Sunday, 21 June 2015

Vintage Treasuries – The Restoration of Leak Valve Amplifiers, TL12 "Point One", Part II

Hello to everybody,

interested following our remote restoration project of the Leak TL12.1's in Tjörnarp near Malmö. As one of the first steps after general optical inspection of the amplifiers it is necessary to work out all defective hardware. The most important parts are the transformers, so here a first concentration of interest should take place. A good start for safeness is to check the dc resistance of any single coil wound with a meter for continuity and dc resistance higher than 0 Z. Here the schematic (published already in part I ) is a great help to identify the numbered contact terminals. Beginning with the primary side of the mains transformer, each section of the input voltage selector between 0, 200, 230 and 250 Volts should be checked for continuity (here indicated in order as no. 1,2,3,4). As well a still working fuse is a good indicator for intactness of the amp, even when its last running service was some decades ago and typically during storage some parts may have deteriorated? So don't connect it to ac power before all checks might have taken place, when you don't know about the history of your amp.

Continuity check with analog meter set to ohms, here shown testing at the first half of the two heater coils between 5 and 12 as grounding center contact (12 and 6 build the other half )

Next check will follow to contacts no. 9, 10 and 11 for the high tension coils with 10 to be the grounding center contact. Final check of the mains transformer (part number TL12/T3, this here is the closed type where /T2 is the older open frame type). Final check of the mains transformer is the heater winding for the rectifier tube between no. 7 and 8.
Next check will follow on to the choke (contact terminals are not indicated with figures) and it is just a single coil. If it shows continuity the check will be forwarded on to the most difficile coils, the output transformer (indicated as here TL12/T2/2). Here two different transformers can be found, the TL12/T2/1 shows a higher output impedance value as option.
Beginning with the primary side of the transformer, it shows mirrored symmetry between terminals no. 9,13,and 10 as first half and 11, 14 and 12 as second half. Continuity should be seen between each contact to the next. This means beginning with 9 to 13, than from 13 to10. The same with the mirrored other side. As well the secondary side will show up with four different coils to be checked: terminals no. 1 and 2, 3 and 4, 5 and 6; as well 7 and 8.  If all contacts are checked at both amplifiers without any detected failure a first deep breath can be made, the continuity check was successful and everything irreplaceable seem to be intact. But this does not say that all isolated coils will work well with high voltages and realistic currents in the circuit?!

Here a 500k potentiometer was customized built in to keep the high sensitivity of Leaks input design variable. The high sensitivity was necessary in the early 1950's with low output components like tuners, tape recorders etc.. But today it is not any longer useful, since it makes a lot of unwanted side effects Today it might me more reasonable to go for a 100 to 250k stepped attenuator for the best sound.

One amplifier has got additionally equipped with some old style input socket and a attenuator for the input signal. Both parts are not original and might be rejected for this reason and for perfect matching of both amps. A much better way is to design such a attenuator for the input grid in a separate housing with octal plug in order to match physically the input socket (octal) of the amp (originally used to match the preamplifier until RC/PA/U. Here the common octal plugs (as used with Leaks connecting cables) or octal cases as used for relays are good components to build up for such a attenuated input circuit in combination with a rca socket. A excellent value here will be a stepped attenuators made from switched resistor pairs in order to keep only one exactly defined value in the signal loop instead of lossy carbon potentiometers. Another benefit will be that you can do different input modules for different reasons,  like one with symmetrical input transformer to match signals from studio equipment. So both variants will be plug&play!

Standard input module with RCA socket and 100k stepped attenuator with bulgin bakelite knob in separate octal bakelite housing, originally dedicated for relays.

For perfect care of the chassis it might be necessary to dismantle the hardware of both amplifiers further. If you want to go the whole way you should do perfect photographs from all sides in particular from all cable connections of the transformers (with flashlight) to identify later all connection again before you desolder these.
If you really have dismantled the chassis from all transformers, it can be cleaned perfectly. I am not a proponent of a complete dismantling with all parts like tube sockets, component board and cable tree,  or even further with new enamel and silk screen printing, because it might happen that working parts might brake and it will get difficult to find replacements matching the chassis. Octal sockets for example have to be the same type used by Leak itself, otherwise the sizes of holes will not match, very difficult to be found todays. Component boards need to be made completely by hand if necessary, but it is manageable. Aging parts need definitely to be replaced, so all electrolytic caps and lots of resistors need replacement, because their values have got out of range. As well here originality will be first choice and in almost all cases a better solution than modern "improved parts".

After dismantling the transformers and the capacitor housing the chassis need a good clean to get exempted from dirt, dust and grease of 60 years of service.

As already mentioned Leak did use for the smoothing and filtering rail of the power supply huge paper capacitors which were made to order by TCC in the 1950ties. Four capacitors of 4 uF/600V got sealed in one box with grease and oil as isolating liquids. The first one is the loading capacitor following the rectifier tube and heading a smoothing choke, followed by two of them in parallel to form a 8 uF filter unit with another 4 uF capacitor at the end of the rail. These paper capacitors normally will not fault, but after 60 years of service, storage and move, after heat, coldness and physical punishments almost all of them show oil at their base. This is a indicator of "leaks" which commonly will be accompanied with hefty audible hum. These leaking capacitors can be measured for capacity, but more important will be their esr (equivalent series resistance) factor. The esr can be measured only with disconnected capacitors with a special instrument, the most will not have such an option. The pure presence of grease might be reason enough to exchange the blocks, because the leak of oil will dry them out. And it will be pure pcb poisoned oil used within these days, so take care with such leaks, for yourself and for the environment!
At the well known heritage site for Leak amplifiers is shown how to replace the outworn capacitors with modern non lasting electrolytic types. By the one hand it is as already said extremely unpleasant to open up the original cans and clean them from all the pcb poisened grease and capacitor components. On the other hand the exchange of the exceptional sounding oil paper caps to modern electrolytic types degrade the sound characteristics to a mediocre level. I can not recommend using such components and would strictly rate to use paper capacitors again, if you want the full advantage these amps can show up with. But you will not find the matching sizes/values in needable amounts to drop into the cans. You might order them customized made with Jensen Capacitors in Copenhagen. They will be able to make them for you, but don't ask for costs. So what to do?

Capacitor block dismantled, showing clearly lots of leaking oil and grease as indicator of
a defective irreplaceble component. 

I hope I can show up within the next article of this thread about a authentic solution for all lovers of these amplifiers. A new set of perfectly matching cans made from sheet copper with paper capacitors built in perfect values of 4uF/8uF/4mF/600V might be ready to be shown than. Even better, I might be able to deliver these on request to people which only can accept a optical perfect original looking replacement with perfect sound characteristcs for their beloved amplifiers in order keep the desperate climbing values of these amps in mind.

Read on soon, Volker

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