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Cobra 29 LTD Classic Schottky Diode Mod

Discussion in 'CB Radio Modifications' started by Shadetree Mechanic, Apr 19, 2018.

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Is Schottky Receive Mod Worth Doing?

  1. Yes it Makes a Noticable Difference.

    2 vote(s)
    28.6%
  2. Maybe, it Depends.

    2 vote(s)
    28.6%
  3. No Not Worth Doing

    3 vote(s)
    42.9%
  1. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Hey guys,
    I have a Cobra 29 that I am going to use in my garage station and will be using it with an amp so I plan on lowering the dead key. I figured while I am in there, I could do the Schottky diode receive mod.
    So my question is: Is this mod worth doing on this radio and would I notice a difference?
    Any thoughts and replies welcome.
    Thanks, 73s


     
    #1 Shadetree Mechanic, Apr 19, 2018
    Last edited: Apr 19, 2018

  2. sp5it

    sp5it Master of puppets

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    What is shot key?
     
  3. Robb

    Robb Yup

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    Yeah, sure - why not?
    Schottky diodes are diodes that are really fast at switching on/off with a low threshold turn on voltage. IOW, when they do switch on/off, they do it with accuracy and are sensitive. Not as sensitive as the germanium diodes turn on voltage they replace, but pretty close. This keeps extra low voltage noise out of the receive, and allow the noise blanker circuit becomes more stable/accurate. They are usually put in both the receive and noise blanker circuits.

    I've seen a few radios where somebody had put a bunch of Schottky in other circuits. Didn't see any harm done, AFAIK. Dunno if it really helped - either
     
    #3 Robb, Apr 19, 2018
    Last edited: Apr 20, 2018
    Shadetree Mechanic likes this.
  4. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Sorry I don't know how to spell. I fixed it. Now I will never forget.
     
  5. sp5it

    sp5it Master of puppets

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    I would put 1n60 into AM detector. Weaker signals will be detected.
    Mike
    Edit: According to schematic http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/index.htm , there is already germanium diode 1k34a from factory installed. No need to replace.
    Also NB detector have it, do not touch.
     
    #5 sp5it, Apr 19, 2018
    Last edited: Apr 19, 2018
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  6. 543_Dallas

    543_Dallas Sr. Member

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    The most important part is that you align the receiver. That alone make a big difference for me. These radios sometimes come out of the box with horrible receiver. My signal generator is a piece of crap I built. I can't imagine how much better of an alignment I could do with real equipment.
     
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  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    (prolly gonna catch h*ll here too for this but here goes...)

    Greetings!

    Well, if the radio is quite old, more than 10 years, then those diodes may to be swapped out. Age, surges, vibration and static can affect the diodes crystalline structure and create punctures in the barrier or can induce a reverse-bias performance hit in the PN junction of the older diode that can add noise and distortion effect in both the DC and AC realm that many young people can hear as "shot noise" or a rattle in the speaker - so if you have that issue with the 29 now, recapping and diode swaps are prolly needed...

    To use Schottky? I always get a kick out of the Wiki definition calls the "Cats whisker" a primitive Schottky Diode ... considering the era - there was a lot of hard work done during those earlier times...

    If your 29' uses the TA7222P audio amp versus the 5-pin 1022 - then Schottky is a personal choice, yes they are silicon, and they removed 1/2 the junction issues so they have a lower forward voltage drop - but the later-year (2007/2008 and later) models already use derivatives of the Schottky in the 1N4148.

    The earlier year models used the Germanium type but just some specific areas. Othrwise they used 1N4148 / 1N914 types which were high-speed switch types for ANL and NB - but they too were less "purified" than the Schottky has in it's pellet today...again as technology went ahead, the older analog designs get left behind.

    If you decide to use Schottky - you miss out on the performance curve the Germanium types provided the receiver in both detection and in AGC response.

    The earlier 29's were designed with older, slower - less sharp diodes and had a receiver quality unlike that which is hard to duplicate in todays slilica-based radios today. The best way to put it would be the RIAA - warmth in bass and low-tone clarity that many older enthusiasts enjoy versus the crispy, nearly snappy sound of silicon based. SMD types you are seeing more and more of. - the not-so-sharp rounding of the audio detected out of a Gemanium Detector can still be discerned by many radio listeners.

    The older 29's compensated for that. The newer ones just added tonal caps to offset the crispy nearly krinkly sound of potato and Dorito chips versus the more milder deep fryer Mickey-D's fries. The capacitance they use to get the spectral roll-off generates a hit, or loss of signal, that many want back in their radios - so they try schottky and it does bring back the predictable performance the older 29's have - you still have the losses incurred by the engineering of the tonal filters used in the audio chain - you don't get the audible warmth the older 29's responded to when it came to audio punch...

