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Old 24th April 2015, 07:12 AM   #1
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Default leak stereo 20 channel imbalance

I have posted about this before. my Leak stereo 20, on occasion, goes real quiet in one channel and super loud and distorted in the other,
I have swapped everything that could be swapped from valves to ins and outs. i haven't swapped transformers but have changed taps,
previously a good sharp slap would either start or stop it
Now the remedy is to put the positive of the RCA into each channel, one at a time so each buzzz very loudly with ungrounded signal and alls good.
Back to a lovely balanced amplifier which i treasure.
seeing both anodes on the first valve share a common filter cap, could one be sometimes hogging all the current, make one channel really loud and distorted and the other nearly unhearable. BTW already basically replaced and checked everything RE components and earth and wiring
if so how to check, how to fix? A diode cleverly placed?
or maybe some clever thoughts from (really) the world of diyaudio
this is driving me nuts. I wont consistently do it so its very hard to plug it in and measure
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Old 24th April 2015, 08:00 AM   #2
nec3 is offline nec3  Turkey
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It happen in same channel ?

Check the volume pot ?

Edit:

Just checked the schematic, no pot is used. So, the only thing can increase loudness may be a loose connection of the NFB RC network from 16 ohms output to the input valve cathode.

Last edited by nec3; 24th April 2015 at 08:07 AM.
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Old 24th April 2015, 09:06 AM   #3
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i will check the NFB
still curious about not earthing the signal, making crazy noise, fixes it
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Old 24th April 2015, 12:00 PM   #4
zelgall is online now zelgall  Canada
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I have a stereo 20 as well that was intermittent - sometimes. If you haven't replaced the tube sockets, you might want to at least give them a thorough cleaning or better yet replace them. They don't last forever and they're over 60 years old now.
Also, check the 1/4W carbon comps at the sockets. They are subjected to a lot of heat / cool cycles over the years.
Really, apart from the general design, the output transformers are the best part of the amp and everything else can / should be replaced without changing the overall sound of the amp.
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Old 25th April 2015, 05:41 AM   #5
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i have replaced everything with 2 watt resistors, except the "fuse"resistor. i also thought about perhaps changing the design, but they bring around 1200 on ebay in OZ and this one is pretty nice. i will never sell it tho, i like it to much, even just to look at "
dirty sockets would not explain one super loud distorted channel and one about 10% channel.
i mean at about 2 on the preamp super loud. and why just putting the positive in the RCA socket to make a racket fixs the problem??
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Old 25th April 2015, 11:40 AM   #6
nec3 is offline nec3  Turkey
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"goes real quiet in one channel and super loud and distorted in the other"

I overlooked this.

Becasue when the malfunction comes up, it kills one channel and excessively drives other, It must be something used in common for both chanels. Which is the first pre-amplifier stage.

Zelgall may be right. You may check V1 tube socket. I have seen these bakelites can be conductive by time. Even bakelite boards with little pins that components are soldered on may be conductive.

You can also compare the supply voltages of each channel when problem occurs.
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Old 25th April 2015, 12:12 PM   #7
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its like schrodinger's cat. if you put it on the bench it stops!
what about the uneven current draw from the first filter cap theory (mine)
and why does partially inserting the RCA input to make, like a ground loop thingy, lots of noise etc, fix it immediately?
i am not an EE but i know enough that this has been bothering me as to WHY for two years at least. i cant see a logical explanation.
before you ask, i have changed, swapped, resoldered, cleaned, poked..you name it
thanks for your thoughts
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Old 25th April 2015, 12:50 PM   #8
nec3 is offline nec3  Turkey
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"why does partially inserting the RCA input to make, like a ground loop thingy, lots of noise etc, fix it immediately?"

"previously a good sharp slap would either start or stop it"

I dont know. Looks like an oscillation caused by a michrophonic tube.

Last edited by nec3; 25th April 2015 at 01:03 PM.
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Old 25th April 2015, 01:07 PM   #9
nec3 is offline nec3  Turkey
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Microphonic tube.. like this:

https://www.youtube.com/watch?v=KtoWCE4fYXA
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Old 26th April 2015, 12:13 PM   #10
zelgall is online now zelgall  Canada
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"before you ask, i have changed, swapped, resoldered, cleaned, poked..you name it "

If you have done all of that, did it include replacing the resistors? If so, it may be your workmanship that is in question. Check the schematic and make sure your connections are correct and the values are right.
I rebuilt mine to the original spec and it has worked fine for over 20 years. There is nothing in the amp that could cause the problems you mention without there being a user related issue.
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