UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

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

Closed Thread
 
Thread Tools
Old 20th Mar 2018, 10:54 pm   #1
Martian Ind
Retired Dormant Member
 
Join Date: Mar 2015
Location: Norfolk, UK.
Posts: 9
Default Leak TL12 and one plus one

Hi Folks.

Quick introduction as a newbie. Designed and built DBR's for braking trams, trolley buses, HST's, loco's, off road vehicles, designed and built HV NER's for protecting trans, and generators. All of the EHV and HV Megawatt variety involving humungous amounts of energy. What I know about milliamps you can fit on a postage stamp. We had a rocket scientist that worked for us that dealt with millivolts, he previously worked on rocket re-entry systems, so he was actually a rocket scientist...

When it comes to millivolts I try and kid myself now and again that I know what I am doing, normally resulting in something black and charred. Anyway attached are a couple of pictures which do not contain anything black and charred at the moment.

Couple of questions: The components that are currently fitted are not original, at least not all of them. Is this a correct assumption? Do they look correct for the 70's or 80's. I will check them all out, and they could be fitted NOS, but I know this one hasn't been used in a minimum of 10 years.

I'm going to have a go at checking this out as

a/ The components are not the stupidly small discreet type, and you can actually physically get at everything.

I have been reading pretty much all the threads involved in the restoration of these things, and it appears that every bridge that can be crossed, has been crossed. so i will follow the rules that suit my style, which is, if it aint broke don't fix it.

The point one plus pre-amp i understand is not the best in the world but it has groovy knobs and a light which illuminates the leak sign. Can someone point me in the right direction regarding connections and kit of parts connection diagram so i can plug one of them dreary modern musical producing phones and notebooks and stuff into it. I am not going to drill holes or add anything to the pre-amp body, but i will have a flying lead coming out of the base somewhere or through an existing slot or something.

I am a bit of a traditionalist, and like to keep things externally as original as possible.

Great informative site by the way.

Thanks Martin
Attached Files
File Type: pdf IMG_1557.pdf (1.36 MB, 215 views)
File Type: pdf IMG_1556.pdf (1.32 MB, 174 views)
Martian Ind is offline  
Old 20th Mar 2018, 11:57 pm   #2
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Leak TL12 and one plus one

nice TL12 plus.
Ok a quick shufty at the underneath picture suggests that it's been "refurbished" and not very well as the 2 white coffin resistors which are the cathode resistors on the output EL84 valves are the wrong value. They should be 270 ohms not the 220 ohms they seem to be. Standard 2W metal or carbon Films will be perfectly adequate here.
I'd replace all the electrolytic capacitors on the board as they look either original or old enough to cause problems. Likewise the paper in oil TCC things, these will almost certainly be leaky both physically and electrically. Substitute a decent modern plastic film capacitor here of 400V DC rating.
The rest of the resistors i'd check and if still within tolerance leave them in, otherwise 1/2W or better carbon or metal film again will be good.
You may be ok with the main PSU caps if not bulging they may reform nicely. and the photo suggests this could be the case.
Under no circumstances replace that 100 ohm green wirewound resistor if its still in tolerance. It is designed to run hot, some leak power amps had the tag cut away to allow the resistor to drop out if too much current was drawn as a crude protection device. Yours seems not to have that feature, I wouldn't worry though.
It looks to have the later 8778 output transformer, and a pencilled manufacturing date of 1962 suggesting that the grid leak resistors on the EL84 will be 470k and the coupling caps to the phase splitter will be 2 x 0.1uF.

The point1 preamp, no need to make irreversible changes and these days it's undesirable as they are starting to fetch good prices from collectors and enthusiasts. As you say not the last word in fidelity but as a line level preamp they can be very pleasant. If a later version which it should be (brown perspex plastic front?) then use a line input, say "tuner" which should have an attenuator pot on the input which allows modern line level devices to be matched to the leaks highish sensitivity.
Again, inside swap out any old electrolytics and paper in oil caps, leave the rest. And change any resistors that are out of tolerance.

link here to steve spicers leak website. He is possibly one of the best possible authorities on leak products. And if you can lay hold of a copy of his book "Firsts in High Fidelity", it's a goldmine of really useful information including circuit diagrams for all the leak products.
http://44bx.com/leak/

Circuit of later TL12+ from same website.

http://44bx.com/leak/Leak/Circuits/TL12Plus3.gif

Good luck

Andy (Leak Enthusiast and fan!)

Last edited by bikerhifinut; 21st Mar 2018 at 12:20 am. Reason: Extra info
bikerhifinut is offline  
Old 21st Mar 2018, 12:08 am   #3
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Leak TL12 and one plus one

You're right. Some of the components are original while others have been replaced.

