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Hey all. A friend of mine picked up an acoustic 125 combo at a pawn shop for pennies because it sounded like what I would call an acoustic 30 at best. First thing I did was check the output transistors. I got to one that didn't switch, so I changed all 4 with some that I have in my stock. This is where I screwed up... The ones I used didn't have the voltage rating that was necessary and instantly shorted taking out the 2 60 ohm resistors next to them. I replaced the resistors, bought suitable replacement transistors (MJ15003) and The thing sounds like an oversized headphone amp now. I ended up finding 2 shorted diodes and a shorted Q4. Fixed those, no difference. I feel like it would have been fixed if I didn't blow it up that first time. I've checked every semiconductor on the board for shorts and every resistor seems to be ok tested in circuit. Any idea of what I could me missing here? Thanks everyone. I don't really know much about solid states, but I feel I need to learn to be more well rounded. These circuits are starting to make a little sense to me.
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Acoustic 30 at best pretty well describes it. Although IIRC there are 4 x TO-3 big output transistors, the power at clip leaves a lot to be desired. How much I forget but likely 50-60W into 8 ohms. Acoustic, being the marketers they were, picked a model number that suggests a much higher power figure but - 125 is just a model number. Maybe, with the wind at its back, and falling over a cliff, it could produce 125W into 2 ohms at 30% distortion for a second just before it blew up. And the tone? Skreeeek, fingernails on a chalkboard. If you can get the power end to run reliably, maybe you can adjust the preamp EQ so it's not so automatically annoying. Good luck!This isn't the future I signed up for. -
were the shorted diodes C10/12? - also if you use a DMM to check semiconductors it does not guarantee that you still don't have a bad device. The 125 was rated at 100W into 4 ohms at 5%. Do you have a scope and load to check the output level? Check voltage levels at Q3,5,7,etc. when the amp is powered up. My guess is that Q3,5 or 7 may be walking wounded. Search here for replacement/alternatives for these RCA transistors. unofficial acoustic control corp forum Forums Also double check any thing you replaced and make sure you have the correct pin out or device type on the board.Comment
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Hey, thanks for your suggestions. The shorted diodes were D7 and D8, and they were definitely shorted. I pulled a leg up to check. I do have a scope and a load and figured the amp was around that wattage. Unfortunately the output is so low that I'm not really to that point yet. I would say that the output is at most 5 watts. The weird thing is that it isn't badly distorted, just quiet. Anyways, I'm going to do more poking around and see what I can come up with.Originally posted by gbono View PostComment
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Would you be able to tell me why exactly I am looking for and how to test it? I put a sine wave into the power amp and it comes out the other end very square and without any voltage gain. I tested All of the DC voltage points on the schematic and they all come out good. I'm not sure where to go from here.Comment
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Jumping right in: what input voltage are you using?
The schematic indicates 140mv's into the power amp.
And with that 140 in, what is the measured voltage at the output into a load & also without the load.Comment
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Hi Jazz P Bass. Putting a 700hz sine wave in the front of the amp and measuring 140mv at the input of the power board, I get 77mv with a load and 65mv with out. I feel like if I knew what the output of all of the transistors in the circuit was supposed to look like, I'd know what the problem was. Thanks for your help!Originally posted by Jazz P Bass View PostComment
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Originally posted by antieatingactivist View Post
Try and download the service manual for the 150 here - it is the same PA board and it will be easier than threading you through it..........
http://acc.homeunix.net/images/manua.../150manual.pdfComment
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65mv output?
Millivolts?
You have a divide by circuit, not an amplifier.
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This doesn't appear to be my power amp board. What should I be looking for?Originally posted by gbono View PostComment
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So sorry brain fade - for some reason I was thinking you had the 140??Originally posted by antieatingactivist View Post
Have you chased the signal through the power amp board? What is the signal look like at the base of Q1? The output (collector)of Q7? I suggest you look at 1983 Electronic Troubleshooting PB by Jerome E. Oleksy........Comment
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So I think I have some useful data here... I put 140mv into the power amp. On the output of both drivers Q5 and Q7 (I think thats what they are), I get about 1 volt. Seems about right. The Sine wave looks good by this point too. I'm getting about 1 Volt on the output of the power amp too!! Not right. Its like the signal from the driver is just passing right though un-amplified. Its possible I got something wrong when I replaced the Bias transistor (I think thats what it is) I used a 2N3904 for this. I don't really understand how this circuit works, but I'm pretty sure my problem is at the very tail end of the power amp.Originally posted by gbono View Post
Edit: I'm not sure why my readings on the output changed rom my previous post, probably user error.Comment
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The output transistors are there to amplify current, not voltage. That is how they can drive a low resistance load.
You should probably have about 10V coming out of the drivers, not 1V.Originally posted by Enzo
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Makes sense. So should I be seeing 10v coming out of the driver transistors as well?Originally posted by g-one View PostComment
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That's what I said
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Originally posted by Enzo
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