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Cobra 2000 no modulation.
- Thread starter Brian Gosper
- Start date
What's everyone think of to use this as a replacement for VR12?
This one?????
Or this one????
This one?????
Or this one????
Either one of those will work fine. One has a shorter height, but otherwise they are the same.
Since you have it working now, there is another option.
That the pot you got in there now - works and you will find yourself at a point where you no longer need to "play" with it and just use it. In that case just measure the resistance across those terminals that solder to the board - then remove the variable and use a FIXED resistor - solder it in the place of the variable and be done with it.
You can then use that variable back in the other radio you took it from to try and make this radio work.
You can then keep both radios working or serviceable to a degree.
That the pot you got in there now - works and you will find yourself at a point where you no longer need to "play" with it and just use it. In that case just measure the resistance across those terminals that solder to the board - then remove the variable and use a FIXED resistor - solder it in the place of the variable and be done with it.
You can then use that variable back in the other radio you took it from to try and make this radio work.
- (...If you decide that the 2000 is going to be up for sale - then perhaps a variable is needed so the next owner can play with that radio as needed for their own entertainment and enlightenment...)
You can then keep both radios working or serviceable to a degree.
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As far as i can tell, With a S9 signal into it, the squelch kill's the sound around 2 O'clock, But I've only got the Synthesizer alignment done so far,
Well this thread has helped me eliminate a lot of work but after following the thread and the info I am still stuck with no modulation in am ssb or pa, well it has ever so slight modulation on my scope and in pa if I crank the mic gain on the d104 up and the dynamic up on the radio. Same issue if I remove tr24 -tr25 we have modulation again . I’ve replaced and checked so many parts in circuit but am still at a loss ? Hope one of ya guys can clue me in to other areas to inspect . I’ve replaced a lot around tr24-25 but no luck .
I found this format in my old teaching aids from my golden days at community college, I went ahead and modified it some it may help and i may not but I offered it anyway. If you find mistakes feel free to correct them.
I will take a look at the schematic and see what could be the issue with TR24,25 circuitry it would be good to know what you have replaced in that area.Summary of Possible Issues & Circuitry for No AM/SSB Modulation
The most probable cause is a failure in the microphone amplifier stage, as this is the common point for the audio signal before it splits to the AM and SSB modulation sections. Always start by testing with a known good microphone to rule out a bad mic element or cord.
Primary Suspects (Highest Probability):
- Microphone Amplifier Circuit: This is the most likely area of failure inside the radio. The audio from the microphone is first amplified here before being routed anywhere else.
TR31 (2SC945AQ) and TR32 (2SC1675L): These two transistors are the core of the microphone pre-amplifier circuit. If either of these components fails, all audio from the microphone will be lost.
TR34 (2SA733P): This transistor acts as a transmit/receive switch. It provides voltage to the microphone amplifier circuit only when you are in transmit. If TR34 is faulty, the mic amp will not receive power and will not function.- Capacitors and Resistors: Check for failed coupling capacitors around the mic amp, such as C171, C172, or C175. A bad capacitor can block the audio signal from passing through the circuit. Also, check the surrounding resistors for correct values.
Secondary Suspects (Lower Probability):
- Audio Power Amplifier IC (For AM):
IC6 (TA7222AP): This is the main audio power amplifier. While a failure of this IC would definitively kill your AM modulation, the audio for the SSB section typically taps off before this stage. Since both modes are out, this is less likely to be the root cause, but it is part of the overall audio path for AM.
Balanced Modulator (For SSB):
IC2 (AN612): This is the balanced modulator that creates the Double Sideband Suppressed Carrier signal for SSB operation. A failure here would only affect SSB, not AM.
Power Supply Issues:
- Verify the presence of the 8-volt supply (often marked on schematics as an 8V or R8V rail) at the microphone amplifier components (e.g., the collector of TR31) during transmit. A lack of proper voltage points back to a switching issue, likely involving TR34.
- Physical Issues:
- Bad Solder Joints: Carefully inspect the solder connections on the microphone socket, the pins for TR31, TR32, TR34, and the surrounding components. Over time, vibrations can cause these connections to crack and fail. Microphone Connector: A dirty or damaged microphone jack on the front of the radio can also cause an intermittent or complete loss of audio input.
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To troubleshoot the modulation issue, let's try removing only transistor TR24. Based on the schematic, TR24 is the only component directly linked to the microphone, so removing TR25 should not be necessary at this stage to reinstate modulation. I still could be true that TR25 circuit is at issue here but lets take a step at a time.
Please let us know if removing TR24 reinstates proper modulation, and we can determine the next steps from there.
Before you remove it, if possible, check the voltages on TR24. You should measure approximately:
Keep us updated on your findings.
Please let us know if removing TR24 reinstates proper modulation, and we can determine the next steps from there.
Before you remove it, if possible, check the voltages on TR24. You should measure approximately:
- Base: 0.05 V
- Collector: 0 V
- And Emitter 0v
Keep us updated on your findings.
Really sounds like someone clipped the wrong resistor, thinking it would increase the audio level.
Sure makes it sound as if something it turning on TR24 when it should not. This requires base current into TR24. There are two sources of base current for TR24, D59 and TR25. In 47 years since this model went on the market I don't remember either of those going bad for no reason. Unsoldering one end of D59 and pulling the lead out would tell is if that side of the circuit is the culprit. And if the mike audio is still missing with D59 unhooked, this leaves TR25.
There are two sources of base current for TR25. The AM AMC transistor TR26 and the SSB ALC transistor TR34. Unsoldering the collector (center) lead of each TR25 and TR26 will settle the question of which one has gone rogue, tripping the limiter transistor.
Maybe.
73
Sure makes it sound as if something it turning on TR24 when it should not. This requires base current into TR24. There are two sources of base current for TR24, D59 and TR25. In 47 years since this model went on the market I don't remember either of those going bad for no reason. Unsoldering one end of D59 and pulling the lead out would tell is if that side of the circuit is the culprit. And if the mike audio is still missing with D59 unhooked, this leaves TR25.
There are two sources of base current for TR25. The AM AMC transistor TR26 and the SSB ALC transistor TR34. Unsoldering the collector (center) lead of each TR25 and TR26 will settle the question of which one has gone rogue, tripping the limiter transistor.
Maybe.
73
