Vintage 70's Midas Channel Strip Problem PRO5A

[telehumbucker]

Hope these help. In the third picture you can see the two layers of board, the top one folds back revealing the eq pots.

 

[telehumbucker]

Lightened up the images.......

 

[telehumbucker]

Two pict. First one is the fader below which is the treble pot....I pulled the fader off to the right to reveal the treble pot in the second picture, with the three solder joints - on top of the blue plastic......with the blue rubber sleeves pulled back. Can't see an obvious break in the wires.

 

[Ike Zimbel]

Could be at the other end of those wires, too. I think you will have to pull off some more sleeves and keep looking. You could also try a light tug on each wire. I've had a couple of older units in the past few years where a wire had broken off, but was so stiff from age it just stayed right where it was, so it Iooked ok, but wasn't soldered. Also, those sleeves with the drops of goo on them should definitely be removed.

 

[ccaudle]

almost thumping sound

There is a behavior sometimes referred to as "motorboating" which can be a little like that. It is caused by a feedback path through the power supply between stages. On a previously working design it can indicate that the electrolytic caps have dried out and are no longer effective bypass capacitors.
Do any of the electrolytic caps have bulged tops?
Do you have any way to measure the capacitance on the power supply rails?

 

[telehumbucker]

There is a behavior sometimes referred to as "motorboating" which can be a little like that. It is caused by a feedback path through the power supply between stages. On a previously working design it can indicate that the electrolytic caps have dried out and are no longer effective bypass capacitors.
Do any of the electrolytic caps have bulged tops?
Do you have any way to measure the capacitance on the power supply rails?

That thumping sound isn't happening now....When I turn up and peg the treble there is just one pretty soft thump...but it's not motor boating.

 

[r2d2]

Sorry if the post do not add more help to your issue,
(that I hope will be fixed asap !!)

being the first time i see this cool Midas "piece",

with "little" and relatively simple pcb topology,
it is single side type ?

The master out is with transformers or ic ?

it really invites a lot to know which would the ch strip cost of the components set
including the ch strip metal bar, ready for the diy assembly,

also replacing the fader in the eq section with a less expensive potentiometer,
freeing up space for possible other functions,
or further compact the strip as well,

thanks in advance for any about,
and cheers.

 

[envelope]

I am in the middle of a PR05 console restore, if I can some up with any productive insights for you, I will post. Do you happen to have an extender card so that you can take measurements of the energized circuit?, Or if not, do you have a test jig to 'power it up with audio in/out' on your workbench?

 

[telehumbucker]

Could be at the other end of those wires, too. I think you will have to pull off some more sleeves and keep looking. You could also try a light tug on each wire. I've had a couple of older units in the past few years where a wire had broken off, but was so stiff from age it just stayed right where it was, so it Iooked ok, but wasn't soldered. Also, those sleeves with the drops of goo on them should definitely be removed.

Looked closer at wires on bass pot, treble pot, and cut and shelf switches.......couldn't see any breaks, the rubber is still soft, wires in good shape......did see that the pegging stops when the shelf switch at the treble knob is at 6khrz or the cut switch for the bass pot is at 120 hrz......or the eq section is shut off with the eq switch.

 

[telehumbucker]

I am in the middle of a PR05 console restore, if I can some up with any productive insights for you, I will post. Do you happen to have an extender card so that you can take measurements of the energized circuit?, Or if not, do you have a test jig to 'power it up with audio in/out' on your workbench?

I do have an extender. I will have to have a tech do the testing as I am not that advanced........

 

[telehumbucker]

Sorry if the post do not add more help to your issue,
(that I hope will be fixed asap !!)

being the first time i see this cool Midas "piece",

with "little" and relatively simple pcb topology,
it is single side type ?

The master out is with transformers or ic ?

it really invites a lot to know which would the ch strip cost of the components set
including the ch strip metal bar, ready for the diy assembly,

also replacing the fader in the eq section with a less expensive potentiometer,
freeing up space for possible other functions,
or further compact the strip as well,

thanks in advance for any about,
and cheers.

I believe the master channels have transformers.

 

[SIXTYNINER]

Sorry if the post do not add more help to your issue,
(that I hope will be fixed asap !!)

being the first time i see this cool Midas "piece",

with "little" and relatively simple pcb topology,
it is single side type ?

The master out is with transformers or ic ?

it really invites a lot to know which would the ch strip cost of the components set
including the ch strip metal bar, ready for the diy assembly,

also replacing the fader in the eq section with a less expensive potentiometer,
freeing up space for possible other functions,
or further compact the strip as well,

thanks in advance for any about,
and cheers.

+ 1
.... and replacing that fader with a pot
more room for a second parametric eq section .

