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
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
Final Solution to the problems
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.
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
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
This also applies to the Stabilock 4040, at least as far as I can tell.
© Mike James 5th October 2009