PCB Boards -- Simple Machinery

[Spectric]

That is the type of kit that I grew up with, large and heavy with real buttons. In another thread I was looking at suggestions for a new logic analyser to replace my old Techtronix unit with CRT screen and now they seem to be all based on either mixed signal scopes or Pc based and not much bigger than a fag packet. When it comes to audio I would say that digital has not yet matched the ability of analogue, valves give just a much warmer sound, literally and I would have been of the age where valves were on there way out and transisters coming in, the old OC71 etc so I dare say we are of a similar generation.

 

[Sideways]

This is my design & test bench at home from a few years ago - it's rather more modern now as I've moved some of the equipment from analog to digital...

That puts those of us with a handfull of meters and couple of scopes to shame

You find the most interesting talent hanging out on workshop forums !

 

[Spectric]

You find the most interesting talent hanging out on workshop forums !

I think it is also a generation thing, we grew up having to use our hands and make things rather have parents who buy everything and fix nothing plus we were not fixated by a smart phone, that would have soon been dismantled to see how it ticked.

 

[deema]

That’s a highly impressive setup. I would have given my right arm for that gear when I earned my living as an electronics engineer designing medical equipment for Ohmeda.

 

[deema]

I really appreciate @nickds1 taking time to contribute to this thread, it’s always a pleasure learning from masters of their art.

@JohnerH Id second taking up Nick on his kind offer and send the PCB to him.

 

[JohnerH]

I wasn't kidding when I said that sending me the PCB would be far faster. e.g. if the 4017 or the SCR are dead, it's something that someone with the correct test equipment should do. There may also be dry joints (particularly on the spade connectors) due to the vibration the board would have experienced over the years.

@nickds1 thank you ever so much for the offer, I'll get in touch via DM...

Push comes to shove, I'll just revert back to the hard wire solution.

The big power amp to the right is a Perreaux 2105B class A 400W/channel into 4ohm. For years this used to either warm my coffee to a nice 60C or act as a space heater to keep frost out of the workshop! I rebuilt and realigned it and it was my reference amp for speaker builds, but I was told by Someone Who Must Be Obeyed that I had too many amps (really? 12 seemed good), so it was sold. Besides, I design and build my own amps, mostly valve...

I wrote up the rebuild of one of my current amps at https://nick.desmith.net/Electronics/QUAD_upgrades.html

I think you and I are going to be friend... Welcome fellow audiophile! In the next few year I'll be dewelving back into that world. For now, I'll stick with my STAX's (I do however miss my QUAD/Cyrus setup I once had)

@JohnerH Id second taking up Nick on his kind offer and send the PCB to him.

Agreed...

Let's see what happens.

 

[JohnerH]

Apologies for the lack of traction here, work, garage prep et al got in the way.

Hopefully I can get some cabling done tomorrow and send the board off to @nickds1 shortly.

 

[nickds1]

Completely re-worked schematic layout - revised as I've begun to make sense of it.

My understanding of the circuit is:

It seems that the PCB is live whenever the unit is plugged in. Horrid. Found several dead active components, dry joints and cooked resistors (R1,R2,R3). There was a 13A slow blow fuse in there when the board says 10A fast blow, plus some other stuff.

P5: Always LIVE
P1 & P2: Always NEUTRAL
P6: Live when unit is OFF, NEUTRAL when unit is ON and all safety switches engaged (safe)
P3: Start capacitor
P4: Starter winding

Like many such systems, to allow high-side switching, logic DC "ground" (Vdd) is actually the same as AC LIVE with the +12V DC logic supply (Vcc) referenced to that. The logic supply is derived via a half-wave rectifier, D1 and a Zener, D5 - some resistors, R1, R2 & R3, limit the current through the small Zener to about 14mA. 14mA though 15K6 gives a dissipation of about 3W, which is why those resistors cook as even if the unit isn't running and is still powered, they'll heat up.

U1, the 1/10 counter, is always running, clocked at 50Hz by a half-wave via D1 clipped by D5 but not smoothed as C2 is isolated from it by D2. The jumper block sets how many 20mS intervals U1 counts before resetting - this is the switching pulse for the thyristor, Q4, and comes from the Q1 pin of U1 to the base of Q1 (the transistor) and thence to the gate of Q4. This is not PWM, its just making sure the thyristor is triggered for the correct time.

The gate drive for the thyristor, Q4, is buffered by Q1.
When the start button is pressed, after a delay of about 200mS, Q2 turns off allowing pulses from U1 through to the base of Q1. About 5 seconds later Q3 will turn on Q2 stopping the pulse train. You need pulses to keep the thyristor on as it'll turn off every half cycle as the current through it drops to zero.

All this basically does is turn on the starter winding of the motor for 5 seconds after the "start" button is pressed whilst honouring the various interlocks. It's all a bit of a palaver ! Meh.

 

[nickds1]

I'll update the above post as and when I make changes to the schematic etc., so that there is only one copy of the PDF, i.e. the latest.

Just completely revised it.

