My 450watt Power Mixer - right output failed

[mikeykhhung]

Dear All,
During a gig, there was a power outage. After power was on again, my power mixer had no sound both left & right output. So, I shut it down that night. The next morning, at home I connect everything to check it - no sound still. Then I opened the cabinet of the mixer to see if there was any blown fuse. Looked at the two almost identical PCB - left and right. There was one blown fuse on the left PCB and all components looked good with no burning marks. So, replaced it with a 3A fuse as original. Power it up, the left has output but the right was still dead. Each PCB has its transformer, a floppy disk look alike cable runs from right PCB connecting the left PCB. The difference of the two is that one the right there were a few more components such as 1 x KA7818, 1 x MC7918, 2 x R27 5W. One each board, there are 2 x LM3886TF, a transformer output +35v/-35v, 1 x KBL04, 1 x 1k 250, 2 x 4700uf 63v, 6 x 100nf,1 x 10uf 50v, 2 x 10uf 63v, 1 x 220uf 63v, 2x 3A fuses, and some resistors - 47k, 330ohm, etc.As I am a rookie to electronic circuits,I only know how to replace damaged components. Would some experts give me some rough ideas about what went wrong. The mixer is ETEK Notemix MA400, made in ITALY but discounted more than 10 yrs. No way to find the schematics on the web. Please help!

 

[crutschow]

It's very difficult to troubleshoot a circuit without a schematic. You might try probing the two boards with a voltmeter and look for differences in the same circuit nodes. An oscilloscope is handy for this purpose, if you have one. Finding the problem could be a slow, painful process. Good luck.

 

[unclejed613]

it would also help if we knew the model of the mixer. maybe somebody (like me for instance) has repaired one of these, or one similar to it. it would also help in searching for a schematic and service manual. and you want to get that model number off the nameplate (where the serial number is) because what's silkscreened on the front of the unit is often not specific enough (it might say on the front panel "KLUGE-O-MATIC 1200" but the model number might actually be "KM-1252").

 

[Mike odom]

the classic way to troubleshoot an amp is to inject a signal and follow it through all the stages...
number one, check power supplies. Since the right board has the 7818 and 7918 (+/- 18v supplies), and the left board works, we can probably assume those are ok, but still, you need to check them and check power to all chips and bias resistors. Don't forget to check grounds locally... at the chips, not at a common ground point.
The KBL04 is a diode rectifier bridge, and the LM3886 sounds like an audio amp (without checking). If you can't find a board schematic, you may be able to find schematics for all the parts and do an imperical check (check all the pieces, and check connections between pieces)... for instance, you see a signal going into your transformer, but not coming out??? Like the man said, check for the same signals at the common points from one board to the other.

 

[unclejed613]

LM3886 is a 65W amp chip, which would make 130W/ch bridged. i am assuming each channel is bridged, because there's 2 LM3886's per channel. the data sheet should give you a decent clue as to the circuitry used in the mixer.

 

[4pyros]

The other fuse may also be damaged and you can't see it.

 

[unclejed613]

fuses should be measured. fuses have two failure modes, visible, where you can see that it's been burned open, and not-so-visible, where the element breaks free from the solder blob in the ends. it looks ok, until you either measure it, or take it out and "ping" it with your fingertip and see the element vibrating inside the glass. i have seen the not-so-visible failures often enough, that it's part of my routine to measure them, even if they look ok. it does happen often in music systems because they get bounced around quite a bit during setup, teardown, and transportation.

also, part of my routine with dead amps is to measure from each VCC rail to the speaker rails at the amplifiers. with regular outputs this is from the junction of the emitter resistors, or from the emitters, to the collectors. with a chip amp, measure from the output pin to both rails. you will usually get a diode drop from one rail to the output and open circuit (you actually may get a low resistance reading at first that climbs to open circuit as the power supply caps charge up from the ohmmeter current) if an output pin shows a short to a rail, you have found the bad chip. even if you find one on the first try, test the other one too. if in doubt, you can compare your readings to the working channel. (one of the advantages to troubleshooting a stereo amp, is you have two identical amplifiers you can compare).