PTBM121D4X (Tristar 747) Distorted Audio

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Phuture
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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.

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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.

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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.

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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/

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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.

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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...

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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. :)

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Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Phuture » 23 Jan 2018, 18:42

Hi ibcomms, Black Spirit and all,

My apologies for dropping this one for a while- things just got to busy around Christmas, NY and a flu in the first couple of weeks of January.

Anyway, attached are two recordings in MP3 format: one of the Tristar 747 and the other of my Major 588. I've divided the recordings into to two volume sets - the first at a medium volume level and the second at a low volume. On "OOOORRRRRRRRR!" sounds it's more obvious on the Tristar that the audio is clipping but unfortunately the recording doesn't really highlight that as much as I'd like. The sound is improved when the volume is lowered. It's obvious, however, that the bottom end of the audio on the Tristar is lacking when compared to the Major 588. It would be nice to improve this as well if possible. The Major 588 recording does also have some audibly perceptible clipping on louder sounds but it doesn't sound as obvious (to me at least here). The mic is the factory one provided with the Tristar but used on both radios for the recording. I tried various mics which demonstrate the same thing more or less. Everything was set up identically for each recording, including ibcomms attenuator resistor circuit.

I also lifted the base of Q4 as per Black Spirit's suggestion but that made no difference. Tested the transistor out of circuit in a component tester and it appears fine.

Wondering if perhaps a cap has gone high in value and needs to be replaced.
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Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by lbcomms » 28 Jan 2018, 13:09

It's a bit hard to tell from that recording, but I'd expect an old Cybernet to sound a bit cleaner than that.

Next thing to do would be to determine if the distortion is in the detector (before the volume control) or power amplifier (after the volume control) stages. To do this, locate the "hot" pin of the volume control and temporarily remove the wire from it.

The "hot" is the outer terminal of the pot what is not a common earth connection. The other outer connection will be earth, and is connected to the shields of two screened wires in most Cybernet SSB radios.

Connect a line level input to the pot. Ground to the earth connection (no need to remove the existing wires), and audio / centre connection to the hot terminal of the pot (i.e. the terminal you removed the wire from).

Play some audio and adjust the volume. Is it distorted or is it clean sounding?

If it's clean, the problem is in the low level / detector stages.

If it's distorted, the problem area is around the audio power amplifier IC or the IC itself. Most Cybernets use a Toshiba TA7205AP audio chip. A common fault is the IC on abused (overvoltaged or overloaded) radios. Less common are leaky capacitors near that IC, or the diodes that turn the ALC circuit on and off turning themselves into resistors...

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Re: PTBM121D4X (Tristar 747) Distorted Audio

Post by Phuture » 29 Jan 2018, 20:49

Hi ibcomms,

Just to confirm, this is distortion on the transmitted audio only from the Tristar. The audio files are recordings on the receiving radio, though it's the same for any radio receiving the Tristar's transmission. The reason I'm confirming may be my misunderstanding but I just wanted to be sure that I was making it clear that the distortion is not on the Tristar's receive audio.

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