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I use a big ceramic resistor. Mine is a 10ohm 10w resistor. I touch it to the power cables that go to the amp from the power supply filter caps.
Another trick, is to wait a while, most amps have a build in bleed resistor that basically does just that, but can take a few minutes. Measure the dc voltage at the power supply to ground, for both the positive and negative sides of the power supply.
sent from Utah.
The thing is, to test for acv passing into through the amp (source of the hum) the amp needs to be powered on. Be careful and deliberate with your placement. Don't just think, I oughta be able to get it in from this angle, it'll be fine... Zap! Don't touch live electrical components with your fingers. A zap with a slipped probe will possibly ruin components, and make a zap, but that's about it. A zap with a finger, ouch.
sent from Utah.
This thread is amazing. I have a ten year old Boston Accoustics sub that shouldn't be broken, but only plays bass from one channel input only I should dig into someday. How do you not kill yourself around the capacitor exactly? Aren't they still charged even when unplugged?
Funny side note, I bought a small room sony speaker set from DJSapp around 15 years ago that I still have. I hard wired surround sound in my house when I remodeled it, but still haven't prioritized funds to replace the system with appropriately sized speakers. It still sounds alright for what it is.
Holy crap, I remember that system. It was a Costco special if I remember correctly. The sub I'm trying to fix was part of the upgrade to that system.Quote:
Originally Posted by Gerome
So here's what I got
First pic is the top board. The black schmoo feels like hard candle wax and is over the back of the rca jacks and some of the board. Some browning of the wiring harness plastic. Volume and xover controls on the right
Second and third pics are the middle board. Power in from the xfmr, speaker out and a bridge to the first board. Caps appear ok, big white blocks (ceramic resistors?) have some browning, I'm assuming heat related. Power in and sub out wiring harness plastic has definite heat browning.
Last pic is the whole thing. I've disconnected the board on the left as that is the high level input and output. I never used that anyway. The wires coming in and out of the xfmr look to be ok. No signs of heat damage that I can see
Without an obvious suspect, I don't even know where to starthttps://uploads.tapatalk-cdn.com/202...e9e9721092.jpghttps://uploads.tapatalk-cdn.com/202...5bf190aff2.jpg
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Give me a model number on the sub...
sent from Utah.
Here's my current little project. It's been sitting on the shelf for 6 months. Yamaha r2000 150wpc monster receiver. Supposed to be nice, but was all fucked up when I got it. Someone fried the output boards and the input stage of the amp. Someone also tried to fix it, and there were all sorts of wrong parts on the boards, And burned traces. Junk show. So it sat. But for some reason I pulled it out, and started working. Almost done rebuilding output boards. I bet it still doesn't work... But we'll see.
Attachment 348486Attachment 348487Attachment 348488
sent from Utah.
It's an AV 123 x-series x-sub. I didn't find a wiring diagram on a simple google search.Quote:
Originally Posted by basinbeater
They're out of business now or else I would have contacted their tech support. The av123 website has an Asian hottie there cyber squatting for someone, so that's nice.
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Yeah it does! Did you scan the qr code though?Quote:
Originally Posted by DJSapp
I can't find a service manual either.
Start at the output, measure between the two output plug prongs, look for acv when the amp is powered up. Jumper the input. Ac there? Thought so.
sent from Utah.
Wait, what?Quote:
Originally Posted by basinbeater
Let me see if I understand what you're saying here: First, you want me to connect everything except the sub and test for AC on the sub output? Then you want me to test for AC across the transformer output (blue and black wires) while everything is hooked up? I think I can manage that without frying myself.
And no I didn't scan the QR code, I figured that would just give me cyber herpes.
Yeah, that's pretty much it. You only want to power up the amp. Jumper the input. Nothing connected to output. We want to figure out what stage to hunt for the problem. First. Let's find rail voltages. After the large filter caps, you should see traces going to the amp section of the board. You have a positive rail, a negative rail, and ground. Positive dc rail voltage ought to be somewhere in the range of 35 to 45v. Negative rail should be in the same range, just negative. Always measure with respect to ground.
