Those are beautiful photos. Actually, the two photos align up correctly as you wanted when viewed on my iPad.

Our objective is to do what it takes to stop smoking anymore components.
After that, we can move on to take voltage measurements. The correct expected voltages are well documented in the circuit schematics. There are still bad components on the board that result in the wrong voltages and blowing components. Each of these bad components has to be located and corrected before moving on.

For starters, C425 1μF/100V must be replaced.

Which flat cables at which connector have been removed?
If there was smoke coming from the front panel then don't plug those in yet.

 

@Dodgydave you are correct, my DDM leads were set to measure Amps

Happy the pictures turned out that way

There are three flat cables going from the main board to the front board.
You can see three white connectors in three groups to the right in the bottom picture.
Ok, I will leave them disconnected.

So I will now replace C425 again.
But, when C425 blew up, R414 was smoking at first, should i replace it too?

 

Leave the flat cables disconnected for now. (Of course, the VOLTAGE CONTROL light will be off).

Replace R414 100Ω ¼W for now and hope for the best.

(How come you happen to have all the right parts available? Resistors, capacitors, transistors? Do you have fairly wide stock at hand?)

 

Ok.

So now i replaced the capacitor C425 and R414

Turned it on.
No smoking, or blowing up the capacitor.
It seems stable.
The front pcb was not connected, so no voltage control light as you mentioned of course.

I do not have so much components, when i looked over the main pcb the first time i ordered the reistors,capacitors and transistor
i saw needed replacing.
I ordered from Ebay, and they sell in such large quantum and costs almost nothing.

 

Glad to hear that the smoking has stopped.

Now the fun begins with meticulously checking that all the voltages are correct.

How good are you at reading circuit diagrams and finding the corresponding node on the circuit board?

It is imperative that you take great care with your meter probes in order to avoid creating a short with adjacent leads and components.
Do you meter probes

Do your meter probes have proper pointed tips?

You can reduce errors by using an alligator clip on the end of the -ve (black) probe and clipping this to the chassis.

For the +ve (red) probe, you can use a normal pointed tip that comes with the meter or you can use a miniature grabber hook as shown in the photo.

This would allow you to hook on to resistor leads and free up both hands after clipping.

Make sure there is no way of creating shorts while hooking on to a resistor lead.
As I mentioned earlier, if your DMM is set to read DC VOLTS there is little danger in the meter causing a problem.

 

We will be checking the voltages in both LEFT and RIGHT channels for comparison.

For starters, look at the following nodes:

LEFT CHANNEL
Q413 emmiter - R427 7.8V
Q415 emitter - R429 -7.8V

RIGHT CHANNEL
Q414 emitter - R428 7.8V
Q416 emitter - R430 -7.8V

Next
LEFT CHANNEL
Q427 emitter - R449 0V
Q429 emitter - R451 0V

RIGHT CHANNEL
Q428 emitter - R450 0V
Q430 emitter - R452 0V

 

Thanks for helping.

I am actually a car mechanic, so i have read some diagram's... maybe I have to get things better explained if it gets complicated, but i'll try my best

I will take great care, especially after my little incident earlier..
Doesn't have a little grabber or an alligator clip, only the ones who came with the DMM, I will buy it tomorrow to make it easier for me.

I will check if i can take the measurements on the transistors you have mentioned with the gear I have, or maybe i have to wait to get better probe's.

 

You may find that it is easier to touch the lead of the resistor rather than the leg of the transistor.
If you don't know which side of the resistor to test, test both sides, one at a time, and give me some sense of orientation with respect to the circuit board.

For the second set of measurements, R449 to R452, either leg is acceptable. The resistances are so low one would expect the voltages to be the same on both leads.

 

I am sorry.
We have to go back.
When i took the photos for you of the main PCB, i took out the two plugs that feeds the main PCB with power because they were in the way for the photos, CP607 and CP606

When i tested the amp yesterday, after replacing R513, R414 and C425 ... I hadn't plugged these in again.
I was going to take some further measurements of the Q's you listed when i noticed.

So I plugged in CP606 and CP607 before i turned it on..

R513 smoked instantly, and after a second C425 blew again

Happened nothing to R414 though.

 

R513 and C425 blowing are unrelated.

Do not replace R513 as yet.
Please remove Q505, Q509, Q515, and Q517. Make a note of which transistor is which.
(I know Q509 is already removed).

