Behringer poweramp EP1500 repair - need some help.

[gnd]

To all who know their way around poweramp repair, I need some help please.

I have Behringer EP1500 amplifier, that has fault. It worked fine many years, now I get no sound out of it, both channels are the same.
Please see schematics: https://www.dropbox.com/s/9yoho1h44rn65kw/Behringer%20EP1500_SCHEMATICS_REV_F.pdf?dl=0

Toroid is working, 24V fan powers up, but with full speed (from schematics it seems to me that it should start slow, then increase speed as amp heats up).  Power supply VCC reads +/- 80V as it should.
But I do not get +/- 15V to preamp stages, so there is some fault with 15V part of power supply.

Looking at schematics, page 3, I should get 15V on connector X17 on channel 2, which connects to channel 1 as X5.

Measuring voltages on 15V rail:
X17 pin1 should read -15V, but it reads -0.8 V
X17 pin2 should read +15V, but it reads +1.92V

Any suggestion what could be the fault? How to troubleshoot this?

 

[JohnRoberts]

To all who know their way around poweramp repair, I need some help please.

I have Behringer EP1500 amplifier, that has fault. It worked fine many years, now I get no sound out of it, both channels are the same.
Please see schematics: https://www.dropbox.com/s/9yoho1h44rn65kw/Behringer%20EP1500_SCHEMATICS_REV_F.pdf?dl=0

Toroid is working, 24V fan powers up, but with full speed (from schematics it seems to me that it should start slow, then increase speed as amp heats up).  Power supply VCC reads +/- 80V as it should.
But I do not get +/- 15V to preamp stages, so there is some fault with 15V part of power supply.

Looking at schematics, page 3, I should get 15V on connector X17 on channel 2, which connects to channel 1 as X5.

Measuring voltages on 15V rail:
X17 pin1 should read -15V, but it reads -0.8 V
X17 pin2 should read +15V, but it reads +1.92V

Any suggestion what could be the fault? How to troubleshoot this?

I feel a little dirty looking at a behringer schematic, I think they borrowed liberally from the QSC design.

It looks like -15V rails are developed on each amp channel...  D7 and D9 zener diodes fed through 3k from VCC-

+15V used zeners D5 and D6, but it looks perhaps like some extra power sequencing circuitry for turn on/off thumps or whatever,

If you have 80V on one end of the 3k resistors but not 15V at the other end... something may be hogging all the current... just search for the missing 25mA or so,,,

The obvious suspects connected between +/- 15V are the op amps... I wouldn't expect both channels to fail simultaneously so look elsewhere,,, maybe bad ribbon cable, or fish heads inside.

JR

 

[gnd]

I feel a little dirty looking at a behringer schematic, I think they borrowed liberally from the QSC design.

It looks like -15V rails are developed on each amp channel...  D7 and D9 zener diodes fed through 3k from VCC-

+15V used zeners D5 and D6, but it looks perhaps like some extra power sequencing circuitry for turn on/off thumps or whatever,

If you have 80V on one end of the 3k resistors but not 15V at the other end... something may be hogging all the current... just search for the missing 25mA or so,,,

The obvious suspects connected between +/- 15V are the op amps... I wouldn't expect both channels to fail simultaneously so look elsewhere,,, maybe bad ribbon cable, or fish heads inside.

JR

John, thnx for reply.
Those 3k resistors you mention (R54, R35) both measure 80V on one side, and 3.3V on other, instead of 15V.
Replugged all cables, no change. No fish heads inside either.

Is it that it must be something that is common to both channels, because both are the same? What if I unplug ribbon cables, and leave just 80V rail connected, just to measure if there is difference then? Would I blow something this way?

 

[gnd]

How do I locate components that draw too much current? I have oscilloscope, tone generator and multimeter. I suppose some component would be shorted, thus lower impedance and thus lower voltage on the other side of 3K resistor.

Board is double sided, i need to take it out to reach bottom side. If I disassemble it, I cannot test it with power on.

Can I do test with ohmmeter, without power? Can components be somehow tested without unsoldering?

I never serviced amps, I can DIY fine, but if something needs repair I'm a bit lost.

 

[JohnRoberts]

after running for a few minutes, turn off the power and feel around for something warm (besides the 3k resistors). While 25mA time 3 volts is less than 1/10 watt, so may not be apparent.  The 3k resistor will be almost 2W.

are the voltages sagging on both channels identically?  I don't still have the schematic open but yes look for something common to both. 

If 80V rails are OK that suggests the output stage is not blown (a good thing). Be careful you do no harm that isn't already done.

JR

 

[gnd]

If 80V rails are OK that suggests the output stage is not blown (a good thing). Be careful you do no harm that isn't already done.

JR

Yes, well, while poking around with multimeter, I managed to destroy it even more.  Quite stupid.

I was measuring voltage on thermistor R72, and while turning my eyes towards multimeter, it seems probe slipped and I touched heatsink with a tip of probe, so I guess I basically connected one side of thermistor to ground, and puff, big spark, pcb trace burned out, and amp went crazy. Fan suddenly started working faster, peak LEDs turned red, and that was it.

Now when I turn it on, I don't have +/- 80V anymore, and red peak LEDs are on all the time. And now those 3k resistors on negative rails, R44 and R19 become burning hot after few seconds of operation. Damage done. I feel quite bad.

Do you think amp is still servicable. I dare not turn it on anymore. If I take it to proper service shop, would they be able to fix it?

 

[JohnRoberts]

Yes, well, while poking around with multimeter, I managed to destroy it even more.  Quite stupid.