Now that you have found the rails, switch to ac on your meter, and measure for ac across the rails. So +rail to -rail. What is the ACv there? If it is 0, or very close to 0, then the ac is not coming from your power supply.
Just to be clear, following my advice could lead to frustration and a messed up amp. But it's messed up already right?
sent from Utah.
Dude! I was using a Sony Home Theatre 'in a box' type kit with the satellite speakers for well over a decade. Sounded ok, I was content. Last year the receiver stopped working so I begrudgingly bought a new (pretty low end) 5.1 receiver. Since then I 've been watching kijiji (that's craigslist in Canadian) for deals on speakers and have upgraded the centre and front channels very economically and it sounds about a million times better already. It's the type of thing I really wish I did a long time ago. Just my 2 cents.Quote:
Originally Posted by Gerome
Quote:
Originally Posted by DJSapp
Ok, I think I know what you're asking. When you say 'Jumper the input' you're telling me to reconnect it to wiring harness clip on the second board, correct? I believe the rails you're talking about can be seen in the picture on the left edge. The board in the background, you can see three silver lines turning 90 degrees. The green wire is attached to the middle line but I didn't get a picture of the topside of that board. I believe those lines run from the input power connection through both of the big caps and then... somewhere. Maybe those big resistors?Quote:
Originally Posted by basinbeater
And when I'm checking DC voltages I put the black probe on the ground lug and the red bit goes exploring, right? When I'm searching for AC voltage, I'm on positive and negative DC rails. Just asking to be very clear, I'd rather not let the smoke out from any of these bits.
Other potentially dumb/smart question: Does it make any sense to try and tighten the nuts on the transformer core, or the connection between the transformer and the heatsink?
If anything is loose, tighten it. Jumper the input means you want to jump the signal line to the ground line. Outside ion an rca is ground. Jump the inside to the outside. Prevents any noise from being picked so you get accurate measurements. So if it is an rca, you can cut the cable, find the inner conductor, and ground conductor, tie them together and then you are left with a simple short for the input. Maybe I should have said short the input signal to ground.
Positive and negative are not actually physical rails per se. They are the main power distribution lines on the circuit board. The two large capacitors will meet in the middle one side positive and negative, and that will be ground. The one with negative in the middle, will have a positive side that supplies positive rail voltage. And vice versa for negative side.
Circuit diagram would look something like this. Squiggly lines are your transformer, box with r diodes is rectifier (splits the ACv into pulsing dc). C1-c4 are the main filter capacitors (yours only has two large capacitors and this one shows 4), c5 and c6 are higher frequency filters, your may or may not have something similar) and r1 and l1 and r2 and l2 are just led indicators that the rails have power. Probably not present on yours. See at the end, v+ gnd and v-. V+ and v- are your rails and gnd is ground. It is likely that all grounds in the rest of the amp come back to this ground point. This is called a star ground. Grounding everything back to the same spot eliminates ground loops that would otherwise be present by grounding shit all over the place.
Attachment 348606
sent from Utah.
Black probe to ground on the chassis is fine, or at the star ground point. Attach it so it stays put if you can, then you can start poking around.
sent from Utah.
Transformer to heatsink is not a big deal, but main output transistors to heat sink (and possibly other transistors depending on design) does need to be tight and consistent or you get thermal runaway in one of the output transistors and pop! But I don't think that is your issue.
sent from Utah.
That makes a lot of sense, circuit diagram really helps me even just in concept. Will poke around this evening and will report back. If you don't hear from me by tomorrow, I bbq'd myself somehow. You can have first pick on my quiver.
edit: crap I forgot to jumper the rca jack. Standby
Ok, I think I got it figured out. Checked dc voltage, 41.0v at the red dot, -41.0 at the blue dot. 0.0v ac across them. Wasn't getting a reading off of the rail to the left of the ground rail though.
Did the next thing that made sense to me and disconnected the transformer from the boards. Powered it back up and it's the transformer that's buzzing. Nothing else was attached except the fused power switch. I guess being in the box makes it a bit louder and more noticeable.