As for C425, we need to verify the proper orientation of the capacitor. The most likely cause for it to blow instantly is that it is installed backwards, i.e. with the polarity reversed. There is 60V applied across this capacitor. If it is backwards it will blow instantly.

Is it possible for you to take a photo of the underside of the board so that I can see how C425 is connected?
If not here is a test.
With POWER OFF and AC power cord disconnected, set the DMM to lowest resistance setting. Measure the resistance between the GND and each one of the pads where the capacitor is connected.
You can use the exposed legs of the blown capacitor.
One side of the capacitor is connected to GND. This should be the negative leg of the capacitor. We need to find which leg is connected to GND. The other leg should be connected to R505 (6.8Ω).

You need to bleed off the charge on those big capacitors C606 and C607 by shorting one of your 100Ω resistors between R505 and GND, and then again between R506 and GND, otherwise your meter is going to see 60V and can cause some damage to your DMM while in resistance range.

Make sure you return your meter back to read DC VOLTAGE when you are finished with the meter.

 

Okey

I have now removed Q505, Q515, Q517 and marked them.

When it comes to C425, it's mounted the way the old one stood, the negative connector is shorter and marked with a grey stripe as you can see in my earlier pictures. Could it be marked the wrong way?
See the picture of how the original capacitor looked like, which I replaced.

I also took a picture of the underside on main PCB as you said, C425 is marked with a red dot between the connectors.
Negative connector will be to the right of the red dot in the picture.
If the picture isn't good enough I will take the test as you mentioned over.

 

Hello, I think the capacitor is reversed.

As you can see on the image of the underside of the board, C425 y C426 have a common terminal (-), so you connect the C425 reverse.

best regards

 

Edit: Disregard what I wrote below:

C425 was installed backwards. The -ve lead should be further away from the heatsink.

Strangely, C426 is also installed backwards. Why it hasn't blown, that I have no answer.

You would be wise to replace both C425 and C426. The +ve lead is closer to the heat sink for both capacitors.
C426 is on the -60V supply line. Hence its +ve lead is connected to GND, while the -ve lead of C425 is connected to GND.

I forgot that I was looking at the bottom of the board and the heatsinks have been flipped from the left to the right.

It appears that C425 and C426 are installed correctly after all.
Now we have to analyze why C425 keeps blowing.

 

Until we can come up with a valid reason for C425 blowing (e.g. excessive ripple), leave it out for now and see if you can get those voltages on post #26 to get us moving on.

 

Okey.
Good to know the capacitor is set the correct way, it just couldn't have been the other according to how the old one was set.

Turned it on today to measure voltages.
And suddenly R414 smoked instantly, and after it stopped, R448 smoked out... i have replaced both these earlier, without knowing what killed them, so Ok..
I was going to just take a picture of it smoking when it just went up in "flames".


Just a little question before i measure.
With the text from #26 right below her, do you mean measuring between the Q and R ? either leg on resistor?
or between each of these components and GND?
Just to have it clear.

LEFT CHANNEL
Q413 emmiter - R427 7.8V
Q415 emitter - R429 -7.8V

RIGHT CHANNEL
Q414 emitter - R428 7.8V
Q416 emitter - R430 -7.8V

Next
LEFT CHANNEL
Q427 emitter - R449 0V
Q429 emitter - R451 0V

RIGHT CHANNEL
Q428 emitter - R450 0V
Q430 emitter - R452 0V

 

A node is a junction in a circuit, both on the circuit schematic as well as on the actual circuit board.

Practically all circuit components would have two or more leads connected to the component and therefore would have two or more nodes and therefore multiple points at which to take voltage measurements.

Asking for the voltage at resistor R414, for example, is ambiguous since there are two leads on the resistor.
Specifically, a voltage measurement at a node is always with respect to a reference point. That reference point is always GND unless explicitly told otherwise.

I am going to simplify my instructions in order to make it easier for you and to avoid ambiguity.
It is sometimes difficult to reach the legs of small transistors. Therefore I will ask for a voltage measurement at a resistor. This means that you will take two measurements, one at each lead of the resistor, and measured with respect to chassis GND. It will be up to me to have to determine which measurement applies to which side of the resistor.

Remove transistors Q424 and Q426. Label these and store in a safe place, e.g. in labelled envelopes or taped to a piece of paper and labelled.
Similarly, remove Q406 and Q408 and save and label.

Let us see if we can stop any further smoking with the power on.

