Tektronix 190 A/B Constant Amplitude Signal Generator.
(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:
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.
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.