 

[RoadrunnerOZ]

It seems that you have a couple of separate issues but suspicion always falls on old wiring, old switches that have not been used for a long time, dry solder joints on the PCB’s, failed op-amps, bad connection between IC’s and their sockets from oxidation - although you say you’ve swapped out IC’s from a working channel there may be a problem with cracked old sockets failing from heat or bad solder connection from the sockets to the PCB - I have serviced many consoles that have the IC socket solder points to PCB going bad from heat conduction through the pins.
You really need to meter all the switches to make sure all pole pairs are giving a dead short in the on position both ways.
Try inserting a patch lead between insert send and return in case the switch normal link in the patchbay is not connecting. Have you tried sending from the dodgy channel’s insert send to another channel’s insert return feeding say a mic into the not working channel input?
No signal from both mic and line tends to point to a small section between where they mix and the insert send. The pegging of the meter and thump when you reach a certain point on the high EQ sounds like an op-amp failure or associated supply voltage missing.
Do you have a schematic of the channel card?

 

[JohnRoberts]

While rare I have found wires with breaks in the middle underneath insulation jacket so hard to find.

If the VOM reveals an open circuit replace the wire.

JR

 

[RoadrunnerOZ]

While rare I have found wires with breaks in the middle underneath insulation jacket so hard to find.

If the VOM reveals an open circuit replace the wire.

JR

Those old wires can rot from the cut-off point in the insulation back into the wire (where wire is exposed and soldered) just back from there at the point of flex all the corroded wire strands break off - this can travel all the way through the wire as well - easy to see if the wire is all black or green oxidation - it’s almost impossible to solder that once it’s like that.

 

[telehumbucker]

It seems that you have a couple of separate issues but suspicion always falls on old wiring, old switches that have not been used for a long time, dry solder joints on the PCB’s, failed op-amps, bad connection between IC’s and their sockets from oxidation - although you say you’ve swapped out IC’s from a working channel there may be a problem with cracked old sockets failing from heat or bad solder connection from the sockets to the PCB - I have serviced many consoles that have the IC socket solder points to PCB going bad from heat conduction through the pins.
You really need to meter all the switches to make sure all pole pairs are giving a dead short in the on position both ways.
Try inserting a patch lead between insert send and return in case the switch normal link in the patchbay is not connecting. Have you tried sending from the dodgy channel’s insert send to another channel’s insert return feeding say a mic into the not working channel input?
No signal from both mic and line tends to point to a small section between where they mix and the insert send. The pegging of the meter and thump when you reach a certain point on the high EQ sounds like an op-amp failure or associated supply voltage missing.
Do you have a schematic of the channel card?

Thanks for the response.
Here is what I have. I also attached the original PDFs which may have a better resolution.

 

[RoadrunnerOZ]

Thanks for the response.
Here is what I have. I also attached the original PDFs which may have a better resolution.

Got them ok. Looks like there’s nothing between IC1 and the input except switches a couple of chokes and the transformer - with caps across everything - I’d check the phase invert switch is working (with a meter) see if you get signal with invert on, try with a dynamic mic with phantom off in case there’s a short of the 48V to one of the balanced input lines. As far as the weird behaviour of the high EQ I’d be looking at IC1 - you said you swapped these but check that you did swap it plus check the integrity of the solder joins to IC1 socket and also the surrounding print circuit tracks and solder joints - if the DC supply to the chip goes one-sided you’ll get crazy stuff happening.

 

[RoadrunnerOZ]

IC1 looks like it’s hiding under a shield plate

 

[telehumbucker]

Got them ok. Looks like there’s nothing between IC1 and the input except switches a couple of chokes and the transformer - with caps across everything - I’d check the phase invert switch is working (with a meter) see if you get signal with invert on, try with a dynamic mic with phantom off in case there’s a short of the 48V to one of the balanced input lines. As far as the weird behaviour of the high EQ I’d be looking at IC1 - you said you swapped these but check that you did swap it plus check the integrity of the solder joins to IC1 socket and also the surrounding print circuit tracks and solder joints - if the DC supply to the chip goes one-sided you’ll get crazy stuff happening.

Will do......thanks for the advice, time, etc......much appreciated.

 

[telehumbucker]

IC1 looks like it’s hiding under a shield plate

Switched out the op-amps with all new ones, including V1 under the plate, and it appears the treble, eq, issues have stopped, but can't definitively determine that as still no input signal from the line or mic inputs. Unfortunately when swapping out the op-amp under the shield plate the wire connecting the plate to the board broke, so have to get that resoldered......it looks like a ground wire....was able to test the eq with the new op amps before that happened, and there was no meter pegging.....

.My version of the PRO5 module is the A version and the schematics I posted are for the later C version.....one difference is mine doesn't have the phase inverter.....they seem pretty identical other than that. Is it safe to connect, power up the module in the board with the wire from the shield plate disconnected from the circuit board? Have included picts of the plate situation. The bottom picture shows where the wire broke off from; It's the slightly blurry stub just above the white V like figure, and just to the left of the red wire.....