tektronix SA 494P

[nicovan10n]

After being on the shelf for a year I wanted to use my analyzer again
unfortunately it doesn't work anymore and i have to deal with the following problem.

After switching on the device, the digital readout no longer works
only a dot in the center of the screen .

So I started measuring with the power supply separate from the analyzer, all
necessary voltages are present as soon as I put the power back on the analyser
appears that the -15 volts is no longer present.

how do i fix this problem

[RogerEvans]

The service manuals for the 494 are available at w140.com/tekwiki, volume 2 includes the schematics.

Failure of one of the low voltage power supplies is most frequently due to a short circuit inone of the tantalum capacitors used to provide local decoupling on each of the circuit boards. Finding the exact location of the failed part is difficult and the most suitable method for you depends on what equipment you have available (most of us can skip the next two paragraphs).

If you have a thermal (ie infrared) camera you can look for the failed component with power applied to the 494. The failed capacitor should be warmer than others nearby. This may not work if some of the circuit boards in the 494 are not accessible with the power on

If you have a precision low resistance meter (four wire Kelvin connections) you can measure the resistance across the individual capacitors decoupling -15V to ground. As you get closer to the failed component the resistance will decrease slightly as it no longer includes the inter-board wiring and the PCB traces. On some Tektronix instruments this doesn't work well because there is a local series resistor of a few ohms associated with each decoupling capacitor.

Most of us don't have the above equipment and the usual route is to systematically remove the cables that supply the low voltage DC to each circuit board until you find the board that has the short circuited component. Then you have to unsolder one lead of each suspect capacitor to test it. The capacitors which are most prone to failure with age are the ones shaped like a tear-drop with the value encoded in bright coloured bands.

Best of luck.

Roger

[tony brady]

it sounds like there may be a tantalum capacitor shorting the -15V line. with the power lead removed measure the resistance between the -15V line and ground using the 200 ohm range.

if you get a low reading try removing the plug in boards on the right hand side one at a time. ( there are long bolts holding them to the chassis and a bar at the side) if the resistance reading you get goes higher then you may have found the faulty board. then you will need to check the manual to see which capacitor is across the -15V line or just measure across each of them in circuit to find the culprit

[nicovan10n]

Hi Roger

I already have the manuals
I have a HP 3457A multimeter but i never use it with 4 wires
I all ready removed the cables and also the boards from the motherboard exept the z axis board where i measure the voltages available

[Diabolical Artificer]

Sounds like a short somewhere putting the -15v line into protection. I had this problem with a +15v rail in a Tek 7704A. It was a case of pulling all power supply connectors to every board whilst checking to see if the short went. In my case the SC mysteriously vanished, maybe from cleaning PCB's?

As regards the infamous tants on Tek gear if you go them with a meter on diode test some will beep, but that's because there's a low resistance in the same circuit. I've concluded I might as well get rid of all the tants and re-cap the whole thing while I have it apart.

Andy.