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ldervish

Hum in Scott LK-48

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I am new to this community and hope someone can inform me about my LK-48.  It is a brown face, no headphone jack, 7199 phase inverter model which I have been unable to find an exact schematic for.  It was completely rebuilt several years ago, at which time it developed a hum that is most noticeable on startup.  The rebuilder was unable to clear it.  After warming a bit the hum subsides but remains.  There are balance trimmers between the output tubes, one for each pair.  I have no idea how to use them and have not tried.  On this model the output heaters are connected in series to provide the bias voltage and so far as I know there is no "hum balance" in that part of the circuit as there is in some guitar amplifiers.  So can someone enlighten me as to where to look for the source?  I have some electronics training and basic tools including variac, multi-meter and etc. (no scope or signal generator though) and know about safety 'under the hood'.  The s/n is 107460 if that helps.

 

Many thanks for your interest!

Edited by ldervish
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Correction:  The bias is controlled by the pre-amp tube heaters, not the output tube heaters.  

 

I have emailed Craig per JJ's suggestion and am awaiting his comment.  Thanks.

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Much appreciated, Lucky.  I did a search but missed that one.   Symptoms are the same, so as time permits I will head to the bench to check potential culprits.  Biggest odds are on the multi-caps.  One can was obviously replaced during rebuild but the other 2 could be original.  I'll have to look under the chassis to see if new components were mounted there.  

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hope it works out.. my rule of thumb is if it hums, usually cap related, if you hear crackling from your speakers, transistor related. I think you have those two main filter caps in middle of chassis under the cans. Sometimes they don't get replaced due to cost  Original Mallory'? That Scott is a sharp unit. Good luck !   ron

LCR meters are a good investment. 

 

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The Mallory has been replaced below deck, but the bias filter quad 75uf looks to be original.  After checking, tightening, searching for ground loops and judicious use of deoxit it still had the hum so I followed a couple tips Craig passed along, and ended up  swapping the 7199's with a known good (and quiet) pair.  Then balanced the outputs "by ear" and was able to reduce it to a barely discernable level.  It's not "dead quiet" but the very next thing to it so I'm pleased with the result.  It's nice not to hear that hum between tracks of music.

 

I have a Wavetek digital multimeter that performs some of CR functions, but it has limits.  Is there a modestly priced  LCR unit that I should look at?

 

Thanks for the interest, all of you,  I appreciate the help and advice.

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