Spindle Error (PCNC 1100 Series 3)

**Eli_Ben** · 04-11-2017, 05:09 PM

My PCNC1100 Series 3 from 2015 has developed a fault. The symptoms have changed slightly since the problem started last night. Here's the situation with the machine:

The spindle will not work in auto mode. At first it was erratically turning on and off/changing speeds by itself, now it doesn't respond at all. The VFD does not turn on in auto mode. BUT it does in manual mode. In fact manual mode works perfectly (initially it didn't, now it does after updating PathPilot to the latest version).

I've checked every cable/jumper for loose connections, plus the spindle door switch (which seems to be a common cause). I've also reinstalled PathPilot from the disk, and updated again to the latest version.

Tormach advised me to measure the voltage of wires J1-1 and J1-2 on the main control board. This should change as the spindle speed command from PathPilot changes, but I measured a constant 0.22 VDC. In manual mode this voltage changes between 0.76 VDC and 5 VDC at the spindle speed is increased.

Page 200 in the troubleshooting section of the user guide details some things to check:

"Ensure you have 115 VAC measured from wire 100 to wire 106."

Actually measured 126 VAC.
"Make a jumper wire and momentarily jumper wires 106 and 107. If contactor C2 pulls in (you will hear an audible clunk) while you have the jumper on but drops out as soon as you remove the jumper, the holding contact on C2 is defective. If C2 stays powered on, the control board is not pasing the run signal to the circuit. Make certain you are commanding the VFD to run. If so, the control board is defective."

C2 does not respond at all to the jumper. Initially it did turn on and stay on, and the VFD also turned on (with no error code) but now there's nothing.

I'm in the UK running on mains voltage (measured at 241 VAC). I have the power drawbar and foot pedal, but no ATC.

If anyone has seen something similar, has ideas on what to test next or would like more information, please let me know! My mill is now out of warranty, but have had some replies from Tormach. Fingers crossed they don't give up on me.

EDIT/UPDATE: Tormach suggested removing the J3 ribbon cable from the control board as a temporary fix, but this had no effect other than to disable manual spindle mode.

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**Zetopan** · 04-11-2017, 09:36 PM

Tormach spindle speed control is done at the computer end by generating a series of pulses that are sent to the control board inside the mill's electrical cabinet. The width of these pulses remains constant while the frequency varies. On the control board those variable frequency constant width pulses get integrated (i.e. filtered) into an average DC voltage. That average DC voltage is what goes to the spindle drive inverter to command the spindle speed. When you switch to manual spindle speed control the spindle speed control on the control cabinet front panel gets selected as the spindle speed control going to the spindle drive inverter input, and the control board average DC spindle speed control output gets disconnected.

Based on the symptoms that you have indicated, any of the following could be at fault. If I was there with one of my oscilloscopes I could help you determine which one of these is at fault, but that is not the case so an indirect method must be used instead.

You need to check the pulse output coming from the computer, inside the mill's electrical cabinet where it enters the control board. When the spindle is being commanded to run you should see a pulse train there, and the frequency should vary with the commanded RPM (higher RPM = higher frequency).

1. If this pulse train does not exist then:
1.A.1 The mill connector end of the interconnecting cable could be at fault.
1.A.2 Or the interconnecting cable from the computer to the mill could be at fault.

1.B.1 The computer connector end of the interconnecting cable could be at fault.
1.B.2 The source of that pulse train inside the computer could be at fault. This pulse train is generated by the Path Pilot card.
1.B.2.A The Path Pilot software is not functioning correctly.
1.B.2.B Or the Path Pilot card is defective.

2. If the pulse train exists but the frequency does not vary with different commanded spindle speeds then:
2.A The Path Pilot software is not functioning correctly.
2.B Or the Path Pilot card is defective.

If the pulse train exists and the frequency varies with command RPM changes then check the output signal on the control board going to the spindle drive inverter.
3. If the average DC voltage does not vary with commanded RPM changes then:
3.A If the output is stuck at some solid voltage level (including ground, if the output is at zero volts) then there is a short:
3.A.1 On the control board.
3.A.2 Or in the interconnect to the front panel switch or the spindle drive inverter.

