Behringer BX600 optocoupler?

[spy]

Hi,
I'm trying to repair a Behringer BX600 bass amp. The problem started when the amp sounded a little bit fuzzy. A weird robot-fuzzy noise was added to the note when the amp was cold. When the amp got warmer it disappeared!
I changed the LM3886 and the two main filter caps but the problem was still there then, I decided to replace an Optocoupler that was at the main amplifier board. But there's no any code on it! Which Opto is this? It looks like a VTL5C1 but it's smaller. I put a VTL5C1 on it but the sound became more robot-fuzzy!
The sad is when I was trying to take off the Opto, one leg broke. It was so sensitive and I'm not clumsy! Anyway, I must find a new one...
Ok, don't laugh with me! It was an accident!
I'll appreciate any help!

By the way, I did contact by e-mail, with Behringer and dealers and service centers but noone helped me. Only one replied but didn't know which Opto is this.

EDIT: I forgot to say a big HELLO to this part of the forum as I'm a new member here! Only some posts to the pup winders... so, HELLO amp techs and gurus!

[Enzo]

Why did you decide it was the opto?

The opto is just a compressor like many amps have. it samples the output, and if it gets large enough, it turns on the opto which attenuates the input to the power amp IC. it is a prtective device, the amp doesn't need it to operate. How does the amp sound without it? There is also a J112 JFET, part T2. it is parallel to the opto and is the power up mute. You could remove both those and see how the amp sounds.

Amp doesn;t sound good.

Isolate the problem.

PLug a signal into the power amp in jack. Does that sound OK or is it fuzzy?

Likewise connect the preamp out jack to the input of some other amp - how does the preamp out sound over there?

Most likely only one of those has the problem. Fighting fuzzy in the power amp is totally different from fighting it in the preamp.

WHich one is the problem?

Power amp? Make sure the cutout contact on the headphones jack are real clean. MEASURE the resistance of each - it better be under half an ohm.

And monitor the voltage on pin 8 of the LM3886. Does it come up to voltage smartly a second or two after power up, or does it slowly get there - does it take the similar time as your warm up period?

Preamp? Does turning the shape on and off make a difference? Other than tonally of course.

[Enzo]

OK, as far as I can determine, their opto is home grown.

it comprises an LED and a photoresistor

The LED is type W03335RSC-6S from Waitrony.

The photocell is type KE-50910 also Waitrony.

If you just google each part number, multiple places to download the data sheet on each is presented. They seem common enough to me.

However, our own John Frondelli, on this forum, stocks many Behringer parts, so before making one, see if he has them.

But I'd find out what is really going on before ordering things.

[spy]

Enzo, you're great! Really thanks for your help!

Why did you decide it was the opto?

Let's say... newbiness!
And not only that but now it has a missing leg!

I checked the board and there's no J112 JFET, part T2! There is a free space on the board but the T2 isn't there. Maybe in this version they don't use it...

Anyway I removed the VTL5C1 and the amp without it, sounds fuzzy again.
At the output of the preamp the sound is clean, so the problem isn't at the preamp.
I plugged a signal into the power up in and it's fuzzy, I coul say more robot-fuzzy. Nice strange effect by the way!
Line Out is fuzzy, too.

Make sure the cutout contact on the headphones jack are real clean. MEASURE the resistance of each - it better be under half an ohm.

I don't understand this....
I measure under half an ohm between L and R channel. When I measure L channel with ground or R channel with ground there's infinity. What does this mean?

And monitor the voltage on pin 8 of the LM3886. Does it come up to voltage smartly a second or two after power up, or does it slowly get there - does it take the similar time as your warm up period?

I did it and the voltage comes a second after power up. I have -40V there and +40V at pin 11. In both pins the voltage comes in a second.

[spy]

OK, as far as I can determine, their opto is home grown.

it comprises an LED and a photoresistor

The LED is type W03335RSC-6S from Waitrony.

The photocell is type KE-50910 also Waitrony.

If you just google each part number, multiple places to download the data sheet on each is presented. They seem common enough to me.

However, our own John Frondelli, on this forum, stocks many Behringer parts, so before making one, see if he has them.

But I'd find out what is really going on before ordering things.

Again, thanks for the info! You're great....
The Opto is home grown!? That's why it doesn't have a code on it.
I checked about the LED and the photocell. Are these in the cell I have? Interesting! It'll be cool to build one by myself...

[Enzo]

They may have decided the mute transistor was a waste of time. And money. Those half-a-cent transistors add up after a while.

I don't understand this....
I measure under half an ohm between L and R channel. When I measure L channel with ground or R channel with ground there's infinity. What does this mean?

