Powered Speaker Amp repair

[RubiconJoe]

I am attempting to repair a Mackie (TH-15A) powered speaker's amplifier. After opening her up and referring to the schematic I found a few burnt components. It seems a resistor in the +-28V supply just off of the step-down transformer is slightly burnt, no other visible issues in this part. See attached image1 (R121). FYI main bridge rectifier is providing correct 300V DC.

Also, the final amplifier chip (TDA7294) as well as the resistor on its output is burnt. See image2 (U200 & R212)

I have ordered replacements for the burnt components, but don't want to slap them in to see them burnt again. I figured I would isolate the power supply portion first and make sure I get the correct voltage there, before connecting up the amp board.

My question is, would the power supply fault cause the amplifier chip to blow, or the amplifier chip fault cause the power supply to blow? Just looking for some insight from someone with more experience in these types of repair. Also, what is the best way to determine if any other parts of the circuit are damaged. Unfortunately, I only have minimal test equipment at the moment in a DMM.

Thank you for anyone who can help!

Joe

 

[davenn]

I would also be checking the speaker(s) that are on that channel for faults

 

[RubiconJoe]

I checked resistance of the + and - from the speaker wire that connets to the burnt chip's output pin & burnt 4.7 ohm resistor. It tested at 4.3 ohms, not shorted.

 

[Audioguru]

There is no 300V.
R121 does not come off the transformer.
The rectifiers are not a bridge, they are simple half-wave.
The TDA7294 IC has short circuit and thermal protections.

 

[davenn]

There is no 300V.

he will be referring to the primary side of the SMPS transformer .... not shown

The rectifiers are not a bridge, they are simple half-wave.

again ... he will be referring to the primary side of the SMPS transformer .... not shown

but just letting us know that the primary side voltage is OK

 

[davenn]

R121 does not come off the transformer.

no, not directly

The TDA7294 IC has short circuit and thermal protections.

but the damage being done suggests that it has failed internally

I checked resistance of the + and - from the speaker wire that connects to the burnt chip's output pin & burnt 4.7 ohm resistor. It tested at 4.3 ohms, not shorted.

that sorta indicates that the speaker may be OK

try that speaker on the other channel at low volume setting and be prepared to power off quickly if there are any signs of stress
bad speaker noise ... hum etc or smoke from IC or resistors previously mentions


I still suspect that the audio amp IC has most likely failed



Dave

 

[RubiconJoe]

Update: I disconnected the power supply board from the amplifier board and have verified there is actually nothing wrong with the +-28V, +-15V or 3.3V rails. The fault on the amplifier board must have been pulling down the supply voltages. I guess the 10R resistor with small brown mark on it was not actually blown, it tested fine. I will replace the amplifier chip and burnt resistors on the amp board and hope all other components are still good and not blow the chip again.

 

[davenn]

Update: I disconnected the power supply board from the amplifier board and have verified there is actually nothing wrong with the +-28V, +-15V or 3.3V rails. The fault on the amplifier board must have been pulling down the supply voltages. I guess the 10R resistor with small brown mark on it was not actually blown, it tested fine. I will replace the amplifier chip and burnt resistors on the amp board and hope all other components are still good and not blow the chip again.

good testing

keep us informed to what you find

 

[RubiconJoe]

Good news: I replaced the main amp chip and output resistor...and they didn't blow up again when powered on!

Bad news: there is still something wrong with this thing! The unit powers up, the LED that indicates the unit is powered on lights up but then dims almost to off, then back on periodically. I checked the +- 28V rails with the amplifier board connected and it is fluctuating up and down. I then disconnected the amplifier board and the 28V supply still is fluctuating. Now thinking there may be a bad cap or something in the power supply? Odd because it had solid rails when tested during troubleshooting previously. Any suggestions on where to look next?

Thanks again!

Joe

 

[RubiconJoe]

Been a busy the past couple weeks and haven't been able to troubleshoot this issue much. Today I checked the power supply again, and seemed to still be fluctuating. I then pulled out the connector that powers the pre-amp board to easily check the +-15V and 3.3V rails. They all seemed to be solid, and now the +-28V rails are solid again now too! Looks like a fault now in the pre-amp section causing fluctuation in the voltage rails! I'm chasing faults all over this thing!

I pulled the can off and examined both boards that seem to be part of the pre-amp, and do not see anything obviously damaged. Any suggestions on how to proceed? Cannot seem to test supply voltages because as soon as these boards are plugged into the supply, it makes all the supply fluctuate.

Thank you to anyone who can assist!

Joe

 

[RubiconJoe]

Any suggestions? I'm kind of stuck here with this repair. I do not know what to test or suspect next.

Thank you in advanced!

 

[davenn]

Any suggestions? I'm kind of stuck here with this repair. I do not know what to test or suspect next.

Thank you in advanced!

Please post a schematic of the preamp board

 

[RubiconJoe]

Thank you for the quick response! Attached is a .pdf of the preamp section.