I have two TRC-1007, fixed one and it works nicely. The other one almost works. TX and RX is fine, its just the squelch control doesn't work. It never cuts out the fm noise.
Does anyone have a circuit diagram for this radio? ( Rick? )
I assume the LM358 at the bottom of the pcb by the audio chip controls the squelch?
What had happened to this set was a small metal retaining clip had broken off inside it and probably shorted something out. The voltages around the LM358 read the same for both sets, and the votages on the squelch control pot itself are the same too.
If I touch one of the components that leads to the FM reciever chip (which probably stops it working) with the squelch fully up, it makes the squelch turn on/off tick from the speaker. So perhaps the squelch switching circuit is ok and something had changed value?
What is the supple voltage like at pin 4? You can find out if the chip is ok by isolating 12 and connect a 10K in series with a 50 K pot back to the supply voltage this will let you know if have a control fault.
I forgot some radio using this chip may have a cap from pin 12 the ground if so check to see if it?s ok.
I swapped the MC3357 IC and there was no change, and the IC works in the other unit, so the chip is OK.
The squelch pot is ok too, approx 3V is across the pot and the pot contact voltage varies in the same manner between the two units.
I experimented lifting the voltage on pin 12 by connecting a 100k between pin 12 and Vdd. Making and breaking the contact open and closed the squelch, however I could not get a signal to break the squelch while it was operated.
There was an electrolytic cap, C16 1uF which was ok too. This was connected in series with a 2.2k resistor from pin 12 to gnd.
I double checked the voltage from the pot contact, it varied from 0 to 3.3V. There is a reistance measured between the contact and pin 12 of around 11.5k. The voltage at pin 12 varies from 0.18 to 0.4V, which is low. The squelch pot is 50k.
If you can not control the voltage at pin 12 then the squelch will not work. The squelch part of the chip works like so, a sample of the audio is taken of at pin 9 and and go through (PD = 2 resistors) back in to pin 14 and the demod is also taken of at this point, the squelch control gate is pins 12 and 13, when the gate closed pin 14 is taken to pin 15 which is ground, so the fault must be with the gate control. I noted you say the supply is at 9.38 vdc this sounds a bit hi. Hope this helps.
You would not believe it. The fault was an original manufacturing defect - the squelch never worked on this in its life! Scattered around the board were some solder through holes, one of them had not made contact with the other side of the board, although it was soldered.
Exactly how long did I spend faultfinding around the FM chip?
To think I was considering transplanting parts there in case there was a component that failed under voltage....
Thank you for your assistance Rick much appreciated.
Very pleased with both of these much better size than the breeze-block of trc-1001!
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