Vintage 70's Midas Channel Strip Problem PRO5A

[telehumbucker]

My PRO5A Midas channel strip is having some issues. It is not receiving a signal from the Mic or Line input. There is something wrong with the eq section. When the treble knob is turned all the way up the led meter pegs in the red......if the treble is turned down the led meter goes down. If the bass knob and treble are worked together, different combinations will peg the meter, i.e., bass all the way up treble down.....if the high 120 hz cut is engaged the pegging stops.......The input gain has no effect on the eg pegging led meter issue. When the meter is pegged there is an odd almost thumping sound that is picked up on the monitor. Sometimes there is a very slight mic signal, if the gain is all the way up and the treble is up, but not enough to peg the meter. Is there a way to clean these types of knobs, switches with deoxit or something similar.......hopefully you can tell from the pictures the types they are. Any ideas welcome.

 

[NoisyIndividual]

My standard procedure for tackling Op-amp circuits is first to measure voltages at the Op-amps.
Make sure you have +/- 15v, 18v, or whatever it is meant to be at the power supply pins. (8 and 4 for typical dual op amps)
Then measure each op-amp output pin. They should all measure pretty much 0v. (pins 1 and 7)
Any output measuring full scale deflection almost always signifies a dead Op-amp.
Similarly, if each output is at 0v, the op amp is good.

All this can be achieved in one minute.

 

[Ike Zimbel]

Is this channel strip in a console?

 

[telehumbucker]

My standard procedure for tackling Op-amp circuits is first to measure voltages at the Op-amps.
Make sure you have +/- 15v, 18v, or whatever it is meant to be at the power supply pins. (8 and 4 for typical dual op amps)
Then measure each op-amp output pin. They should all measure pretty much 0v. (pins 1 and 7)
Any output measuring full scale deflection almost always signifies a dead Op-amp.
Similarly, if each output is at 0v, the op amp is good.

All this can be achieved in one minute.

Thanks for the reply. I swapped out all the op amps from a channel strip that is working. Not sure if that covers the op amp potential issues...

 

[telehumbucker]

Is this channel strip in a console?

Yes.......Hadn't used that channel in a while.....

 

[NoisyIndividual]

OK, so my next procedure would be to inject a signal into the Insert jack socket and see if the channel strip works with that.
Most channel faults seem to be at the input circuitry before the Insert point.

 

[Whoops]

Thanks for the reply. I swapped out all the op amps from a channel strip that is working. Not sure if that covers the op amp potential issues...

Well, you can swap OpAmps but if they aren’t receiving the voltage supply then they will not work.
Do as was suggested before, measure positive and negative voltages at the correct pins in all the opamps

 

[telehumbucker]

OK, so my next procedure would be to inject a signal into the Insert jack socket and see if the channel strip works with that.
Most channel faults seem to be at the input circuitry before the Insert point.

Inserted a signal and it works.

 

[telehumbucker]

The mic plugged into the pre-fader insert produces a normal clean signal. However, if I turn the treble up bast 3/4 the led meter still pegs. If I switch the 120 hz cut on the pegging sometimes, but not always, stops....also if I switch to eq out the pegging stops. The mic input still does not get a signal, nor the line input. I've never measured op-amp voltage as was suggested, so will have to wait on that, till I have someone else do it or I get a better understanding of how to do it. Does the insert send success have any bearing on the op-amp situation?

 

[NoisyIndividual]

The Insert point should be at line level, not Mic level, so try it with a line signal, not a microphone.

The EQ seems to be after the Insert point, where it normally is, but it sounds like you are describing 2 seperate faults - the EQ feeds back AND there is no Mic or Line input getting through to the Insert point.

You wouldn't normally find 2 separate faults occuring at the same time but it is not impossible.
You still need to establish you have the correct power supply rails before going any further.

Quite often, the power supply rails come into the PCB via 2 resistors, often 10 ohms, that are often stood off the PCB so they can burn out safely in the event of a catastrophic failure. Look for these resistors, and you should be able to measure +/- 15v, 18v, or whatever the power supply uses for the audio rails, on both sides of them.
If one or both are burnt or discoloured then therein is a large clue.

