Last updated: 19 December 2006
Subject: repair report ETX-90 Sent: Monday, December 11, 2006 05:25:18 From: a. toonen (firstname.lastname@example.org) Repairing a Meade ETX90 with Motor Unit Failure. Downloaded printlayout of the panel PCB via Mike's website. Found reference to US Patent Office for the Meade ETX-90 patent. Downloaded and studied the patent to understand how the ETX-90 works (I2C-bus is used for communication between Autostar and both motor units.) Tried internal batteries. Tried external power supply. Tried #497 Autostar. Tried #494 Autostar. Tried hand controller. All gave the same result: MUF. Internal inspection: encoders and fototransistors are clean. Visual inspection of all PCB's showed no burned components. Checked supply voltages on panel PCB and both motor units PCB's: all OK. Exchanged the order of the motor unit cables on the panel PCB. Still MUF. Both motor units defective? Disconnected both cables: MUF. This is very strange, looks like bad contacts. Checked all interconnections from panel PCB to Autostar connector, HBX and AUX connector. All OK. Checked interconnections from panel PCB to both motor units. BINGO: ground line was not connected. So both motor unit got no power! Why was the supply voltage check on both motor units (see above) OK? Because I used a common ground on the panel PCB and the +5 and +12 were OK. Problem must be in the panel PCB. Removed (horizontal) motor unit PCB and motor to get access to the panel PCB. Found out (Mike's website) that this panel PCB is snap-fit and glued, so cannot be removed. Used the downloaded printlayout to check at the back of the print where the connection was broken. Located two "isolated islands", one connected to the panel connectors and the other to the motor units. Soldered one tiny isolated copper wire to both islands and that was it. ETX-90 worked perfectly again. Conclusion: as I have no proper access to the component side of the panel print, it is not possible to find the real cause of the bad connection (bad soldering joint, broken PCB-trace etc.) Next time I will check supply voltages on satellite PCB's with two probes on the PCB (and not the common ground on an easy accessible place). Arnold W. Toonen
Subject: repair report ETX-90 - Repairing a Meade ETX90 with Motor Unit Failure Sent: Saturday, December 16, 2006 11:05:50 From: Niall J. Saunders (email@example.com) Welcome to the wonderful world of repairs - and to the tormenting problem of missing 'earths' or 'grounds'. If you do any kind on electronic 'investigation', using meters and oscilloscopes, you become so used to hooking the 'black lead' somewhere convenient, and then probing around the circuit with the 'red lead'. However, this tunnel-vision approach means that we often forget to even consider that the zero volt line, track, plane, or whatever, may itself be 'broken'. A very high percentage of my most insidious repairs will usually have a broken ground circuit, and I always kick myself for not checking them first. It is an easy enough task to do - just don't use the 'black lead' as your common reference on the DMM or Scope - instead, use the red lead onto the supply line, and probe around the circuit with the black lead instead. Now, instead of looking for 'supply level' type voltage indications, you are looking for 0 Volt readings (when you are checking for broken, or missing, earths). Anything other than 0 V, and you can start being very suspicious !! In the ETX (and LX) scopes there is a very common point of failure that manifests itself as a 'missing / broken ground' fault. This occurs when the scope has been used on an external power source, with a 5.5mm/2.5mm power plug being inserted into the Aux Panel. When this is done, a switch contact inside the power socket is opened (by the outer barrel of the inserted plug). This switch opens in order to disconnect the internal batteries (if fitted) - to prevent the external power supply from 'charging' any internal batteries - which would have almost certain disastrous results. However, I have come across several scopes now that no longer work on internal batteries - because the socket used by Meade fails, and the switch contact does not close properly when the external power plug is removed. As you have discovered - this is a major problem, simply because of the difficulty in removing the auxiliary control panel to get access to the power socket. The socket itself would cost mere cents to replace, but you will have a major struggle ahead of you just trying to get the panel out. It seems that, in your case, the problem was not associated with the power socket, but was a poor ('dry') solder joint elsewhere on the PCB - hopefully you were 'lucky' in being able to get to it and cure it. By the way, another problem that can cause MUFs - Motor Unit Faults - is the HBX connectors themselves. If you get any kind of tarnishing, or corrosion, on the gold wires that form the pins of the HBX socket, you can end up with one, or more, axes that do not respond to Autostar control signals. I have experienced this, and find that one cure is simply to insert and remove the HBX plugs many, many, many times - until the contacts wipe themselves clean once more. Sometimes a 'miracle cure' can be observed just by swapping the curly-cord lead 'end-for-end' - but there is no guarantee that this will work. I have also had an Autostar 'fry itself', on my LX90, as a result of pins in the (unused) Aux connector socket(s) having been bent (long before I bought the scope - because I have never had to use any of the Aux sockets on my scopes). Repairing this problem was also a case of several hours of work with dental picks and a big magnifying glass !! (The Autostar remains dead, until such time as I can find another 'dead' one to donate suitable components!!) Hope this helps. Cheers, Niall Saunders Clinterty Observatories Aberdeen, SCOTLAND
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