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realistic TRC 427/shottky/rx meter issue
- Thread starter Uncle Ronnie 336
- Start date
My thoughts are - if everything else went well, the Recap didn't.
The dead RX meter - it needs AGC so if the radio seems to receive - it may get pinched up in really strong signals from a cap put in backwards.
As far as a D7 - 8 Schottky - this is a grey area. For the receive level it had - the thing uses a 25-based design - so the IF as well as TX side uses the same coil - so this means the thing may be "trigger happy" and although you may hear RX - does not always mean the RX lines up like it should - this would affect S-meter too.
So I'd look at reversing out the D 7 - 8 see if it recovers, more than likely it might not - it 'means something done earlier in the surgery didn't go as planned.
The real Schottky mod would be for D5 (RX Meter) and D4 (D10 and 11 on a 427) - not D7 or D8 - even D6 - these are DIRECTLY in line with audio and do affect the "color" or make it sound more tinny than the customer you may have to return it to - would appreciate. C30? That is the ANL Filter cap - installed the wrong way would kill receive and even blow from strong signals.
C39? Is it C30?
IT's the dangers of mixing one schematic with another - if you work by layout - only - then the rest seems to fall into place pretty easily.
D4 and D6 act as a rectifier to "boost" and augment the RX receive (The Detector) for pushing thru any signal into the ANL filter and smoothing cap to get to the RX Volume pot.
D7 is the ANL Limiter Diode
D8 is the TX Meter power sample Rectifier for TX
D5 is your RX meter sample Rectifier
Or is it?
Try
D4 / D5 part of the 1st IF receive - part of a bridge network
D7 - what D4 is in a Cobra 25 - D8 is what D6 is in same.
Thanks for @1iwilly for remembering the SAMS
A lot of tight spaces in there so look for soldering issues, not just shorts but lack of solder - simply due to work being done may have changed some connections (desoldering to remove but resolder only pads of the cap not everything else that could have been affected).
"Ok so I did that" - but it may mean the caps used to refresh the unit - one may not have wetted well enough to take solder and make the type of connection the original one had - so reflow fresh solder with a Rosin core flux around your work. Why do I say that - the AGC cap C34 or C37 - both 4.7uF caps, if not soldered or poorly soldered - will kill the S/RF meter for RX but not TX - because they affect the RF Gain control and the PIN section along with C2 - they act as a "lag" or they help to slow down the Speedier RF Gain from being a regulator control if they were not in place.
As you'll see in the differences - they have even the same parts in places - but the numbering is different - like C2 - is it C1? Go take a look...
The dead RX meter - it needs AGC so if the radio seems to receive - it may get pinched up in really strong signals from a cap put in backwards.
As far as a D7 - 8 Schottky - this is a grey area. For the receive level it had - the thing uses a 25-based design - so the IF as well as TX side uses the same coil - so this means the thing may be "trigger happy" and although you may hear RX - does not always mean the RX lines up like it should - this would affect S-meter too.
So I'd look at reversing out the D 7 - 8 see if it recovers, more than likely it might not - it 'means something done earlier in the surgery didn't go as planned.
The real Schottky mod would be for D5 (RX Meter) and D4 (D10 and 11 on a 427) - not D7 or D8 - even D6 - these are DIRECTLY in line with audio and do affect the "color" or make it sound more tinny than the customer you may have to return it to - would appreciate. C30? That is the ANL Filter cap - installed the wrong way would kill receive and even blow from strong signals.
C39? Is it C30?
IT's the dangers of mixing one schematic with another - if you work by layout - only - then the rest seems to fall into place pretty easily.
D4 and D6 act as a rectifier to "boost" and augment the RX receive (The Detector) for pushing thru any signal into the ANL filter and smoothing cap to get to the RX Volume pot.
D7 is the ANL Limiter Diode
D8 is the TX Meter power sample Rectifier for TX
D5 is your RX meter sample Rectifier
Or is it?
Try
D4 / D5 part of the 1st IF receive - part of a bridge network
Rectifies the 27MHz signal then remixes it to form a 1st IF signal for 455kHz downmix - strange but effective - for you capture the audio in the channel - the whole mess - then decode that.
Why do it this way?
The problem with IF downmixing is the bandwidth product - not all the channel traffic even those on Xtal sets with FIXED TX frequency can even be found let alone heard - the "Bridge" concept simply decodes the audio - then remixes it to form a secondary image for IF - then you don't have to concern yourself with band pass - the 455kHz can easily work this itself - it's when the IF image doesn't align with the 455kHz peaking - does the hiss and other unwanted products rear its head.
D6 is in the 455kHz IF Filter - keeps a DC presence from getting out of hand (a biasing/alasing problem)D7 - what D4 is in a Cobra 25 - D8 is what D6 is in same.
Thanks for @1iwilly for remembering the SAMS
A lot of tight spaces in there so look for soldering issues, not just shorts but lack of solder - simply due to work being done may have changed some connections (desoldering to remove but resolder only pads of the cap not everything else that could have been affected).
"Ok so I did that" - but it may mean the caps used to refresh the unit - one may not have wetted well enough to take solder and make the type of connection the original one had - so reflow fresh solder with a Rosin core flux around your work. Why do I say that - the AGC cap C34 or C37 - both 4.7uF caps, if not soldered or poorly soldered - will kill the S/RF meter for RX but not TX - because they affect the RF Gain control and the PIN section along with C2 - they act as a "lag" or they help to slow down the Speedier RF Gain from being a regulator control if they were not in place.