 

[jensenmann]

It´s quite likely that there´s a (or more) wire(s) broken. The wiring isolation hardens under the rubber tubes and then breaks the copper. That´s a typical vintage Midas problem.

 

[Dreams]

It´s quite likely that there´s a (or more) wire(s) broken. The wiring isolation hardens under the rubber tubes and then breaks the copper. That´s a typical vintage Midas problem.

This. Look specifically at the treble and bass pots, likely there's a broken wire there. Had this problem before, turned out to be a leg had broken on the treble pot. Was able to tack the wire back on and the eq problem cleared up. The input signal problem will likely be something similar

 

[telehumbucker]

The Insert point should be at line level, not Mic level, so try it with a line signal, not a microphone.

The EQ seems to be after the Insert point, where it normally is, but it sounds like you are describing 2 seperate faults - the EQ feeds back AND there is no Mic or Line input getting through to the Insert point.

You wouldn't normally find 2 separate faults occuring at the same time but it is not impossible.
You still need to establish you have the correct power supply rails before going any further.

Quite often, the power supply rails come into the PCB via 2 resistors, often 10 ohms, that are often stood off the PCB so they can burn out safely in the event of a catastrophic failure. Look for these resistors, and you should be able to measure +/- 15v, 18v, or whatever the power supply uses for the audio rails, on both sides of them.
If one or both are burnt or discoloured then therein is a large clue.

Thankyou for the detailed reply. Sorry I didn't make it clear, but the mic was going through a mic preamp that was then fed into the insert send, so was a line level signal. Oddly enough, the insert send appears to bypass the eq section, as I tested this on a properly working channel......

 

[telehumbucker]

This. Look specifically at the treble and bass pots, likely there's a broken wire there. Had this problem before, turned out to be a leg had broken on the treble pot. Was able to tack the wire back on and the eq problem cleared up. The input signal problem will likely be something similar

Thanks for the suggestion. The problem does appear to be directly related to the treble eq.......How can I access the eq pots on the PRO5A? Does one have to fold back the top layer- section of the board (which is held in place and thus tugs on the wires when opened up), or can the channel tbe disassembled some other way and the eq pots accessed from behind?

I had been warned that pulling back that top plate can lead to broken wires - it also makes it more difficult to reach some of the op-amps. There are a lot of screws on that channel, but it isn't clear what they are holding in place......and whether or not their removal allows for easier access to the eq pots.

 

[Ike Zimbel]

Can you post some better pics? The ones you posted up-thread are very low res.

 

[telehumbucker]

Can you post some better pics? The ones you posted up-thread are very low res.

Will do later today. Hopefully they will be better.

 

[Philbo King]

Soumds to me like you have oscillation in the EQ going on

 

[Ike Zimbel]

Thanks for the suggestion. The problem does appear to be directly related to the treble eq.......How can I access the eq pots on the PRO5A? Does one have to fold back the top layer- section of the board (which is held in place and thus tugs on the wires when opened up), or can the channel tbe disassembled some other way and the eq pots accessed from behind?

I had been warned that pulling back that top plate can lead to broken wires - it also makes it more difficult to reach some of the op-amps. There are a lot of screws on that channel, but it isn't clear what they are holding in place......and whether or not their removal allows for easier access to the eq pots.

I don't think you will need to do that. To start, I would start by lifting the blue rubber Hellerman sleeves and see if any wires have broken off. There's a thread here somewhere about those sleeves becoming conductive as they age...you can test for this by simply probing them with your DMM, set to the Ohms range. With both probes on the same sleeve, they should read open circuit. If they give you any reading at all, they've gone bad and should be removed. Just slice them open with a sharp blade and toss them. they do not need to be replaced.

 

[telehumbucker]

Here is a video of what is going on. You can see the pegging as the treble is turned past 3:00.......The pegging stops when I switch off the eq....

 

[Ike Zimbel]

Here is a video of what is going on. You can see the pegging as the treble is turned past 3:00.......The pegging stops when I switch off the eq....

I meant a better picture of the module. I totally believe you about the fault! Need a better pic of the module to be able to say "Go here, and look at this..."