If no more signs of smoke, measure the voltage at R427 and R429

 

Sooner or later, you will have to make a decision on how much time, effort, and money you want to spend on fixing this. I will give you a synopsis of the situation.

Both LEFT and RIGHT channels of the power amplifier are blown. This is because the DC balance in the amplifier circuitry is out of whack.

At this stage, I would say that you have two options in repairing the amplifier.

Option #1 is the shotgun approach where you remove and replace all the transistors and IC401 and IC402 in the power amplifier section and then hope for the best. This is not my style of diagnosis and repair.

Option #2 is to remove all the associated transistors in the power amplifier stage and rebuild the amp from left to right, one channel at a time. We would be doing ohmic test on the original transistors before attempting to reinstall them into the circuit.

In either case, this would require having a stock of replacement transistors on hand. It may also require having to purchase replacement components when deemed to be faulty. This could become a long drawn out process as we wait for replacement parts to arrive.

 

Thank you for the overview of the situation in the amp.

I think this is interesting and I'm learning something new every day, so if you can bear with me and are willing to help me out further i want to go for option #2, but of course, it can take some time to get the right components shipped, but i have no rush.
It's not about the money over what it's worth either, i just want to fix it, if i could do it with your great help i would be very excited over it.

So you are telling me to remove some transistors to stop smoking.
But the way it sits now, after it smoked R414 and R448 yesterday, it's no more smoke.
Has been on for a while now.

Since no smoke, I measured the voltage at each foot at R427 and R429 to GND, no voltage at all on neither of them.

Or should i remove the transistors you mentioned anyway?
Q424, Q426, Q406 and Q408


By the way, could i take out these two signal PCB's to get better access to the main PCB ?
Or do they have to be in there?

 

I believe most people troubleshoot that way. It's the way I was taught and used for years. For those who are familiar with electronics, a much easier and faster way is an octopus. (google oscilloscope octopus) This was necessary because there was no documentation at all........the manufacturers didn't want anybody to be able to repair the boards.

When I was first introduced to this....I was leery because I believed the interpretation of the patterns was a little on the iffy side......trying to decide if the pattern was a normal pattern in that particular circuit or not. But that is not where the value is. The value is being able to determine the validity of a gate or PN junction with just a probe touch. And the same with caps and coils. The pattern doesn't verify the value of a cap or coil.....but it verifies the function. All inputs gates and output gates on all solid state devices.

After a while.....one can intuitively recognize much more. But what makes the tool the easiest and the best......is not to be able to interpret a pattern........it's whether the pattern is present or not.

A super, no doc, no power, in circuit, troubleshooting tool. I haven't used one for quite a while....and with the new chips.....one might want a lower voltage setting.

Sorry....I know this doesn't help you, Marius83........but you could spend a lot of time on that dog. But you should be able to solder pretty good by the time your done. Mind the heat on the board trace. If you like electronics....you should consider a scope.

 

I don't mind helping out. Fixing systems such as this one is where I get my kicks.

I will try to explain why such a complex system as this is so difficult to diagnose.
The entire section of the power amplifier stage is DC-coupled and is stabilized using negative feedback. Multiply this by two for two channels, LEFT and RIGHT and we have double trouble.

Negative feedback is a good thing because it keeps things in check and prevents run-away or going into extreme overdrive. Negative feedback, in mechanical terms, is like a speed controller or a centrifugal governor on a steam engine. When the engine tries to speed up, the governor weights move outwards and close the steam valves, thus attempting to reduce the speed of the engine.

That is the same with this amplifier. That works well when everything is working. However, when something is out of whack and the negative feedback is no longer functioning, the amplifier goes into overload and something has to blow. In other words, every component in the system has to be doing its job right otherwise the entire system doesn't work. Now it is our job to find what is not working and to correct it.

The plan is to start at Q401 and Q403 on the LEFT CHANNEL and work our way from left to right in the circuit schematic and make sure all the voltages are in an acceptable range as documented in the circuit schematic. We do the same on the RIGHT CHANNEL.

If you look at the schematic, you will notice two power rails, one at the high side of Q401, Q405, Q409, Q413 at +60V.

On the low side of Q403, Q407, Q411, Q415, we see -60V. That is a total of 120V across the entire circuit and that is a lot of voltage that can produce a lot of smoke and sparks as you have already experienced.

In between the high side and the low side we would be looking for about 0V. If we don't see 0V or what is documented in the circuit schematic, then the DC condition of the circuit is out of whack.

If there is no smoke, then we proceed with caution.