4. If the average DC voltage does vary with commanded RPM changes then:
4.A The average DC speed control voltage from the pulse integrator going to the spindle drive inverter is not making it through the interconnect to the spindle drive inverter. Check the same signal at the control panel Manual/Auto spindle speed control selector switch.
4.A.1 If the correct signal does not exist at the input to the switch then the interconnecting cable is at fault.
4.A.2 Or if the correct signal exists at the input to the switch but not the output when the switch is toggled, the switch is defective.

The above list is not completely exhaustive but it should be sufficient to debug your problem. You will have to refer to your Tormach mill schematics to find the correct test points to examine. My Tormach is quite old (late series I with Mach 3) so I doubt that the schematics would agree.

**Eli_Ben** · 04-12-2017, 06:37 PM

Wow, thank you for your amazingly detailed response! I really appreciate you taking the time to help me.

It appears that the pulse train does exist, based on the flashing green LED on the control board (labelled 'D10' and 'Activity') flashing at different frequencies as the RPM command from PathPilot is changed. It flashes more slowly at low RPMs, and faster at higher RPMs. I don't have an oscilloscope unfortunately. The output from the control board is stuck at a constant voltage of 0.22 VDC, regardless of the RPM command in PathPilot. (In manual mode (which works fine), the output ranges between 0.76 VDC and 5 VDC at the spindle speed control knob is increased.)

Tormach have advised, based on this information that the control board is faulty and needs to be replaced. That will set me back $470 (plus shipping and 25% tax/import fees) so I'd like to try and repair it if at all possible. I've double checked for loose wires/debris etc and sadly found none.

Tormach are unable to offer more support than telling me to replace the board. In their words, they don't have the 'technical aptitude' to offer further advice. Apparently the person who designed the board was their founder Greg Jackson who sadly passed away in 2015.

Right now I'm looking at 2 options to repair the board:

1. There is one chip on the board installed into a socket, which I'm assuming is a micro controller loaded with some proprietary code. I've asked Tormach to sell me just this chip.

2. It looks like there is possibly some very minor heat damage to 2 other chips on the control board. These look like off the shelf parts so I'll try to find replacements.

In the meantime I suppose I could use the mill, using a tachometer to set the spindle speed in manual mode for each and every operation.

**Eli_Ben** · 04-13-2017, 03:55 PM

I've found one of the chips that appears to have minor heat damage available online at RS in the UK: PS2502-4 Optocoupler (the chip appears to be obsolete and has been replaced with the PS2502-4-A). Only £3 delivered so I've ordered one to try.

The other chip that might have heat damage is labelled:

R
2501
NJ412

I haven't had much luck finding this yet.

No idea if these chips are relevant to the issue at hand, just something to try.

**Zetopan** · 04-13-2017, 04:15 PM

I do not have any control board schematics. However, I would expect that the pulse train being supplied by the computer eventually goes to a one-shot multivibrator (a constant with pulse generator, used in case the computer's output pulse is not exactly constant) followed by a filter to average the pulse train to a DC voltage. The filter could involve an op-amp since that would provide for better filtering and a buffered output. It would require tracing the runs on the EC board and using an oscilloscope to determine how the filtering is done and where the problem actually is on your control board since the schematics are not readily available.

"Apparently the person who designed the board was their founder Greg Jackson who sadly passed away in 2015."
Or it could have been designed by a third party that does not provide any schematics, etc.

The blinking LED is surely being driven by a down counter since the actual pulse rate would be way too fast for visible blinking. If you can locate that counter then the input pulses should also go to the integrator. Also note that you can buy an inexpensive small hand-held oscilloscope that would be adequate for this type of testing for around $100 to $200 US. Having been a designer of full sized oscilloscopes many years ago I have lots of oscilloscopes (costing up to $23K for one), but that does not exactly help you any in this situation. Since you are in the UK and I am on another continent there is also little chance that I could ever see your control board.

**Eli_Ben** · 04-13-2017, 05:05 PM

Would this cheap DIY oscilloscope be suitable, assuming it actually works? I'd love to get a better one, but this is the only thing I would use it for and money is tight.

