Repairing Fm92 Vcos - Philips FM92E Manual

39: Repairing FM92 VCOs

by VK7ZRO
Over the last year, I have been involved in servicing about 70 FM92 E band radios. It soon became clear that
these radios have a major problem with their VCO's. From my observations, the majority of radios have a VCO
problem to some degree, particularly the older radios. Symptoms are varied but generally involve noise,
cracking, microphony and in bad cases, the VCO dropping out of lock.
I test the VCO in the following way. Open the case and hinge open the receiver board. Power up the radio and
connect a power meter. Monitor the radio on another receiver. Connect a voltmeter to the VCO test point.
Press the PTT and note the power reading and the reading on the VCO test point. Gently tap the VCO with the
back of your knuckle. With a good VCO there should some microphony but there should not be any cracking or
frying noises. Bad VCO's are usually highly microphonic. Watch the meter on the VCO test point. It should
remain quite stable, if not you have a faulty VCO. Watch the power meter. If the output drops to zero, the VCO
has temporally gone out of lock and is definitely faulty.
After hearing various stories about the repair of the VCO's, I decided to develop my own method. I was in the
fortunate position of having a number of radios that I could cannibalise for VCO's should my efforts fail. So far I
have repaired about 15 VCO's with a 100% success rate. This includes both 10.7 and 21.4 IF VCO's. I still
have a heap more to repair. The repair takes about half an hour including taking the VCO of the board, which is
the hardest part. The repair method is quite drastic and is probably not for the faint hearted although it is quite
easy to do.
Slide a sharp trimming knife under the label and remove it. Place the label to one side. The cover plate under
the label has a number of notches in it. Place the point of the knife in one of the notches and slide the knife
along under the cover, breaking the solder joint as you go. The cover plate should come off easily. Make sure
the cover is flat and straight and place it to one side. Examine the VCO. Using a small screwdriver VERY gently
lever the circuit board upward in the area of the coil on the end of the board. Look very carefully at the soldered
joint under this coil. The odds are you will see movement indicating that the soldered joint has broken. Do this
very carefully. On all boards that I have diagnosed as needing repair I have found that this joint has broken. I
have also found other joints broken but not as often. I have had two cases where the board has been
completely loose in the case when I have removed the VCO from the receiver board. I'll leave it to the experts to
determine why this joint breaks.
Now for the interesting part. Clamp the VCO in a vice. Clamp it between the ends with the exposed board
upwards. Only clamp the VCO lightly as it possible to distort the case. Now heat the VCO using a hot air gun on
high. Use an ordinary heat gun, the type used for paint stripping available from most hardware stores. You'll
have one for shrinking heat shrink tube anyway. Aim the gun at the VCO at an angle from underneath and at the
side away from the pins. This is done to minimise heating of the board. Gently touch the pins with a small
screwdriver. When the case is hot enough, you will see the board move as you touch the pins. This takes about
30 seconds to 1 minute. Remove the heat gun. Lift the board out of the case with a pair of long nosed pliers.
This is a bit tricky. Be very careful, as it is quite likely that you will have melted the solder on the board and the
components will be loose. Don't worry, they seem to survive. Put the board aside to cool. Immediately re-tin
the solder pads in the case while the case is still hot. Leave plenty of solder on the pads. When the board has
cooled, turn it over and re-tin the board where it solders to the pads in the case. Leave plenty of solder on the
board. You could find that some of the silver plating has come off the board. This is why I recommend using
plenty of solder. The aim is to get the board soldered to the case in as many places as possible, particularly the
pad under the end coil. Put the board back in the case. Position the top edge of the board against the top of the
case. Position the board laterally so that the gap between it and the case is the same both ends. This is
important, as you won't be able to mount the VCO onto the receiver board unless this alignment is correct. It is
critical so make sure its right. Re-heat the case. After about 30 seconds gently push down on the centre of the
board clear of any components. When the board is hot enough, the board will sink down onto the pads as you
push gently on it. Remove the heat gun. It is important that the board is pushed down otherwise the alignment
won't be correct. Be careful not to touch any components as you may have melted the solder on the board.
Now place the cover onto the case while the solder is still melted. Hold the cover down onto the case as the
solder sets. Wait a couple of minutes and while the case is still warm (not hot) put the label back on. Let it cool.
Done properly, the VCO should not look like anything has been done to it.
Put the VCO back onto the receiver board, align the VCO coils and retest as above. I think you will be amazed at the
difference.
Have fun. Ron VK7ZRO.
P.S. Since writing the above, I have had one repaired VCO become very microphonic after about 6 months. Running the
tests that I outlined above showed that the VCO was OK apart from being very microphonic. It was so microphonic that
mechanical feedback from the speaker produced an audio howl. The most sensitive area was around the Rx coil. When I
removed the slug an extremely small piece of broken slug fell out. This fixed the problem. There must be a moral here
somewhere.
Philips FM92E Conversion to 6 Meters: Version 3.9 Page 78