    So look over the radio and decide, Schottky can help with noise floor and an overall rebuild of the Diodes used in the RX strip in the older chassis can always be a benefit - but it's splitting hairs. Older radio Germanium and Silicon is different than todays replacements and you'll notice the difference.

    I am seeing a resurgence (SIC) in people wanting more of an Analog-era feel and Germanium is leading the way in many of the audio tracks musicians are doing these days. The amplifier Fuzz effect in original germanium based "pedals" is noticeably different than their silicon counterparts - even when they use the schottky low-voltage junction drop diodes to replace the originals.

    In a way, you are doing that same thing...requesting advice on how to get a 29's receiver back to a time - when they were known more for their reproduction of audio from the carrier that used Germanium than in the skew and slew improvements in distortion, noise floor and recovery rates the silicon types present versus using the older signal diodes.

    Just wanted to help remind us all of the time radios were the #1 means of communication.
    Regards!
    :+> Andy <+:
     
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  8. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Greetings!

    Ok Part II

    Here are some Travel Lodge Postcards to help you find those special spots along the way...

    These Photos from the Brochure, CHAPTER XXIII - The Diode's Way...

    This one is for the Receivers L9 and Noise Blanker - out of a Cobra 29 KEPC - 225C about 2007' Vintage (ST)
    D4 5 6 NB 24.jpg

    And here is the next one on page 3 - SWR and Means To You...

    D1 2 3 SWR Cobra29.jpg

    Hope the above help you in making a decision on choosing the right part to replace those older weakened parts...

    Regards!
    :+> Andy <+:
     

    Attached Files:

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  9. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    The radio is only a couple of years old at the most. So the Schottky diodes won't help because it already has Germanium diodes? Just making sure I got this right.
     
  10. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Greetings!

    No , er well - if you want to replace their diodes with Di-Lithium Crystals or even the more refined Folgers' Models ..... We'll see if they can wake up that radio...
    I'd prefer Dark Roast myself...

    Seriously_Mode=On...

    Sorry had to re-set a flag - Saturdays around here get a little crazy...

    They, more than likely, use Schottky based 1N4148's - which although may sound like were violating a law in IEEE or ISO - they're really not ,due to their proposed purpose.

    So (scenario) say the "factory" that makes most of the Semiconductors used in the Cobra 29 are from Factory "X" - and they produce Schottky types for general commercial use and Cobras' get these 1N4148 made to a Schottky standard - doesn't mean they are Schottkys' just produced used materials dies and process that are also the same for Schottky. (Like Ford Sources to Mexico, but that doesn't mean it goes great with Lime Juice or a free 6-pack of Corona is included - it's just that you get an instruction manual that can teach you how to identify the vehicles parts in Spanish - translated to English - and the real meaning behind why "Open Other End" is printed/embossed on the BOTTOM glass on the bottle of Cronoa...)

    Just so you know, Schottkys are a major standard manufacturing process now so you'd be hard pressed to find mainstream parts with Germanium as a main ingredient. OR to locate a factory that still makes PN junctions to older standards...(hint: Pb-free)

    Which then would indicate that Cobra 29s' are SOURCED from Factories that may not make, use or develop Germanium type of Semiconductors - which than means that Cobra / Dynascan has to re-outsource to locate the detector diodes to meet a specific standard.

    But do they?

    Cobra29KEPC1141.jpg

    I've got LX and LEs laying over that are all 1N4148 derivatives - meaning not one of the diodes' have a voltage drop below 0.4V - which is typical of Schottky being silicon - Germanium ones from the 2007ST (Shown earlier) are 0.3V or so voltage drop.
    You are not guaranteed the same level of quality or performance EVEN THOUGH THEY ARE ENCAPSULTED TO APPEAR AS SUCH - does not mean that the IEEE standards are violated, just they are low-power capacity capable and the Glass enclosure is merely for outline and packaging purposes as Small Signal Diodes. IT does not mean they are REQUIRED to be germanium in there - just they meet a spec that makes them suitable for it and Cobras' Management says - go ahead...

    I've got 687 and 787 from Uniden that are all SMD and all are Schottky derived.

    DiodesUniden687.jpg

    As technology goes ahead - we may find ourselves more and more disadvantaged when it comes to enjoying the sounds the older chassis provided.

    So enjoy and keep, respect and cherish, revere and remember - these old radios are classics and may never come back.

    Regards!
    :+> Andy <+:
     
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