Krell KSA Repair

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#21
Tested ok. Problem fixed. What a challenging experience. Took me several days before I nailed the problem. I called this episode as “hunting for gremlins”. Haha. Overall very satisfying. So I thought I should share this as this amp is a collectible item and there are many of these amps still in operations.
 

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#23
KweeSong Lim said:
Throughout my more than 50years of amplifier building and repair I have never encountered such a failed mechanism.
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I have....... more than once. As soon as the PCB turns into a carbon/coal-like substance, it starts conducting. I have actually seen
circuits, where the PCB-material has been constantlly glowing red before the fuse finally gave up.
Nice, thorough repair. Well done. Congratulation with the result. A fully working Krell is one of the good things in life 👍 🙂
 
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#25
KweeSong Lim said:
I wired a jumper to bypass this defective PCB trace. Re-solder the capacitors. The short has been located and isolated.

Subsequent test of the repaired channel after reassembly showed that the problem is fixed.

Throughout my more than 50years of amplifier building and repair I have never encountered such a failed mechanism.

Would like to hear if anyone else has encountered such an incident.
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I've seen similar faults on some Arcam equipment. The failure mechanism on those was that the capacitors leaked electrolyte onto the multi-layer PCB, which then over time caused corrosion. This corrosion on multi-layer boards can cause internal leakage which then starts to get warm due to the resistance. With heat the board starts to become conductive which can then also affect the traces. Once they short, they can catch fire in spectacular fashion.
 
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#27
KweeSong Lim said:
Dear Boydk,
thank you for your note and sharing your story too. So it may not be a multilayer board with ground plane but a deteriorated substrate? interesting indeed.
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You´re welcome. Although I can´t be 100% certain, the KSA 250 is from the beginning of the 90´s, so I seriously doubt,
that they are multi layer boards. Bigger chance, that @kramtweeter is correct, with some substance of leaked electrolytic fluid has soaked
the pcb, which then has started conducting, and from there only gotten worse day by day 😉
 
#31
I had a repair on a Marshall TSL100 amp that had a partially conductive circuit board. Turned out that tracks around the output tubes would leak plate/screen voltages into the grid, so the bias would just climb until the outputs red-plated. Took some diagnosing. much as you did. It wasnt dead short, but as voltage diffs are a few hundred volts there, and the grid bias high-impedance, it didnt take much leakage....

A similar method to yours fixed it. (Cutting tracks, isolating grid pins, adding wire links.)

Apparently quite common with earlier Marshall DSL / TSL amps. Marshall changed the PCB material on later revisions.

Cheers, and regards,


Ant
 
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#32
KweeSong Lim said:
Received a Krell KSA 250 for repair from Kuala Lumpur Malaysia. The owner is a friend of my friend in Singapore. The owner couldn’t find anyone to repair his amp.

One day he turned on the amp, the relays clicked and then he saw spark coming from the left channel and then relay clicked off. Subsequently couldn’t be turned on again.

It was a September 1991 date of manufacture so it’s 32 years old. View attachment 1206260
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The Malaysian friend of your friend in doesn’t happen to have the first name Najib? 🤣
 
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