CAPI VP312 troubleshooting

[tgs]

I'm having some problems with a CAPI VP312 since quite a while back, it's intermittent though so it's been a bit difficult to troubleshoot. I'm getting sort of scratchy noise on the output. Did a quick recording of it:

Noise

One of the first things I did a while back was to go over all solder points and reflow. Same on the opamp. Next step, I swapped the opamp. When I've done this, the noise has usually gone away for a bit, only to reappear after a while. I've suspected the Mill-max connectors and I've cleaned them as thoroughly as I could but the problem persists.

Increasing the input gain also increases the noise. Output dial attenuates it. Mute button mutes the signal including the noise. The pad button doesn't change the noise though. It does pad the signal at input, but the noise stays the same.

I've tried to trace the noise, I'm seeing a lot of it on the - side of the opamp. Switching SW6 between 16V and 24V reduced the noise. The weird thing though: the switch doesn't affect the voltage at the opamp pins (neither on pos or neg side), it stays at 24V+/-. I pulled out another VP312 to compare. Same thing there actually. I checked at the diodes before the switch but there all voltages are fine. What's going on here?

(VP312 RevA, 51X version, stepped gain)

 

[Whoops]

If I remember correctly +-24V is too much for the 2520...

You should post in the VP312 project thread:

https://groupdiy.com/threads/build-...icial-support-thread.38786/page-8#post-972304

 

[tgs]

That's funny, never even bothered to check that but indeed it's rated at 20V max. I have several VP312, meaning a bunch of them are running too hot in that case (not all are 51x so some have been spared).

Still doesn't explain why I'm getting 24V regardless of switch position though.

EDIT: Figured out the constant 24V presence at least. I was measuring without any opamp present so C3 and C4 probably stayed charged up when I switched. Measured with an opamp in place now, seeing a good 16V +/-

 

[Whoops]

That's funny, never even bothered to check that but indeed it's rated at 20V max. I have several VP312, meaning a bunch of them are running too hot in that

sorry mate, I don’t know what is funny…
I never did any project without knowing the recommended voltages and getting the correct PSU rails for that project, neither I see any other way of doing a project.

That’s my only recommendation to you.
that and also posting on the proper project thread instead of doing a new thread

good luck

 

[tgs]

sorry mate, I don’t know what is funny…
I never did any project without knowing the recommended voltages and getting the correct PSU rails for that project, neither I see any other way of doing a project.

That’s my only recommendation to you.
that and also posting on the proper project thread instead of doing a new thread

good luck

I built these very early in my DIY days, I set them at 24V indeed without knowing better. In a couple of later builds, I did the same, based on that things were working fine. Been running four of them at 24V for 5-6 years now. But actually looking at the Gar2520 specs now after you mentioned it, I have set them all to 16V. Only having problems with one of the 312s and it doesn't seem to be related to rails voltage, so thanks for pointing it out but I'm still to find the source.

 

[pucho812]

usually most 312 style circuits faults come down to a faulty DOA. worth replacing and you are good to go

 

[tgs]

usually most 312 style circuits faults come down to a faulty DOA. worth replacing and you are good to go

That was my initial suspicion but after changing DOAs multiple times, including swapping between different preamps, the problem still persists. Sometimes it has temporarily made the problem go away though which has made me suspect the sockets. But reflowing the solder, cleaning them, nothing has helped so far.

 

[bjosephs]

If the problem comes back after swapping opamps, and follows opamps that have exhibited the issue to other preamps, isn’t it time to consider that the over voltage condition has damaged the opamps?

 

[tgs]

If the problem comes back after swapping opamps, and follows opamps that have exhibited the issue to other preamps, isn’t it time to consider that the over voltage condition has damaged the opamps?

Not ruling it out entirely, but DOAs that have been in this particular module are working fine in others. I can also have a problem with one DOA in, take it out and do some testing, put the same one back in again and the problem is gone. Then at some point, it reappears out of nowhere.

That said, I'm now running it at 16V. Still had the noise but swapped to another DOA I had laying around and so far it's good, except that it has a bit higher noise floor than its siblings. And again, the other modules that I've stupidly have been running on 24V for years have had no problems whatsoever.

 

[pucho812]

Ok if we feel confident that the doa sockets and doa is ok, trace the circuit. See if you can use an extender module to work on the unit outside of a 500 rack. See if you can recreate the problem there, then scope it.
maybe the edge card pins are wonky? That could be the slot or the module.

 

[tgs]

Ok if we feel confident that the doa sockets and doa is ok, trace the circuit. See if you can use an extender module to work on the unit outside of a 500 rack. See if you can recreate the problem there, then scope it.
maybe the edge card pins are wonky? That could be the slot or the module.

Got an extender module indeed. Been doing some tracing already but didn't have time to sit for as long as I needed (in the middle of a recording so troubleshooting only on off-hours). "Unfortunately" the problem is gone at the moment so I have to wait for when it starts again. I'll check edge card pins then for sure, plus check the rails in general. I did see what could be a bit of ripple on the scope yesterday but I had the band walking in so I had to stop. But the caps should take care of that before it reaches the DOA, right?

 

[jsteiger]

Classic symptoms of a bad solder joint(s) somewhere on the build.

 

[tgs]

Classic symptoms of a bad solder joint(s) somewhere on the build.

That was my first suspicion some years ago, after which I think I reflowed all joints. I know I did it for the opamp that was in at the time but I cannot swear that I also did it for the whole card. I'll check it when I have some time. I know I reflowed the mill-max sockets at least but the rest...not entirely sure.

The preamp has been nice and quiet all day though. Always welcome, except when you want to troubleshoot.