Wednesday, August 31, 2011

Tektonix Oscilloscope Repair

Alain found a couple oscilloscopes at a yard sale and called to see if I was interested. The Tektronix Digital Storage 100 Mhz scope sounded real interesting. (Model 2232 produced about 1990.) He bought the scope for $40. At home the trace triggered and displayed but all was out of focus and the focus knob did not go far enough in the improving direction.

I was able to download the 366 page PDF Service Manual off the internet. This manual has the description of operation and also the schematics! What a pleasant find!

On page 285 I found the focus circuit schematic.

I then took the cover off the scope and eventually found the series resistors chain on the bottom board and they were well marked. Using an ohm meter I found R890 open instead of reading 510 k ohms. Since the failure was so clear I did not even remove the circuit board to observe the failed resistor. I simply scabbed a new resistor on the bottom of the board. Remember the training (KISS = Keep It Simple Sam)?

I cut an opening in a plastic high voltage protection plate to make room for the new resistor. Now all works as designed. At least the focus is good, the traces display and the triggering works. Now to read the manual to figure out the many other features of this rather powerful scope.

Also I should replace a missing screw that holds the back cover on.

Friday, August 19, 2011

Viking Husqvarna Model 6440 Sewing Machine

Failure and Repair.

Linda’s 33 year old, ‘vintage’ sewing machine started smoking the other night. The machine was just sitting there when the motor started by itself and smoke emerged from the machine! They unplugged the machine but there remained a ‘nice’ odor in the room. This has been an excellent sewing machine over the years.

We plugged it in the next day and the motor started at a slow speed without depressing the foot pedal. After concluding the machine is not worth much anymore I started the disassembly. I was able to eventually get to the motor and the little terminal box that held some electronics. There was capacitor that had blown up leaving a moist oil like substance on the terminal board. The cap had a label indicating it was 0.1 uf, 250V ~ MP, fo = 3.6 Mhz, PME 271M. This turned out to be a interference suppressor used, I think, to reduce electrical noise caused by the motor.. I looked for a replacement capacitor but found only one half the size that sold in lots of 200 for about $2 each. ($400).

The motor and terminal board assembly.

The blown capacitor. Its belly is missing.

I didn’t want to leave the machine disassembled too long for fear I may not get it back together correctly so I completely removed the capacitor. Now the sewing machine works just fine. Maybe when the motor is running we will hear interference on our radio or tv. I may look further for a replacement part as time permits?

Here the beauty is.

I wonder how many more years of service this machine will provide? There are some new machines out there even in the $150 neighborhood that may outperform this vintage sewing machine. Who knows?