe will then give me one color for + a different color for - and nothing if open. It does light leds so it helps in checking if they are inserted correctly.

This is not detailed but intended to give you the idea.