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Old 05-18-2018, 04:13 AM   #1
llonen
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Default Belcom LS120l radio (crystal filter)

I am in need of a replacement crystal filter for this set, 7.8mhz not sure if these are repairable as it seems the set is passing both side bands and carrier through the filter. The problem is intermittent, I have addressed the bad soldering around the input and output pins which immediately helped with the problem (well the heating did) but after a few hours am back to the same intermittent failure.
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Old 05-18-2018, 04:31 AM   #2
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Default Re: Belcom LS120l radio (crystal filter)

if heating it helps, it may have electrolytic caps in it.
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Old 05-18-2018, 06:09 AM   #3
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Default Re: Belcom LS120l radio (crystal filter)

Opps I'm sorry I got this into the wrong forum, could some one please move it to
Troubleshooting Game Consoles & Other Weird Devices please, There wont be any electrolytic s in there, maybe some ceramics in there though, casing appears sealed though. Will have to scope it, annoyingly it seems to be working at the moment.
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Old 05-18-2018, 08:50 AM   #4
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Default Re: Belcom LS120l radio (crystal filter)

Just a crystal in it... or a bunch of crystals. Yeah unfortunately you'll need to locate an identical unit or substitute, there should be no electrolytics in it.

Likely one of the wires or plating fell off a crystal, and getting it back on would be tough...
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Old 05-18-2018, 08:53 AM   #5
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Default Re: Belcom LS120l radio (crystal filter)

in the past maybe, now we have carbon-loaded conductive glue.
very cheap on ebay.
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Old 05-18-2018, 12:34 PM   #6
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Default Re: Belcom LS120l radio (crystal filter)

Would that affect the crystal vibration frequency with additional mass?
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Old 05-18-2018, 01:58 PM   #7
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Default Re: Belcom LS120l radio (crystal filter)

no idea.
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Old 05-18-2018, 04:52 PM   #8
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Default Re: Belcom LS120l radio (crystal filter)

From my understanding of the lattice crystal filters, they are made up from a series of crystals strung together, each one sealed along with the housing itself sealed with solder. The amount of thermal mass and heating required to disassemble the outer casing will probably cause more damage. I think I have located several candidate crystals rather than attempt to repair this one.
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Old 05-18-2018, 05:15 PM   #9
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Default Re: Belcom LS120l radio (crystal filter)

Yeah probably difficult to open, especially if it's crimped and soldered together. I wonder how much of a loss it would be if you replaced it with an RC filter, alas the Q is going to suck.

Getting a pile of 7.8MHz crystals will be tough too, I wonder why they don't make them anymore? I feel for you, this is worse than my crystal problem for my 2m radio...

I need to really figure out SSB someday...that modulation technique doesn't quite make sense, well, specifically, to encode and decode via analog. The SSB concept easy to understand; implementation, that's a whole another ball of yarn...

I have no SSB tuners/transmitters...
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Old 05-18-2018, 05:43 PM   #10
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Default Re: Belcom LS120l radio (crystal filter)

Quote:
Originally Posted by eccerr0r View Post
Yeah probably difficult to open, especially if it's crimped and soldered together. I wonder how much of a loss it would be if you replaced it with an RC filter, alas the Q is going to suck.

Getting a pile of 7.8MHz crystals will be tough too, I wonder why they don't make them anymore? I feel for you, this is worse than my crystal problem for my 2m radio...

I need to really figure out SSB someday...that modulation technique doesn't quite make sense, well, specifically, to encode and decode via analog. The SSB concept easy to understand; implementation, that's a whole another ball of yarn...

I have no SSB tuners/transmitters...
get yourself one . had fun with mine in the 1980.s . covered large distances at low power no problem ..was like being local . i was using directional antennas though . no way to move easily so was stuck with same contacts for months . worked best at night for long distance . local was 24hr . cant remember mhz now about 38 mostly rings a bell .
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Old 05-18-2018, 06:30 PM   #11
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Default Re: Belcom LS120l radio (crystal filter)

Eh, nah, not going to buy one. Has to be "MAKE" or "MAKE INTO"
So theoretically I can hack one of my CB radios to the 10m band (of which I have the schematics to!), add on an SSB frontend/decoder ... hmm!

And I suppose using a CB radio is QRP as it is ...

Alas I want to figure out how to get onto 2m band without buying a radio first. I do have an old 2m radio but not the right crystals to communicate with a repeater. Task #1 is to get this to work...

Then I'll have acquired the knowledge to reliably generate/receive the 10m frequencies needed to hack the CB radio - the radio I'm thinking about hacking is also using crystals. 10m should be around 28-30MHz I'd guess as 11m is 27MHz CB.

Step by step, incremental abilities! (then again, still haven't gotten my license yet, and antennas will be a problem...)
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Old 05-18-2018, 07:20 PM   #12
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Default Re: Belcom LS120l radio (crystal filter)

i used 3 19ft dipoles in tandem style in the attic .. parallel . like a large beam type thing .. had to make sure nothing interesting on tv as neighbours were not happy otherwise . just had to key up and watch his front window to see if he was shaking his fists .. was a cb radio with lots of channels .fm am too . thinking back there was too much going on in a small space to consider interfering with tv.s etc .
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Old 05-18-2018, 07:51 PM   #13
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Default Re: Belcom LS120l radio (crystal filter)

Ideally it wouldn't affect others TV, shouldn't even be within frequency range of VHF/UHF TV! Alas yes, HOAs and antennas is sadness. Need to build so-called stealth antennas, I do have a fence I can play with, as long as the fence stays dry...
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Old 05-19-2018, 05:11 AM   #14
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Default Re: Belcom LS120l radio (crystal filter)

The problem as I understood it was not related to the VHF / UHF frequencies used for broadcast television but rather one of the IF stages / frequencies used within your stock consumer television. These included IFs of 38.9 MHz vision, 33.4 MHz sound applied to Systems B, C and F. System E used 38.9 MHz vision, 27.75 MHz the latter of which being right in the middle of the CB / free radio allocations for 11 meter.
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Old 05-19-2018, 06:55 AM   #15
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Default Re: Belcom LS120l radio (crystal filter)

I would hope the sensitive iF parts would be shielded in the TV... Might be better in new tv's where the IF section is tiny and thus cheaper to shield.

Interesting...
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Old 05-19-2018, 09:42 AM   #16
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Default Re: Belcom LS120l radio (crystal filter)

modern tuners have no intermediate frequency.
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Old 05-19-2018, 01:39 PM   #17
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Default Re: Belcom LS120l radio (crystal filter)

If double side band happens in Kenwood radios, usually there is a leaking cap and the cap juice went underneath the Xtal and maybe some trace / through hole repair.
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Old 05-19-2018, 01:56 PM   #18
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Default Re: Belcom LS120l radio (crystal filter)

filter here ..https://www.ebay.co.uk/itm/Uniden-7-...YAAOSwI-BWIQ7P
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Old 05-19-2018, 06:31 PM   #19
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Default Re: Belcom LS120l radio (crystal filter)

lol... that meter had a 15K resistor soldered, and it's still NOS? Hmm.

I wish that guy showed a picture of the pins on the filter... might have been removed poorly.
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Old 05-19-2018, 06:34 PM   #20
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Default Re: Belcom LS120l radio (crystal filter)

Thanks Pete, that was one of the candidate filters I located, the other being a US vendor who wanted 40$ just to put the thing in a jiffi bag and ship it. Well I have ordered the https://www.ebay.co.uk/itm/Uniden-7-...YAAOSwI-BWIQ7P and hoping it fits the pcb and more importantly works lol.

will keep you updated
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