When you turn up the squelch does the noise still exist?
- It may serve you better, to know that as these radios age, many "noisy" problems and "quirks" in performance of the Receiver show up so you may find yourself replacing a lot of components - this is normal - you'll know why as you work thru the rebuild/repair process..
- In nearly every case, Age of the radios' more sensitive filtering parts, like the electrolytic caps and many of the Mylar (Green Chiclet) caps are failing - drying out and or losing internal cohesiveness - its' simply "old" and they're showing their age.
The Tips earlier - after going thru this thread, are, to me, workable but you need to follow an order of steps that after I've done this - helps me locate the problem "parts" faster.
Start at the Audio Chip itself. Isolate its' input from the rest of the board - this way, if that noise still persists - you know it's not generated by something further up the Receiver chain, or the Mic amp sections.
You do that by removing C110 0.1uF or 104M/K cap - from the receiver Pre-amp and C82 0.047uF (473) - Mic Amp / AF Switch (this cap number may differ - but it comes from TR27 part of the Mic Amp chain going to the AN612)
Power up the radio and let it warm up - listen for that noise - be patient at this step, for the radio is not injecting noise, hiss or any sort of background audio into the amp to help it warm up faster - this takes time to bring the conditions back that give you the noise.
If you can, and are EXTREMELY careful - reheat soldering joints CAREFULLY - DONT CAUSE ANY SHORTS - it's is better to wait ...
But - if pressed for time - just apply small amounts of heat - while it is powered, you'll near hum and static - it's normal - this helps you in locating the faulty parts.
- In the reheating process you may locate the poor soldering area and the condition goes away, you may have found the "barely able to make contact" solder pad - but then too, reheating these parts can also recondition them into working temporarily - so you will have to do this several times - letting it cool down after each attempt.
- Pay attention to the detail of where the noises encountered during this reheat process are the strongest
- - in the Audio amp chip itself has many feedback and Equalization pins so they all can add to this problem in one fashion or another.
The next process, is to reattach the Receiver Preamp - reinstall C110 that 0.1uF cap (or whatever value it had) BUT REMOVE C108 from TR36 - this takes away the Volume control but you want to know if this pre-amp is ADDING to the noise... Repeat the above like you did for the Audio Amp but for the Receiver Preamp...leave the Audio Amp alone - it may be fine but then too - you may find it "acting up" again, then between these two sections is the problem.
If not, as in it's all too quiet...you'll have to proceed - by removing C108 of the Pre-amp and now reconnect the Mic amp AF Switch C82 that 0.047uF (473)
This may sound tedious - and I agree, but it is the only way you can locate isolate and rebuild the noisy sections of the radio.
Nine times out of ten, this noise is related to just the Audio amp and its' support - replacing parts with fresh is not the easiest job but this set of steps helps me locate the noise sources faster.
Now, to cut to chase...
Locate the limiter - has it been removed? You might want to put it back in - if only for your receiver. In this radio the Limiter ENGAGES in Receive (RX) mode thru D41 - which shunts any noise in the mic cord to ground before it's amplified - the Mic amp itself is powered all the time.
For your benefit, if "defeating the limiter" is necessary, keep the limiter in place, and locate R103 - 1 Meg (105 or Brown - Black - GREEN ) . Replace it with a 560 ohm resistor (Green - Blue - Brown). Locate R125 -1.5K ( Brown - Green - Red ) and replace it with a 3.3K ( Orange - Orange - Red ) or LARGER value
When you switch to TX mode, the D41 stops conducting and the Limiter opens up - this also occurs for the PA preamp too - thru D40 - if they are disabled - noise can be picked up in the mic amp and sent into the Audio amp skewing this test - so it's why I recommended the removal of these two Caps C110, C82 listed above - to isolate the Amp and rule out any potential problems where the biggest source of noise AMPLIFICATION comes from - the Audio Amp itself.
You can apply this to any radio to help isolate the sections from themselves by removing their connection capacitors that keep the DC Bias (if any) back in their respective places. The Audio amp itself can be the main culprit and isolating the other sections helps you determine this.