Tried to tighten the nuts and screws on the transformer coil, but they're right and would require excessive force to move, so I didn't push it.
Just a bad/noisy transformer then? How can I go about figuring out what to replace it with, and where do I find one?https://uploads.tapatalk-cdn.com/202...5102a4cf16.jpg
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Laminations could have come loose in the transformer. So it hums? That my guess. Rail voltages look good, so I am assuming then that power supply is good. You could try using a glue gun to reglued the transformers laminations. I have not run into this problem.
sent from Utah.
Ok, jumpered the rca with a grounding wire from a romex scrap. Good and snug in there.
So no difference in the dc voltages, and the AC still found its way to zero, but it seemed to take a bit longer with the jumper than without the jumper. Like 3 to 5 seconds to arrive at zero volts with the jumper
Edit: and I did hook it all back up to the receiver and the speaker itself is humming at the same frequency as well. Same pitch as the transformer him, but much louder when the rca is plugged in.
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Does the speaker reproduce the correct tone when you play a signal though it?
Measure the dc offset. Power the amp on, and with the speaker disconnected, measure dc voltage between positive output and ground. Then measure the same from positive output and negative output. Should be less than .1vdc. If it is higher than that, it should be going into protect.
To verify if all the transistors in the amp are ok, power the amp down. Let the main caps drain. Put your meter on diode check mode. Touch probes together, should make a tone. Now you are going to locate a transistor and touch the probes on 2 of the three pins. If you get the tone, bad transistor. If no tone, you should see overlimit or 1, OR you should see a voltage drop number. Play around with different combinations of the probe on the three pins. Any beep on any combo means a failed transistor.
http://www.learningaboutelectronics....t-a-transistor
My next guess is a failed component in the amp circuitry. Bad capacitor somewhere. That is letting some ac through that should be blocked. Testing capacitors is tough. But, if you put the black probe to ground, set meter to lowest ac measurement. Power up amp, and start looking for ac on either side of capacitors in the amp circuit (post large filter caps).
sent from Utah.
Speaker is not producing the correct tone. Not producing much at all actually. It buzzes/hums, but doesn't thump.
DC offset? I thought I did that. 41.0 on the positive, 41.0 on the negative. When I checked again with the RCA jumper, I was getting 40.6v and -40.6v. Was I supposed to have the speaker connected before? I thought I wasn't.
No,. You measured the rail voltage correctly, and those voltages sound good for a sub amp.
Dc offset is how much dc is present at the speaker output. Less is better, my home made amps are all around .005v or so. Most commercial amps are between .01 and .05 v. So, put dc meter on speaker outputs positive probe to positive output, negative probe to negative output (it's really ground) And then put ground probe on chassis ground. Number should be the same. This should have been one of the first tests, but I spaced it. Perform this test with amp powered on, input shorted, no speaker connected.
Some people say shorting the input is not necessary, but I've had funky readings in the past that were due to noise being picked up by a floating input, and best practice is to short it.
sent from Utah.
Also, I could do a video call with you and show you how to test a transistor while it's still in the board. It's harder to explain than to show, and then you can motor through those.
sent from Utah.
IIRC, set the multimeter for max ohms, red probe in the middle, touch each leg with the black in turn. Then switch the black and red probes and do it again. Good transistor and you'll only get a reading on one setup. Bad transistor and you get readings on both or neither. Right?Quote:
Originally Posted by basinbeater
I think my mm has a diode setting as well. Not sure how that setting works though.
Throw it on diode setting. A transistor (oversimplified) acts sort of like a switch. When the base receives voltage, it closes the switch so voltage and current can pass from the collector to the emitter. So, on diode testing mode, with no power to the transistor, if you get any sort of beep, it means there is continuity in the transistor. That is bad. If no continuity, it will either read overlimit or give you a voltage drop reading. The manner in which the probes are hooked up to the transistor to obtain a voltage drop reading depend on the type of transistor being tested. There are two main types of bipolar transistors. NPN and PNP. They are opposites to each other. But none of this really matters. If you play around with the probes, and the meter beeps, you have found a bad transistor.
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