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TRC-490 MB8734 Channel mod question

Discussion in 'CB Radio Modifications' started by slidetbone, Jul 18, 2018.

  1. slidetbone

    slidetbone Member

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    I got a really decent TRC-490 that had it's SSB switch smashed. I was able to replace the switch and repair the knob and the radio works great. I just finished a cap replacement and it works really well. Since this radio has the MB8734 chip, I do not want to replace it and instead will go with a loop crystal mod. It uses the 11.3258 xtal. I read the radio is pretty broadbanded, so I got the xtals to go HH, H, L and LL. I got a rotary switch to fit in the phone jack, so there is no cutting or drilling and the knob blends well, looks factory.

    Here is the issue: I cannot get lock on some of LL and none on HH.

    I get good lock on 26.515 thru 26.955 and CB, but for H, I have to turn the radio off and on to get it to lock for 27.415 thru 27.855 although it will transmit with full 4 watts. No lock for HH crystal. For LL I get lock from 26.505 down to 26.305 and then no VCO lock. If I adjust the VCO to lock on 26.065, I lose the Highs and same issue backwards to lock the High High freqs. I can compromise and leave it like it is since I am not interested in going into 10 meter freqs.

    I read here the thread on the 142GTL mod:
    https://www.worldwidedx.com/threads/poormans-192-channel-mod-cobra-142-gtl.79065/

    Will snipping R71 apply to this radio as well or it is limited to the MB8719 chip? Anything I should look for to get most of the freqs to lock on this rig?

    Thanks in advance!


     

  2. loosecannon

    loosecannon Sr. Member

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    this chassis can be made to cover about 192 channels, and sometimes the VCO needs to be broadbanded in order to do this.

    Cutting R71 will broadband the VCO, but it will have to be re-set once R71 is cut.

    most likely you will only be able to use the low-CB-high crystals and maybe a few channels on the other ones.

    even if you do get the VCO to lock on the extreme edges of your coverage, your RX and TX will be very low.

    LC
     
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  3. Handy Andy

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

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    I'm all for this - set up some data too...

    I don't have any info on the TRC-490 - so I'll need some help from you about that "R71" - it's a 1.5K resistor and serves as a pull up from the PLL's own power feed off of Pin 9 going to Pin 1. Same thing for 8719 stuff only it's referenced as "R71" and 8734, it's called "R95" - both 1.5K - but here's where it gets interesting...

    MB8719R71.jpg

    When you "cut" power - you're affecting the inverter "latching" so when you try to go way out on the limb, its' designed to "stop" and "set" the inverters input high - no further - killing the loop.

    MB8734R95.jpg
    What the "resistor" does is not what many think. Sure it's part of the loop and serves to offer a method to make UHIC007 functional - but it's part of the TP9 "scope/Meter" (Varactor) and also you should keep something on Pin 1 of the PLL IC for the sake of it's otherwise vulnerable input - and since the PLL's uses these things - in the same package they are not exactly replaceable - it's best to use a HIGHER value for R71/R95 like start at 2.7K or even 22K - this would provide enough "Pullup" to keep the Pin 1 power rail forward biased (Ideal) versus running it open in a possible non-polarized and possible reverse polarity - ground fault issues with noise creeping into it. This also keeps Pin 5 UHIC007 pin from going bonkers when Out Of Lock (OOL) is detected. - back to the Pin 1/2 and Pin 3/4 of the loop(s)

    MB8734SAMs.png

    Ok, the above is a precautionary statement - because you are using two inverters running on their own, and if one of those inverters runs away with itself - you can have some damaged parts and a blown IC - so the reason for the R71/R95 issue is more for stability of the Pin 1-2 Inverter running HIGHER than Pin 3-4 Inverter that is slower - between these two is also the Pin 5 "Charge Pump" - being the way the PLL "sees" phase efforts between the two "loops" of Pin 1-2 and Pins 3-4 - (The Whole Kit and Kaboodle) and IC1 UHIC007 - (Pin 5) being the Varactor - key components in all the Oscillation going on - and it needs to do all of that to compare results on PLL's Pin 17 - the 1-2 and 3-4 combo - and the PIn 5 CP from the PLL - is what feeds Pin 5 UHIC007 and it's Varactor..

    MB8734R95L21.jpg
    So when you "disconnect" the R71/R95 - one of those loops simply goes to Pin 5 but it's at the levels of the loops want to run at. So if you get the right balance - you're ok right? Well, that's up to you.

