OK all done
Attached are the simple style schematics for the two versions of 2nd Local Oscillator found in the R3132. I include the very much simplified DC model to the right of the schematic. Your module is a Type 1 early version.
As can be seen, changes have been made around the area of the varicap diodes in the Type 2 version. The changes have improved the design but the simple alignment procedure that I wrote applies to both versions in all respects except where you measure the control voltage on the varicap diodes..
In both versions, the Varicap diodes are in series with the VCO resonant circuit and this is how they influence the oscillation frequency of the VCO. In purely DC terms however, the two diodes are in parallel and the potential difference across them is created by tying one end of the parallel pair to +11.3V whilst the other end is connected to the Control Voltage (CV) line coming from the Phase Locked Loop (PLL) PCB. The PLL produces a voltage at its output in the range -12V to +12V. As such it is capable of providing ~0V to ~23V across the varicap diode pair.
If CV -12V then the p.d. to +11.3V on the other end of the varicap diodes is 23.3V
If CV +12V then the p.d. to +11.3V on the other end of the varicap diodes is 0.7V (but forward bias on varicap)
In my alignment procedure the CV pin settles at +10.3V and the VCO produces 3805MHz. When the PLL is active, the CV line drops to around 0V, which is its mid point of output capability, and the VCO rises in frequency to its specified 3840MHz. In the alignment phase, the p.d across the two Varicap diodes is 1V. When the module is reinserted into the chassis and the PLL is active, the p.d across the varicap diodes increases to approximately 11.3V.
Now if we consider the loss of the -12V rail to the PLL output LM324 chip we have the following potential scenario........
As stated, when the module is aligned in the manner that I detailed it has no PLL control. When the PLL is active it applies approximately 0V to the CV pin, the VCO should be pretty much on frequency at 3840MHz. If however the alignment phase has proven that the VCO cannot be set at 3805MHz, due to component tolerances in the VCO circuit, we may have to settle for something near to that frequency.
The PLL can swing the VCO through a range of -40MHz to +40MHz around its set centre frequency. 3805MHz is the ideal alignment frequency but less than 10MHz either way will not be significant in most cases. That would provide an acceptable alignment frequency range of 3795MHz to 3815MHz. It may even be acceptable to use a frequency as much as 20MHz away from ideal, but this does limit the PLL's lock capability in one frequency drift direction as it is already compensating for the alignment error. This is already explained in my alignment guide.
We know that an alignment frequency of 3805MHz is optimal as when the PLL is active it only needs to apply approximately 0V to the CV line to gain lock at 3840MHz. It is therefore pretty much in the centre of its control range. If however we could only achieve 3790MHz during the alignment, the frequency will be too low and the PLL will compensate by applying the required voltage to the CV line. In this example it would actually apply a negative voltage on the CV line in order to generate greater potential difference across the two varicap diodes. If the -12V power rail is not present at the LM324 on the PLL output, a negative voltage cannot be applied to the CV line. The result will be a PLL trying to pull the VCO onto frequency and failing to do so because it cannot create the required p.d across the varicap diodes. It will do its best though and try to drive the LM324 for negative output. The output will sit at 0V however as the LM324 cannot supply what it does not have, i.e a negative potential.
Well this is my present theory on what has gone wrong but as I do not have the module in front of me, you will need to do some testing to prove whether I am right or wrong about this. I am very tired right now so regret I cannot help further for the moment.
Fraser
UPDATE : PLEASE READIt has become apparent to me that the type 1 module may differ to the type 2 module in ways that I cannot capture as I do not have a type 1 module available to test. The simple schematics that I have drawn come from my type 2 module repair notes, and photographic records of both module types. Please treat these schematics with caution and use them as guidance only as their content IS NOT guaranteed accurate !
Caution should also be exercised when using my simple alignment procedure on type 1 modules as there Are differences in the type 1 module PCB. The principal remains valid however.
Thank You
Fraser