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.
 

I could never determine why this chip develops this problem. 

I have fitted a new chip in a faulty radio and seen the new device exhibit the same symptoms.
 

Symptom:
 No Tx. 

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. 

These whiskers can be seen with the naked eye if observed under a strong light source. 

The usual symptoms of the presence of these whiskers is the Receiver goes deaf, or the V.C.O. becomes intermittent or stops altogether. 

A quick fix is described as "Percussive Maintenance", which consists of elevating the radio to a height of 18 inches above a hard surface covered in newspaper or other covering (we don't want to scratch the radio do we ?) and then letting Sir Isaac Newtons' invention do its thing.(drop it !). 

This brutal but effective method should only be done to somebody else's radio preferably when the owner is out of sight and hearing range. 

The second method is to laboriously, and with a very big soldering iron, remove the offending module from the circuit board, prise the side cover off with a fine bladed screwdriver and then use a small paint brush to thoroughly clean out the inside of the module.

Thats the easy part.
 

Position the side cover back on the module, tack solder it at a number of points around the edge, using a screwdriver to press the cover into place, then flow solder as neatly as possible to reinstate the shielding effect of the cover. 

Its impossible to not dramatically heat the module during this process but it doesn't seem to worry it, because the "Circuit Board'" is ceramic. 

If you are working on the V.C.O. module check while the cover is off, that the ceramic substrate has not broken free from one or more of the mounting posts inside the module. 

This fault is typified by the radio being extremely microphonic on both Rx and Tx.

If this is the case don't try to force the substrate back down, just apply lots of heat to the outside of the module adjacent to the post in question and allow solder to flow into and fill the gap. 

Allow the module to cool and then refit to the radio, not forgetting to solder both ends of the V.C.O. module to the P.C.B. groundplane. 

Although this procedure seems fairly drastic, the module was faulty anyway and attempting this repair is definitely worthwhile, compared to parting with the hard-earned for a new module, if you can even source one now. 

Other problems that can occur include the flexible printed circuits that join the circuit boards at the front of the radio becoming intermittent, especially in early model radios that used paper insulated flexible P.C's.

If they are in a bad way they can be replaced with ordinary ribbon cable, just keep the lengths about the same and maybe apply hotmelt glue to the solder joints after it has been fitted in place, to provide some mechanical support. 

The volume and squelch controls have a printed carbon track and due to the cunning design of the adjustable part of the control, the cup that keeps the dust out of the control will wear away the carbon and cause it to go open circuit. GOOD ONE, Mr. P !! 

I have had no luck finding an "off the shelf'" replacement that doesn't involve a bit of hacking and gouging to make it fit, but a reasonable job can be done with a "gas axe" and only 2 Kilo's of body filler !! 

If water has made its unwanted way into the control head on either the local or remote control versions, the P.C.B. can be removed from the housing, successfully cleaned and repaired. 

I use a cut down artists paint brush about 6mm wide to clean around the components with either metholated spirits or preferably one of the low evaporation circuit board cleaners available from Woolietronics or other suppliers. 

The plastic housing is best just washed in soapy water and thoroughly dried.

A quick squirt of Mr Sheen restores that "just like a bought one" finish to your pride and joy. Be careful though, not to scratch the red plastic lens that covers the 7 segment displays. 

The push-button switches can be carefully prised apart and the contacts cleaned. 

It helps to have obtained a rubber pencil" (made by Stadtler for correcting typing errors) from the stationers to achieve this.

Most solvents/cleaners available will take the carbon film from the moving contact if applied, so use the pencil and then brush the residue out with a small artists brush (the adjective refers to the brush not the artist.)