Here are the specs:

Kuman JYE DSO Shell Oscilloscope DIY Kit with Open Source 2.4 inch color TFT LCD+ Shell + DIY Parts + Probe 15001K (SMD pre-soldered)
Number of Channel: 1
Analog Bandwidth: 0 - 200KHz
Sensitivity: 5mV/Div - 20V/Div
Sensitivity error: < 5%
Resolution: 12-bit
Input Impedance: 1M ohm
Maximum Input voltage: 50Vpk
Coupling: DC, AC, GND
Trigger Modes: Auto, Normal, Single
Trigger Types: Rising/falling edge
Trigger Position: 1/2 of buffer size fixed
Display: 2.4 inch color TFT LCD with 320 x 240 resolution
Power Supply: 9V DC (8 - 10V acceptable), NOT INCLUDED
Dimension: 115mm X 75mm X 22mm
Weight: 100 gram (not including cables and power supply)

**Zetopan** · 04-14-2017, 05:02 AM

It will take me a couple of days of looking at what is available to provide you with a really useful recommendation.

About the kit that you are looking at:
1. 200KHz bandwidth is lower than I would recommend. That would be fine for audio work but a bit short for anything digital. I would recommend at least 1MHz bandwidth if not somewhat higher.
2. The display resolution should be higher in the horizontal direction that what is being shown on the unit that you are looking at.
3. I do not see anything about the number of stored samples. It is extremely useful to be able to capture a waveform while probing so that you can examine it after probing. This is useful because sometimes examining the display while trying to probe a signal in a relatively tight spot can result in your probe slipping and shorting against an adjacent electrical contact. Being able to touch the probe and then examine the display after lifting the probe can be very useful at times. This requires a sample memory and non-auto triggering (single shot triggering) capability.
4. That scope also does not appear to include the power supply, which for maximum utility should be battery powered in a hand held instrument.
5. It also appears to not include any useful probes. Alligator clip leads are not very useful and generally way too large to probe IC pins and runs on the EC board without shorting against something else.

Addendum:
6. It appears to have a single analog input channel when you really need 2 analog input channels for timing comparisons and triggering on one signal while looking at another signal.

More about this topic later.

**Stigoe** · 04-14-2017, 06:37 AM

Somewhat more expensive, but not very expensive, are Picoscope and Bitscope. They're not handheld, but USB-scopes which you connect to a PC and use that screen as the display. I haven't tried Bitscope, but I use a Picoscope at work and it works quite nice as a portable scope.

**Zetopan** · 04-14-2017, 04:48 PM

OK, I talked with a Chief Engineer that I used to work with and he has a handheld scope that he has been using for a number of years, while I never had any hand held scopes (all of mine are much larger full sized portable and bench scopes). The below scope is a far better product than the one that you were looking at.

1. 10 MHz bandwidth.
2. Two analog input channels, with switchable 1X/10X probes.
3. Supports both Y/T (signal amplitude vs time) and Y/X (Y amplitude vs X amplitude) modes.
4. Up to 8,000 samples per analog input channel.
5. Battery powered, with built in charger (uses USB).
6. Can be used with a computer via the USB.
7. Up to 400 V on the analog inputs in the X10 probe setting (the one that you were considering only goes to 50 V).

The only downsides that I see relative to what you were looking at is that the display is only slightly smaller and monochrome instead of color, the digitizing is 8 bits instead of 12 bits, and the highest sensitivity is 10 mV instead of 5 mV. Everything else is much superior and far more useful as a general purpose oscilloscope. The scope that you were looking at is suitable for audio signal tracing, although with a maximum 50 V analog signal input it would not even be sufficient for use with high power audio output stages (which can easily exceed 50 V peak).

https://www.amazon.co.uk/Channel-Vir...ZKI/ref=sr_1_1

**TurboStep** · 04-15-2017, 04:56 AM

In both the manual and auto settings the microcontroller generates the spindle control signal on pin 13. If the spindle runs under manual control then the path from the controller chip to the VFD would appear to be ok.

Originally Posted by Eli_Ben

It appears that the pulse train does exist, based on the flashing green LED on the control board (labelled 'D10' and 'Activity') flashing at different frequencies as the RPM command from PathPilot is changed...

