Atari 800XL with Pokeymax + mixer boot issue

[TJ76]

This one's going to be tricky to explain, but interested to know if makes sense to anyone.  I'm sure something unique to my setup here.

 

I have working 800xl and 600xl here, and recently working on the 600XL, added 64k+U1MB + enabled Svideo following guide from FLC successfully.  Not completely relavant to this, as issue occurs without the 600xl hooked up, however after problems testing with cable i made up myself, that I didn't trust, I bought a HQ audio video cable from Retrolemon in UK, and used for SVID connection, via a SVID/composite to scart adapter into a Sony Bravia TV.  Very nice video.  The audio i had hooked up from the cable into a Soundlab mixer I use, then on to my amp.  All worked well.

 

When I returned to using my 800XL, something odd happened.  I first had plugged in a spare USB PSU I had (wanting to see if it ran it ok, and could have both atari's cabled up in parallel.  The PSU I was using with the 600xl I had previously used for the 800xl and was borrowing it.

 

Anyway, upon turning it on I had a audio blipping and screen also flashing.  Didn't sound good, turned straight off.  Returned to proper PSU.  800xl won't power on any more.  My first thought, I'd fried something in it by potentially using bad PSU.

 

Later, simplifiying by plugging in only essentials - known good PSU and DVI from Sophia into a monitor, it boots fine.

 

Cabled back way things were before (including Pokeymax to Amp connection).  Won't boot again - black screen.

 

Skip to the end......

 

I find that when both the svid and audio cables from the HQ audio video cable setup for my 600XL (which incidentally are not currently connected to anything on the Atari 6 pin DIN side) are hooked up to the TV and the mixer, the Atari 800XL with Pokeymax audio connected to the same mixed will not boot.  Furthermore:

 

If I remove the HQ cable audio connection(s) to the mixer and boot the atari 800xl; if i connect the HQ audio cable to the mixer after boot, the 800xl cuts out.

If I remove the SVID conenction from the SCART adapter taking it into the TV, but HQ audio cables left connected to the mixer's other channel, the 800xl boots fine, no issues.

 

What on earth is going on here.  Something shorting / grounding incorrectly somewhere.  I'm stumped, but something is really off here.

 

My setup if this helps - interested if anyone understands this!

 

 

Edited April 17, 2021 by TJ76

[_The Doctor__]

You should meter it all out, it would be the only way to determine where the level is shifted and the potential difference is...

you could try isolation transformers or opto coupler isolation... you may have ground loops and any number of other issues going on in respect to reference voltages/currents... this shouldn't be taking place... something is clearly mucked up of course.

[TJ76]

Solved my problem here I think, however potentially still something lurking somewhere, interested in thoughts.  Here's what I had originally did and changed:

 

For the Pokeymax installation into the 800XL I was referring to this video from FJC (which I realise refers to 600XL, however unsure differences matter here): 

 

 

At 24.53 you can see a convenient ground is used as opposed to the ground from Pokeymax.  One less wired going across the board, and I did the same after checking continuity to ground from what I believe was same resistor on my board (case closed now, too lazy to double-check).  Recalling this, and on a hunch, I desoldered it from the audio jack and took a wire back to the ground on the Pokeymax itself.

 

Seems this now runs fine, with everything wired as shown in my original post.  So that shortcut somehow caused my issue (though would not be surprised if in conjunction with something else that is still wrong somewhere).  Thoughts?  (and I'll add an @flashjazzcat here, though don't get me wrong, I'd be lost without some of your guides, so just for interests sake here and whether any thoughts re that ground wiring.  In isolation it indeed worked just fine).

 

Have a good day all.

 

Tim

 

[_The Doctor__]

I'd think what you thought was the same convenient ground... wasn't....    600XL vs 800XL. Call it case closed.

Edited April 26, 2021 by _The Doctor__

[flashjazzcat]

Yeah. Without knowing exactly what was used as said convenient ground, it's impossible to say, but presumably 5V was getting shorted to GND via the audio output somehow.