FBT M12X4 preamp - service manual or schematic needed

[tedsorvino]

Hi everyone.

I recently bought a used FBT M12X4 preamp unit and apparently there is a problem with one of the 2 channels (tested in every input).
I have experience on repairing stuff but i can’t find any schematic or service manual online.
Does anyone have the schematic for that unit or any idea of any similar fbt device.
The whole device is based around tl072 op amps mostly (and a few ne 5532). A few vactrols (for switching purposes), along with a comperator and a few transistors are there too.
Could also be something with cleaning, hve tried with deoxit, nothing really changed, but it would be so much better if i could follow a valid schematic.

Thanks in advance

 

[pucho812]

If you have working channels and non working channels you can compare and contrast the working vs nonworking. You can determine where the fault is and fix hopefully.
start with checking power on those channels then scope the audio path with a tone generator and scope.

 

[tedsorvino]

If you have working channels and non working channels you can compare and contrast the working vs nonworking. You can determine where the fault is and fix hopefully.
start with checking power on those channels then scope the audio path with a tone generator and scope.

Thanks for the reply. That’s what i’m trying to do but:

1) the pcb is really messy, very difficult to follow the paths, because there is a lot of switching going on (some vactrols are involved as well)
2) there are parts of the pcb where there is no signal

Checked power rails. Both good on all ICs (positive and negative around 14V).
I suspect as always the few npn, pnp transistors.

 

[NoisyIndividual]

Check all the audio op-amp output pins for DC, a faulty one almost invariably has full Power supply rail voltage as an output.

 

[tedsorvino]

Check all the audio op-amp output pins for DC, a faulty one almost invariably has full Power supply rail voltage as an output.

Thanks for the tip. I will surely do.

Ps. Checked all dual op amps.All outs and ins show 0v (except one out which was under 5mV - i guess it’s fine). All v+ and v- very normal (ne 5532 just under 14v and all tl 072 around 14.5v).

But there is an LM 339N quad comparator - op amp which had full rail voltage on 3 of its 4 outs.

Is that normal?
Could a quad comparator be used as a normal op amp?
I checked the inputs with my scope and indeed both signals are there (but the problematic one is already problematic on one of the in+ pins).

 

[NoisyIndividual]

This unit has a priority system built in, so one channel overrides the next, which overrides the next etc, so there will be a fair amount of comparing/switching going on, the comparator is probably part of that.

I think you are saying that only one side of the stereo output is working, not one or more of the input channels.
Your terminology is a little confusing.

 

[tedsorvino]

This unit has a priority system built in, so one channel overrides the next, which overrides the next etc, so there will be a fair amount of comparing/switching going on, the comparator is probably part of that.

I think you are saying that only one side of the stereo output is working, not one or more of the input channels.
Your terminology is a little confusing.

I’m trying not to be confusing. Thanks for your help so far. Imdo appreciate it.
I receive a healthy signal on one of the stereo outputs.
But the problem occurs somwhere around the op amps of the inputs. It has 8 inputs. 4 mic ones with seperate preamps ( and a priority switch for input 1) and 4 more on line ins around a qued switch.

Can’t really figure how each dual op amp is connected. Really messy pcb because of the switching.

Most dual op amps show the same signal. Healthy and weak. That’s why in need of the schematic. Otherwise i may have to spend a very serious amount of time tracing the pcb.

Should i replace the quad comparator or just check the are before it?

 

[NoisyIndividual]

So are the stereo inputs also only working on one side? As I was assuming?
If so, then it may have nothing to do with the mic inputs, and something far closer to the non working output.
This is where I am a little confused.

 

[tedsorvino]

So are the stereo inputs also only working on one side? As I was assuming?
If so, then it may have nothing to do with the mic inputs, and something far closer to the non working output.
This is where I am a little confused.

I’m afraid this is true. The xlr + side is louder than the xlr - side after the input jack.
On the in jack, on the pcb, both sides are equal.
But this may have to do with the preamp design or they may use 2 different op per xlr in, or something else.

 

[NoisyIndividual]

I still don't know exactly what you are saying.

Are you saying that the signal levels on pins 2 & 3 of the output XLR socket are different levels? And this is the same on BOTH XLR output sockets?

My understanding of "SIDE" is either Left output or Right output. I think you are calling XLR pin 2 a "side", and XLR pin 3 another "side".


Can you just clarify whether sound comes out of both XLR's or not, and whether this is on ALL the inputs, not just the mics?

 

[tedsorvino]

Yes for me pin 2 is + side and pin3 - side. And L is left channel and R is right channel. These channels go to 2 xlr outputs

So:
I have the same problem on ALL 8 inputs.
1 channel works fine the other not.
I ‘ve made the assumption that each op amp seperates the xlr sides ( + and - ) to 2 seperate channels (L and R) since the same happens to the simple rca stereo inputs.

I wish i could make things clearer.

 

[NoisyIndividual]

" I ‘ve made the assumption that each op amp seperates the xlr sides ( + and - ) to 2 seperate channels (L and R) since the same happens to the simple rca stereo inputs. "


No it doesn't do that.
The microphone signals are balanced mono. The op-amps convert this to unbalanced mono, so using just one op amp output per microphone.
These 4 mono microphone signals will at some point be connected to the stereo output bus along with the stereo channels, so equal amounts of these mono mic signals will be fed to each output bus, Left and Right.
Probably the very last stage before the Output XLR's is a simple circuit that converts these internal unbalanced signals into a balanced output on pins 2/3.

 

[tedsorvino]

" I ‘ve made the assumption that each op amp seperates the xlr sides ( + and - ) to 2 seperate channels (L and R) since the same happens to the simple rca stereo inputs. "


No it doesn't do that.
The microphone signals are balanced mono. The op-amps convert this to unbalanced mono, so using just one op amp output per microphone.
These 4 mono microphone signals will at some point be connected to the stereo output bus along with the stereo channels, so equal amounts of these mono mic signals will be fed to each output bus, Left and Right.
Probably the very last stage before the Output XLR's is a simple circuit that converts these internal unbalanced signals into a balanced output on pins 2/3.

Even your absolutelly correct assumption seems slightly simpler than the stages this preamp takes the signal through.
At least 4 opamps and the comparator are involved. The main problem is that the op amps on the pcb haven’t got an obvious order (series - parallel etc).

I think it’s either the comparator or certain components around there. Hopefully…
I don’t dare to throw away the vactrols because of cost, and i have no reason to be suspicious of the op amps.
If nothing change i will remove the transistors for testing.