I have just finished fixing a similar problem, on an 8719 Washington, slowly rising noise in RX with volume off.
I replaced C109 to fix the issue. Strange thing is, the schematic shows it as a 0.1 tantalum. Mine had a 22uf tantalum. I replaced it with a 22uf electro and it’s fine now.
QUOTE="Handy Andy, post: 719536, member: 52309"]Ok, had to give you guys a break...some humor (or humus)
When you "squelch out" and the noise never changes, then you still have to pull those two caps C110 and C82...
- IF you find C110 and C82 doesn't stop the noise, then the problem is at the Audio Amp.
- When the noise is gone, and you reconnect, try C110 first...your receive.
If the limiter has been disabled, you might want to put that transistor back in operation. You'll need it for RX to stay quiet "listening". Thanks to foreign manufacturing putting in a lot of bells and whistles in products, it encroaches in a lot of todays electronics even in the shack, the products' own timers, power supplies and keypress oscillators can produce a PLETHORA of noise.
These newer CFL, LED and even Monitors and Tablets and DSL - emit so much RFI noise, it's easy for them to be interfering with 27MHz band as well as spurs in others.
I didn't want to have you sitting in the dark -literally- because - it's the only way to keep down the noise.
Otherwise, the noise picked up in the mic amp adds to this problem and deafens your ears. Seriously, do what you can to quiet the speaker - I'll show you how to rework sections to help you and keep your neighbors happy at the same time. See above for starters...
But let's look at the Nosie level, in the radio - first...
Remember to:
- Swap - Reconnect - ONE section at a time
- Be PATIENT - Allow time for symptom to reappear.
- If Problem is not in that reconnected section - proceed...
- MIC amp section -
- There are two parts, Mic Amp for TX and PA amp (No TX)
- Noise present in MIC Amp?
- - BE SURE TO CHECK PA SEPARATELY
- Place in PA mode - Remove C88 - 0.047uF (473)
- Does Condition Disappear? - PA amp faulty -investigate R95, R96, R97 and C86
There is also another problem - dealing with AGE of the Radio...
- You already know Electrolytic's need to be replaced
- Mylar's (Green Chiclets) are also suspect
- Power supply FEED - Both Battery and 120VAC line
- A TSB (#1226) was issued to PROTECT MB3756 (IC-5)
- This Diode modification keeps the radio from excessively high Input Voltage Supply spikes from damaging IC-5 - MB3756 and also to prevent REVERSED Polarity problems in Power Up and Down conditions as Downstream Caps charge and discharge can place excessive loading on IC-5's Regulation.
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I happen to have a lot of Respect for the 140-148 Series line from Cobra - as well as Unidens effort with the Grant, Jackson, and Washington. I love working on these - both radio lines deserve a lot of credit for the abuses they have taken from owners wanting more out of their equipment, then realizing - too late - they could have bought Equipment more suitable for their needs than to take it out on these radios.
I can only help you get this corrected - the process is tedious, but rewarding once you've solved the problem.
