powered galaxy microspot monitor audio fault

[philiphtc]

.hi all i'm a newbie and this is my first job repairing stuff and i'm just used to really basic stuff, making up patch leads / setting up / keeping the PA going for our local church ps i am used to fault finding on gas c/h boilers in the uk

i have a problem with one of our 4 galaxy hotspot powered monitors they can be used on 240v or 12v so they may be useful to check voltages at different points

the monitor was dropped and now the speaker cone shows full movement and the ON led pulses on and off, both approx once per second in both 12v and 240 i am obtaining a schematic diagram to aid fault finding

jumping to conclusions is bad but it seems like on overload /shutdown problem to me but i'm just a gasman!!

any suggestions where to start and useful tools to use would be helpful

their tech department were'nt more helpful other than emailing a schematic

thanks

 

[Nigel Goodwin]

If it's been dropped, then it's likely the PCB is cracked?, check near heavy components (like the mains transformer, if it's PCB mounted).

 

[philiphtc]

hi nigel thanks for the quick reply there are 3 small pcb's all solidly mounted onto a cast frame i'll check tomorrow to see if there are any cracks thanks

 

[philiphtc]

checked on the pcb's cant see any cracks as mentioned earlier the pulsing is displayed when using 12v and 240v so was assuming the fault was later in the circuit is the schematic any use to you ,i have on pdf if i could send it somehow?

 

[Nigel Goodwin]

checked on the pcb's cant see any cracks as mentioned earlier the pulsing is displayed when using 12v and 240v so was assuming the fault was later in the circuit is the schematic any use to you ,i have on pdf if i could send it somehow?

Attach the PDF to a post here, click on 'Go Advanced', then 'Manage Attachments'.

 

[philiphtc]

hi this is the schematic sent by their tech dept i dont know enough about electronics to know what readings should be at what positions on the 3 boards 1 appears to be mains power 1 board for filters 1 board for amplifiers hope this all means something to you!

thanks for your time and all help

 

[tunedwolf]

Take a divide and conquer approach and start with the basics. Disconnect the speaker to prevent damage to it. Check the supply rails are indeed +/- 22V at C208/9 as expected. I would also measure for DC on the output of the amp. I suspect there is a fine crack in the print somewhere around the main amp chip U304 (TDA2050).

 

[philiphtc]

thanks for the info i am at my daughters for a couple of days i will check it out when i get back home i notice that the voltage rails are 22v + & - as the unit runs off 12v i presume that the transformer is only for mains 240v and it has a seperate circuit to increase built in shown somewhere on the PDF as the pulsing fault shows on both 12v and 240 your idea about suggested fault location (towards the end of the circuit) would make sense thanks again for your time

 

[philiphtc]

sorrry for the delay Happy New Year with my 2 meters i cant get a stable reading either dc or ac on the 22+ or 22- i was expecting dc though i have traced the pulsing as far back as the 150uF capacitor and the bridge rectifier main power supply board as it is a 1 pulse per sec would you suggest that i take out the capacitor and test that 1st what reading should i expect if i test the board conns where the capacitor used to be 240v dc as it is after the rectifier just as a side question what would the "inductor"?l100 be used for?

 

[tunedwolf]

Happy New Year!

Desolder pins 3 & 5 of U304 (supply rails going to the amp chip) and measure it again. If there's a problem with it and it's oscillating or if it's actually faulty it may well be pulling down the supply rails. If you still can't obtain a stable reading then I suggest isolating just the power supply and measuring again etc.

 

[philiphtc]

after a lot of testing i swapped the amp chip and all is now fixed

thanks for all the help everybody who gave advice

i will be back in the future

 

[unclejed613]

the pulsing is die to the power supply operating in shutdown mode. most likely from a short circuit on one of the 22V rails somewhere. the supply starts up, detects a short, and shuts off. it does this repeatedly, giving the pulsing you describe. most likely the TDA2050 is shorted.


oh, didn't see the last post.... sorry...

 

[philiphtc]

thanks for the reply however late unfortunately it is now only working on 12v i must have blown something somewhere but 12v is better than none