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Old 6th April 2012, 02:14 AM   #1
mikemac is offline mikemac  Australia
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Default Help with diagnoses for leak 20 PLEASE

For the last couple of days my leak stereo 20 has been micraphonic,. Noisy when touched. About two hours ago, I gave it a good tap, as u do, to see what was buzzing and now have all the symptoms of a short O/T, low volume in one channel
I have changed valves, checked the O/T checked for shorts etc. there is no visible damage to any components and no problem up or downstream: I checked source and speakers.
There is sound in both channels until it warms up and then one drops out. I have had a beer or two and don't want to check the voltages at present. Any thoughts?
Easter break and no music, not good
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Old 6th April 2012, 07:24 AM   #2
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Maybe a leaking electrolyte capacitor? No other choice but to measure anode supply voltages. Did you notice excessive heat on the mains transformer? Maybe the anode voltage is getting dragged down by an excessive load such as the cathode resistors of the power tubes. I assume they are correctly biased. Worth checking the negative grid voltage to the power tubes.
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Old 6th April 2012, 12:03 PM   #3
DF96 is offline DF96  England
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Check DC voltages.

What sort of noise do you get when you touch it? Could be RF oscillation, although why it should suddenly start is unclear.
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Old 6th April 2012, 12:42 PM   #4
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Once a transformer goes it is usually gone for good.

If it works for a while I would be looking at caps.

BUT, transformers can work OK if the failed insulation is still keeping the windings apart until they warm up.

You might need to substitute L to R to ascertain if the ouputs transformers are OK.
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Old 6th April 2012, 12:58 PM   #5
limono is offline limono  United States
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When I've got Leak ST-20 all capacitors (I mean all of them) were leaking DC badly and needed to be replaced. The name leak stands for something like used to say Joe Rosen a vintage audio connoisseur If your's is not updated it's definitely a time for it. Happy Easter!
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Old 7th April 2012, 03:42 AM   #6
mikemac is offline mikemac  Australia
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All components replaced about twelve months ago. Thanks everyone
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Old 7th April 2012, 07:23 AM   #7
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Quote:
Originally Posted by mikemac View Post
All components replaced about twelve months ago. Thanks everyone
Including valve bases ?
This amp has a pentode front end, check all connections around it, esp earthing.

BTW. I get alot of faults from today's constructors...please don't use pliers holding the resistor body..instant damage as shown in pic..as the protection of films is very minimal. The thicker phenol body of an older carbon resistor could take this, but not todays lot.

richy
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Old 7th April 2012, 07:36 AM   #8
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You can get thermal runaway with the Leak 20 see:
my leak stereo 20 exploded this morning

My blog:
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Old 7th April 2012, 08:16 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by mikemac
All components replaced about twelve months ago.
So we may be dealing with a fault introduced then, but which has lain dormant for a while. Dry joint affecting feedback stability?
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