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9090 DC spikes on outputs and speakers solved

markkb

markkb

AK Subscriber
Subscriber
I was getting occasional zap-thwat-switht-zpfft sounds from my right channel audible through my speakers. It eventually got to be too frequent to ignore, and I pulled the 9090 out.
I checked the bias and DC offset, during the DC offset check I observed the current spiked up on the voltmeter when I heard the sound. Other symptoms were a UV meter spike at the same time of the sound, and a low right channel output. I had the unit recapped a few years ago so I suspected the original transistors on the driver board. I pulled the cover off and was all set to start my problem scoping thread here on AK.

I pulled the driver board and looking over the components noticed black transistor legs. This ‘sparked’ my synapses. I’ve read here on AK of black transistor legs conducting electric current (carbon buildup?)

I scrubbed the legs with a ‘eurotool fiberglass scratch brush’ and cleaner....reinserted the board and problem solved! I was psyched at both my application of AK knowledge and how painless the fix was!
After, I wished I had done some experiments, like cleaning one transistor at a time. Been running for a week with no recurrence of DC spikes.
 
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markkb said:
I was getting occasional zap-thwat-switht-zpfft sounds from my right channel audible through my speakers. It eventually got to be too frequent to ignore, and I pulled the 9090 out.
I checked the bias and DC offset, during the DC offset check I observed the current spiked up on the voltmeter when I heard the sound. Other symptoms were a UV meter spike at the same time of the sound, and a low right channel output. I had the unit recapped a few years ago so I suspected the original transistors on the driver board. I pulled the cover off and was all set to start my problem scoping thread here on AK.

I pulled the driver board and looking over the components noticed black transistor legs. This ‘sparked’ my synapses. I’ve read here on AK of black transistor legs conducting electric current (carbon buildup?)

I scrubbed the legs with a ‘eurotool fiberglass scratch brush’ and cleaner....reinserted the board and problem solved! I was psyched at both my application of AK knowledge and how painless the fix was!
After, I wished I had done some experiments, like cleaning one transistor at a time. Been running for a week with no recurrence of DC spikes.
Click to expand...
Nice job. However it is only a temporary "fix". You have discovered the cause of the noise, but the transistors will be needing to be replaced before long. The "black plague" actually creeps up inside the transistor and will eventually short completely internally. The next time you may not get such a graceful warning. :D
 
Skywatcher, thanks for the explanation that this is a temporary fix.

Eastpoint, the fuse resistors were all replaced when the unit was recapped, but I’ll definitely plan to replace the transistors and the trimmers on the driver board as well.
 
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