Acoustic guitar preamp not working...

Post by **RichardH** » Tue Jan 16, 2018 10:04 am

The preamp in a friend's acoustic guitar doesn't seem to work. It's a B-Band T35, and I have tried fresh battery etc.

The tuner doesn't light up (I have tested plugging a cable in to make sure it doesn't need a cable in to operate), and no sound comes out through an amp.

I suspect it is just a matter of replacing the unit, but anything else to try before we do that?

Mottlefeeder · Post by **Mottlefeeder** » Tue Jan 16, 2018 8:59 pm

RichardH wrote:The preamp in a friend's acoustic guitar doesn't seem to work. It's a B-Band T35, and I have tried fresh battery etc.

The tuner doesn't light up (I have tested plugging a cable in to make sure it doesn't need a cable in to operate), and no sound comes out through an amp.

I suspect it is just a matter of replacing the unit, but anything else to try before we do that?

If you have access to a multimeter, you could check the resistance between the two battery contacts (with and without a guitar lead plugged in) An open circuit would indicate a faulty connection in that area, and no change when you plug a lead in might indicate a fault in the output jack socket.

David

Post by **RichardH** » Wed Jan 17, 2018 9:40 am

Hmmm - that was helpful David.

I have pulled the battery box/jack socket assembly.

With the multimeter I am getting a connection between B- from the battery box and the GND of the 4 pin connector (which goes off to the preamp etc).

I am not getting anything from B+ on the battery and B+ on the 4 pin connector, whether or not I have a jack plug in the socket (I am assuming that the battery is switched out when no lead is connected).

When the jack is disconnected we get

TAG1 > BATT-
TAG1 > B-

TAG2 > OUT

TAG3 > SW

BATT+ > NOTHING

When a jack socket is connected, I get signal thus:

TAG1 > BATT-
TAG1 > B-
TAG1 > SW
TAG1 > TAG3 (if mono jack inserted)
TAG1 > jack sleeve

TAG2 > OUT
TAG2 > jack tip

TAG3 > SW
TAG3 > TAG1

TAG3 > jack ring (if stereo jack inserted)

BATT+ > NOT CONNECTED

...so it would seem that the jack is not switching the +ve supply through to the 4 pin connector.

Post by **RichardH** » Wed Jan 17, 2018 12:46 pm

...I ran a lead round from the BATT+ to the B+ terminal on the 4 way connector, and it works! It seems that the tuner operates with the lead disconnected, but I think that is switched at the preamp end, so when tuner off the battery is disconnected.

Result! Thanks to David for encouraging me to have a tinker!

oxblood · Post by **oxblood** » Wed Jan 17, 2018 4:58 pm

Sounds like repeated plugging in/out stresses may have caused a hairline crack in one of the PCB tracks - which you've now bridged wid yer wire (sez he helpfully, after the event

).

Mottlefeeder · Post by **Mottlefeeder** » Thu Jan 18, 2018 12:03 am

RichardH wrote:...I ran a lead round from the BATT+ to the B+ terminal on the 4 way connector, and it works! It seems that the tuner operates with the lead disconnected, but I think that is switched at the preamp end, so when tuner off the battery is disconnected.

Result! Thanks to David for encouraging me to have a tinker!

You're welcome. Glad you got it sorted.

tauzero · Post by **tauzero** » Thu Jan 18, 2018 1:03 pm

The normal way of switching active preamps of all types is to connect the battery negative to the ring and the preamp negative/ground supply to ground, so the negative side is switched and the positive side is a permanent connection - or, in this case, permanent disconnection.

Post by **RichardH** » Fri Jan 19, 2018 12:59 pm

Yes, that is what I realised in my tinkerings. Just wondering as the tuner operates without the lead plugged in, but I suspect that is intentional, and the battery is only being used when the tuner is switched on.