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Hi everyone. I recently found 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 device is based around tl072 op amps mostly (and a few ne 5532). There are some vactrols (for switching purposes i guess), a competator. And few transistors.

Could also be something with cleaning but it would be so much better if i could follow a valid schematic

Thanks in advance

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Boswell Wed, 04/10/2024 - 06:05

If it's the box I'm thinking it is, the Vactrols are for automixing, i.e. suppressing channels that are not the loudest.

What I would do is send a 1KHz sinewave at about 20mV from a generator to the mic input on channel 1 (panned centrally) and then chase through where it gets blocked on its way to the output. Use the working channel as a comparison.

Check the voltage across the diode pins on the pairs of Vactrols, comparing with the working channel as you go. The diodes should read something like 1.5V d.c. for a constant input level from the generator and then show a small variation for a step change in input level (indicating a larger current change). If there is a very small voltage (<700mV), then the automix controller is faulty. If there is a relatively large voltage (e.g. 10V), then there is a break in the diode chain, which could indicate a blown LED in one of the Vactrols.

tedsorvino Wed, 04/10/2024 - 06:30

That’s very interesting Boswell. Thanks for the reply.

Of course i tried so far with the 1khz sine signal but it’s pretty tricky to trace that pcb. 
It seems like the initial op amp nakes the balanced - unbalanced and somewhere towards the out it becones banced again. Many op amps, doingbthevsame thing.

There is also an LM 339 where strange things happen - i suspect that ic or around that area (that’s where the vactrols are too) and there are the ever suspicious pnp -npn transistors that i may have to remove for testing (same area too).

The op amp pins and voltages are great. 0v on all in-out pins stable -14v and +14v on - and + voltage sides

Boswell Wed, 04/10/2024 - 09:54

I think you can be a little more confident about what you see on the board. Leave aside the balanced-unbalanced conversion, and look at the unbalanced section between the conversion. However, before you do that, it's worth checking that the balancing amp on the non-working output is not the cause of the lack of signal there.

The LM339 is a quad comparator, and my guess is that it is used to compare the smoothed rectified output of a channel with that from the current active channel. If it is greater for a given length of time, then the current through the Vactrol diode for that channel is increased, lowering the resistance of the Vactrol's light-sensitive variable resistors for that channel and hence increasing its output. Conversely, the diode current of the previous channel is reduced, raising the resistance of that channel's Vactrol and hence reducing its output. The Vactrol resistors would be used, for example, as the signal inputs to a summing amp. It's possible to achieve in excess of four decades of resistance value change by varying the Vactrol diode current. Zero diode current results in an output resistance of many MegOhms.

If you are sure that the behaviour you are seeing is due to a fault, then a signal comparison between the faulty channel and the known working channel can be very informative.

As an aside, Vactrols are unfortunately no longer available in most countries as they contain very small amounts of the element cadmium. This falls foul of waste material regulations for end-of-life disposal. Go figure.

tedsorvino Wed, 04/10/2024 - 10:20

Thanks Boswell for the very informative advice.

i think i have to take it in a more step by step way once again i think. 
i will keep your advice in mind and inwillbtry to see if i will be able to figure out the vactrols workings.

Actually i’ve used photoresistor vactrols in other circuits ( vibrato, filter) but they cost for good things. The good thing is that the ones used in the preamp are cheaper ones, easy to find ones.

so i will first try the opamps, then the transistors, then the comparator and finally the vactrols. 
This is a link of the manual to see the block diagram. I suspect seriously the muting circuits.…

Boswell Thu, 04/11/2024 - 05:53

I've had a look at the manual and the block diagram you linked in your last post. I have to say that it does not appear to describe the model I was expecting from your initial description, i.e. a manual mixer with auto-mixer capability. However, the text says "each channel has its own volume control and 'VOICE' function to adjust speech signals to optimise intelligibility". This may well be the feature that I have been describing as auto-mixing, which would more accurately be auto-switching in this model.

In the block diagram, there are conventional pots shown, marked "volume". There are also wavy lines with a slanting arrow through them, which could well be the Vactrols. The block diagram shows no circuit block to control the diode currents through the Vactrols. Unless it's a part of the box marked "MUTE", the manufacturers may have regarded that as proprietary information and not included it.

I still think that the course of your investigation should be as I mentioned earlier, but with one initial difference. This is to see if there is signal at both the "REC" phono/RCA plugs, or only at one of them. If you find only one channel active here, there is no need to check the balanced driver output. Proceed from there backwards towards the inputs, making a note of the diode voltages on the Vactrols. I would expect a low voltage (<700mV d.c.) on the Vactrols attached to input channels that are not being fed with signal.

tedsorvino Thu, 04/11/2024 - 07:01

As i’ve told it’s way more complicated than a simple pre-amp because of the muting and the switching but i will follow your advice and hopefully i will come to a conclusion….how much easier things would be with a schematic

Boswell Thu, 04/11/2024 - 07:50

Well, I think the area you need to investigate can be shrunk by applying some constructive thinking.

The block diagram shows a MONO switch. If setting that switch to the mono position results in both outputs producing signal, then the whole of the output section feeding the XLRs must be working.

If the STEREO OUT phono/RCA sockets both produce signal, you really have narrowed down the fault area to not very much, possibly only the contacts on the MONO switch. The caveat here is that the stereo outs are shown as having Vactrol-set level controls. These are not involved in the signal flow to the main outputs, but unless the secret mute circuitry is allowing signal to flow to the stereo outs, they will show no signal.

There is an additional secret box labelled BASS HIGH MASTER. This is probably the tone control circuitry, and your fault may well be in one of the two routes through it (L or R). Try increasing the level of your mono signal through the mixer and see if both PEAK leds activate together.

tedsorvino Thu, 04/11/2024 - 11:08

Thanks for the advice Boswell.
i will try again as soon as possible - sometime next week- and i will keep you posted.

Bass high master is the tone control circuit and it seems to work well.