I have a 142gtl strange issue has occurred i get s9 on alignment using signal gen sounds great but when i plug in antenna its very quiet any help would be appreciated.
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142 gtl strange issue
- Thread starter kaos513
- Start date
Alignment procedure is almost spot on I can hear my self with test radio but the recieve is far far far away
Since you're using a different coax and PL-259 with the signal generator than with the antenna, I would look there first. Do other radios receive properly when connected to this antenna? If they do, I would check the SO-239 on the 142. Sometimes they open up a little or get corroded and don't make perfect contact. Especially if the antenna PL-259 is worn a little from being plugged/unplugged a lot.
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The S9 reading on the meter can be totally misleading. Remember that the meter reading adjustment can be boosted far too high. You should never adjust the S-meter trimpot until you are sure it hears a weak signal like it should.
The receiver fails to hear a weak signal.
How weak?
Does it hear a 1uV signal? How about 10 uV?
Might help to zero in on just how bad the sensitivity problem really is.
Alignment is a diagnostic tool. If an tuning slug fails to exhibit a proper "peak" when it's turned, that could indicate a circuit failure near that part.
And if someone has turned the tuning slugs randomly while keyed up and watching the wattmeter, several receiver adjustments may be adjusted totally wrong.
73
The receiver fails to hear a weak signal.
How weak?
Does it hear a 1uV signal? How about 10 uV?
Might help to zero in on just how bad the sensitivity problem really is.
Alignment is a diagnostic tool. If an tuning slug fails to exhibit a proper "peak" when it's turned, that could indicate a circuit failure near that part.
And if someone has turned the tuning slugs randomly while keyed up and watching the wattmeter, several receiver adjustments may be adjusted totally wrong.
73
Then understand, you may have inadvertently tuned for the birdie IF image.
Might want to return to the last IF stages and retune them...
10uV MAX or carrier on a dummy loaded radio in another room to generate the 27MHz signal you want the chassis to hear, not the saturated levels and the subsequent IF images that mirror each other up the strip - the deafness could be from tuning the wrong image...
TR18 is a big culprit in this - it's between two IF filters both set up to deliver an image, but OPPOSITE of each other.
7.8 MHz is used, but look where the conversion occurs, AFTER TR16 - part of the issue here is the use of R190 - it's a 68K resistor which BROADBANDS the L3 output - but it gets the IF in which to DE-Convert from, back at where the 7.8MHz (2nd IF) TR22 is.
For LSB, TR21 turns on - allows it to use L17, AGAINST that 7.8025MHz and the AM Trimmer CT1.
For USB TR21 is off and it uses CT 2 at the 7.7975MHz.
For AM it goes back to the 7.8025MHz - then you have to hope for the best for in-between when in AM - but uses the same Xtal as L17 does - you tweak L17 for the LSB Offset but in AM L17 is OFF.
To tune AM - use CT1 on the same Xtal - 7.8025MHz Gives you the INVERTED image same as USB - but more centered on carrier - hence the offset.
So if it's "deaf" you may have tuned for the wrong image in AM - CT1 and L3 are for that.
I mention TR16 due to a Tantalum used in it's strip and feedback filter. If the capacitor is too leaky, the culprit may be C193 a 1uf Tantalum - acting more like a resistor than a audio filter cap to CAPTURE the audio in the signal. This cap can "fool" AGC by you thinking "S9" is at one level - when it should be another - forcing you to tune on an IF carrier image - but potentially the wrong mirror of the one you should use.
Just "piping in" here..... .
Could this be an AGC problem? Something in the AGC control voltage gone bad and causing low signal through the IF?
Could this be an AGC problem? Something in the AGC control voltage gone bad and causing low signal through the IF?
Good to ask!
This would be a "tough call" because to know this would require the radio on my bench to even listen to it for this performance hit to show up.
Cobra 142GTL AGC detector and Amp
Locate C22 - [by TP4] note it's aspect to D4
It's a "detector" that applies the output on FET1 from L3
D5/D6 D8/D9 are part of a "balanced bridge"
It starts out as "Non linear" amplification - takes a DVM to listen on the AGC to know this compared to the voltage chart - because you're looking at a rising voltage that REDUCES the voltage across the Collector - to Emitter of any point in the RF amp strip - not all places are tapped into that AGC line - only key sections.
