PCB Number: N/A
PLL Number: MC4044 Phase Detector (IC-109)
Test gear available: Complete bench
Symptoms: PLL Unlock causing channel display to go blank Intermittent problem
What is working: Receiver all the time and transmitter when PLL is locked
What has been tried: Loose IC sockets, voltage checks on Phase Detector IC
Did it suddenly happen ?: Yes
Anything else that may be of help?: I do not have the PLL schematic for this radio, if anyone can get it to me, I would be most grateful.
According to the datasheet on the phase detector, if pins 2 and 13 are high, the PLL is locked. When the radio acts up, pin 2 is low and pin 13 remains high.
Can someone give me any ideas?
- Posts: 3
- Joined: Monday 24th May 2010, 9:59
- First Name: Stan
- Location: New York Stste
Question, is the PLL board populated with many 14 & 16 pin chips and transistors? TTL chips, like 74whatever or is there just one... oh, like 20 or 24 pin Motorola chip?
If it's the earlier discrete component board (which was supposed to be field replaced with the newer board) look for one of the transistors being fed from the PLL oscillator and feeding back into the divider string (the logic chips).
It's an FET IIRC and the circuit is a bit dodgy in that if the transistor's gain falls off the least bit you will experience the failure you're describing. Look with a scope to see if you have enough signal to operate the first chip in the string... seem to remember it needing to be at least 2 volts peak to peak.
And that's my memory from the very early nineties... gosh, IT STILL WORKS! And you asked the question that I had an answer to... WOW!!!