High Frequency noise on sound after RGB Mod

[Rickrob]

Hello,

 

First post here, just joined the forum today.

 

I just modded my TI-99/4A with the RGB upgrade described in this thread:

 

The video is working great, but I have a high pitched noise/interference on sound.  I didn't carry sound through the SCART cable, I installed a 3.5mm jack wired direct to the sound out (pin 3 of the old DIN connector ) to plug the speakers into.

Any suggestions on how to find the source of this noise? I have double - checked all the work on the RGB mod, that all looks good.

 

Thanks

[Tursi]

Is this a new sound that you didn't have before the mod? Cause TIs naturally have a relatively high pitched noise - it'll change pitch some as you press keys. Are you maybe just hearing it clearly for the first time?

 

[Rickrob]

3 hours ago, Tursi said:

Is this a new sound that you didn't have before the mod? Cause TIs naturally have a relatively high pitched noise - it'll change pitch some as you press keys. Are you maybe just hearing it clearly for the first time?

 

Maybe I never noticed, just got my first TI-99 -- It seems like its around 1KHz. Rough guess.

[Rickrob]

On the home screen it's about 1Khz steady. When I select Basic or Extended Basic, The 1Khz noise stops, and becomes  a lower pitch  -- then I can hear the noise on key presses. I guess that must be normal behavior. If I turn the volume down, the noise is barely audible and I can still hear the power up beep and the beep entering Basic.

[Tursi]

I hit YouTube for a random video, and you can hear it reasonably well in the startup to this video before Parsec starts, see if that's what you're hearing?

 

[Rickrob]

Yep, that's it-- exactly the same noise on the startup screen.  I never noticed it until I finished the RGB mod.   Good to know that this is normal for a TI-99.

 

Thanks for your help everyone. 

Edited June 11 by Rickrob

[Duewester]

Well I'll be darned, an advantage to being older and hard of hearing. I don't hear anything but the startup tone and beeps when when selecting.🙂

[+mizapf]

Has it been settled what signals exactly crosstalk into the audio line?

[+OLD CS1]

59 minutes ago, mizapf said:

Has it been settled what signals exactly crosstalk into the audio line?

From what I recall being posted around here, the 9901 is a primary culprit.

[Gary from OPA]

The audio on the ti99 is messy one. Some systems are better than others. You have sound coming from 3 sources mixed together the sidecar audio in line for the speech, the gated sound from the cassette port and then finally the sound chip.

 

I find shielding the audio out from the sound chip to the din port helps in some consoles it is.

 

Also changing the power supply can help as well.

 

On my ti99 system I also added a iron bead around the audio cable as soon as it leaves the console and that helped to reduce the sound on my speakers.

 

Also remember the ti99 is mono system, I have found on some setups having the sound only going to one left speaker jack instead of both with a y joiner helps leaving the right speaker jack not connected seems to increase the noise for some reason.

[Tursi]

I emulate it reasonably well in Classic99 by tracking the VDP vertical interrupt and the 9901 keyboard scan. The keyboard scan especially slams the 5v rail since it runs through a 1-of-8 decoder at high speed. I don't think it's the 9901 so much as the TTL logic it interfaces to. Likely the multiplexer adds a lot too.

 

I used to use that sound to know when I had the RF tuned right in.

 

[Rickrob]

4 hours ago, Gary from OPA said:

The audio on the ti99 is messy one. Some systems are better than others. You have sound coming from 3 sources mixed together the sidecar audio in line for the speech, the gated sound from the cassette port and then finally the sound chip.

 

I find shielding the audio out from the sound chip to the din port helps in some consoles it is.

 

Also changing the power supply can help as well.

 

On my ti99 system I also added a iron bead around the audio cable as soon as it leaves the console and that helped to reduce the sound on my speakers.

 

Also remember the ti99 is mono system, I have found on some setups having the sound only going to one left speaker jack instead of both with a y joiner helps leaving the right speaker jack not connected seems to increase the noise for some reason.

I have a new power supply on the way -- It's an open source replacement that uses a buck converter and  allows the use of a 12V 4A power supply.  I can add a ferrite bead on the audio out and see if either of those help.