 

[RobinBHM]

Morning All,

Quick, silly question... Given the simplicity (or at least it used to be) of the mechanical elements in things like jointers and band saws for instance, why is there a need for PCBs in them?

Reason for my ask is that I am troubleshooting a jointer at the moment and will probably have to send the PCB out to get checked, but since this question occuered to me, I thought I'd ask

(not to mentioned in a few month time I'll be opening up all sorts of Wadkin kit which I'm sure won't have any PCBs)

J

have a look inside any industrial woodworking machine and you will see a whole control panel

these days machines have all sorts of safety devices like micro switches on travel limits, doors, machine guards etc etc

 

[JohnerH]

have a look inside any industrial woodworking machine and you will see a whole control panel

these days machines have all sorts of safety devices like micro switches on travel limits, doors, machine guards etc etc

I can't wait to open up my Wadkins, I doubt I'll see such detailed control pannels.

But yeah... with great safety comes great complexity.

 

[nickds1]

So, done a bit of testing to check my hypothesis and:

The jumper sets the mark-space time for the gate pulse - the "mark" (on time) is always 20mS (one 50Hz cycle); the "space" (off) time varies between 20mS when the jumper is in position 8 to 160mS when it's in position 1. This gives a starter winding on-time of between 50% and 11% for the 5 seconds or so it's on. The default is position 4 which gives 20mS on and 100mS off, i.e. 20% power for 5 seconds.

This also works well as the clock for the 4017 is derived from the mains frequency, so the zero-crossing of the supply corresponds with the rising edge of the gate pulse (phase-locked), i.e. the thyristor is on for a whole mains cycle, but being an SCR, it's actually only conducting for half of the cycle, so the above power delivery to the starter winding is half what you might expect, i.e. between 25% (jumper 8) and 5.5% (jumper 1) for the 5 seconds with a default (jumper 4) of 10%. So if the motor is reluctant to start (all other things, e.g. interlocks etc. being equal), moving the jumper towards position 8 just might help.

No wonder later single phase motors simply use a centrifugal switch to do away with all this kerfuffle!

The 'scope doesn't photograph well at slow scan rates, e.g. 50Hz, due to the persistence of the phosphor, however if you can read it, that's the gate pulse with the jumper in position 4 (the default).

 

[deema]

@nickds1 Hi Nick,
Thank you so much for the circuit, I’d welcome your thoughts, and my eduction. I haven’t studied the circuit you’ve drawn, but I did from the manual sketch out how the PCB is connected to the motor. Reading your explanation of what it does I can’t get my mind around it! Here’s my two penny worth of gibberish

A single phase motor can have either one or two capacitors. If it has one capacitor there is no switching of anything between starting and running. If it has two capacitors, one is used to start the motor and then is switched out when the motor reaches a certain speed to run on the other capacitor.

This machine has a motor according to the manual that has a single capacitor for both starting and running. There is no need to switch anything. The capacitor has to be connected in circuit when the motor is running. In your explanation you talk about the live and neutral being swapped around, and permanently connected to L/N which coupled with the motor capacitor not needing to be switched still suggests to me it’s a timed reversal of the motor direction to act as brake. Now, I know I’m probably wrong, but could you sketch the PCB in circuit with the motor and highlight exactly how it’s achieving the starting. I’m being completely thick I’m sure, so I want to thank you in advance for helping the silly person with basic motor theory.

Best J

 

[nickds1]

Hi. The board only controls the starter winding - if you look at the schematic you'll see that pin 3 simply connects one end of the capacitor to N via a resistor - it's a permanent connection. You are correct in that the capacitor is permanently connected to the start winding and doesn't need switching...

However... the starter winding is controlled from pin 4 ("X1-P4" on the schematic) - the other end of both the run & start windings is connected to L when the start button is pressed.

Pin 4 is the connection that has the 5 seconds of pulses on it which are delivered only to the start winding and only just after power is applied.

There is no brake - when the contactor is open, there is no power to the motor - you can see that all 3 winding connections (start, common & run) all go via the contactor though one end of the run winding is also permanently connected directly to N. The only other connection is for the start cap that is permanently connected to N (via a resistor).

 

[nickds1]

Board repaired and returned to owner. It was pretty beat up, but now working fine. An interesting exercise in reverse engineering the control system and understanding the approach they were taking. Unnecessarily complex, which is why later versions used the more normal centrifugal switch to disengage the start winding.

 

[deema]

@nickds1 brilliant, well done and thank you for the schematic and detailed explanation. I really appreciated it.

 

[JohnerH]

Board repaired and returned to owner. It was pretty beat up, but now working fine. An interesting exercise in reverse engineering the control system and understanding the approach they were taking. Unnecessarily complex, which is why later versions used the more normal centrifugal switch to disengage the start winding.

Many thanks @nickds1,

It is dually sitting on top of the jointer waiting for some peaceful time for me to put everything back together.

Its going to take a while given that I'm in the middle of a domestic 3 phase installation, so at the moment I'm property looks like a building site

I will revert in due course.

Again thank you all for the help.