434

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The Tektronix 434 is an analog CRT storage scope with two independent storage areas on the screen.

It has a bandwidth of 24 MHz and uses a tunnel diode in its trigger circuit. The storage is done in the CRT itself, by trapping charge on a special fine grid of isolated pads behind the phosphor where the electron beam is then blocked or released.

There is a chopped two channel system and the fastest sweep time is 20 ns/div. An option for a tube with higher sensitivity at high writing speeds was available (OPT. 01).

Key Specifications

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Internals

The signal passes through rotary cam-operated input attenuators, then passes through an input amplifier that is on a separate small board. The input amplifier contains a JFET source-follower buffer that uses a 2N5911A (Tektronix part number 151-1032-00) biased with a drain current of 12.5 mA and Vgs of -0.26 V. The input amplifier has unity gain, high input impedance, and an output impedance of about 120 Ω. The input amplifier drives a Tek-made differential amplifier chip, part number 155-0050-01.

The original instrument design utilized an all-discrete switching power supply, making this an incredibly light portable scope. Unfortunately, this power supply was extremely difficult to troubleshoot when it went belly-up. At serial number B500000 and later, the design of the supply was simplified to use the same basic circuitry as found in the 7704A using a Tek-manufactured controller chip.

Repair Issues

In general, the 434 is a nice-looking, easy-to-use oscilloscope. However, the 434 was probably one of the worst mechanical scope designs to come out of Beaverton engineering. This was the model that caused bench technicians to think that the factory should send all new mechanical engineers out into the field to work at a Service Center for a year to get an idea of good and bad mechanical designs.

First of all, the 434 design departed from the proven 4-40 steel PCB mounting screws. Instead, it uses 3-48 brass screws in a design that requires a screw-holding driver to get the fasteners in place. Brass screws are non-magnetic and getting them started in some of the more obscure places on the vertical preamp board is extremely difficult. Since these scopes are of an age that one can make modifications without incurring the wrath of Tektronix technicians, it is suggested that these brass screws be replaced with steel 3-48 screws, preferably ones with captive washers like the original brass screws. Most dedicated Tektronix fans and ex-technicians do not like changing any circuit design from the original, but some mechanical changes such as this are for everyone's benefit and don't affect operation of the oscilloscope in the least. In fact, if the circuit boards are removed, the original 3-48 threaded holes in the castings can be drilled out and retapped for 4-40 screws.

If the V/DIV knob skirt lamps go out, the vertical preamp board must be removed to replace them. This is a process involving the removal of a lot of the aforementioned brass 3-48 screws, the removal of some connectors, the unsoldering of a lot of wires and the removal of knobs and shafts all over that area of the front panel. Remember, it is always better to leave those red VAR V/DIV knobs on their shafts and instead remove knob and shaft as a unit by loosening it from the coupler on the potentiometer itself. Since these oscilloscopes are considered obsolete, the suggestion is to replace ALL of the vertical knob skirt lamps with light emitting diodes, choosing either white or yellow LEDs. These lamps already have small-value current limiting resistors on the circuit board that are in series, so replacing these resistors with the appropriate value to limit LED current is an easy process. Be careful that you select an appropriate power rating for the resistors. It is suggested that current be limited so that the panel lighting is sufficient for bright room lighting, but not so bright as to make it difficult to read the numbers on the knob skirts.

The 434 uses only two plug-in attenuators (both ×10) in each channel, and most V/DIV switching involves gain switching of the preamp. The 434 often cannot meet vertical gain specifications and some very low-value 5% tolerance resistors must be replaced to correct this. These resistors are not located in an easy-to-access area of the board and once the defective resistors are identified, the board will need to be removed to allow replacement of the resistors without board damage.

Another common problem involves the mounting hardware for the back casting. Nearly every 434 that was received in Tektronix Service Centers had to have these mounting screws tightened. There is one located under the cover for the line cord and fuse. If that screw comes completely out, it is held captive inside that small space, and causes a lot of damage to the open mains connections on the fuse holder. This is a job for a #2 Pozidriv screwdriver and a LOT of muscle torque. Don't use a Phillips driver for this because it will "cam out" of the screw head with the torque that you must apply, causing screw head damage. Regardless of how much torque you apply, you won't strip the extruded aluminum rails or break the large 10-32 screws. Tighten all of these casting-mounting screws by hand as tight as you possibly can. Of course, do not use high-torque pneumatic or electric tools for this job. Especially don't use the electric impact drivers that are so popular now in the construction industry. The hardware DOES have its mechanical limitations.

Links

• Ken Matheson, Servicing the 432/434 Oscilloscopes. In Tekscope Vol. 4 No. 5, 1972

Pictures

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