Since you say this hasn't been used in a minimum of 10 years please do not power it up immediately (assuming you haven't done so already). Several components may fail if they are not brought back into operation gently. In no particular order:

The electrolytic capacitors will need to have their insulating layer reformed. This is done by running a current into them. Too high a current e.g. the current you'd get just by switching the amp on may cause the insulating layer to start to form but then break down electrically, dumping the stored energy into a very small volume of the capacitor foil. This is usually fatal for the cap - it never works again. The trick is to bleed the current in very gradually - milliamps (!) or even less. The success rate of reforming can be quite high. But the smaller caps may have dried out and if they have then they will never reform properly. If they are in a power valve's cathode circuit they can fail catastrophically and if this causes a short circuit that can damage the valve.

The older high voltage capacitors (known collectively as film capacitors because of the insulating paper or plastic film inside them) may become electrically leaky, either as soon as they are powered up or when they become warm or even when they've been on for an hour or more. The ones connected to the valves' control grids can leak sufficiently to raise the grid voltage and therefore the valve's anode current to fatal values. If you're lucky that only destroys an expensive valve. If you're unlucky it destroys more than one valve and the very expensive output transformer too. There is no cure for leaky capacitors. They have to be replaced with ones which don't leak.

The EL84 valves are prone to evolving gas inside them as they age. With care it is sometimes possible to re-trap this gas in the valve's getter (the shiny silver coating on the inside of the glass). Otherwise once the valve gets hot, or even before, the gas can cause the valve's grid voltage to rise with the same unwelcome consequences as a leaky grid capacitor.

Sorry to sound so worrying about all of this, but it would be sad to blow something expensive unnecessarily.

Some of the resistors may well have drifted in value. If they have then this will degrade the amp's performance. Harold Leak chose the resistor values very carefully.

Last thing to say - don't power up the amp without a speaker connected. And it's worth jiggling the impedance selector plug on the output transformer to make sure it is making good contact before powering up.

The pre-amp is actually more complex inside than the power amp. It is vulnerable to some of the same potential faults as the power amp (it doesn't have EL84 power valves though). Resistors and capacitors which have drifted in value will throw all the tone controls out of kilter. Your main problems with connecting modern sources to it will be a) that it is very sensitive, so unless you can turn the volume from your music source down you will find that you can hardly move the Point One's volume control away from zero before it gets deafening and b) if you only have one power amp then you will have to run the system in mono which means somehow combining the two stereo signals from your modern source into just one mono signal for the power amp.

Oh, and welcome by the way !

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is online now  
Old 21st Mar 2018, 12:18 am   #4
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Leak TL12 and one plus one

Welcome to the forum.

It has had some components changed and there is an odd preset resistor which looks to change the grid stopper resistance to one of the EL84 and makes no sense unless the amp was unstable.

Good news is it looks like the transformers are untouched and the place to start would be to test the resistance of the windings to make sure none are open circuit.

Assuming this works out OK then a full restoration is in order:
Replace all the non-electrolytic capacitors again and go for 630V axial polyester types.
Replace the electrolytic capacitors on the tag strip of which quite a few of yours look to be original with some good quality low ESR types.
Check resistor values and replace any that are out of tolerance with 2W 500V types.
Strip out that preset resistor.

Also floow GJ's recommendation for reforming the large chassis mount eletrolytics.

Last edited by PJL; 21st Mar 2018 at 12:24 am.
PJL is offline  
Old 21st Mar 2018, 12:37 am   #5
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Leak TL12 and one plus one

400V ratings are more than adequate for this amplifier, the HT is of the order of 320V give or take. I have been using 400V rated caps in my Stereo20 and no issues at all over the last 15 years since I fettled it. That said the ECC81 anode DC blockers to the El84 grids on your amp are at a worryingly low 250V rating. Granted they will only see 180V when the amp is warmed up but until the ECC81 starts conducting they could see the whole 320V on them if the GZ34 warms up faster than the rest of the valves.
You only need to worry about high voltage resistors on the anodes and the 1 meg feed to the EF86 screen grid. Anything around a signal grid or south of the cathodes won't see anything like it. If a High voltage did occur in these places you have a lot more to worry about than the resistors! This will give you a wider choice of resistors.

Seconded on G J's advice ref capacitor reforming, a lamp limiter could be your friend!

Andy.
bikerhifinut is offline  
Old 21st Mar 2018, 12:58 am   #6
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Leak TL12 and one plus one

Quote:
Originally Posted by GrimJosef View Post
The pre-amp is actually more complex inside than the power amp. It is vulnerable to some of the same potential faults as the power amp (it doesn't have EL84 power valves though). Resistors and capacitors which have drifted in value will throw all the tone controls out of kilter. Your main problems with connecting modern sources to it will be a) that it is very sensitive, so unless you can turn the volume from your music source down you will find that you can hardly move the Point One's volume control away from zero before it gets deafening and b) if you only have one power amp then you will have to run the system in mono which means somehow combining the two stereo signals from your modern source into just one mono signal for the power amp.
GJ I am pretty sure the mono version of the contemporary Point one pre to this TL12+ has an attenuator pot on the line level inputs (and magnetic disc input too for some bizarre reason), My Point One stereo and varislope stereo preamps certainly have this useful feature.
Yes, a bit of thought to how to combine the stereo output of an iPod type device will be needed, perhaps buffer resistors in the connecting lead to combine both channels if my experience with a creative labs Zen media player is to go by.
One of the very first refurbishments I did was to the Stereo20 and accompanying Point One preamp and by patiently replacing each out of spec or worn out component, one at a time, like for like, I managed to successfully get the preamp working very well indeed. I was pleasantly surprised how well it sounded from a line level source.
Nice feedback Baxandall tone controls I thought at the time.

