Commtron Cxx / Nat0 F.M capacitance issue

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dodgeyvanner
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Commtron Cxx / Nat0 F.M capacitance issue

Post by dodgeyvanner »

hallo

I have one of these but it looks like i need to add a litle more capacitance to the VCO to get it bang on frequency across the 3 bands
(US1-40, US+1-40, UK1-40) It reads as follows for example:-

26.965 is actually reading 26.966?
27.60125 is actually reading 27.60180?

My frequency counter is calibrated but to be on the safe size i checked it on my other rig and it read 27.60125 exact.

No adjustment of the relevant CT trimmer will get the values any lower, it will incresase in frequency ok but won't tune down any lower than this.

Could anyone tell me the ID for the capacitor I need to experment value wise with please?

Thanks!
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Otter
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by Otter »

C212 and C245 are the capacitors that set the frequency of the 10.240 reference. Only look at the 26.965-27.405 frequencies when adjusting the reference. After that there is a separate trimmer for hi band, and seven trimmers to cover the UK frequencies.
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dodgeyvanner
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by dodgeyvanner »

I will try some different capacitors and see what it does, may try a new ct pot if the capacitors dont work. Will let you know how it turns out, thanks for the help!
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by bigpimp347 »

have you checked the 10.240 is right ??

i sold a couple of these radios due to being a pain to set up and no information out there..
i have a XII on 10mtrs i want to put back on 11 but nothing out there to help me figure it out so it's sat in the junk box.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by Otter »

bigpimp347 wrote: 01 Apr 2021, 15:03 i have a XII on 10mtrs i want to put back on 11 but nothing out there to help me figure it out so it's sat in the junk box.
There's a schematic. It's just another LC7120 Maxon chassis. There's an extra wafer on the channel switch which selects one of 7 crystals in the right place when it's in on the UK band. There's also a standard 15.810 for high band. If you need / want the schematic pm me your email address. If someone has removed the UK crystals you will be looking for another scrapper to get them from.

X401 is 15.810MHz and adjusts the Hi band.

These are the Xtals and the UK channels that will change when you adjust them

Xtal UK Channels Xtal Freq

X402 1-3 15.99625 MHz
X403 4-7 15.98625 MHz
X404 8-11 15.97625 MHz
X405 12-15 15.96625 MHz
X406 16-19,24,25 15.95625 MHz
X407 20-22, 26-40 15.94625 MHz
X408 23 15.92625 MHz
Last edited by Otter on 01 Apr 2021, 21:30, edited 2 times in total.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by Otter »

Wait - you said XII not CXX. That should just have a couple of extra crystals for low and high - 14.910 and 15.810. Nothing special.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by bigpimp347 »

Otter wrote: 01 Apr 2021, 20:31 Wait - you said XII not CXX. That should just have a couple of extra crystals for low and high - 14.910 and 15.810. Nothing special.

that's the thing it's a normal maxon board but the crystal board isn't like a CXX/Nato board,
also it's half it's size, only has one crystal currently on it and some pleb has disabled low and hi band off the band switch.

so i need to repopulate the crystal board, which i should be able to do but also reinstate the band switch,
but i'll look at it again one day..


if you need any mic inserts let me know i'l PM you a link to a really good 600ohm insert that fits tight.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by Otter »

600 Ohm inserts link would be great. Thanks for that. Isn't the Commtron XII the same as the Colt 510? There's a schematic on cbtricks for that.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by dodgeyvanner »

Its sorted!!!
changed the 56pf (C212) to 100pf and its bang on now across everything, would have been back sooner but had to order the capacitor needed. Thanks for the help its been great!
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by ChubbyChops »

Hi All
I need a mic insert for one of these (Commtron CXX).
Would appreciate the link too.
Also need to start with getting it working again.
It's totally dead except for the PA circuit.
Any ideas?
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by Otter »

The first thing to do is check for the correct voltage (roughly 7.8V) on the PLL chip pin 16. Then check for 10.240 oscillation.
"To this day the words Stirling bridge conjour pride in every Scotsman's heart, while to an Englishman those same words conjour literally no feelings at all." - Cunk on Britain S1 E1
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by ChubbyChops »

Pin 16 is showing 0.2 volts.
All other pins on PLL are 0 volts.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by ChubbyChops »

Just a quick thank you for the info on this thread.
I've got mine working and on frequency using the info posted here.
I just need to peak the TX circuit but still need a mic insert @bigpimp347 if you wouldn't mind letting me know where to get them from please.
Thanks again chaps.
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by ChubbyChops »

Does anyone have the receiver alignment procedure for this radio please?
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Re: Commtron Cxx / Nat0 F.M capacitance issue

Post by ChubbyChops »

Figured it out, I think.
RV101- Squelch
RV201-AM Modulation
RV501-FM Deviation (sub board)
RV103- Rx "S" Meter
RV202- Transmit Rf Meter
TX - L304, L305, L306, L303, L302, L301. L306 peak then anticlockwise to 4.0w
RV201-AMC, D203--AMC Defeat
RX - L204, L101, L102, L103, L104, L105, L106, L107
FM detector on sub board next to case.
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