Did a new measument with my multimeter and actually it got 1v set probes to black and grey.
Look at these pics down below
Attachments
Always use scope probes in x10 setting unless you have good reason otherwise. x1 setting has large capacitance and can provoke oscillation when probing sensitive nodes.
I noticed with a thermal camera that resistors R529, R531,R530 and R532 are getting quite hot compared to other components. Not sure what that can indicate.
That's way too much, but probably not enough to trip the overcurrent protection. Have you checked the bias current? What's the DC voltage across the test points when there's no load connected and no signal? You should have less than 100mV across those two.
I think the four transistors all the way to the rights form an overcurrent detector. You could measure the voltage between base and emitter on T125 and T122 to check whether the overcurrent protection has tripped. All of this should be done with no load and no input.
Speaking of input. Does the amplifier even pass signal through it?
They are dropper resistors and can get a bit warm. They should definitely not be hot enough to make it unpleasant to touch them. Measure the voltage across them so we can get an idea how much current they carry. It shouldn't be a lot. They're just powering a low-powered unput section.
@njswede
After letting amp rest I got 2,86mV at test pins and Tr525 I got 2.23mV and Tr522 i have jumping values 0-2mV and them 0-5mV just jumping around. Relay is out and no load or signal. After powering off amp off and on for making measuments my voltages are down to 74v and now when I do new measuments I’m not getting any voltages at all, only jumping up and down 0-5mv. Speaker output I’m now not getting 1v only jumping values. 🤔
After letting amp rest I got 2,86mV at test pins and Tr525 I got 2.23mV and Tr522 i have jumping values 0-2mV and them 0-5mV just jumping around. Relay is out and no load or signal. After powering off amp off and on for making measuments my voltages are down to 74v and now when I do new measuments I’m not getting any voltages at all, only jumping up and down 0-5mv. Speaker output I’m now not getting 1v only jumping values. 🤔
Well all were already replaced with other parts when I bought the amp. I have some original transistors but many were smoked. So I decided to go ahead and replace components with same parts that another user on audiokarma successfully manage to do and got a lot of help but we came to a dead end with my amp so I decided to se if I could get any other help. Behavior on my amp has almost been the same since I got more or less working my way replacing component step by step and hoping for some positive results. I do understand this might not be a good way to work but that’s is were I’m at right now.
@njswede
And please don't tell me you got the replacement transistors from eBay or Ali!
And what does the output look like on the scope?
And please don't tell me you got the replacement transistors from eBay or Ali!
The outputs are 4 npn as a set to do half the wave, and 4 pnp to do the other half. You can assess If the amp is DC here, or at the speaker terminals.
I think you said you tested at the speaker, and got OL on your meter. What does OL mean on your meter. Nothing, or everything? It sounds like a manual ranging meter, that need shifting up a range to get an actual read. I suspect DC in the 80v region, which is what njswede is trying to find out.
You carried on to say moving 0-50mv, but is that OL changed state, or your rails were unsteady?
I note you speak of cn6 a lot, and it seems to control a relay, that brings power to an area you see hot resistors on the + side only. Where this area feeds out to the opamp 501 was it? there as a DC test point of interest. 2 mins have passed since I saw the drawing, but it was a TP on the left opamps input almost. Just a component away.
If this came to you broken, and someone had already swapped a lot, you can be forgiven if you do slide it off the bench onto the floor. There are non standard parts everywhere, so you are tasked with checking the value and perhaps orientation of everything they touched.
Thankfully that drawing is covered in test points to rapidly move through from left to right, to see how early in the chain things go wrong. How far have you got with that?
IC501 and TR551 look like dc offset control. The comparitor is looking at the signal from the pre, and the speaker out. It's output effects the rails around the 551 circuit. These rails are involved in the DC biasing of the power amp input pair. I think the idea is it can shift that around.
It's 6am, and though a good session, I really shouldn't be up still, and certainly not making mistakes like this lol
I need to look at the other chip still. My first reaction was why all this circuitry. It's like they did the discrete preamp to excess. Then did it all again with chips.
It's 6am, and though a good session, I really shouldn't be up still, and certainly not making mistakes like this lol
I need to look at the other chip still. My first reaction was why all this circuitry. It's like they did the discrete preamp to excess. Then did it all again with chips.
The problem is the i'm able to make som measurments if amp has rested for some time but after powering on and off to make measurments my voltages drop to about 74V and my measuments start to jump and become unsteady. The only way i'm able to make good measurments is when CN6 isnt conneted or CN13 wirch is supplying voltages to that part of board.
Something that i noticed for some weeks ago is that somone replaced capasitor C001 with 0.47uf instead of 0.0047 witch service manuel states.
I have replaced it with correct value. Considering this i was thinking maby this wrong capasitor has stressed the Bridge Rectifier just next to it. So I connected my scope to positive and negative lead on it to se if i had any ac ripple or oscillation there. What I cant understand is that the waveform has trouble getting steady, it is jumping up and down all over, look at video down below. I also did same measurment after filter capasitors but still got that jumping waveform. When i place probes on CN-13 to check i'm not getting same problem there. Is this jumping waveform a normal behavior?
Both rectifers read good on multimeter.
I do have CN6 disconnected when measuering this.
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Have you tried to test the power supply in isolation? Disconnect it from the amplifier and make sure it delivers the right voltages without too much high frequency garbage on top of it?
Again, you should only connect the ground lead to ground. It sounds like you connected it to a negative rail. Don’t do that!
And no, it should not jump around like that, but it’s hard to tell what’s going on unless you probe correctly.
Unfortunately I think you need to learn some measuring and troubleshooting techniques before you can proceed with this. The unit obviously didn’t work when you got it and it appears the previous owner did a botched attempt at restoring it. There could be dozens of issues all masking each other. Add to that the fact that the architecture of the amplifier board is a lot more complex than most.
I would try to have someone local look at it. You’re in Sweden, right? There’s a Facebook group called “Elektronik som hobby” with some very knowledgeable people. If you’re lucky, maybe someone can pop over to have a look. I think your chances of tackling this by yourself are slim. I hate to be the guy who tells you this, but I’m afraid it’s the truth.