As you say, the PWM command is reaching the board. It first gets buffered by U21 before reaching the counter U17. The counter drives the activity led so the PWM signal must be getting this far. The manual/auto switch on the front panel appears to be monitored by the controller (I haven't reverse engineered the entire board) and it drives the reset pin of the counter (pin 2). I presume this must be working if the activity led flashes but you can test the voltage on pin 2 when switching between manual and auto. Manual should be at around 5V and auto around 0V. The controller board has several grounds so I'd use pin 7 of the counter as the 0V reference to be sure. Pin 12 of this counter feeds the PWM signal to pin 11 of the microcontroller. This should provide a variable frequency square wave with 50% duty cycle. You don't need a scope to verify this, just measure the average voltage with a multimeter. It should show around 2.5V on both the counter and microcontroller pins. As I said, I haven't reverse engineered the entire board and there are some other components attached to the PWM signal that I don't yet understand, but I suspect that either this path is interrupted or the microcontroller has a defect. As the microcontroller seems to be working at least partially and is very well buffered from the outside world I find this to be unlikely.
In order of probability:
1: IC socket (signal on counter pin 12 but no signal on microcontroller pin 11 - carefully pull the microcontroller out of the socket a little and then re-seat it. You don't have to pull it out completely, just move it a little to remove any corrosion between the contacts).
2: Bad solder joints (also signal on counter pin 12 but no signal on microcontroller pin 11 - remove the board and re-solder the affected pins).
3: Microcontroller (signal is measured on microcontroller pin 11 - the microcontroller MIGHT be defect. If Tormach will send you one this would obviously be very easy to test).
4: None of the above

The issue does appear to be on the controller board - maybe those parts I haven't been able to explain yet. Let us know how you get on, perhaps we can get a little further....

My boards have 2 2502 ICs in the row of chips above the microcontroller (my boards are fairly old - series 2 and early series 3 upgrade). I "suspect" these are to isolate the motor drive step and direction outputs and "probably" not related to the spindle drive. I'd be surprised if these had any heat damage. The numbers are very difficult to read, but that's normal. If you're referring to these chips the 2501 just has transistor outputs whereas the 2502 has darlingtons. Maybe someone wanted to increase the output speed by using the faster 2501s or perhaps they just ran out of 2502s? I wouldn't start replacing these unless you had a very good reason.

Good luck
Step

**Eli_Ben** · 04-15-2017, 05:12 PM

Zetopan and Stigtoe, thanks for your scope recommendations. The Bitscope BS05 and SHINA DSO 094 are similar in price, but the Bitscope is available in the UK whereas the Shina has to come from China. I'm keen to get the mill back up ASAP so if the Bitscope is suitable I'd prefer that. Do you think it would be sufficient?

Based on TurboStep's excellent advice above I've taken measurements with some pretty strange results.

Auto Mode, RPM set to 500 in PathPilot:

Chip U17 Counter

Using a multimeter with PIN 7 as ground, here are the voltages measured on each pin of U17:

Pin 1: 2.48V
Pin 2: 0V
Pin 3: 1.8V - 2.1V Fluctuates back and forth slowly (every 3 seconds, roughly in time with the audible spindle speed fluctuations).
Pin 4: 2.42V - 2.53V Fluctuates every 2 seconds.
Pin 5: 2.47V/2.48V (Fluctuates)
Pin 6: 2.47V/2.48V (Fluctuates)
Pin 7: GND
Pin 8: 0V
Pin 9: 2.47V/2.48V (Fluctuates)
Pin 10: 0V
Pin 11: 2.48V
Pin 12: 0.46V Probing turns spindle on/off sometimes (see below)
Pin 13: 0V
Pin 14: 4.97V

Probing PIN 12 (positive probe on PIN 12, ground probe on PIN 7) momentarily will sometimes start the spindle. Probing again will sometimes stop the spindle. When the spindle is on, the speed audibly fluctuates every 3 seconds, and is usually much lower than 500 (I don't have a tachometer yet but it is obviously lower than 500 RPM). Setting a higher speed (600 RPM and above) in PathPilot stops the spindle.

If the spindle is turning, holding one probe to PIN 12 (with the other probe not touching anything) causes the spindle to speed up and slow down quickly (about 3 times per second).

In auto mode, turning the spindle on in PathPilot usually does nothing. But when the spindle command is left on in PathPilot, the spindle will sometimes turn on and off at random. Turning it off in PathPilot will always stop the spindle and keep it off.

Twice I noticed the VFD briefly display: "AC.lt" before turning off. The Tormach nor the Emerson VFD documentation list this code, but they do say that "lt.AC" is an over current error on the spindle motor.

Does the above offer any clues as to what I can test next? I can provide videos if anyone would like to see/hear this weird behaviour.

**popspipes** · 04-15-2017, 10:40 PM

Possibly a bad ground? This will cause all sorts of odd behavior.

