Not sure if this is entirely appropriate for this forum or not. I'm working on fixing the amp in a Gemsound BA200 PA speaker that a friend gave me. It amplifies the sound to a properly loud level, the tone controls work, and the line out is clean but the sound coming out of the speaker itself is distorted and choppy - almost sounds like a dying OP-Amp. I swapped amp modules with a working one to make sure the speaker and crossover were good, and they are. I also installed a new tl072 in place of the 5532 op amp on the amp board (page 2 of attached schematic) and no change. No visible problems either. Before I start replacing parts somewhat blindly - does anyone have any tips on where to possibly look for the problems? I'd assume power transistors would be the first things to replace?
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any dc across the speaker output?
all the rail voltages are normal?
then yes check output transistors.
when you where in there swapping the op-amp you did confirm that it's supply voltages are correct?
all the rail voltages are normal?
then yes check output transistors.
when you where in there swapping the op-amp you did confirm that it's supply voltages are correct?
Don't think in terms of what should you replace first, think in terms of finding where the problem is.
No circuit works right without good power supply, so check the main rails, are they both clean and up to the same voltage? Since you have a good unit, you can find out in it what those supply voltages should be. They only need to be close, don't sweat a volt or two. The op amp in the power amp needs good power. I can't read the voltage on the two zeners, but make sure pins 4 and 8 have good clean voltage on them.
Turk already said that stuff.
Check the transistors in place, looking for shorts and opens.
Do you have a scope? It would be informative to look at the output and describing the distortion. Is it clipped, one side missing, modulated, etc? What does the signal look like on the output pin of the op amp?
No circuit works right without good power supply, so check the main rails, are they both clean and up to the same voltage? Since you have a good unit, you can find out in it what those supply voltages should be. They only need to be close, don't sweat a volt or two. The op amp in the power amp needs good power. I can't read the voltage on the two zeners, but make sure pins 4 and 8 have good clean voltage on them.
Turk already said that stuff.
Check the transistors in place, looking for shorts and opens.
Do you have a scope? It would be informative to look at the output and describing the distortion. Is it clipped, one side missing, modulated, etc? What does the signal look like on the output pin of the op amp?
seeing as there is correction to zener orientation with respect to the filter cap on the schematic i'd look to see if the board was reworked and that things are correct.
the cap's(100 uf 16v) are suspect if the op-amp supply rails are off...
the zener's should be 15 volt no? N4756 they and the cap's are the supply rails for the op-amp also any signs of heating on or off tolerance resistance in the 5.1k 2w resistors feeding these supply lines?
need measurements please...
the cap's(100 uf 16v) are suspect if the op-amp supply rails are off...
the zener's should be 15 volt no? N4756 they and the cap's are the supply rails for the op-amp also any signs of heating on or off tolerance resistance in the 5.1k 2w resistors feeding these supply lines?
need measurements please...
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I'll measure the 2w supply resistors. The rails on the op amp seem good -15 on pin 4 and +15 on pin 8. I've been meaning to check some things with the scope - will do so this evening and report back. In checking the transistors for shorts/opens I should just look at the schematic and verify their connections and make sure there isn't continuity between legs?
test them (transistors) out of circuit then check for open emitter resistors at the same time as well as the 10 ohm base resistors.
Finally got a chance to scope some things. Now it seems the speaker relay isn't turning on - not entirely sure why that has happened or what triggers it to turn on.
On the power amp page, the relay coil is lower left. The thing either won't time out from, power up, or it is detecting DC on the output. Find those 0.22 ohm 5w resistors near the output transistors. Is there any DC on them? If they are within maybe a quarter volt of zero, that is OK, but if there are several volts or more, that is likely why the relay is off.
For power up, look at that 33uf cap, what voltage is across it, and check that 6.8v zener.
For power up, look at that 33uf cap, what voltage is across it, and check that 6.8v zener.
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