Vintage Audio: Tektronix 310 Portable Oscilloscope

An earlier edition of Found in the Attic examined a surplus OS-8/U oscilloscope, and we noted that this was one of the first portable general purpose scopes. The honors for the first portable laboratory-grade oscilloscope goes to Tektronix, who introduced the model 310, which is the subject of this installment.

While the 310 eventually had a wide range of field service applications, it began in 1954 as a project contracted by IBM, who needed such a device for servicing its main frame computers. Initially, Tektronix didn't expect much demand for the 310 beyond fulfilling IBM's needs. Soon however, production could barely keep up with demand. Manufacturing of the 310 and 310A continued through 1971, a 16-year production run. When it was over, tens of thousands of these oscilloscopes had been sold.

Despite the parts density of the 310, they are a joy to work on. Once the side and bottom covers are removed, the entire chassis hinges open from the back. Circuits are logically organized, and most components are easily accessible for servicing and voltage measurements. As with most Tektronix documentation, the service manual for the 310 is outstanding.

Our 310 had been a workhorse for contract engineering jobs for over 20 years. Despite countless hours on the road and hard use, it held calibration well and never failed. After sitting idle for about 6 months however, the trace was dim and would not focus, sure signs that the high voltage section had failed. It was mothballed for several years, and a newer Tek 465M took its place in the field. Now is a good time to investigate and get to the bottom of this. The manual for the 310-A is available as a free djvu download from the BAMA website.

Many people are afraid to tackle the high voltage section of vintage Tek scopes. Some fear electrocution, some are unfamiliar with the circuit (which looks a bit intimidating), and some don't have a high-voltage probe. With an understanding of the circuit, knowledge of common failure points, and a bit of patience, this section of the 310 not that difficult to figure out and repair.

All vintage Tek scopes from this era, be it the mighty 535 or the diminutive 310, use the same basic high-voltage regulation circuit. On the 310, V701A/B is a 12AU7, and functions as the feedback amp for the high-voltage supply. The grid voltage of V701B is compared with the cathode, which comes from the regulated -150v supply. The grid of V701B should be around -155, which results in around -2 volts on the grid of V701A, which puts the plate voltage around 50 volts. This is applied to the screen grid of V704, a 6AQ5. It, along with HV transformer T700 and C705 comprise the HV oscillator.

The pivotal point for troubleshooting vintage Tek high-voltage supplies is that grid voltage on V701B (or the equivalent 12AU7 on other models). If it is not at -155 v (for the 310), that suggests the feedback loop isn't working, and points to HV oscillator components, the 6AQ5, HV transformer, or C705, which forms a resonant circuit along with T700. Socket voltages on the 6AQ5 may also be bad.

If, on the other hand, we see a solid -155v on V701B, that tells us that the feedback loop is working properly, and we need to look upstream from the grid to find the problem. That usually points to the voltage divider, R740-744 on the schematic. If these components are original, then they are at least 45 years old. Carbon resistors are notorious for drifting upwards with age, and this is another common failure point on these scopes. Don't forget about HV adj and focus controls R741 and 743, which can also change value. If you can't locate exact replacements, you can shunt a resistor across the pot, using Ohm's Law to find the right value. The pot won't work exactly right, but will be close enough.

As a practical matter with equipment that is this old, it is usually a case of ‘death by a thousand small cuts', and there may be bad components on both sides of the grid. Still, the divide and conquer approach will get you there quickly. Our 310 had bad oil-filled caps on the HV side, as well as out-of-tolerance resistors on the voltage divider side.

All of the Tek service manuals from this era stress substituting known good tubes during the troubleshooting process, rather than relying on tube testers. This is very good advice. Most tube testers evaluate tubes on the basis of their ability to amplify 60 Hz sine waves. Many tubes can pass this test with flying colors, but fail miserably in amplifying high-frequency or pulse waveforms.

That being said, the reverse logic also applies. Any tube that cannot amplify 60 Hz signals or flunks the shorts or leakage tests in a tube tester definitely won't work in a Tek scope.

The 310 was among the first Tek scopes with a calibrated time base. Previous scopes were individually engraved with timing nomenclature during final production testing. A precision wideband sweep generator and calibrated X amplifier, both designed by Tek engineer Dick Ropiequet, delivered consistent performance, and with a few tweakers in the circuit, could be easily calibrated at the factory, and kept accurate in the field. The circuit was originally used in the 315 scope.

One of the Tektronix 'firsts' with the 310 was the use of printed circuit boards. And not just ordinary PC boards, but double sided ones. This innovation was driven by the need to pack a lot of electronics into a very small space. The boards were used for the high-voltage supply and vertical preamp circuits. But these boards soon proved to be a nightmare for Tektronix and its 310 customers.

The technology of plating through holes to join traces on opposite sides of the PC board did not exist at the time. Instead, these boards used copper rivets to do the job, forming a mechanical joint. It wasn't long before corrosion, expansion and contraction with temperature cycling and the stress on the boards from tubes being removed and inserted all took their toll on those joints. They soon went intermittent or failed completely.

Tektronix must have taken a bath on warranty repairs resulting from failed PC boards. The end result was a complete mechanical redesign of the Tek 310 – the 310A, introduced in 1959. It had essentially the same electronics as the 310, only the circuit boards were replaced with the more conventional (and reliable) ceramic terminal strips and point-to-point wiring. Many Tektronix innovations were runaway successes, but this clearly wasn't one of them.

There are two Tek 310s in the attic. Both were college surplus grabs. The subject of this article gave many years of great service, the other had suffered badly at the hands of the military. It came minus the bottom panel, quite a bit of hardware, and one of the shields on the preamp. It also showed signs of high-voltage issues. Someone had also removed the AC socket from the chassis and hardwired an extension cord directly to the power transformer leads. Bringing this one back to life will take a bit of time and effort.

Parts have been ordered to repair the high-voltage sections of both scopes. The first 310 will probably be placed in semi-retirement, used as a backup when the 465 is out for calibration.

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