So sorry, this one goes to the PIN section - but to know how that PIN section is working - you look at R17 - the 1.8K - What sends me back to the IF strip is from this - D4 can detect the IF as a Carrier and push down the signal - any signal - which would be the IF strip's own "carrier" - you have to tune the strip for the audio side - and if you'll notice One of the Balanced Bridge Diodes - D8 is acting like an RF Switch - but only in SSB modes C16 is a 4.7uF cap that controls the SSB delay of AGC.
When you put the radio in AM mode - R50 drops, D8 is cut off - stopping C16's influence - when it's off - C17 works the circuit exclusively. C17 is much smaller lower value - but since caps in parallel ACROSS a power line - ADD together - in SSB mode C16 is added in, making C16's influence as much much higher slowing down the AGC - and so you'd see S9 a lot quicker - thinking you're done...
But my head scratching came from the L3 - and the Tri-Tied 3-stage amp section - uses a Tantalum to "slow it down - so you can hear weaker signals - if you remove it - you hear very compressed audio - but if the Tantalum is too old, it's ESR then accelerates the amps response to a point that it's very pinched and words get cut off and really poor noise floor performance appears in the speaker - then that Tantalum is suspected - change it to see if the strip recovers. The ESR of the cap - increases the drive output level - so it will pinch - you don't hear anything because of that effect - overdrive the output - put the cap in or use a higher value one to replace it - CHANGES this IF sections response to incoming signals.
Found more than one tantalum in different radios in their IF strips going bad from being leaky.
Locate D14 and Pull one leg to disconnect the PIN Attenuators from the input so the "Signal" from the Signal Generator can get in there and get processed...
In this scenario, I treat this like I'd tune a PC-122 - REMOVE a PIN DIODE that feeds the AGC to the section - or at least find the resistor and disconnect the AGC power - helping you localize and properly center the tuning of the IF Strip so it's (Radios') AGC is not "defeating" your effort to capture the signal and THEN let AGC determine the Selectivity by affecting the sensitivity.
That would be D14 - takes out the influence so the Signal Generator doesn't get attenuated.
So NOT APPLY (See my post above ) More than 100uV to the input to reduce the potential for damages in over amplification.
So I've given you a lot of notes to you and to @kaos513 - let's see if any of this precipitates out...
This would be a "tough call" because to know this would require the radio on my bench to even listen to it for this performance hit to show up.
Cobra 142GTL AGC detector and Amp
Locate C22 - [by TP4] note it's aspect to D4
It's a "detector" that applies the output on FET1 from L3
D5/D6 D8/D9 are part of a "balanced bridge"
So sorry, this one goes to the PIN section - but to know how that PIN section is working - you look at R17 - the 1.8K - What sends me back to the IF strip is from this - D4 can detect the IF as a Carrier and push down the signal - any signal - which would be the IF strip's own "carrier" - you have to tune the strip for the audio side - and if you'll notice One of the Balanced Bridge Diodes - D8 is acting like an RF Switch - but only in SSB modes C16 is a 4.7uF cap that controls the SSB delay of AGC.
When you put the radio in AM mode - R50 drops, D8 is cut off - stopping C16's influence - when it's off - C17 works the circuit exclusively. C17 is much smaller lower value - but since caps in parallel ACROSS a power line - ADD together - in SSB mode C16 is added in, making C16's influence as much much higher slowing down the AGC - and so you'd see S9 a lot quicker - thinking you're done...
Found more than one tantalum in different radios in their IF strips going bad from being leaky.
Locate D14 and Pull one leg to disconnect the PIN Attenuators from the input so the "Signal" from the Signal Generator can get in there and get processed...
In this scenario, I treat this like I'd tune a PC-122 - REMOVE a PIN DIODE that feeds the AGC to the section - or at least find the resistor and disconnect the AGC power - helping you localize and properly center the tuning of the IF Strip so it's (Radios') AGC is not "defeating" your effort to capture the signal and THEN let AGC determine the Selectivity by affecting the sensitivity.
That would be D14 - takes out the influence so the Signal Generator doesn't get attenuated.
CAUTION:
- Do Not Transmit - remove TP 7 and TP 8 wires to disconnect the Driver And Final BEFORE you remove D14
- D14 is part of a Protection Circuit that prevents excessive RF power from damaging the Receiver side.
- Restore D14 BEFORE you reattach TP 7 and TP 8
So I've given you a lot of notes to you and to @kaos513 - let's see if any of this precipitates out...
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