**Zetopan** · 04-16-2017, 05:34 AM

The problem with using a DC multimeter to measure a digital signal is that at best it can generally only indicate the average voltage. If the digital signal is changing very slowly the multimeter may be able to at least partially track those changes (literally less than at a few Hertz), depending in the display update rate. However for many if not most digital signals the frequencies are well outside of the multimeter's DC measurement capacity to track and it can only approximate the average voltage, and even that may not be the true average due to aliasing, how the multimeter digitizes the voltage, etc. Also, did you check pin 2 of the counter with the Manual/Auto switch in both positions? That is a DC test where the multimeter would work fine and the results are useful information.

The part that you are examining is a 7 bit binary ripple counter (ripple = relatively low speed for a counter) and examining it does not really provide any useful information since we already know that the LED that it ultimately drives blinks at a rate that is proportional to the commanded spindle speed. Since this is a 7 bit down counter, it can divide the input frequency by a maximum of 128 (2^7) on pin 3. Since pin 12 is the first counter stage output (divide by 2) and that apparently goes to a microcontroller (I am assuming that TurboStep is correct about the function of that part) any excessive loading on that pin could mess up what the microcontroller sees since the binary counter has a relatively low external loading drive capability (that binary counter is a quite old design, not really TTL drive compatible). I have attached a schematic showing the pinout of this counter.

The socketed 28 pin part that TurboStep has identified as a microcontroller would surely be a CMOS part so make sure that you ground yourself before futzing with it in any way. Carefully lift each end of the part to extract it at least partially from the socket (trying to do keep it fairly level with the socket to prevent bending pins) and ensure that none of the pins are bent underneath the package. When the part was first plugged into the socket it is potentially possible for a pin to slightly miss the socket hole and fold underneath the part, potentially making partial contact with the metal in the socket pin hole. If all the pins are straight simply reinsert the part fully into the socket. If a pin is bent, fully extract the part being careful to not bend any of the pins and straighten out the bent pin with a small pair of smooth jawed needle nose pliers. You must stay grounded during this operation since CMOS can be destroyed by static discharge. If the commanded speed works after reinserting the part fully into the socket you will have discovered where the electrical problem was located (bad socket connection). If not, we would need to reverse engineer a bit more to decide if the 28 pit part needs replacing. It seems unlikely that the optoisolators are damaged, since that would require a quite severe faulting condition and I would not expect the control board to work at all after that.

Since I do not have any direct experience with the BS05 or the DSO 094, I can only go by the specifications and one prior review provided to me by someone that I have known for many years, who is also extremely competent.

Firstly, unlike the DSO 094 the BS05 isn't really an oscilloscope. It can sort of act similar to an oscilloscope in some *limited* respects when coupled to a computer. It can also do some things that some oscilloscopes cannot do, like generate signals to inject, but it also cannot do a lot of things that a self contained general purpose oscilloscope can do without requiring an external computer or power supply.

The BS05 and DSO 094 are not really directly comparable since they are actually aimed at performing quite different tasks. The BS05 Is not really a general purpose oscilloscope. You *must* use it with a computer for any measurement setup and to view any of the measurement results and it is also powered over the USB. It is apparently limited to -6.5 to +9.2 VDC nominal input voltage levels on the signal inputs. To measure larger voltages you will need to purchase a pair of suitable probes. The lowest cost switchable probes for it cost about $25 each so that adds about $50 to the base price. In contrast the DSO 094 has a much larger input voltage range, it can run stand alone, is self powered with a battery, allows for user setup and waveform viewing without a computer, and it comes with a pair of switchable probes.

For low voltage analog and digital measurements the BS05 offers more channels (6 digital and 2 analog or 8 digital), built-in signal generation, serial bus decoding, programmability, and more. But this requires connecting to a computer for power, setup, control, and waveform viewing. You cannot use this with higher voltages without adding at least one attenuating scope probe. Also note that a 10:1 attenuating probe still does not drop the analog input voltage enough for examining a 240 VAC power line without damaging the BS05 (I assume that the is standard voltage in the UK).

As a result, you will need to decide if you want an instrument that is stand alone, self powered, for general purpose 2 channel analog/digital, or one that is limited to low voltage 2 channel analog and 6 channel digital, with serial bus decoding, etc. and can only be used by attaching it to a computer.

