Improving TX Audio Quality on a Transcom GBX4000

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26VT01
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Improving TX Audio Quality on a Transcom GBX4000

Post by 26VT01 » 14 Jan 2019, 17:40

I have a 2955 that works well & a vintage tektronics oscillioscope but arent sure if thebscope works or how to use it, so as well as the "fix" to imrove the TX audio, how would one use an oscilliospoe ( assumimg thats the right tool) to measure / compare TX audio quality on a CB.

Its not muffled, just unclear compared to say a Uniden 400. I've a Fidelity 2001 thats the same. Have set dev to 3khz. Not tested at factory or legal dev spec. Mic's are a coffin from a rotel 240 & a cobra /highgear power mic. I am just starting out learning to repair my own CB's.

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ch25
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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by ch25 » 14 Jan 2019, 18:14

First of it all get rid of doubtful quality dynamic cartridge and use electret capsule.
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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by The Collector » 15 Jan 2019, 18:53

The Transcom 4000, Fidelity 2001, Amstrads etc use the same board. I've always found the audio to be clearer than that of the bassier Uniden-boarded radio, which sometimes makes the Unidens sound louder. A lot of people prefer the bassier sound though.

Turning the audio trimmer up a bit inside may help matters, or using say a Ham International Relax power mic' or a Cobra Dynamike on it, or something similar with a bit of clout to it.

I've also heard of people messing with resistors or something on the radio's circuit board to get the outgoing audio frequency slightly lower, but you'd have to ask someone like Black Spirit or Cubwolf etc about that kind of thing :)

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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by 26VT01 » 16 Jan 2019, 13:37

Thanks Collector. Its a friends radio the Transcom, & she is getting poorer audio reports compared to her Uniden 400,. To me it sounds like less treble but i cant define it more than that. Compressed maybe?!

She has 2 mic's & same are used on both radio's & the 2 sound good on the Uniden 400.

I may pull out the cap's in the audio section, & test them, but am new to this so was wondering if anyone had come across it before.

& CH25, i have ordered an Adafruit 1063 electret board & a batt to put in a stock mic housing, so will test & report back on that one. I'll also get the girl to test my Fidelity 2001 from her QTH.

Also - If anyone has ears on north west london & could assist with some on air tests that would be champion.
Last edited by 26VT01 on 16 Jan 2019, 15:37, edited 1 time in total.

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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by ch25 » 16 Jan 2019, 14:41

Adafriut is OK, I played with that in the past.
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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by The Collector » 16 Jan 2019, 17:41

Just tell her to stick to the Uniden if she's got the choice of the 2.

A bassier Uniden will probably make a girlie voice sound louder.

The Transcom may pick up distant signals an S point better though, but unless she's into DX'ing she won't notice much difference at all, and the louder audio of the Uniden may help her actually TX better over distance. "Swings and roundabouts" methinks.

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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by 26VT01 » 18 Jan 2019, 13:00

She loves the transcom, it was her 1st radio back in the day. She has a really good location & low QRM level so the extra sensitivity will be of use.

Its sensitivity is 14 microvolts @12db sinad , so pretty good, just after an alignment.

If anyone has ears on (west) london, & can assist with some on air testing , please let me know.

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Re: Improving TX Audio Quality on a Transcom GBX4000

Post by 26VT01 » 15 Feb 2019, 19:41

Well the Adafruit electret was pants.So I recapped it the whole thing with mainly Nichicon from RS. I found 3 cap's at about 20% high ,& one next to to the PA socket dry jointed & loose. All other's were close to spec.
Added 2 x 100nf ( electrolytics ) across C29 & C31, which upped the dev by about 50% & I adjusted it back down to suit. No idea if the 2 x 100nf caps just raised dev level or did something else not as i still haven't learnt how to measure audio signal composition or interpret a schematic lol, but & its brighter & louder now & she ( the owner ) is very happy.

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