Behringer EP2500 trying to get it working (channel 1 short probably)

[dutch_anykey]

Hi guys,

I have gotten great help on my DDX3116 repair on this forum thank you for that. Right now I'm trying to repair a Behringer EP2500 PA. But I just can't get my head around the working of this PA. So my first conclusion for my self is that PA's are really something else to repair....
I know there's a lot of people that sya that this PA is a POS. But it won't stop me from trying to repair is (yet). I got the schematics from the forum somwhere here.

I bought it dead, so it is not really clear what history it has. First is checked the output resitors, a few were blown. Also a few smal transistors where gone (A06 and A56 on the pos. step driver part on ch. 1)
When I turn on the amp (no load and pot meter turned down) The fan starts blowing, but no power led lights up. And the heatsink starts to warm up around T19 to T21)
I measured the bias voltage on this channel ( I thought maybe this would be the cause) And it is around 2 volts So that practically means something else is at fault?

Perhaps someone can give me some pointers on how to proceed and what to measure and check?


kind regards,
Maurice

[dutch_anykey]

o.k.

Found a blown trace under one of the big capacitors. soldered a wire on it to restore the trace and the amp is powering up now
Instead of testing if the amp is working correctly now, I choose to end this day on a high and take this win for the evening It's far too late and I need to sleep. Will test the output tomorrow and try to figure out how this broken trace matches the symptoms.

br
Maurice

[dutch_anykey]

Hi,

Very busy with other things at the moment. But yesterday I managed to connect the amp to a set of speakers. Sound was o.k. But channel 1 is getting hot (volume set to 30% I estimate) Eventually it was even shutdown after 5 minutes. (when it cooled it is working again)
So one problem solved but still not a usable amp. I quickly measured the bias voltage, this is within specs.
I have to figure out where to start from here.

br
Maurice

[Jazz P Bass]

"I quickly measured the bias voltage, this is within specs. "

And what did that measure at??

[dutch_anykey]

Hi,

80 mV (Like the other working channel)

br
Maurice

[g1]

You replaced outputs but not drivers and now it overheats under load? Static DC conditions look normal (same as good side)?
Then replace the drivers.

[dutch_anykey]

Hi g1,

Actually it already get's warm/hot without any load attached within a few minutes. Also no signal attached to the input.
you throw in a few terms that I have to figure out first. Static DC conditions > Is that checking there is no DC voltage on the output? Or do you mean the rectified 55 and 110 volt in general?
And the drivers are the transistors that control the output transistors? (they seem to be fine, at least no short)
Sorry for the questions. I have knowledge on electronics, but i'm a noob on PA's

br
Maurice

[Enzo]

Static conditions means stable readings with no signal present. it means any measurement. It means checking the output for DC. There ought to be zero DC there, but checking is checking. Other points in the circuit should NOT be zero volts DC. For example the transistor bases of the output transistors, on one side I might expect about +1v, and on the other side -1v. Other static DC readings might be the power rails - do you have both +40 and -40 volts (or whatever it uses), and are they clean? Do your op amps have clean +15v and -15v? And so on.

The reason we may want to know driver voltages can be because we think they are faulty. Your meter may not be able to detect that just testing the transistor. Your meter puts a very small voltage on a transistor at low current. But a transistor that checks good that way may in fact not be able to act that way at 40v and carrying 3 or 4 amperes of current. So we take readings while it is in an active circuit.

An amp that wworks and sounds OK, but gets hot even while idling, usually has mis-adjusted bias.

[J M Fahey]

Please post or link schematic here to be able to offer useful suggestions.
Otherwise itīs hard to go beyond "it must be some bad thingie in there"

As of:

I know there's a lot of people that sya that this PA is a POS.

.... how would THEY know?
Typical ignorant comment by know-nothing people who feel they must write "something" anyway.

You wonīt find that here

[dutch_anykey]

Hi,

I really can't complain on the behringer stuff I have here. For what I use it for it's more then sufficient. Of course there is way better equipment, just not in that price range.

Anyway regardig the ep2500. I will start measuring this evening when i get back from work. I only managed to check the voltage on the outputs this morning (no load, no input) and there is no DC voltage at all on the outputs. So that looks good.
I will have to do some in circuit measurements like Enzo said.
One thing I noticed is that the 1943 output tansistors has one toshiba branded and 3 fairchild branded. Not sure if this is bad and perhaps there is a rule to have them matched? at least of the same brand?


attached the schematic:

behringer_europower_ep2500_schematics.pdf

br
Maurice

[Enzo]

As long as they are real, I don't care about brand. Behringer sure wasn't matching transistors when they built it.

[Jazz P Bass]

This is a 'rail switching' amplifier.

In the off chance that the switching circuit is bad, check the output transistor emitter voltage.
At idle it should be + 55Vdc /NPN & -55Vdc /PNP.

If the switching circuit is at fault it will read +-110Vdc.

[dutch_anykey]

hi Jazz,

Measured the emiter voltages, they stay at +-55 volts. so no switching rail problems for now i guess.
I observed with an infrared thermometer that al 8 output transistors warm up pretty much evenly. Not sure though whether the heat sink is distributing the warmth quickly and evenly or if all transistors are generating heat equally.
Got home late from work and not much time to measure around. Will continue tomorrow.
I want to see next if the transistors are signalled to be open somehow, but it's hard to measure on the backside of the pcb But thinking of it, if they are signalled to be open it shouldn't be a problem anyway and not generate heat. It can only be some kind of load or short that will have them working and generate heat right?

br
Maurice