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Training the hand and mind since 1982.

Saturday, March 23, 2013

New O-Scope in the shop

Order 2 new O-Scopes GW INSTEK GDS1052-U. Initial test were great, bandwidth was much better than the published 50Mhz, got up to 100Mhz before dropping out. Better than our old GDS-2062 that only got 10 MHz above the maxim bandwidth before dropping out. The screen is much sharper and there is less noise at the floor of the newer scope Price was $299, cheaper than the older GDS-2062 at $899.

So far I'm very impressed, Cheaper, Better measured specs and smaller to boot. I also liked the mechanical on/off switch, the only down side is the sampling rate is lower at 250M Sa/s but for our use it does not matter to much.

Screen Comparison

Noise Floor (same settings)

 70 MHz Signal
 100 Mhz Signal

   Compare Size

Digital Side CAR

On with the repairs again for the digital side car. Major design flaw with this board! there is suppose to be a fuse to protect the input, it is on the schematic but not on the board. There should be a PTC fuse CB1 it there to protect Q1, but it did not on this board and a reverse battery polarity blew the SH!T out of it, because there is an internal diode, if the diode was not there it probably would have been fine since the FET only turns on when VBATT is the correct polarity, I replaced it with a through hole MOSFET without a internal diode. I'm so surprised this design got out to the kids. I did find a CB2 but it connect to another part of the circuit.

TI JAGUAR Motor Controller Repair

Here is a repair I did the other day a TI JAGUAR Motor Controller. For the Norwell, MA Robotics Team 348.

Pretty easy to work on, fully published schematics. Fix four so far, most of the problems were in the H-Bridge MOSFETS shorting out. I used a UDB1105S Mini Function generator, definitely a must for servo systems. Made troubleshooting a lot easier than trying to use a regular function generator. The UDB1105S is around $60 the lowest cost DDS function generator I've seen. It takes a bit to learn curve to step thought the menus but once you get past the learning curve it is very easy to use.

The H- Bridge uses two parallel MOSFET per channel (4 Channel total) to get the current high enough for the SIM motors. MOSFET when they fail they will usually short (most of the time) there is also a internal diode to the power MOSFET to dissipate inductive kick from the large motors, best way to check the MOSFET is out of circuit with an ohm meter, I just check it in circuit and take out the shorted pair. I've also noticed that they have different part numbers on the JAGS depending on model, so keep an eye on it when swapping (steal from Paul to pay Pete) when in a bind. If you can order new from a reputable parts supplier like mouser or digikey.

So far 4 out of 6 were MOSFET shorts. One JAG was the SMPS for the 5 volt rail, impossible to solder with a regular iron. I will have to use the IR SMT reflow on that one, there is a heat sink connection on the bottom and there are no VIAs to solder on the copper side. I bypassed the 5 volt SMPS with a bench PS and it came back to life so it's definitely the IC. The last JAG I could not find the problem, totally dead.

BTW you need to install the jumpers on the limit switch. Both vertical. the manaul and schmatic are here http://www.ti.com/lit/ug/spmu130c/spmu130c.pdf 

Next time I get a few more of these I will post a trouble shooting video, I had to get them out ASAP so I could not take a video this time.


 JAG Bottom

UDB1105S Mini Function generator and JAG