A.
bikerhifinut is offline  
Old 21st Mar 2018, 10:35 am   #7
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Leak TL12 and one plus one

Quote:
Originally Posted by bikerhifinut View Post
Seconded on G J's advice ref capacitor reforming, a lamp limiter could be your friend!
The problem with lamp limiters is that they're usually designed to stop already bust kit from drawing catastrophically high currents. If the kit isn't bust (yet) then they won't do much limiting at all and the cap will try to charge (reform) in a very few seconds which is dangerously fast.

If you want to limit HT current to the milliamp or sub-milliamp regime then your bulb resistance will have to be so high that the heater on the rectifier will never warm up.

Much better as a current limiter is a simple (large) resistor. I have a few wired in series on a choc-block which easily allows me to reduce the value as the capacitor charges up. You will need a separate HT supply of course. You could disconnect the current cap, tack a known good one in its place and then use the existing supply for this. A simple arrangement with solid-state rectifiers mounted on an octal plug could be used in place of the GZ34 for this (you'll also need a croc-clip to ground).

There are plenty of other circuits for capacitor reformers if you have the time to build something more sophisticated.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is online now  
Old 21st Mar 2018, 10:01 pm   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Leak TL12 and one plus one

Jut a silly thought, for a high resistance reformer, it would be quite easy to load two silicon diodes and a suitable resistor onto a recycled valve base and have a simple plug-in reformer.

Pull out all the valves, plug in the reformer and let it get on with its job.

Add a couple of sockets so you can use a voltmeter to check on progress.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 21st Mar 2018, 10:53 pm   #9
Martian Ind
Retired Dormant Member
 
Join Date: Mar 2015
Location: Norfolk, UK.
Posts: 9
Default Re: Leak TL12 and one plus one

Just a quick note to say thanks for all the prompt replies, I will digest what has been written over the weekend and take stock. Interesting comments regarding the preamp. This forum is a mine of information. Thanks again for all your inputs, will get back to you with questions, comments, and progress in due course.
Martian Ind is offline  
Old 21st Mar 2018, 11:01 pm   #10
Martian Ind
Retired Dormant Member
 
Join Date: Mar 2015
Location: Norfolk, UK.
Posts: 9
Default Re: Leak TL12 and one plus one

Forgot to mention, the Leak was part of a house clearance and the gentleman switched the unit on just before telling me it hadn't been used for 10 years, the output leads to the speakers were attached but the speaker was missing. All valves did glow, up to the point when I switched it off, all of about 10 secs. Whether he had previously been " playing with it " is uncertain. Everything was connected including a Goodman's crossover. I will be checking everything out in due course.

Thanks again.
Martian Ind is offline  
Old 27th Mar 2018, 8:30 pm   #11
Martian Ind
Retired Dormant Member
 
Join Date: Mar 2015
Location: Norfolk, UK.
Posts: 9
Default Re: Leak TL12 and one plus one

Quote:
Originally Posted by bikerhifinut View Post
The point1 preamp, no need to make irreversible changes and these days it's undesirable as they are starting to fetch good prices from collectors and enthusiasts. As you say not the last word in fidelity but as a line level preamp they can be very pleasant. If a later version which it should be (brown perspex plastic front?)
Hi Andy, Thanks your comments, trying to locate a copy of Spicers (leak) book, no luck. I prefer paper copies, but I assume someone has pdf'd it somewhere on line, which I presume can be downloaded for a fee? any pointers?. Attached pic of cleaned up pre-amp. Thx again. Best M
Attached Thumbnails
Click image for larger version

Name:	IMG_1562.jpg
Views:	126
Size:	67.3 KB
ID:	160110  
Martian Ind is offline  
Old 28th Mar 2018, 8:37 am   #12
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Leak TL12 and one plus one

Quote:
Originally Posted by Radio Wrangler View Post
Jut a silly thought, for a high resistance reformer, it would be quite easy to load two silicon diodes and a suitable resistor onto a recycled valve base and have a simple plug-in reformer.

Pull out all the valves, plug in the reformer and let it get on with its job.

Add a couple of sockets so you can use a voltmeter to check on progress.

David
That's an excellent idea but how about a constant current source instead of the resistor.
PJL is offline  
Old 28th Mar 2018, 9:03 am   #13
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Leak TL12 and one plus one

That's a good idea too. There have been long debates in the past about cap reformers though. In the end they often boil down to choosing between operational convenience and circuit simplicity. One advantage of a very simple circuit is that it's generally less likely to blow up, either on its own or when a partially reformed capacitor suddenly fails (as they occasionally do) dumping its stored energy into the surrounding electronics. Simple resistors are very reliable.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:35 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.