Taming the TFT Model 884 FM Modulation Monitor

TFT 884 FM Mod
              Monitor
There appears to be an ugly intermittent that was designed into this unit that can not only be frustrating for the engineer that relies upon it to help keep the station in compliance with FCC rules, but, also frustrating to the on-air staff that uses it to externally monitor their product.  If your 884 is intermittent in operation then it's very likely that the below modification will solve this.  Since most radio stations are on a tight budget the best option is probably to repair your monitor versus buying a new costly one.

The illustration below is a view of the circuit board (PWA) that requires this modification and is held in by only six screws.   Carefully remove the screws and four Molex connectors.  There is one coaxial cable that is soldered to the circuit board that shouldn't be a problem for you if you lift the front edge of the board upwards and to the rear of the 884.  The last illustration on this page shows this.

The frequency synthesizer IC (a Motorola MC145152) divider chain is loaded by data that is fed from two CMOS 27C32 EPROM's depending on how their address lines are set up by the frequency selection thumbwheel switches located on the front panel. It was discovered that the designer/s left three crucial inputs to these devices unconnected (as in floating)! These EPROM's are shown in the illustration below and are labeled 844A-1 and 844A-2.
Synthesizer section

Further investigation reveals that as long as pin numbers 18, 19, and 20 are held at a logic low the monitor operates normally.  Touching the probe of an high impedance DC voltmeter to any of these three pins will show that the voltage is unsteady or floating. It's pretty important that any unused input pins of a CMOS device are tied to either ground or +Vdd or you can end up with illogical logic. Note that the pinout diagram for the EPROMs illustrated below shows these three pins to be the Enable line, Address 10 line, and the G bar (select) line! It's no wonder the unit took time off whenever it felt like it.
27C32 EPROM Pinout

At this point it was time to get out the soldering station and wire wrap to get these inputs tied to ground.  As you can see I didn't do the prettiest job, but, as long as care is taken to leave some space between the circuit traces that run below the wire wrap lead (illustrated below) then everything should be fine and no short circuits will result. Note that this must be done to both EPROMs. Pins 18, 19, and 20 get tied to pin 12 (ground) of each EPROM. Also note that the red jumper at the top left corner was already there before I even disassembled the unit.  All in all this turned out to be a fairly simple job.
The solder job

Wider view of the modified circuit board (PWA) as illustrated below.
Wide view of the PWA

Reinstall the PWA and verify the operation of the modulation monitor by connecting an antenna to the low level RF input, selecting a strong local station, and listen with a pair of stereo headphones. With the top cover is still off, it's a good idea to check all power supplies for ripple and ascertain that all regulators are giving you the correct voltages. I found +12V, -12V, +5V, and +8V regulators in various locations on two of the circuit boards. The final thing I checked was to turn the whole thing upside down with the top cover off to see if there was anything loose inside. Sure enough a small screw fell out. This can happen because small objects can inadvertently fall through the vent holes in the top cover of the unit's enclosure over time.

Happy broadcasting!



Contact:

Mike
(888) 265-6193
(574) 222-3796
South Bend, Indiana
email: wb9ypa@fourway.net





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