    This a Galaxy rendition of the infamous UHIC007 chip - note two varactors - one for PLL side and the other for the L21/L13 side of things (it's Pin 9).
    GalaxyVCO98VHP.gif

    I hope you're paying attention to C72(8719) / C90(8734) and the values used. Remember this needs a "delay" and smoothing filter too - select your components wisely. Because what comes out of the UHIC007 on Pin 10 in mixed in with the entire mess from L18(8719) / L21(8734) which goes to PLL Pin 17 - so if you know what works - in oddballs for one - use caution but try it with the other for some success.

    Can't do anything about the L13 and L19 - same part but unless you use some thing else say a LA165 - even then - no guarantees...

    Have fun.
    Remember - You are on your own - Good Luck...
    :+> Andy <+:
     
  4. Dmans

    Dmans Well-Known Member

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    Another awesome in depth triage!
    Thanks Andy!!!!!

    73's
    David
     
  5. loosecannon

    loosecannon Sr. Member

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    Thank you Andy!!!

    I knew there was some more complex issue with the resistor removal (Sue had taught me about it), but i couldn't remember what the issue was.

    now that i remember, what do you think of putting say, a 10K resistor there?

    man am i glad you decided to come over to this forum!
    LC
     
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  6. Handy Andy

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

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    Well, you could, just something to help the thing "start up" - there's like the "high speed side (1-2)" and the Low-Speed side (3-4) and then there's the "skip/add - a - beat" pin 5.

    In you look into "op amps" the Varactor needs something to "start" so that R71 / R95 helps the Pin 5 keep the Varactor biased with some capacitance - so just removing power may make the thing refuse to fire - just plain quit.

    The other aspect - and I hope Robalo can forgive me - is that R104 / C90 combo - 1K and a 0.47uF on the 8734 while the 8719 - uses 1K - calls it R80 and the cap is now a 10V TANTALUM rated at 4.7uF and it's called C72.. These are the SMOOTHING filters - so remember; the smaller the capacitance, the higher the Xc resistance is across it at a given frequency, meaning it moves the knee of the hi-cut filter this (circuit snippet) is, to a HIGHER roll-off (cut) frequency.

    For those that are playing at home ,we are discussing the high-end cut-off of where the VCO hit's when L13/L19 meets their full-stop for tuning range...

    I'll Take Emphasis For 500 - Jack

    With the above being said, look at R207, (or R72 for the 8719) that's 2.7K - that resistor is the current limiter for whatever is inside the UHIC007 pack - and look back at R71 / R95 -being 1.5K - I'd use NO LESS than 2.7K and 10K is well within the limits even with using 27K for R95 or R71 - you just need some capacitance - just not a lot of current - which is why they chose the 1.5K as the "lower limit" (Read Max Voltage) when it came to power and the cutoff "skirt" - so it is getting some voltage already but what resistance is provided offset it internally? Only taking it apart can we figure that out. We really don't need to - just keep something there R95 / R71 - it doesn't need a ton of it.

    Just remember - there is a varactor in there and their maximum effective capacitance is within a very small range of voltage presence so be delicate...


    :+> Andy <+:
     
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  7. 2315 Robert

    2315 Robert Grateful

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    Handy Andy, are you available for other questions? Normally you post something here and get a brand new discussion that has nothing to do what someone asks. Example I have high reflect. Gets "wow that is way to high". The recommendations are sound but I loose alot in the discussion. I have got great advice and parts from this site.
     
  8. Handy Andy

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

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    What do you need to know?
    (but before you post it in this thread - start up a new one - just post a title here - then post the question in your thread over there - so I can find it - and hope it's something I can help you with...)
     
  9. slidetbone

    slidetbone Member

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    Incredible insights here! Thanks to everyone here!

    Here is where I start out: I managed to get 148 channels out of the stock set up using 4 xtals.

    27.885Mhz is the high limit, is 26.265Mhz is the lower useable limit. Below 26.265Mhz the VCO voltage drops very fast and loses lock.

    I will try the R71 change and readjust. As soon as I do I will post results.
     
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  10. Martian

    Martian Active Member

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    This PLL design has an easy to trip on dead zone.
    It works fine for what it was intended but its really easy to trip with those crystal kits.
    But those crystal kits cause other instabilities that contribute.

    Cobra altered the design of the PLL in the front mic 148s and they had to incorporate a small circuit to deal with the PLL going into a weird state.
    Probably because they could not work out why it was happening so they would give it a jolt (literally) when it went out of lock.

    It's on my list of things to look at one of these days and fix that design flaw just for grins.
     
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