Modifying amplifier trigger board PCB and deriving 12V supply

[tiff_lee]

Hello all new member seeking some advice/help although I don't expect to be spoon fed just nudged in the right direction as i'm looking to learn

Right the problem, my home cinema audio processor (Audiolab 8200AP) has a trigger system to turn on additional amplifiers/gadgets whenever it's turned on (via 3.5mm mono/stereo jack cable), the problem with this trigger is that instead of a 12V DC output which seems to be standard amongst most AV equipment it uses a open/close circuit system whereby it shorts the 'pin' (would be left audio on a stereo jack) and the sleeve/ground. To make matters more confusing audiolab produce their own range of amplifiers some of which use a 12V trigger but only 1 which uses this open/close system so I have 3 output triggers and nothing to use them, cheers Audiolab.

The trigger board itself is a standalone PCB as can be seen here




As you can see there is 3 x 3.5mm sockets, when the relays energise 2 pins of the socket are shorted hence the open/close loop. Now my idea was to break the circuit between the socket and the relay (supply to sleeve of mono jack) earth the socket side so the sleeve is always earth and then route a 12V supply to the other side of the circuit so when the relay energises the pin of the socket/jack becomes live.

Here is the service manual diagram and my proposed mod (i'm not sure if con1/3 is the tip/sleeve but will be easy to find out with a meter the image is just to illustrate my idea).


Is it even possible to break the tracks/trace on a laminated PCB? or will I need to physically disconnect the socket/relay from the board and mod it? no great shakes just looking at options.

The next step would be deriving a 12VDC supply within the unit to supply this circuit but i'll leave that query on the backburner for now and try to solve this problem first to avoid confusion.

 

[chuckey]

If you are careful, modifying this board would be doable. Your suggested circuit modification won't work, you show the +12 shorted to earth. What I would do is to cut the track between the trig_2bx point and the relay contacts and feed the +12 V in to the relay contacts, and the +12V earth to the connector earth. If the current drain on the +12V trigger line is very low, then you could generate the +12V from a 5-> 12V converter. It could actually work with the +5V, depending what the input circuit it was that the line is feeding.
Frank

 

[tiff_lee]

Cutting the track between trig_2bx and the relay contacts isn't that the same as what i've illustrated but on the other side of the relay contacts? Ah unless you're referring to the pinout of the 3.5mm socket. I triggered my power amps on using a 12V supply and they draw 5.5mA.

What would be the best way to cut the track on a board like this?

- - - Updated - - -

Just had another look at this board and checked the actual tracks that need to be modified, one of the three triggers has the track that needs modifying running on the underside of the board which would make it alot easier to mod as that's all there is.

 

[chuckey]

Ah yes, that diagonal line means a cut track! Traditionally the sleeve of a jack is the earth, I think in your scenario, you are actually putting the negative side of the 12V out to the tip, with the positive side earthed to the sleeve via the relay contacts?
Frank

- - - Updated - - -

To cut the track, use a craft knife cut through the track at two places a couple of millimeters apart and peel away isolated section. Use a PCB drill with a burr annd grind away a bit of track. Gently with a drill very slightly larger then the track width, put a countersunk hole in the PCB, so the copper as just broken (can't be done if track is too wide).
Frank

 

[tiff_lee]

Yes, you'll have to forgive my crude microsoft paint edited diagram!

The last picture I uploaded of the underside should be correct I did check with a 3.5mm cable and multimeter, as the jack is inserted the contact closest to the edge of the board is the sleeve and the one furthest in is the tip.

Tools wise I don't actually have a PCB drill in fact i'll need to google that to even see what one is! Could I perhaps cut/grind away part of the track with a dremel? not the most precision tool I know.

Edit: With regards to the 12V supply and what you said earlier about boosting the 5VDC I guess I could use something like this
**broken link removed**

it would cost me more to order the parts to make my own! it's even cheaper straight from China, £1.84

As I mentioned earlier when I triggered one of my amps (Audiolab 8200MB) with a 12V supply the current draw was 5.5mA, the amps also have an "Out" trigger as well so they can be daisy chained but i'm pretty sure once turned on they provide the 12V supply for this rather than drawing from the processor. I'd have to take another look at the service manual/rig the amps up with my meter to check this.

 

[chuckey]

Yes the Dremel is the tool most often used, you do need a steady hand, the bit tend to run away under its own steam! - Practise on one of those big blank areas of the PCB first.
It could be worth checking to see if any of the amplifiers can be triggered by 5V, 12 V at 5mA is not enough to power a relay, so the input circuit must be a transistor /IC, which depending on the circuit could be powered by anything over .8V. 12V is just used for historical reasons or to power stuff down really long lines like at a rock concert or opera house.
Frank