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Noisy cassette output when video is plugged in

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Hi all, I'm new to retrocomputing and recently purchased a beige model TI-99. Today some components arrived and I was able to build a composite A/V cable and a cassette cable (the latter according to http://mainbyte.com/ti99/hardware/cables/cassette.html).


I've been able to load .wav files from my computer onto the TI-99, but was having trouble saving - the output was really noisy and the TI kept saying no data when trying to play it back. While troubleshooting, I unplugged the A/V cable and the noise went away. I was able to both save and load a "hello world" BASIC program by unplugging the A/V while saving.


I monitored the cassette output in Audacity and tested a few configurations. I get noise when:

  • TI-99 is switched off and unplugged, A/V connected to an AV2HDMI converter
  • Power OR HDMI is unplugged from the AV2HDMI
  • Any of the 3 RCA plugs are grounded to my PC case
  • With the RF modulator, when the coax cable is grounded to my PC case
  • With the RF modulator, when the coax is grounded to one of the AV2HDMI's RCA jacks

I get no noise when:

  • Both power and HDMI are unplugged from the AV2HDMI
  • All RCA plugs are disconnected, DIN side still connected to the TI
  • RF modulator plugged in to the TI, with coax disconnected
  • Nothing plugged into DIN


I also saw the issue when plugging the RCA into the A/V switch attached to my downstairs TV, and using a Tascam digital recorder in place of a cassette player, so it's not just the PC or the cheap AV2HDMI converter that's causing trouble. Seems like connecting any device to the ground pin on the DIN socket does it.


Has anyone seen an issue like this, or know how to troubleshoot further? I've double-checked for shorts on both of my cables - could there be an issue with the console itself? I don't know the history of the machine - it looks brand new and came complete in box with an unopened stack of manuals, but I also took the case apart to look at the internals and noticed there are some wires soldered to the PCB.

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What are your power sources?  I found noise on my cassette audio when using a USB power supply for my nanoPEB, and also when I used a TV system on a different power outlet than the TI and cassette drive.

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Yeah, be wary of ground loops, especially if your TI and your PC are on different outlets. 


It's probably not the console, but that's just opinion.


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Posted (edited)

Good idea to connect all AC devices to the same OUTLET/POWER STRIP.
Most homes/buildings are wired so loads are out of phase with respect to neutral, this helps to present balanced loading to the neutral wire. 
Newer T.V.s/monitors with polarized plugs are intentionally designed to ground the chassis through a bleed resistor to discharge static charge from the HV and to trip GFCI breakers when power travels in from the CATV to ground.

Too many such devices connected to different AC LINE OUTLETS and CATV or VIDEO/AUDIO switchers can leak AC to chassis GND.

If your running signals all over the house... from units connected to different electrical outlets, this should be considered.

This can also become a problem if the polarization is defeated on any device.


A ground issue in the cassette line or internally is also suspect.:ponder:


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Upstairs, I have the TI, PC, and monitor plugged into the same power strip. The AV2HDMI is USB powered from the PC. Downstairs, the TI and TV are plugged into the same power strip, and the digital recorder is running on battery. The TV has a two-prong plug, I can check when I get home whether it's polarized or not. In both cases there are lots of other things on the same power strip. AC power is still black magic to me but I'll read up on ground loops.



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