The phones jack cuts off the speakers. There are contacts in it that open when a plug is inserted. When there is no plug, they have to come together to complete the circuit to the speaker. By measuring the resistance of each set of contacts, we determine if they are making good contact electrically. SO resistance to ground is irrelevant.

AT this point it seems to be in the power amp. The power amp is pretty much the LM3886 chip. But the signal does travel through one section each of TL074 IC3 and IC4. I'd be thinking bad 3886 myself, but I'd try to verify the location of the problem with a scope or signal tracer.

If you do replace the LM3886, be aware they come in regular versions with the metal tab, and LM3886TF where the tab is plastic and thus insulated. You ought to find the type you need. You can mount the metqal tabs in place of the plastic ones, but then you have to also add a mica washer, and plastic shoulder washer, all to keep the metal tab isolated from the heat sink.

[spy]

The phones jack cuts off the speakers. There are contacts in it that open when a plug is inserted. When there is no plug, they have to come together to complete the circuit to the speaker. By measuring the resistance of each set of contacts, we determine if they are making good contact electrically. SO resistance to ground is irrelevant.

Ok now I got it!

If you do replace the LM3886........

Yes I did replace the LM3886 with a metal tab version like the old one. I used the mica and washer that where allready used with the old LM 3886.
The PCB doesn't have too many parts, it's pretty weird!
Also, I can't find IC3 and IC4... there are only two SMD transistors T1 and T3.

I'm wandering why with the Opto the fuzzy sound was extremely lower and after 10 to 15 minutes disappeared and now without the Opto is much more louder... don't know what happens after 10-15 min.... I'll check..

[spy]

Ok, the amp is on the bench. The noise disappeared again (without the Opto) after some minutes. At the input of the board there's no noise (or I think so). At the pin (+) of the Opto there is! The problem is probably btw input and Opto, but where?

By the way, can I replace the Opto with a VTL5C1? I have one spare...

EDIT: I found IC3 and IC4, they are at the preamp board...
EDIT2: Ok, the "input" of the Opto is at the output of the LM3886....

[spy]

Ok I'll go crazy! I have a board with "ten' components on it and can't find the problem!
I have some updates, if anyone would like to help...
The problem is definetely on the amp (&PSU) board. I'm sure because I have disconnected the preamp board and I have connected the signal generator directly at the input of the amp board.

The weird is when the amp is warm enough the signal is clean but every 5 seconds it comes and goes....

Enzo, do you have any idea what the T1 and T3 do? They're at the feedback of the amp. Are these the components of the "dynamizer" circuit?
As I can see on the PCB they're marked as A06 and A56. Can I replace them with any equivalent non-SMD transistors?

[Enzo]

T1 and T3 are in the power up mute circuit, nothing to do with feedback. They control, pin 8 of the LM3886 - the mute. Before you try changing them, just monitor the voltage at pin 8. If it is bobbing up and down with your symptom, then they could be involved. If that voltage is stable, then they are not. Even then, I'd suspect some of the passive components first.

I think the full part number is MBTA06/56 - tiny versions of MPSA06/56.

Does this symptom also show up on the headphones and the line out?

[spy]

I monitor the voltage at pin 8 and I see the signal with a steady pulse 1Vp-p around 100Hz. It doesn't change with input variations.
When the amp is warm enough and is starting to bobbing down and up, pin 8 has the same symptom. The pulse remains but the signal not.
The sympton is also on the headphones and the line out.

[spy]

Enzo, do you have the schem? What does the el. capacitor C9 do?!?!
IT WAS THE COMPONENT THAT CAUSES THE PROBLEM!

Please, I need an answer of one of my previous question,
if anyone can answer I'll be more than happy
"...can I replace the Opto with a VTL5C1? I have one spare..."

[Enzo]

C9 is the filter for the negative supply for the power up reset - the circuit of those two transistors.

I have no idea if that vactrol would replace the stock item. Look up the data sheets on the parts I dentified and compare them to the data sheet on the vactrol. I imagine the LED part is not an issue, the resistance range of hte photocell is where to look.

[jemigo]

BX600 Loud hum (40W to speaker) no other sounds

There's 40 watts at X1 with amp on that is going straight to speaker. Do you know if that would be the amp chip lm3886t? Does anyone know what the in/output of the individual pins should be? Thanks!!

[Jazz P Bass]

40 Watts

What does 40 watts at X1 supposed to indicate.
40 volts on the speaker is not good.
Anyway, here is a link for the LM3886 power IC.
Link:LM3886 - High-Performance 68W Audio Power Amplifier with Mute