720 - atari series 2, +15v on audio and video board..where does this come from?

720fan

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720 - atari series 2, +15v on audio and video board..where does this come from?

so, im still stumped on my 720 machine... so far here is what happens when i turn it on.
all lightbulbs light up on coin doors and marquee.
monitor turns on and goes white when i adjust the "screen" control.
there is NO sound at all.
the video board and the cpu board light up red, and when measured i adjust my switching power supply to get to 5-5.1 volts... still nothing.
on the audio board when i switch to and from self test mode, nothing happens.

i measured the voltages (AC) coming off the transformer and they are 12, 12, and 18, so i now know it is not a bad transformer.

the only other power to measure is the +15vdc off PIN 7 of the P13 connector on the video board which goes to the +reg on the audio board - where do i measure this and what am i hoping it is? where does it come from?
and there is also a +15/-15vdc coming to the cpu on the P15 connector that on the other end is coming off the audio boards +reg/-14vdc.

when i measure on the back of the audio board when all is connected this +reg and the ground only show me about 5v...why am i missing the other 10 volts? is the +15 taken from the 18 that comes in and stepped down on the audio board?

this is all i can think my problem is, but any other ideas/suggestions on what to check, where to check are greatly appreciated...

thanks
joe

i have tried this machine with 2 separate boardsets (both believed working) and 2 different audio boards.. and still nothing.
 
I'm no expert, for sure, but I was in a similar state. I pulled the board set and checked all the EPROMS and 8 of them were bad. I ordered an EPROM burner and it just arrived yesterday. I burned the replacement EPROMs and installed them. I'll be testing again tonight.

But in general with no sound and no video, with power verified, then it could be wiring harness problems (both to sound AND to the monitor), but more likely it's problems with the boards. Could be a dead CPU chip or bad EPROMs or broken traces or a whole lot of other things.

Burning EPROMs is pretty easy to do, so I'm crossing my fingers that that will solve the problem with my board set. I can also replace the CPU and test voltages at the pins of the CPU and other chips, but beyond that my odds of getting the board set working goes down significantly.

If you're familiar with checking EPROMs with winromident or something like that, I'd say try that first.


so, im still stumped on my 720 machine... so far here is what happens when i turn it on.
all lightbulbs light up on coin doors and marquee.
monitor turns on and goes white when i adjust the "screen" control.
there is NO sound at all.
the video board and the cpu board light up red, and when measured i adjust my switching power supply to get to 5-5.1 volts... still nothing.
on the audio board when i switch to and from self test mode, nothing happens.

i measured the voltages (AC) coming off the transformer and they are 12, 12, and 18, so i now know it is not a bad transformer.

the only other power to measure is the +15vdc off PIN 7 of the P13 connector on the video board which goes to the +reg on the audio board - where do i measure this and what am i hoping it is? where does it come from?
and there is also a +15/-15vdc coming to the cpu on the P15 connector that on the other end is coming off the audio boards +reg/-14vdc.

when i measure on the back of the audio board when all is connected this +reg and the ground only show me about 5v...why am i missing the other 10 volts? is the +15 taken from the 18 that comes in and stepped down on the audio board?

this is all i can think my problem is, but any other ideas/suggestions on what to check, where to check are greatly appreciated...

thanks
joe

i have tried this machine with 2 separate boardsets (both believed working) and 2 different audio boards.. and still nothing.
 
is there a good way to check the wiring on a machine other than just checking continuity from every connector?

i dont think its a board problem because i am getting the same problem with 2 different sets of boards...
how did you check the EPROMS to find 8 bad ones without a burner?
joe
 
is there a good way to check the wiring on a machine other than just checking continuity from every connector?

i dont think its a board problem because i am getting the same problem with 2 different sets of boards...
how did you check the EPROMS to find 8 bad ones without a burner?
joe

I had an older burner. I tested them with it. But after erasing bad ones, I went to burn them, and it failed to burn. So I ordered some new EPROMs and tried burning one of those, and that failed as well. So I ordered a new burner, and that succceed (at least they read out as the right thing on both the old and new burners).
 
is there a good way to check the wiring on a machine other than just checking continuity from every connector?

i dont think its a board problem because i am getting the same problem with 2 different sets of boards...
how did you check the EPROMS to find 8 bad ones without a burner?
joe

The boards are not the problem. I had a chance to look at the schematics. My guess is the problem lies with Q3 on the audio 2 board.

The 18vac comes from the transformer though fuse F4 on orange wire to pin 7 Jacdc of the audio board through Q3 to pin4 of the same connector which carries the now 15vdc through the gray wire to pin 7 of p13 video board and pin 2 p15 of the cpu board. On video board test for 15vdc red meter lead pin 7 black pin 10 (white wire). On Cpu board red lead pin 2, black lead pin 1. Problem could also be bad ground connection. With power off test for continuity between white/red wire coming off transformer and pin 10 video board as well as pin1 cpu board. This ground travels from the transformer to pin6 Jacdc of audio board through c1,c2,c3,c4 of audio board to pin5 of same connector white wire and travels to pin10 of video board(as stated above) and pin 1 cpu board(as stated above).
 
thanks vic
i think you are right and it is on the audio board. i have a new one on the way and will let you know what happens. thank you again for all the help and the boards.
joe
 
"+15v on audio and video board..where does this come from?"

It comes from a planet called Dune and is generated from an element called spice.
 
Just in case you're in the same situation as me. At some point during the monitor repair my Super Sprint got switched and will not make any attract sounds. She was working as expected before, and suddenly silence. power LED's illuminated, coin lights were on, voltages measured as expected. Since my monitor is dead, I assumed the audio and/or PCB had died also. Man was I mad...

While investigating, I inadvertently pressed P2 start and she made the "I've joined in" rev up and then started a race. It seems that it's skipping past the "insert coin" screen and sitting on "waiting for others to join" screen in silence. I'll T/S more when I get the monitor working again.

HTH
 
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