As you'll see in the differences - they have even the same parts in places - but the numbering is different - like C2 - is it C1? Go take a look...
Last edited:
It's the point I was trying to make...
Thanks for the "equalizer" - now all questions get answered...
Uncle Ronnie,
thanks for the reply.
the reason we are interested is that the term "NPC" gets used as a synonym for "variable deadkey" a lot of times.
LC
thanks for the reply.
the reason we are interested is that the term "NPC" gets used as a synonym for "variable deadkey" a lot of times.
LC
What gets called "NPC" these days was originally the brainchild of Billy D. Ward, now long passed away. His version of it was "NPC/RC" meaning Negative Peak Compression Reduced Carrier".
No need to speak ill of the dead, but his idea only applied to radios that use a big PNP transistor to modulate the radio's final/driver stages for AM. His trick was to horribly distort the circuit's negative feedback and make it "swing" in a more-pleasing way. Some folks swear by it, others swear at it. Mostly I'm content to let people decide what they like when they hear it. This has led to having the "NPC" diode removed from most radios that show up with it installed. Turns out that there is a "sweet spot" for the carrier wattage. Lower than that and audio gets choppy. Too much carrier and the audio just gets weak.
Too much like playing a violin for my tastes. The only reason to have a carrier control on AM radios is to match the needs of a linear amplifier. No other valid reason on God's green earth to have it. And if the 'sweet spot' carrier level for your radio is too high or too low for your amplifier, it's not so useful. The radio should sound good no matter how high or low the carrier level is set. NPC doesn't quite make it over that bar.
Naturally the term "NPC" has become generic. Folks who couldn't guess what the three letters mean apply it to any AM radio that can still deliver good audio with the carrier power turned down. Billy's ghost just never goes away.
I vaguely remember posting a "how to" carrier control for that kind of radio using the aformentioned "TIP120" NPN Darlington power transistor. Gotta track that down. Don't think it was more than ten years ago.
73
No need to speak ill of the dead, but his idea only applied to radios that use a big PNP transistor to modulate the radio's final/driver stages for AM. His trick was to horribly distort the circuit's negative feedback and make it "swing" in a more-pleasing way. Some folks swear by it, others swear at it. Mostly I'm content to let people decide what they like when they hear it. This has led to having the "NPC" diode removed from most radios that show up with it installed. Turns out that there is a "sweet spot" for the carrier wattage. Lower than that and audio gets choppy. Too much carrier and the audio just gets weak.
Too much like playing a violin for my tastes. The only reason to have a carrier control on AM radios is to match the needs of a linear amplifier. No other valid reason on God's green earth to have it. And if the 'sweet spot' carrier level for your radio is too high or too low for your amplifier, it's not so useful. The radio should sound good no matter how high or low the carrier level is set. NPC doesn't quite make it over that bar.
Naturally the term "NPC" has become generic. Folks who couldn't guess what the three letters mean apply it to any AM radio that can still deliver good audio with the carrier power turned down. Billy's ghost just never goes away.
I vaguely remember posting a "how to" carrier control for that kind of radio using the aformentioned "TIP120" NPN Darlington power transistor. Gotta track that down. Don't think it was more than ten years ago.
73
yea, i prefer it adjustable, and every time ive seen the simple parallelled r/c the resistor heats up and the audio is just.....sloppy compared to the darlington based adjustable dk with the 100uf. i make the unit on perfboard and sometimes will pop a hole to mount the TIP120 right to the case all slick
Yeah Nomad your diagram is on the forum in a few places at least.
Im sure ive seen it recently somewhere.
I remember Billy quite well.
he and i were corresponding pretty regularly towards the end.
It was a bummer when he died as it seemed he was just getting back on his feet.
I agree about the NPC mod in the ranger chassis having limited use.
I think he knew that and that's why he always told people to set their carriers at 2 watts.
seemed to work best right around there.
I feel like the mod got a bad rap because too many shops were doing it without checking the result on a scope. I can say that i never ended up using a 100 ohm resistor because i always used a variable trimmer first to get the right value and then replaced it with whatever value i measured.
pretty sure its never been a 100 ohm resistor that i ended up using.
LC
Im sure ive seen it recently somewhere.
I remember Billy quite well.
he and i were corresponding pretty regularly towards the end.
It was a bummer when he died as it seemed he was just getting back on his feet.
I agree about the NPC mod in the ranger chassis having limited use.
I think he knew that and that's why he always told people to set their carriers at 2 watts.
seemed to work best right around there.
I feel like the mod got a bad rap because too many shops were doing it without checking the result on a scope. I can say that i never ended up using a 100 ohm resistor because i always used a variable trimmer first to get the right value and then replaced it with whatever value i measured.
pretty sure its never been a 100 ohm resistor that i ended up using.
LC
yeah, if something comes across my bench with the r/c version i'll automatically change it for a darlington or undo it completely
ok revisiting this one. changed back the diodes to stock, still have no RX meter movement, TX meter is working, and I reflowed the caps that were swapped in that area. hmm, what else did I mess up? i don't want to return this with no rx meter