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Tektronix 190 A/B Constant Amplitude Signal Generator.

April 19, 2014

Tektronix 190 A & 190 B
(Please click on this picture to view it properly. I think that the meter on the 190 A is better looking than the meter on the 190 B.)
I acquired a clean 190 B sans attenuator which is essential to the function of this design. One of my friends at VintageTek said that they had a unit on the scrap pile that had retained its attenuator so in due course that unit, which turned out to be a 190 A, arrived here. Even though it was dirty and missing the covers, one tube was missing and another tube had a failed heater (which is rare in my experience) I could not resist checking it out. In addition, two of the PSU capacitors were dry and I initially reformed them, then later, replaced them. Having put in the missing 6C4 oscillator tube and replaced the dead 12AU7, being Tektronix of course it worked, and worked properly too. The unit also had a 4 pin Jones socket mounted on the back, presumably so as to use it as a regulated power supply? I could not tell for sure since the socket had been disconnected.
As luck would have it, an attenuator turned up on Ebay for a reasonable price so I now have a working 190 A and B! As always, I applied Deoxit to all tube pins and switch contacts.

The basis of the unit is a Colpitts oscillator with 5 switched ranges covering 350KHz to 50MHz which at the time, was sufficient to support bandwidth testing of oscilloscopes. There is also a fixed 50KHz output. An output attenuator is provided at the end of a lead that has a male UHF connector mounted on it for direct connection to an oscilloscope. The attenuator is coupled to the generator by a special purpose Cannon connector that has a VHF coaxial connector enclosed with 3 other pins that support feedback of a DC signal that is linearly related to the peak-to-peak amplitude of the HF signal at the attenuator. The attenuator has 7 ranges from 0.1 to 10Vp-p, (a constant variation control is provided on the generator). The attenuator contains diodes that sample and rectify the HF output, resulting in a negative DC voltage that is close to the peak to peak amplitude of the HF signal, and that is linearly proportional to the HF signal. This DC signal is returned to the generator via the special Cannon connecter, and is used to control a regulated power supply that feeds the plate of the Colpitts oscillator and by this means, maintains a constant amplitude at the point of application, the attenuator output. The manual states that if the shunt capacitance at the output is less than 50pF, the output amplitude will vary less than +/- 2% from 50KHz to 30MHz and less than +/- 5% from 30MHz to 50MHz. Using my 200MHz Tek 475, both units appear to be very much flatter than specified. The output impedance is 52 Ohms. Here is the special connector:
Tektronix 190 Special Cannon Connector

The DC sample voltage is also used to drive a calibrated meter that indicates the output amplitude in p-p volts, it also shows when the generator is being operated within its controlled amplitude envelope. The frequency is indicated on a vertical drum with a separate scale for each and an vertical illuminated cursor line in a window on the front panel.

The insides of the B unit:
Tektronix 190 B Tubes
Tektronix 190 B

In earlier equipment, the sampling diodes were a dual diode 6110 tube, later replaced in the B version by 1N87 silicon diodes. Both my attenuators have the 6110 tube since the B unit was divorced from its original attenuator. If you are looking closely, you may have noticed an empty hole near the top of the chassis; the A version had a 6AL5 double diode in this location. The meter is connected across the cathodes of a dual triode, one grid of which is connected to the sampling diodes, and the other that was connected to the 6AL5 that is connected in the same configuration as the sampling diodes. The intention was to minimise thermal drift of the thermionic diodes being registered by the meter. I suppose that I could replace the 6110 in the attenuator that I have connected to the B unit with 1N87s but it is more likely that I will instal the 6AL5 compensation circuit instead, if I do anything. Interestingly and fortuitously, the B unit retains the heater supply to the attenuator.

