PTBM121D4X (Tristar 747) Distorted Audio

A forum for the discussion of matters of a technical nature. All such activities are undertaken at the readers discretion and own risk. If you don't know what you are doing, don't blame us if it all goes wrong!
Phuture
Super Member
Super Member
Posts: 102
Joined: 04 Nov 2012, 14:01

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Phuture » 24 Nov 2017, 17:30

Yes, I can do that and just have but curiously I don't get as discernible a difference in the recorded audio between FM and AM. What I can hear in the recorded audio would be more or less acceptable if it were the case through the speaker, which is far more obvious. I've swapped new cable and speakers in and a couple of different mics to rule them out and it made no difference. I'm kind of surprised as the distortion doesn't sound like it changes by varying the deviation or if I move away from the mic to reduce the volume of audio that way. It looks to me like the audio is becoming distorted before it reaches the audio output IC. Anyway, I'll play with the recording level on my laptop to see if I can get it into an audio file, else I'll just have to record the speaker output.

lbcomms
Super Member
Super Member
Posts: 244
Joined: 04 Oct 2015, 08:10
Location: Sydney, Australia

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by lbcomms » 24 Nov 2017, 19:25

If you are connecting to the speaker side, play safe and use an attenuator:
Image
The resistor values are not critical, anything close will do.

The 10 ohm resistor is just there for safety in case you reverse the supply polarity or if the power supply negative is not the same voltage as PC ground.

Don't connect anything to the outer pins if the ext speaker socket, use PCB ground for the radio side ground connection. That way, both Tx and Rx audio will be recorded and you won't have really loud clicks when you key up and release the PTT.

When it's all up and running, record the distortion on SSB, FM, and AM to compare the differences. If you have one, use a dummy load for the tests (to reduce RF interference to the recorder) but don't disconnect the base of the driver as before because RF energy is sampled to operate the limiter circuit.

Phuture
Super Member
Super Member
Posts: 102
Joined: 04 Nov 2012, 14:01

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Phuture » 28 Nov 2017, 01:34

Hi ibcomms, apologies for the delay replying. I've not had a chance to do a new recording yet, as per your advice above. I'll try to o it tomorrow evening. I think the recording software may have been affecting the recording so I'm going to use something less sophisticated to record the output of the radio. Thanks very much for spending your time on this with me so far. I'll come back to you tomorrow evening I hope.

lbcomms
Super Member
Super Member
Posts: 244
Joined: 04 Oct 2015, 08:10
Location: Sydney, Australia

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by lbcomms » 28 Nov 2017, 06:43

No probs at all, whenever you are ready. In the middle of a biggish project myself (the updated 2824+ board), so any reply would be delayed by a day or two anyway. If you get interference on the recording, try it again with a dummy load (a proper 50 ohm type, not one of those light bulb things) connected directly to the radio, i.e. no patch cords or SWR meters and the like.

If you are reasonably proficient with computers, try the Audacity audio recording / editing app: http://www.audacityteam.org/

User avatar
Black Spirit
Veteran
Veteran
Posts: 3961
Joined: 16 Mar 2009, 19:30
Location: S.Wales

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Black Spirit » 28 Nov 2017, 09:44

Out of curiosity try isolating the centre pin of Q4 and see if the AM/FM TX audio clears up [Q4 is at the front of the board near the FM deviation VR], This transistor puts the deviation VR to ground during RX to stop RX howl when volume is increased, If this switch is bad it will cause poor TX audio.

lbcomms
Super Member
Super Member
Posts: 244
Joined: 04 Oct 2015, 08:10
Location: Sydney, Australia

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by lbcomms » 28 Nov 2017, 13:21

Off subject slightly - say your wanted ad for the Colonel channel switch and board. I've got a couple of those somewhere here, left over from a rotary encoder conversion. The channel switch and display boards were replaced in those radios to give them continuous coverage from 26.5 to 28.5 or so, and the A/B/C switch used to select 1K, 5K, or 10K steps. This meant replacing the whole channel selector, board, and display assembly.

If you can't find one locally, I'll have a dig through the junk box for them. Can't remember turfing them out so have probably got two. Postage from Oz could be a killer though.

Replied here as the forum wouldn't let me do a reply in the section where your wanted post was.

Now, back on subject...

Phuture
Super Member
Super Member
Posts: 102
Joined: 04 Nov 2012, 14:01

Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Phuture » 03 Dec 2017, 03:48

Thanks very much for your advice and replies. Unfortunately this last week has been especially busy and I've had no time at all for the radio. I'll be trying your suggestions (ibcomms, Black Spirit) on Monday night, as that'll be the first opportunity I get. Thanks again for taking the time to reply and suggested advice. Always appreciated. :)

Post Reply

Who is online

Users browsing this forum: Google Adsense [Bot] and 2 guests