Possible solder bridge causing short in Mic Pre

[telehumbucker]

My 70's mic pre is having some issues. It's the same pre (from vintage Midas board) that was discussed in the linked thread below. I had a tech look at it and in the process of trouble shooting he re-soldered some joints, during which, I think two of the joints were bridged. The posted picts show a working channel and the one with what looks like bridged joints.

Before the soldering was done, the mic pre input -both mic and line - was not functioning. The pre would power on but not receive any signal through the mic or line input.. After the soldering, I found that a wire had broken and was hanging loose on the input module. Had that resoldered and thought that would solve the input problem. But, instead, what happened was when the module was powered up, it melted one of the op-amps and shut off one of the power rails.

When I removed the module the power supply returned to normal functioning....but couldn't power up the problem
module without melting an op-amp and shutting down the power. So I visually inspected the solder job and it looks like in two places the solder joints are bridged. Any opinions on the situation welcome.


https://groupdiy.com/threads/vintage-70s-midas-channel-strip-problem-pro5a.85650/page-3

 

[Khron]

Those "bridges" are nothing more than solder on top of already existing traces (the almost-golden looking strips), so the problem is elsewhere.

Could the dead opamps themselves be the cause of the issue?

 

[telehumbucker]

Those "bridges" are nothing more than solder on top of already existing traces (the almost-golden looking strips), so the problem is elsewhere.

Could the dead opamps themselves be the cause of the issue?

Thanks for the reply. I swapped out the opamps a couple times with the same meltdown and power shut down results. The three events that happened before the op-amp meltdown symptom, was the re-soldering of the loose wire to the mic/line input and the resoldering of some of the board. The red wire is the one that was resoldered. There were also a couple wires from the transformer that had to be resoldered as the tech took out the transformer to make sure it worked but didn't resolder some of the wires so I had a different tech do it.

 

[Khron]

Did all the wires get resoldered to the correct spots? And how certain are you of that?

 

[RoadrunnerOZ]

Did the right wires get resoldered to the right places from the transformer etc. and is the transformer itself ok? Have you compared the wiring from a known good channel to the “repaired” one?

 

[telehumbucker]

Did the right wires get resoldered to the right places from the transformer etc. and is the transformer itself ok? Have you compared the wiring from a known good channel to the “repaired” one?

I think so. I checked it, but will check again tomorrow..........I compared it to a good channel. The tech swapped out a transformer from a good channel....so that should be o.k......

 

[RoadrunnerOZ]

Is the “loose wire” that was resoldered common to other channels and in the same place from the same source?

 

[RoadrunnerOZ]

Before starting any work I photograph everything - all point to point wiring, component orientation etc. Especially when working with loom style wiring it’s easy to have broken/rotted wires in vintage gear so a preliminary photo set allows me to check the integrity of wiring with confidence, as some may break off during manipulation or board removal. A/B every solder joint - as Khron mentioned those “bridges” appear to be just solder over existing tracks anyway but you need to check the whole board against a known good one. Zoom-in photos with good light is a good way to A/B.

 

[Graham Nystrom]

Without looking at a schematic, I would say to test for any shorted tantalum (or other, but tantalum caps short out when they fail) capacitors and look for any diodes that don’t test correctly. You’ll likely have to lift a leg to test. That would be my dmm probing starting point. And resoldering the broken wire of course.

 

[telehumbucker]

Replaced three of the op amps....including the one that was melting.....and the melting/power shutoff has apparently been fixed.....Don't know why, as I'd replaced them before....but now, I'm back to the original problem. The Mic input and line input jacks are not working...Plug in a mic and there is no signal...run a line signal into the line input and there is no signal.....Run a signal into the return and it works......The loose wire from the mic/line input switch that was resoldered appears to not have been the problem...The transformer was swapped from a good channel, so that should be o.k....

 

[Khron]

Got at least a block diagram of the thing, if not a schematic? Odds are the return connects somewhere downstream of the input stage.

 

[JohnRoberts]

op amps installed backwards can sometimes look like a dead short between the + and - PS rails

[edit- you can plug in the channel strip with the op amps removed to confirm the stip does not load down the PS /edit

JR

 

[pucho812]

Replaced three of the op amps....including the one that was melting.....and the melting/power shutoff has apparently been fixed.....Don't know why, as I'd replaced them before....but now, I'm back to the original problem. The Mic input and line input jacks are not working...Plug in a mic and there is no signal...run a line signal into the line input and there is no signal.....Run a signal into the return and it works......The loose wire from the mic/line input switch that was resoldered appears to not have been the problem...The transformer was swapped from a good channel, so that should be o.k....

You need a signal generator and a scope.
Can test and probe to find where the issues lies.

 

[CJ]

If you fix the line in I bet the mic jack will work too.
You can run music through the line in if you do not have a SIG gen.
You can use a voltmeter on AC to follow the music through the preamp.
Work your way down stream and you will find the culprit.

 

[telehumbucker]

You can run music through the line in if you do not have a SIG gen.
You can use a voltmeter on AC to follow the music through the preamp.
Work your way down stream and you will find the culprit.

Got at least a block diagram of the thing, if not a schematic? Odds are the return connects somewhere downstream of the input stage.

Here are the schematics I have. My module is PRO 5A, the schematics are for revision C...which has a polarity switch......they should be basically the same.

 

[Khron]

Mic/line switch shot? Dirty send/return jack contacts (and/or faulty wiring to/from that)? I'm assuming the latter is a TRS 1/4" jack socket? Or are the send & return on separate connectors?

Do you get any signal out of the Send output?

 

[Ike Zimbel]

+1 to what Khron said. If you can get a Mic/Line signal into the module and out the Send jack, then the fault is most likely bad (or missing) "normals" on the Insert jacks. In most cases, the switching is on the Return jack.

 

[telehumbucker]

Mic/line switch shot? Dirty send/return jack contacts (and/or faulty wiring to/from that)? I'm assuming the latter is a TRS 1/4" jack socket? Or are the send & return on separate connectors?

Do you get any signal out of the Send output?

No signal out of send output on faulty module when applying signal to line in. Other modules work in the slot for the faulty one, and the faulty one doesn't work in other slots.......The patch bay/return jacks all appear to work when using functioning modules where the faulty module is usually placed. - so it appears to be something in the module...Mic/line switch seems like a good possibility.......

 

[RoadrunnerOZ]

Or the phase switch is faulty

 

[telehumbucker]

This version of the module doesn't have the phase switch option.