Need Help!- Hobby CNC

[John Tribe]

I purchased a Hobby CNC kit because although I like to build things I am not and don't pretend to be knowledgable about electronics.

I have "carefully" followed the written instructions on how to put in all the bits and pieces that came with the kit and examined all the solder points under a magnifying glass and cannot find any bridges in my soldering - however:

The cooling fan requires 24 volts to run and it is not running. It gives a little vibration when turned on and then sits still. If I put a voltmeter across the leads of the fan I get 2.4V. The other checks that I intuitively can make look OK. The leads to the main switch are 120 volts. The transformer is receiving 120 volts and putting out 42 volts to the rectifier which is also putting out 42 volts into the circuit board. From there I am stuck because the circuits get all hidden the the PCB. If it tap into a 24V area of the circuit board the fan works fine.

I have obviously done something wrong but have no idea what - any help?

[pminmo]

Couple of pointers first:
1. If your putting 42Vdc into the HCNC board it's won't last long in all probability.
2. Your heat sink needs som hole in it to allow airflow and reduce a thermal gradient.

As to your problem, reflow your solder joints for you power connector, voltage regulator and components nearby the voltage regulator. In all probability you have a cold solder joint.

Is your 5V ok when the 24V out is low?

[John Tribe]

Thanks Phil
None of my components get hot in fact they don't even get warm to the touch. I can certainly reflow my solder joints - probably starting at the "U" components?
I can also drill holes in the heat sink - no problem although it hasn't had much to do so far.

[pminmo]

The issue at 42V is a couple of things based on the SLA drivers 46V max rating. #1 if your reading 42V and if it is a linear PS, you probably have some ripple so you may have some slightly higher peaks. #2 you will have some motor back emf which will add to the 42V during certain times, and that is awfully close to the max rating. The SLA's are big heating elements, depending on your motor currents, and they have no thermal protection in the chip.

So I just thought I would give you a heads up as to the potential issue.

[vulcom1]

I have the same board and using it on my foamcutter. I also have a NS board that I stripped the components off and checking the traces I have a question. What voltage are you getting on U5 (LM317)? It should be 24volts. The trace runs from the right pin of U6(7824CV) to the right side of U5 with the fan connector in the middle. U5 drops the voltage for the circuits to 5 volts and if your fan is not getting 24volts then your circuit won't have the 5 volts to run either.
On the same topic I would like to know where you can purchase some SLA7078MR chips. Digikey does not have them or maybe I have to go back to Hobbycnc for them. I am working on a board for an experiment to drive a couple of steppers.
Thanks, John

[John Tribe]

Hi Vulcom,
I put the meter on Right of U5 to Right of U6 and I get no voltage at all. And you're right I don't have 5 on the circuit either. So I will have to search before U5 or U5 is fried?? - Is there a circuit diagram anywhere that can be used to trace the fault?

I wish I could help on your search for SLA7078MR but I'm already over my head - I would have to go to an electronics supplier and get their help.

[pminmo]

Using a DC voltmeter negative lead to the fan connector negative, measure the voltage at center pin of U7 (positive output of U7). Also measure the right pin of U7 (positive, incomming voltage from your power supply connection).

Unless your willing to buy a significant quantity of SLA7078's, your pretty much limited on where you can get them.

[John Tribe]

Hi Phil,
The voltage check from the negative of the fan connector to the center pin of U7 gives about 3.6 volts (estimating from my 5 volt analog scale) The Voltage from the right pin of U7 (estimating from 125 volt scale) is about 35 volts, (and will run the fan if I touch the positive lead to the right pin of U7. Now what?

[pminmo]

Start be resoldering the three pins on U7, R14 and R15. If that doesn't do it then check that you have the correct values for R14 and R15.

[raddy]

Hi Vulcom,

You interested in the SLA7078 chips? i can supply them to you at small quantities.
How many are you looking at?

Thanks
Conrad.
raddy@singnet.com.sg

[pminmo]

I can sell SLA7078MPR's for $13 each + shipping.

Phil

[John Tribe]

I am progressing along the CNC road.

I am having some problems getting and efficient gCode.

The raw entry from DXF to Lazy Cam has an unmanageable number of "rapids" as it moves all over the place. A conversion to Layers greatly reduces the movement of the router to a number I could live with, except in the detailed parts of the item (gear teeth)

However there is a large number of chains - 180 in the first layer and 191 in the second layer. Still I can work with this although it is time consuming. Things get really messy when I get down to entities. I can't do anything with the entities in terms of reordering them to eliminate some rapids. When I simulate the cutting of the gear the router spends more time in the air than it does doing work.

Is there a way to reorder entities to avoid the rapids?