131/Repairs

The power supplies for the 131 Model 4 and the 134 look alike and use the same output connector, but they are not interchangeable. The center pin of both is more positive than the outer connection, but the 131 supply has the center pin grounded, while the 134 supply has the outer connection held close to ground with a pair of anti-parallel diodes.

The 131 supply is more complex, having filtering and Zener regulation in addition to full wave rectification.

Using the wrong supply will short circuit the supply output. Tektronix claims that a short circuit will not damage the 134 supply and will not blow its fuse. I don't know what happens with a 131 supply.

I have added a label to my 131 supply: "For Type 131".

Nevertheless, the 134 supply adds two features that appear to be good things to add to the 131 supplies: The anti-parallel diodes to ground in the negative side of the output holds that side CLOSE to ground, instead of having a direct ground. This prevents ground loops. It may be tempting to leave neither side grounded, but in that case, a primary-secondary short would lead to injury, a hot chassis, or severe instrument damage.

The 134 power supplies also added a 100 mA fuse in the hot lead of the 115 V_AC power input. This is expected to blow if the power supply transformer develops a primary-secondary short. Make SURE the fuse goes into the HOT side of the power input. And also check your home and shop outlets for proper polarity.

I added both of these features to my 131 Model 4 power supply using a pair of 1N4002s, and the 131 works just fine with this modification. This was very cramped work, but it can be done. Use the smallest 100 mA fuse you can find. I assume the same additions will work equally well in Models 1, 2, & 3.

Also, if you ever take one of these supplies apart, note that there is a problem with each of the grey plastic parts. It appears that the plastic shrinks over time, and this leads to cracking. This is especially common in the coaxial output connector. If you catch this early, you can let some super glue soak into the crack. Do this with the plastic cover removed, or you'll glue the cover to the connector and never get anything apart again.

Also, open up the hole for the output connector in the cover with a tapered reamer so it is not under stress when installed. This will help prevent future cracking.

I managed to find a late style locking BNC output connector to replace the UHF connector on my 131. This seems like a really worthy improvement, but, alas, to do this, the whole 131 would have to come apart, including dozens of solder joints on the PC board. Faced with this prospect, I stopped. I may do it someday, but only if this [[131] needs other serious repair.