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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 24th Jan 2021, 6:53 pm   #1
yestertech
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Default LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Really strange 'timed' fault on this one :
After being left on for anything up to 2 hours, in quiescent state, the left channel starts to crackle moderately at first and then becomes louder which builds quickly into a loud 'crashing and banging' and much movement of the test speaker cone. Due to volume of said noise, it becomes necessary to hit the off switch fairly quickly!
After a few seconds at 'off', if switched on again the noise returns at full tilt !
If the amp is left to cool, all is well again and a gentle hiss (from the disc channel ) shows the amp is still working, no smoke or blown fuses.
Clearly a thermal problem, (which takes an age to manifest itself ) I played a heatshrink gun around the output transistors , LS coupling cap, PSU caps and the LH driver board. Not a peep !
Amp is still a happy chappy ...

Measuring with the ESR meter , the LS coupling electrolytic on the LH channel all seems fine. Ditto the electrolytics on the driver board ( albeit measured when 'cool' )
I then swapped the driver boards between LH and RH and left the amp idling again. Sure enough, after a couple of hours the problem repeats but this time it's on the RH channel.
This I would take to indicate the problem is on the LH driver board ( now in the RH channel slot )
It would also seem to absolve the O/P transistors and the LS coupling cap on the LH channel.
On the driver board There is an ancient looking preset which for good measure I will replace, as well as the 4 electrolytics and 2 'tropical fish' caps for starters.
Of course it could be anything on the board !!( no dry joints or broken tracks in evidence ) so any suggestions most welcome.


Andy
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Old 24th Jan 2021, 7:09 pm   #2
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

The 'tropical fish' caps generally don't give problems. I would be more suspicious of the ancient electrolytics breaking down. There's an outside chance it might be one of the driver transistors....I had a BC143 fail but that went short and was an easy trace. There is also the possibility of a resistor becoming noisy but that would probably show up as a rustling noise that varies in volume.

The pot sets up the quiescent current which you adjust by putting a meter in series with the collector of the 'top' output transistor....think it's 30mA.
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Old 24th Jan 2021, 7:36 pm   #3
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

I had this is a Delta 70, think a Lockfit transistor was responsible. I found it with freezer spray.
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Old 24th Jan 2021, 7:49 pm   #4
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

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Originally Posted by Nickthedentist View Post
I had this is a Delta 70, think a Lockfit transistor was responsible. I found it with freezer spray.

Ahh yes....good thinking! If it's got BC147/8/9 lockfits in place of BC107/8/9 then they could well be the source of the trouble. Lockfits are almost 'replace on sight' if there are intermittent faults present.
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Old 25th Jan 2021, 2:11 am   #5
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

"Almost"....?

David
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Old 26th Jan 2021, 7:02 pm   #6
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Here’s a pic of what’s on the driver board - no lockfits as far as I can tell.
First stop will be to change the 220R bias preset, and the four electrolytics
Following the comment on the tropical fish caps I will leave those for now

A.
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Old 26th Jan 2021, 7:19 pm   #7
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Have you tried tapping the skeleton pre-set pot?
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Old 26th Jan 2021, 8:30 pm   #8
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Andy, have you removed any heatsinks from the metal cased transistors?

These work at a high-ish voltage and power dissipation in the Leak 70.
The driver transistors are fitted with heatsinks but sometimes fail.

The Leak 30 Plus works at a lower voltage and may not need them.
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Old 26th Jan 2021, 8:37 pm   #9
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Those resistors fitted with end caps can go intermittent. Worth tapping around them.

Don't think the 30 has the heatsinks
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Old 26th Jan 2021, 11:03 pm   #10
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

Quote:
Originally Posted by Silicon View Post
Andy, have you removed any heatsinks from the metal cased transistors?

These work at a high-ish voltage and power dissipation in the Leak 70.
The driver transistors are fitted with heatsinks but sometimes fail.

The Leak 30 Plus works at a lower voltage and may not need them.
No, there's no heatsinks fitted - the board is as found.

A.
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Old 26th Jan 2021, 11:06 pm   #11
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Default Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise

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Originally Posted by Vintage_RC View Post
Have you tried tapping the skeleton pre-set pot?
No, not tried that - I'm wary of doing it in case the bias voltage changes catastrophically and damages the output stage.
It will be replaced with a better quality one.
A.
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