**TurboStep** · 04-16-2017, 06:00 AM

Originally Posted by Eli_Ben

Pin 1: 2.48V
Pin 2: 0V
Pin 3: 1.8V - 2.1V Fluctuates back and forth slowly (every 3 seconds, roughly in time with the audible spindle speed fluctuations).
Pin 4: 2.42V - 2.53V Fluctuates every 2 seconds.
Pin 5: 2.47V/2.48V (Fluctuates)
Pin 6: 2.47V/2.48V (Fluctuates)
Pin 7: GND
Pin 8: 0V
Pin 9: 2.47V/2.48V (Fluctuates)
Pin 10: 0V
Pin 11: 2.48V
Pin 12: 0.46V Probing turns spindle on/off sometimes (see below)
Pin 13: 0V
Pin 14: 4.97V

Probing PIN 12 (positive probe on PIN 12, ground probe on PIN 7) momentarily will sometimes start the spindle.

Not exactly what I was expecting but that happens all the time

Pin 1 is the input to the IC derived from the input to the board. Your measurements imply this is stable - that's a good start.
All output pins (12, 11, 9, 6, 5, 4 and 3) should have a square wave with duty cycle of 50% and decreasing in frequency in the order I listed them. All should therefore measure around 50% of the supply voltage on a DVM with the exception of pin 3 which is loaded by the activity LED. I checked on my machine and I get a similar voltage on pin 3 and I also see a fluctuation on the DVM which is not unexpected because the frequency at 500RPM is only about 8 Hz.
Pin 12 on my machine definitely averages around 2.5V, the same as the other outputs.
I would not expect a DVM to affect the output in any significant way. If your DVM was maybe defect and was pulling pin 12 down it would also pull the other outputs down in the same manner.
As Pin 12 connects to the microcontroller it's theoretically possible that the input pin on the microcontroller is pulling pin 12 down. If that were the case then the additional impedance of the DVM is very unlikely to affect the signal.
Just to be absolutely sure I measured the signal on pin 12 with 3 cheapo DVMs and a scope and the spindle speed was not disturbed by either of them.
It seems to me that this counter IC has a defective output stage on pin 12. That said, "remote debugging" is always difficult and I obviously don't want you needlessly replacing any ICs on the board - there's always a possibility that the board might get damaged.
If you do decide to replace this IC you'll obviously need to remove the board. I've discovered that this is actually not quite as painful as it might seem because I've just realized (yesterday

) that the connector blocks are actually plugable. The smaller blocks come away fairly easily but the longer block takes a little more persuasion.

Spindle Error (PCNC 1100 Series 3)-pcnc_connector-jpg

The Kuman scope is better than nothing (if it works) but as Zetopan says with only 1 channel it's very limited. Scrolling down the Amazon page I found this Hantek scope for just £56 - can this be the correct price?
https://www.amazon.co.uk/d/rj2/HANTE...ZBCJM13VVKQ4GS
At that price I'd be tempted to buy one just for the workshop so I can leave my bench scope in my office (if only they shipped to Switzerland).
Good luck
Step

**TurboStep** · 04-16-2017, 06:38 AM

Originally Posted by Zetopan

...and that apparently goes to a microcontroller (I am assuming that TurboStep is correct about the function of that part)...

Your assumption is correct

Step

**Eli_Ben** · 04-16-2017, 01:24 PM

Originally Posted by popspipes

Possibly a bad ground? This will cause all sorts of odd behavior.

Could you elaborate on this? Do you mean a ground short somewhere in the circuit, or an issue with the mains supply etc? How would I check for this?

**Eli_Ben** · 04-16-2017, 03:40 PM

Thanks again guys, really appreciate your continued assistance.

Originally Posted by Zetopan

...did you check pin 2 of the counter with the Manual/Auto switch in both positions?

Just checked this:

Auto mode: 0V regardless of spindle command from PathPilot or if the spindle is actually turning.
Manual mode: 4.98V regardless of spindle on/off/speed.

Thanks very much for the pinout!

I was able to remove and replace the socketed microcontroller, sadly with no effect. There was no evidence of misalignment or bent pins etc.

Originally Posted by Zetopan

you will need to decide if you want an instrument that is stand alone, self powered, for general purpose 2 channel analog/digital, or one that is limited to low voltage 2 channel analog and 6 channel digital, with serial bus decoding, etc. and can only be used by attaching it to a computer.

This diagnosis is the only thing I'll be using an oscilloscope for, but it seems difficult to make a decision as we're dealing with a lot of unknowns. The standalone unit seems like the better choice, but the long delivery time is unappealing. It's also possible to hire better oscilloscopes for around the same price (£100) for a week.