Author Topic: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)  (Read 15851 times)

Offline awemawson

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #400 on: September 14, 2018, 07:57:27 AM »
In anticipation of the correct capacitor arriving, I made up an extension cable, in order that the OMM could be be driven by the MI12 on my desk for testing purposes.
Capacitors arrived and sure enough they are the right size this time, so I removed the OMM from the machine, dismantled it and removed the circular PCB. Hey guess what  that's NOT an exploded capacitor - it's a blob of paint  :bang: :bang: :bang:

Never mind, couple it up to a 10 volt bench supply and do some more testing. PK's tip about the ICL 7660 chip is very interesting. It takes in +5 volts and apparently should 'mirror' this to provide a -5 volt rail, whereas it's producing -3.5 volts.

I identified the two capacitors that it uses to perform this magic (68 uF @ 6 volts), and following PK's tip that they might be low capacitance tried paralleling them one by one with a 22 uF capacitor - it made not a jot of difference. So probably PK's other suggestion of  it being over loaded is the case  :scratch:

Now much of the circuitry of this PCB is hidden under a screening can, and to progress it's going to have to come off. Only held down by seven through PCB tags neatly soldered on a very crowded small PBC  :bugeye:

Digging out my 'electric pump' de-soldering sucker it looked ridiculously big compared to the PCB, but with a delicate touch, a bit of flux, and adding some low melting point solder to the last persistent lug eventually got it off with no apparent damage.

Now I was rather fearful that the cover might conceal some encoding / decoding logic but in fact it all seems to be analogue OP Amps comparators etc so the full chip count is as follows:

Reverse Side:

LM78L05 (2 off): +5 volt regulator
ICL 7660 : Negative rail generator (should be -5 is -3.5)
LM393M  : Op Amp / Voltage comparator
DS8921M : Differential output driver (gives SIGNAL and SIGNAL

Plus 3 off SMD transistors

On the Front Side (under the screen):

OSD 15-3TRB : IR receiver
LM833 (2 off)  : Audio Op Amp
LM311M          : Voltage Comparator
AD848             : High Speed Op Amp

Plus 4 off SMD transistors
On the front side (not under the screen) ZTX 450 discrete transistor

To go much further I'm going to have to make a rig to mount the circuit so that I can safely probe it without doing damage, and also to make up an IR transmitter that sends a train of pulses so I can trace through the circuit.

If indeed one of the circuits is pulling down the -5 v it's going to be a pain to find as the LM833's, the LM311, The AD848 and the LM392 all take the negative rail voltage.

Won't be much more progress today as I have to play host to some guests arriving this afternoon.
« Last Edit: September 14, 2018, 11:33:32 AM by awemawson »
Andrew Mawson
East Sussex

Offline awemawson

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #401 on: September 14, 2018, 09:01:53 AM »
I made up a mounting jig to stop the PCB skidding all over the desk when testing !
Andrew Mawson
East Sussex

Offline awemawson

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #402 on: September 15, 2018, 03:36:24 PM »
I came across this Youtube video where Tom has done quite a bit of investigation into the OMM. Not all relevant but some useful stuff there:


&t=2184s
Andrew Mawson
East Sussex

Offline awemawson

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #403 on: September 16, 2018, 07:30:38 AM »
OK so the problem is to try and work out which component or components is drawing too much current from the negative rail, and dragging it down from -5v to -3.5 volts.

I hit on the idea of looking at the board with my infra red thermometer - now it's by no means easy to be sure exactly where it is pointing, but looking at each integrated circuit the hottest was the ICL7660 - not surprising as we know it's probably over loaded. (I think that it can supply  about 20 mA) - it was running at about 39 degrees whereas the other ICs are at about 26-28.

But scanning the board in general one of the capacitors was running at over 40 degrees - is this our trouble maker? It's the 68 uF / 6 volt one next to the famous one with the paint blob and is the one used by the ICL7600 to do the voltage inversion.

It's position on the PCB and (relatively) large size allowed me to remove it with a fine tipped soldering iron and substitute an axial one for testing.

. . .so what was the result, nothing, zilch, nowt  - voltage remains at -3.5  :bang:

Never mind, it was a good idea. Carefully put the SMD capacitor back and scratch your head a bit more  :scratch:

Now if this was a conventional sized board with DIL I/C's I'd carefully lift the -VE rail input legs on the I/C's one by one and monitor the voltage on the negative rail to see who's causing problems - not really possible at this scale.

Another technique is to construct a probe with two very sharp tips close together but insulated from each other, and with it connected to a sensitive millivolt meter, probe the PCB tracks leading to the supply pins in question. The tracks have a finite (but very low) resistance so a small voltage is developed across the probes that is proportional to current. Brilliant in theory but not too good at this scale as you can't really get at the PCB tracks.

. . . a bit stumped at this stage  :scratch: :scratch:
Andrew Mawson
East Sussex

Offline russ57

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #404 on: September 18, 2018, 08:07:27 PM »
Are you able to inject -5v separately? If something is dragging the line down, that may make it more visible (eg hot), or show it is the converter itself at fault.



Russ


Offline awemawson

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Re: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
« Reply #405 on: Today at 10:45:32 AM »
Russ,

Yes I had considered that but I'm scared to do more damage. I've dug out a x20 microscope and have been looking round the circuitry, but it is very confusing - the SMD markings are rather coy about what they really are - I have found a document that helps somewhat in the translation (attached) and I have found what appears to be a transistor type 5B (MMBT4123) that seems to be s/c base / collector. It appears to buffer the output of one of the LM833 op amps.

Now I'd like to just remove it and / or the LM833 but these things are sized like a grain of rice and I'm just not equipped for this size of component  -
I did seriously consider getting myself a hot air re-work station, but even if I did, I don't have the skills working at that scale  :bang:

. . . time for a re-think  :scratch:


oops - the .pdf was too large have a link instead:

http://www.marsport.org.uk/smd/mainframe.htm
Andrew Mawson
East Sussex