LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise - UK Vintage Radio Repair and Restoration Discussion Forum

yestertech · 24th Jan 2021, 6:53 pm

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

Sideband · 24th Jan 2021, 7:09 pm

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.

Nickthedentist · 24th Jan 2021, 7:36 pm

I had this is a Delta 70, think a Lockfit transistor was responsible. I found it with freezer spray.

Sideband · 24th Jan 2021, 7:49 pm

Quote:

Originally Posted by Nickthedentist

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.

Radio Wrangler · 25th Jan 2021, 2:11 am

"Almost"....?

David

yestertech · 26th Jan 2021, 7:02 pm

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.

Vintage_RC · 26th Jan 2021, 7:19 pm

Have you tried tapping the skeleton pre-set pot?

Silicon · 26th Jan 2021, 8:30 pm

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.

Sideband · 26th Jan 2021, 8:37 pm

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

Don't think the 30 has the heatsinks

yestertech · 26th Jan 2021, 11:03 pm

Quote:

Originally Posted by Silicon

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.

yestertech · 26th Jan 2021, 11:06 pm

Quote:

Originally Posted by Vintage_RC

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.

24th Jan 2021, 6:53 pm	#1
yestertech Octode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 1,718	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

24th Jan 2021, 7:09 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 5,969	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. __________________ On Spike Milligan's headstone......I told you I was ill...

24th Jan 2021, 7:36 pm	#3
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 15,375	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.

25th Jan 2021, 2:11 am	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 16,717	Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise "Almost"....? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Jan 2021, 7:02 pm	#6
yestertech Octode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 1,718	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. Attached Thumbnails

26th Jan 2021, 7:19 pm	#7
Vintage_RC Hexode Join Date: Sep 2018 Location: Twickenham, London, UK. Posts: 263	Re: LEAK stereo 30 PLUS - Intermittent 'crash and thump' noise Have you tried tapping the skeleton pre-set pot? __________________ Alan G6PUB, BVWS

26th Jan 2021, 8:30 pm	#8
Silicon Octode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 1,316	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.

26th Jan 2021, 8:37 pm	#9
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 5,969	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 __________________ On Spike Milligan's headstone......I told you I was ill...