Philips Fm 900 Series Service Notes - Philips FM92E Manual

34: Philips FM 900 Series Service notes:

[compiled by Andy Beales VK4KCS]
Symptom:
Radio keys up when P.T.T. is operated but although the "Tx" light stays on, only a brief moment of power out is
observed.
The loudspeaker makes a faint "squeak" whilst this output power is present.
Check:
Using an oscilloscope check to see if Pin 6 of I.C. 201 on the R.F. board shows signs of oscillations while the
P.T.T. is operated.
This I.C. takes care of switching the radio from Rx to Tx and some other housekeeping chores.
If oscillations are observed then this is what is causing the problem.
Fix:
1/. Fit a 0.001uF ceramic or polyester capacitor from Transistor Q 203 B to E. This is successful about 80% of
the time.
2/. Fit a 0.1uf polyester capacitor from I.C. 201 Pin 6 to Pin 16 (ground). This is almost always successful.
Only one radio I ever encountered needed both these fixes added.
Symptom:
No Transmitter.
Check:
After eliminating all the usual causes for no power out, check that there is switched supply voltage at the Emitter
of Q 209 on the R.F. board.
Voltage should be present while P.T.T. is operated.
Fix:
If this transistor is faulty then either leave it in place and short-circuit Collector to Emitter or if a replacement is
available then change the device.
General Notes.
Anytime that the lids are off an FM 900 then check the condition of C 432 and C439, 10uF 50v. electrolytics in
the Voltage doubler circuit on the Logic board.
This part of the circuit supplies about 17 volts to the P.L.L. circuit, this voltage eventually being used as the
Steering voltage to control the frequency of the V.C.O.
All manner of strange symptoms can be observed if this voltage is not stable and fed from a low impedance
source.
These capacitors operate at an elevated temperature in the radio and are prone to dry out long before any other
components show signs of distress.
I have measured caps removed from a faulty radio as having a capacity as low as 0.5uF.
One of these capacitors is inside the metal shield and one just outside.
Quite often the P.C.B. is is coloured in the vicinity of these capacitors.
V.C.O. and Rx. Front end Modules.
These two modules both suffer a common manufacturing problem in that when the module housing was being
plated, they were not cleaned properly of the acid used to initially clean the base metal.
This acid slowly reacts with the plating and over a period of time, Dendrites or very fine metallic whiskers can
grow inside the module and eventually short out to the pins on the edge of the ceramic substrate that supports
the components or to the components themselves.
Philips FM92E Conversion to 6 Meters: Version 3.9 Page 66