KONIG PA 10000 (2 x 500Watt) amplifier, no output in one channel repaired
This amplifier was given to me for repair after an unsuccessful repair intervention of another technician, who unfortunately left permanent traces of vandalism in its PCB trying to repair it without even using common sense in the methodology he used (if there was any). You will see why I am saying this below… This is a general view of the amplifier’s front panel and inner circuitry:
And this is the inner circuitry block of the pre-amplifiers and the power stages:
Below you can see the rest of it, namely the PSU PCB along with its toroidal transformer and the loudspeakers’ connection PCB.
As you can easily understand, this is a monster amplifier and therefore it needs a huge heatsink to cool down its output transistors which become very hot in a few minutes, even having the loudness potentiometer fully closed. This is due to the high quiescent currents used to correct audio distortion at such high power design. That’s why the system does not relay on conventional cooling exclusively, but instead it is supported by a blower which works permanently and automatically. The reason is obvious and needs no further explanation.
The ventilator is located at the left corner, right after the heatsink, in the back side of this assembly and it is in line with it in order to achieve the highest degree of cooling efficiency. See below:
And also below:
And here below you can see (among other points not shown by photos for the sake of space economy) the reason I spoke about vandalism during the repair attempt of my unknown colleague…
Seen once again these photos, let me please give you a piece of advice when working with assemblies like this one, in order to avert any vandalism that might result from your improper intervention:
- Be patient and reasonable. This is of utmost importance and above all.
There is no reason to hurry up when you face difficulties. Doing otherwise will mislead you and deprive your ability to see things clearly and find proper solutions to the problem you have in front of you. This in turn might bring with it negative feelings of frustration pressing you either to follow wrong steps that will end up to great trouble indeed or even force you to quit your attempt, failing to withstand the battle and perhaps rendering the device irreparable any longer…
- Use your imagination, which I am sure you have (at least just because you are involved in electronic repairs).
- Treat the device gently, especially it’s PCB(s), and in such a way as you would like your surgeon doctor to treat you during a surgery on your body. Try to remember this analogy of actions every time you have your soldering iron in your hand and before using it…
- Keep finally in mind that the purpose of your intervention is not only to bring the device back to life, but to secure its longevity after that. Not to put a death date on it.
In case you are a professional, the first above will bring you more customers in the future, while the second will very soon spread a bad reputation about you resulting in serious loss of customers, perhaps to the level of total business failure in case your business is situated within a small community.
As regards money making, let this follow your attempts and not (mis)lead them, due to the aforesaid reasons. And my technical advice for these cases is:
- Having such an assembly in hand, remove the heatsink first from it and then the semiconductors from the PCB. This will save the integrity of the PCB. So,
- Remove the screws retaining the semiconductors on the heatsink
- Remove the screws retaining the heatsink iself (if any. In my case there were no screws at all. The retainers of the heatsink were simply the semiconductors themselves).
- Free the heatsink from the assembly
- Now, with the PCB free in your hands and at your own pace, locate the defective power semiconductors.
If you already did that, it is now very easy and safe to remove them, despite the fact that they are soldered in the reverse way than the normal, that is, they are soldered directly at the solder side of the PCB and not at its component side, this being the cause of the need to remove the heatsink first from the assembly.
My colleague tried to remove them directly, trying first to desolder them, but this is impossible as they are not fully accessible from this side. He did not use desoldering braid as well, working probably with a sucking pump. But even working with an electrical desoldering pump, the back side of the power semiconductors remains soldered. No way to use the desoldering braid there either.
Moreover, his action had as a result the overheating of the PCB’s foil traces up to the point of having foil trace lifts in many points of it.
The explanation is again obvious. There was a high level of heat transfer from the points he worked through the terminals of the semiconductors which ended in this huge heatsink. So, in order for him to melt the solder, he kept increasing the soldering iron’s temperature probably to its maximum which caused those foil lifts…
- Replace the defective semiconductors with the new ones, taking care of the soldering distance of their terminals. Use the heatsink as a guide, making sure that the screw holes of the semiconductors coincide with their respective ones on the heatsink. Solder them afterwards one by one.
- Install the (new if possible) insulators putting thermally conductive paste wherever needed and
- Upon completion, reinstall the heatsink putting every screw back in its place and you are done.
Coming back to troubleshooting and the reason that I disassembled the unit, I did this first because of the obvious vandalism signs and then because all the fuses were intact and I needed to know what was the general condition of the solder joints of the PCB. Along with that, it was impossible to replace anything except the fuses without this disassembly…
With the PCB in hand I checked all the transistors of the circuit without to find anyone faulty. I also swept it for bad caps with my ESR meter, again without to find anything. Then I resoldered all the joints just for prevention against any “non typical failure” and cleaned the circuit using flux removing spray.
And the question was still there. What was the cause for this failure?
Well, there were not many suspect components left on it. I removed the two protection relays from the PCB and tested them out of circuit using my bench PSU. The first I took in hand was stuck. I could not hear any clicking noise when applying excitation voltage on its 12V coil terminals. The culprit was there. I decided to replace both the relays. The blue ones you see are the new ones.
After their replacement and the following reassembly of the entire device, the monster woke up with both its eyes wide open this time!
Finally it’s a pity that my colleague did not manage to complete the repair, as he did the most of it replacing all the defective semiconductors he found and missed a simple relay failure…
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Note: You can check out his previous repair article below: