Hamble RadioZone

Schlumberger Solartron Stabilock 4040 

This machine appears to be from about 1987. It is built without regard to how much the circuits that make it up would have cost. There are tens of ICs controlling the front panel : on two separate stacked PCBs covering almost the entire front panel area. It is beautifully built.
It consists of a RF signal generator coupled with a radio receiver.  It also has audio generation and analysis capabilities that can be used without the RF section along with a true RMS volt and ammeter.

It  had a problem that it could recieve signals from a 2 metre handheld transciever indicating its power but it was unable to count its frequency. When signal generating , it would lock onto some frequencies. It was usually wrong in frequency. Looking at the spectrum analyser output it was often producing a sweep of frequencies: either FM or AM on even if the modulation was off.


It could be disturbed by tapping the screening with a screwdriver. Some frequencies appeared to tune backwards or hop. Rather like a malfunctioning n/n+1 prescaler device.  Taking the screens off, you could make it lock using fingers poked around the decade synthesiser board. As it had many PLL synthesisers interlocked it was difficult to find the root cause of the problem. Nothing seemed particularly wrong. Just there was not much signal on one of the prescaler input pins in one of the synthesisers. But there was some signal there, enough to make it count and put me off the hunt for failure.

Final Solution to the problems

November 2006
On and off over a year or so, I poked around. Eventually I discovered an SL560 amplifier IC which had less signal coming out than going in. It was being used in an AGC amplifier configuration with a PIN diode controlling its gain in a clever way.   At first I thought that the opamp controlling it was turning the SL560 gain down. Then I realised the PIN diode was modulating the emitter impedance in a common emitter amplifier and if the opamp put out a large voltage it was turning the gain to maximum. The SL560 also had a couple of solder blobs between pins : obviously somebody before me trying to get more gain from bypassing some of the internal resistors. The SL560 is long obsolete, but I was able to download the data sheet.
Fortunately the SL560 has only 3 transistors in it. So I simulated the schematic using SPICE substituting BFR91 transistors for the unknown devices in the the SL560. The resulting circuit  seemed to be likely to work.  It is also quite clear that shorting the output of the SL560 to ground would destroy it.

I decided to go for two transistors rather than three and replaced the Darlington output stage with a single device, as it seemed not to need the full 30dB gain possible from the SL560.  I then used the BFR91 transistors I had rescued from a throwout session at work to build a dead bug construction amplifier over the pads of the SL560 and its supply and coupling components.

The circuit worked and the AGC was operational: putting my finger over the VCO feeding it showed a distinct change in the output of the opamp in the AGC loop. I put the screens back on and it still works.

With the aid of a Stabilock 4039 user manual I was able to put the unit through its paces with a Yaesu FT11 handheld VHF. It seemed to be able to measure and generate the expected signals.

All of the harmonics of this transmitter on low power (0.44W)  are at about -74 dB with respect to the carrier.

 I also ran the selftest program (ON 31). It ran to the end and did not report any errors.

To Do

I may need to go back in and replace an opamp elsewhere in the synthesiser board with a FET input type instead of the 741 I was trying to substitute around the board to see if it was a dead opamp at one stage.

Now all I have to do is replace the broken meter movement : a rather nice 100uA torsion wire movement which is used for the right-hand voltmeter function is broken. Fortunately all of the moving coil meters are backed up with a 4 digit LED based display showing the reading.
The meters are driven to auto-scale in a 1-3-10 stepping by the control processor from a DAC so I can replace them with different sensitivity meters or recalibrate a repair to the meter by changing a resistor somewhere on the front panel PCBs.

Scanned Stabilock 4039 User Manual

I have scanned in my copy of the user manual , and after a recent server destruction^H^H^H reorganisation (dont stick random Linux boot disks in your LVM home file server)  I can now offer the file for download.

Scanned User Manual

This also applies to the Stabilock 4040, at least as far as I can tell.

Page Mike James 5th October 2009
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