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3 Comments
  1. Grid2 permalink

    How timely… and encouraging to hear of your success! I recently acquired the 190A version – fortunately came with the attenuator. It cleaned up nicely and my pre-power steps included replacement of the paper bumble-bee caps on the 50khz tank and also two other B-Bees; power supply reformed nicely. The 12AU7 was simliarly dead – which I suspect is because it serves as the dc regulator and get’s heavy current…it was replaced. Other that the 12AU7, all tubes tested good on the Hickok.

    On power-up, got a nice signal and tracked well with the calibration. However after about 10mins the fuse popped. Investigation found that the 5Y3 rectifier had developed a short from plate to heater on once side. Fortunately – the transformer is still ok. I haven’t checked anything else yet… still to do.

    Some thoughts on the failure:

    1) the 5y3 is mounted horizontally. There’s guidance in the tube data about what pins should be horizontal (or vertical). In my unit the rectifier is 1/8 turn out of alignment per the guidance. So I’m thinking this may have been an aggrating factor. Interesting – as found, the 5y3 was the ‘newest’ of the bunch dating to 1965 or so. Meanwhile all the other tubes had mid 1950s date codes. Hence, has been replaced at least once before. One thought is to figure out how to ‘rotate’ the socket such that the 5y3 plates are vertical as called for in the tube data.

    2) When running, I was examining signal on the .1VPP attenuator setting. However it was much greater than that on my scope. I dont’ recall exactly, but 5-10x high seems is what I recall. I suspect this isn’t regulating correctly and requires some component examination b4 further power-up. If indeed this is the situation, then this may have been the prime fault (& possibly overdriving the rectifier as a result).

    A manual observation – Where, for instance, should one set the output level with reference to the meter indication? The manual doesn’t really go into depth here. Maybe this is related to point #2 above. Perhaps you can comment on that.

    Best regards,
    Grid2

    • Hi Grid2. Does that make you a “scream grid”?
      Unusually, the bumblebee caps in my units are OK. They surely won’t stay that way but OK for now.
      The 5Y3 failure is most certainly due to drawing too much current. Check the meter calibration procedure (summarised below). I would also check the electrolytics for leakage current. I don’t think the 40 degree alignment “violation” is the cause.
      The meter calibration instructions on page 4-3 seem clear to me. Zero the meter and set the unit up to display 10Vp-p at 500KHz then adjust the meter calibration to indicate 10Vp-p on the meter, then re-zero and re-check, repeating until the zero and 10V are correct.
      I did not experience any problems with amplitude, the attenuator is reasonably accurate.
      Let me know how it goes!.
      Best,
      Richard

  2. Grid2 permalink

    UPDATE: Your suggestion to calibrate the meter was on target. Thanks! The 5Y3 and fuse was replaced, meter calibrated. Now the unit is stable and provides nice constant sine waves. The red illumination for the frequency dial window and cursor line is really neat! I suspect not many vintage signal generators can provide amplitudes at this magnitude (10vpp).

    What went wrong: This unit was dormant for a long time. For most instruments I usually don’t attempt calibration-steps until I get it basically up/running to see what’s up. I did zero-set the meter per the operating instructions. However for the 190s it seems meter calibration is also an essential early step (since before calibration the meter was under-reading by 23%!). Thus when initially operated I was unknowingly running it past red-line (the right-hand red zone past 10v). I’ve also since determined the 10vpp maximum is easily reached with only 1/3 rotation of the output amplitude control*. Given all that, I’m sure in the initial check-out it was operating well past that maximum and little wonder the 5Y3 failed. The manual does not caution on the risk of over-driving the circuit (which the operator controls allow even when calibrated/ there’s no limiter). Indeed a learning experience and I’m happy the loss was extended to just the 5Y3.

    *BTW same result with as-found weak V15 (12au7) series-pass regulator or a strong replacement.

    One last observation: The manual mentions having adequate side clearance to keep from overheating. The 5Y3 throws off a lot of heat and as seen in your photo it sits vertically under many components. Since the 190s don’t have a fan I’m thinking for extended operation it might be best to take the cover off the tube-side.

    Thanks again,
    Grid2

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