losing voltage when testing with meter

[Simon_m74]

Hi,
Problems with my VR machine again..


The problem I have affects the logic system, which is mostly controlled by a 74LS139 (2line- 4 demux)

I notice that its getting the correct signals in, but giving nothing out.
So I changed the chip..
Same problem.

One of the issues is I get very quiet, but heavily distorted audio.

What I tried, is remove the chip from the socket, so to be able to check the voltage going to it.

Upon machine boot-up, I got fully working audio.

But when I put my meter on the pins to check the voltage, I only got about 1.8v, instead of 5, and it dropped over about 3 seconds to less that 0.5v
Also, at the time of contact, I lost the audio back to the distorted - so I guess that the audio voltage is dropping too.

Any ideas why I`m getting a voltage drop, especially when I use the meter.
And seemingly completely when the IC is in place.

Simon

 

[Simon_m74]

Sorry all

Fixed!

I just traced where this 5v comes from, as other chips are stable at 5.2v. The 5v to feed the 74LS139 and 5 x AD7111`s ( for the digital audio control) are powered not from the onboard PSU, but from the 5v out of the joystick ports from the Amiga - that I have not had connected as I had to re-wire due to different joysticks - bypassing the need to plug them to this PCB - but direct from the amiga to the joysticks!

 

[kellys_eye]

But when I put my meter on the pins to check the voltage,

Which pins?

 

[Simon_m74]

Hi,
Was checking voltage from P8-Gnd, to P16-Vss, on the 74LS139

I have a spare PCB that I have `salvaged` by removing most components, so I can trace line hidden by IC`s. So I traced the power lines, and found the supply was not from the same place as most of the rest of the 5v logic.

So plugging the joystick cables back in, gave the 5v supply, from what would be been for an `auto-fire` circuit in the joystick
(keep in mind that these systems are about 25 years old)

All sorted now - I just need to re-wire the joysticks I have used on my machine, to not bypass this PCB.

The original joysticks would of had the option of being analogue - and they would be been connected to this PCB, that has circuitry built in for trimming the max/min/ctr point of each axis. These joysticks connected to the board via a 15pin D plug.
Once trimmed, the output is sent to the Amiga computer via the 9-pin Dplug joystick ports. I bypassed the PCB and plugged my `new` joysticks direct to the Amiga - not realising that the Amiga powered some of this PCB!

All this work I`m doing is for the VR machine, that I`m sitting in on my avatar pic.

Next is to sort the video output.... its not clear...

FYI
This is the same kind of PCB that I have the voltage drop on the 5v also, on another thread I posted.
Different problem on that board though, as I`m loosing 5v direct from the PSU, which affects everything else.


I have an ex-employee of the Virtuality company, who is also an electronics guy, coming over tomorrow night to have a look at it all.

Thanks .

Simon