Behringer Ultratone K900FX

[dwatki]

Hi, I am new to this Forum.
I am looking for a schematic for a Behringer Ultratone K900FX, made about February year 2010.
It is blowing the 1.6 Amp power input fuse.
Behringer most unhelpful and want more than the item cost just to examine it, quote 30 day warranty.
Any help or assistance most welcome.

[Enzo]

Probably a bad power amp IC, TDA7293. Output device shorted is almost always first thing on the list of fuse blowers in any amp.


Until we know the amp is stable and working, DO NOT connect a speaker or load.

To save fuses and prevent further damage, look up "light bulb limiter" and make one and use it. Very simple.

Other than the IC, possible shorted rectifier in main power supply.

It is very unlikely, but the power transformer could be shorted. UNplug the transformer wires from the circuit board, leaving only the mains wiring. If it still blows fuses, it is bad.

[dwatki]

Hi Enzo, thanks for swift reply

Looks as though I have serious problems. I have unplugged the transformer wires and it still trips.

I suspect the transformer is duff AND expensive !!!!

There are no signs of arcing or such on the PCBs. When the power is connected to the equipment the LEDs come on briefly, then it trips.

I have connected a fast 2 Amp circuit breaker in the supply input to save blowing fuses. This is faster than the 2 Amp fuse that I temporarily uprated to.

Thanks a lot, I will go away and have a cry.

dwatki

[Enzo]

Seriously, look up light bulb limiter. It is just a light bulb wired in series with your amp. It is a very simple, very effective way to not blow fuses and also n0t send shok waves of failure current through the amp.

If the secondary wires are sticking in midair and the fuse goes, that is a bad sign. But one more step, just to be sure, disconnect the transformer primary and fire up the empty chassis. It is possible your mains primary wiring is shorted, or perhaps one of those capacitors across the mains has shorted. And the other half of the equation, clip mains to the primary wires directly, with the transformer no longer connected to the amp in any way. If it still draws excess current, it is gone. This is where the bulb limiter would shine. (pun intended)

[dwatki]

Thanks Enzo, I will do as you suggest. Had a bit of trouble sourcing a 100W Tungsten Bulb, but eventualy found one. I will investigate the problem futher in the next few days.

I contacted Behringer Technical Support for info. on the Toroidal Transformer (Part ID TR-TOR-LPA190-EU) and received the following reply:-

‘ I am unfortunately unable to provide you with schematics, information regarding or replacement internal components, or repair instructions.

Behringer's non-flexible policy states that we only release schematic diagrams to our Authorized Service Centers.

Adhering to this policy guarantees that all repair work is carried out to factory specifications, and that warranties are maintained where applicable. It also ensures quality work by factory-authorized technicians, and keeps us away from any liability issues. ‘

Does anybody know the rating of the beast? Looks to have two secondaries one centre tapped, 0-5v and 15-0-15v perhaps?

Thanks again, will post how things pan out.

[Enzo]

I understand, I am a service center, and I am bound by agreement not to distribute their schematics. But I can answer questions.

I looked for this model on Elektrotanya, they are often a good source for Behringer schematics. Didn't find it.

Two windings, one is for the +5v supply with enough current to run the DFX circuit and the LEDs. 8-10v maybe? Other center tapped winding for the main power rails, and the +/-15 are derived from those rails. I don't know the voltage of those main rails, it is not specified on the drawings, so 30v is a reasonable estimate, so maybe 20-0-20vAC? 24-0-24? And enough current for the output power of the amp.

[dwatki]

Hi, some progess but not good news from my point of view.

I built a bulb limiter and tested the transformer. Performed the tests in darkened room, with strange results. The limiter bulb illumunates fully with no secondaries in circuit, bad news. However I could see little sparkles of light inside the toroidal winding, hmmm.

Prior to testing the primary winding was about 85 Ohms (low I thought) on a low voltage multimeter, after test with current limiter it is now in the order of 13 kOhms. The input coil is now well cooked. Outputs resistances remained at about 2 Ohms and 0.9+0.9 Ohms

I have looked for a replacement, but can not find anything like a close match to i/p 220v, o/p 0-8v and 24-0-24v. The Manufacturer’s Part ID TR05-0022 (R1L) and Behringer Part ID TR-TOR-LPA190-EU Can anybody decipher these codes or suggest where I might find a similar beast.

Alternatively, it strikes me I could use two Toroids, one for 0-8v op and the other with two 0-24v secondaries wired in series to produce 24-0-24v output. Has anyone experience of trying this? Comments most welcome.

[Enzo]

Behringer Part ID TR-TOR-LPA190-EU

TRansformer
TORoidal
for LPA190 power amp

That is what you'd have to get from a Behringer parts supplier. Our friend John Frondelli's DBM Pro Audio is one such place:

Welcome to dBm Pro Audio/Music Services


If you want to roll your own, there is nothing cosmic about a transformer. You need the voltages your circuit wants, and they need to provide sufficient current for the power needs.

[dwatki]

Thanks Enzo

Quick update. Costed a replacement Transformer and would be cheaper to buy a complete replacement Amplifier, which is what we did. Not another K900FX.

Not to be beaten, however, thought I would try the Two Toroidal approach. I found a suitable 18-0-18v in a box and got a 9v 60VA Toroid from Farnell UK complete with fixings for £14.00. Used the original mounting for the 18-0-18v and drilled a new hole for the 9v Transformer in spare space on the other side f the chassis. Quick bit of rewiring on the Supply side to the LPA190 Amplifiier Board and it all looks fine now.

I think the original got zapped by a mains surge at a gig in rural France.

Thanks for all your help. My daughter now has a Backup.

[gcmillgc]

Enzo, I have a K900. Works fine and has for over 3 years. Early-on, channel one went out with just distortion. A mic into the XLR jack works for about 60 seconds but that then fizzels out in distortion. I would like to fix that channel one problem. I am thinking probably an op-amp has gone bad on the input. I am an experience electronic technician so I can replace an op-amp no problem. Before I crack it open, can you tell me if the inputs/pre's use standard op-amp chips (I have a lot of these, 8 pin DIPS, utra low noise, 4558's, etc). Also before I crack it open is the preamp board surface mount technology? Not sure if I can fix it if that's the case. I certainly don't need a schematic if it is an op-amp on the front end, been there and done that. You advice would be appreciated. Thanks! G

[Enzo]

Hi. Almost all the circuitry is surface mount. The dual op amps are all common types, and they also like to use quads, all of which are TL074 as far as I know.

I would not be surprised if you have an op amp with its output moving to DC. Could be a bad IC or could be an open at an input pin so it loses reference.