Schlumberger 4002: Overview

After quite some time, I am finally starting to check my 4002 signal generator in-depth. The first thing I want to do before really starting this project is to get a good idea of the system layout, hence the “part 0” thing above. I will link from here to the different components as I wriggle through the unit and check them. As there is no service documentation freely available, I will go deeper into critical parts of the circuit along the way.

This will also help me simplify things later when trying to figure out which line went where, if things go wrong.

Clock source (Decade stage)

OCXO top compartment. Oscillator on the bottom right, distribution on the top right. 10.7 MHz mixer (10.625 MHz + 65-85 kHz) and filter in top center. Quad VCO and PLL for widerange signal in bottom center and left.
Fig. 1a: OCXO top.

Contained in the bottom RF block, consists of a styrofoam-encapsulated 10.000(00…) MHz oven-controlled precision oscillator and some clock distribution buffering. This part sources the main TTL clock which is also available on the backside ports as an instrument reference. The picture shows the whole top side of the module block, but the actual OCXO and distributor PCB are on the right.


OCXO bottom compartment. Oscillator on top right, 10.625 MHz and 100 kHz ref generator in bottom right corner. 65-85 kHz generator in center and control logic circuits on left.
Fig. 1b: OCXO bottom.

There are three additional circuits in this module: The 10 MHz TTL buffering and switchover for external references, a 10.7 MHz IF generator (PLL+VCO+Mixer) and another phaselocked VCO for a derived widerange signal (49.3-70.7 MHz according to the marking) which is used to fine-tune the RF synthesis stage.


FM modulator

FM modulator
Fig. 2: FM modulator.

Contains the FM modulation frequency generator for FM and PM, the mixing circuit and a XTAL+PLL octupler to provide an exact 80 MHz TTL reference from the 10 MHz base ref. The actual frequency and phase modulation is mixed onto the 10.7 MHz IF signal coming from the OCXO and then sent off to the VCO. Also hidden in this block is a downmixer that shifts an input between 80.1 and 100 MHz down to 0.1 to 20 MHz by mixing an external input with the 80 MHz reference. We’ll see what that is good for later on.

Voltage-controlled UHF synthesis

VCO unit
Fig. 3: VCO unit.

This module features 8 separate VCOs starting at frequencies of 480, 560, 600, 640, 760, 880, 920 and 960 MHz. That’s right, no direct digital synthesis (DDS) in this generator – well, with the exception of the modulation base-band signal. The VCOs are switched according to band and mixed with the 10.7 MHz IF from the MOD unit (containing the f./p. mod.) as well as the wideband variable signal from the OCXO, forming the unrefined RF output. You may have noticed that the VCO does not span the entire output range of the device, which hints at several other tricks which must have been used here. An important feature for diagnostics on this module are the two red/yellow LEDs on the PCB strip next to the right-most coax port. These signal the unlocking of oscillators and should be off at all times.


Fig. 4: Filter+AGC.

This is where a lot of the magic happens. The 4002 uses three different methods of synthesis for different frequency ranges: Direct VCO output, integer dividing/filtering and downmixing of the VCOs. An RF switching circuit on the right PCB selects the appropriate source/mode via PIN diodes, followed by a matching LCLCLCL lowpass filter to suppress eventual harmonics. The integer division is also included here. Passing the filters and the bridge to the left PCB, several buffer amplifiers, two power metering circuits and a PIN-diode based attenuator stabilize the amplitude of the output signal to +13 dBm. Instead of being fixed, the reference level can also be set from outside the module, providing around 20dB of linear attenuation range if this feature is enabled by pressing the button “CONTINUOUS” on the front panel. The mechanical attenuator is then kept constant while the PIN attenuator does all the work.

Output attenuator

Fig. 5: Attenuator.

A typical mechanical attenuator which switches pi- or T-type attenuator segments using magnetic actuators. I did not look inside since these devices are easy to break and mine is working. The attenuation values are conveniently written down, as is the internal signal path. The one special thing about this module is the pair of send/return ports which allow extracting and reinjecting the signal to/from another module if the corresponding switch is enabled. The output doubler block is attached to this port. From the output of the attenuator, a semi-rigid cable runs directly to the RF front panel output.

Frequency doubler

Frequency doubler
Fig. 6: Freq. doubler.

A frequency multiplication circuit which I have not yet inspected too deeply since it seems to work. I expect to find the usual doubling circuit using an over-driven nonlinear component and appropriate band- or lowpass filters at the output. This module most certainly contains another AGC stage and is enabled when frequencies above 1080 MHz are selected. Note that the 4002 will not allow the user to input frequencies above 1000 MHz if the module is not connected.

All of the modules are plugged into a backplane carrying the power and control lines, which are transferred by ribbon cable from a second backplane in the left side compartment. This one connects the control circuits on up to eight euro-format PCBs. In my case, six modules are inserted:

  • Microcomputer (MC6802/MC6821/MCM68A10)
  • DDS (generates the modulation audio)
  • D/A interface for voltage-controlled circuits
  • Display interface
  • Memory (battery-buffered RAM for stored profiles)
  • IEC interface

So that’s it for right now, stay tuned for updates while I try to get this organized!


2 thoughts on “Schlumberger 4002: Overview

  1. *
    Hello Mario,
    A 4002 recently followed me home with similar issues to yours, the symptoms are exactly the same and also get the modulator error message. I came across your blog and based on reply’s and suggestions by your followers will attempt to track down the fault. Thank you for all the valuable info you have made available. Was the service manual ever made available for download?

    1. Hi Renato,
      congratulations :-) I still think it is a very capable generator, although it really has a tendency to develop new and unexpected errors.
      I did not find a complete copy of the service manual so far, but I have an incomplete version of the original schematic somewhere. I’ll dig it out and send you what I have – might take a day or two. I never found the time to complete my reverse engineering of the entire unit, unfortunately. Let me know if you need any help, since my 4002 can serve as a comparison. I also have a complete dump of the system EPROMS, just in case.
      Best regards

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