I was measuring voltage on thermistor R72, and while turning my eyes towards multimeter, it seems probe slipped and I touched heatsink with a tip of probe, so I guess I basically connected one side of thermistor to ground, and puff, big spark, pcb trace burned out, and amp went crazy. Fan suddenly started working faster, peak LEDs turned red, and that was it.

Now when I turn it on, I don't have +/- 80V anymore, and red peak LEDs are on all the time. And now those 3k resistors on negative rails, R44 and R19 become burning hot after few seconds of operation. Damage done. I feel quite bad.

Do you think amp is still servicable. I dare not turn it on anymore. If I take it to proper service shop, would they be able to fix it?

Somebody should have warned you.... Oh I did.  :-[

It probably still can be fixed, but now you have even more to fix, what may have been a minor problem before, and now components that have released their magic smoke. Depending on how badly the PCB was burned, that may be simple or very difficult.

If you take it to a shop, don't tell them you blew it up or they will charge you more (I would). IIRC those amps were pretty cheap new, so I don't know how much big dollar per hour repair labor is justified. Fixing it yourself, just got to be even more educational.

JR

 

[gnd]

I took the amp to repair shop, guy kept it for a day, then called me that it is not worth repairing the amp. He also said that all power transistors are burned and that one costs 15 euro.
So this raised some doubt in him, because at Farnell transistors (2SA1943, 2SC5200) cost approx 2.50 euro each, which is much less than what he stated.

I looked at what I burned, and trace is burned just on one side of PCB, so I will patch it with some wire. Thermistor measures ok.

Probably I burned led in optocoupler, but that is just for fan speed control.

I measured all small transistors and diodes without unslodering them, all seem fine.

I measured also power transistors on PCB, without unsoldering. 
All base-collector measure 450 with diode meter, and it says GOOD on multimeter. But shouldn't it be in 600 range? Could they be good, but reading is a bit off because they are not unsoldered?
I cannot measure properly base-collector, it gives me BAD on diode measurement, and 22,2 ohm both ways, but thats probably because that 22ohm resistor and 0.22ohm current sharing resistors on collectors.
Would it be enough to unsolder those 4 x 22ohm resistors, to get proper measurements? Or do I need to completely unsolder all power transistors to be certain.

 

[gnd]

Also, lets say all power transistors turn out to be burned.  There are 4x PNP and 4x NPN on each channel.

Amplifier has power 700W per channel at 2ohm, and 450W per channel at 4ohm. But I don't need that much power, my subwoofers are 250W 4ohm. So 200W into 4 ohm per channel would be good enough for me.

Could I replace 2x4 burned transistors with just four instead of eight, 2xPNP and 2xNPN, and leave other four empty? This would give me some 200W at 4ohm. Is this correct? Would it work? Would I need to change anything else in amp?

 

[JohnRoberts]

I wouldn't suggest trying to redesign it on the fly without a more intimate knowledge about how it works and what everything does, so fix the whole amp and keep it stock (IMO).

It may not be worth repairing paying professional labor rates and retail parts cost.

Fixing it yourself, if you can, will surely be cheaper if you don't charge for your time, and you will get an education. You are already a little smarter about being careful when probing around inside with power on.

It is handy when repairing power amps to use a variac, to slowly bring up rail voltage to identify problems before releasing (more)  smoke.  Another handy trick is to wire a 100W light bulb in series with the line cord to current limit the mains draw at about 1A, until it is operating properly.

It will cost you something to replace all the blown parts, and it will take you some time to figure this out and get it back in working order.  I don't want to scare you, but it is not uncommon when repairing power amps to blow them up again if you didn't fix all the broken parts.

JR

PS: I've seen new modern switching amps for only a couple hundred dollars..That may factor into the calculus about repair justification.

 

[Andy Peters]

Yes, well, while poking around with multimeter, I managed to destroy it even more.  Quite stupid.

I was measuring voltage on thermistor R72, and while turning my eyes towards multimeter, it seems probe slipped and I touched heatsink with a tip of probe, so I guess I basically connected one side of thermistor to ground, and puff, big spark, pcb trace burned out, and amp went crazy. Fan suddenly started working faster, peak LEDs turned red, and that was it.

You want these probes.

 

[gnd]

It is handy when repairing power amps to use a variac, to slowly bring up rail voltage to identify problems before releasing (more)  smoke.  Another handy trick is to wire a 100W light bulb in series with the line cord to current limit the mains draw at about 1A, until it is operating properly.

It will cost you something to replace all the blown parts, and it will take you some time to figure this out and get it back in working order.  I don't want to scare you, but it is not uncommon when repairing power amps to blow them up again if you didn't fix all the broken parts.

JR

PS: I've seen new modern switching amps for only a couple hundred dollars..That may factor into the calculus about repair justification.

Yes, I was thinking about 100W lightbulb trick. I use 3x100W lightbulbs and timer contactor on my entire studio main line, as power-up surge limiter - bulb is in series with mains, but gets shorted automatically after few seconds by timer contactor. We get a lot of power outages here.  And I have similar contraption for my other amps, which I turn on and off all at once by remote switch - inrush current otherwise tripped fuse.
Only to get 1A current in Europe, I'll need to use 200W light bulb, or two 100W in parallel.

I bought this amp for some 150 euro second hand. I will not buy another, rather I'll DIY something.  One solution I was thinking of is to simply take guts out of this amp, and put in TDA7294 blocks. I have 4 TDA7294 lying in drawer since long time, so building it would cost me almost nothing. I could probably keep existing transformer, it has quad primary (120/240V) so I could possibly get 40V out of it, which is kinda ok for TDA7294. I can use two ICs in parallel, connected via 0.1ohm current sharing resistors, and get 100W to power 4ohm speaker this way. Maybe ...