Leak Stereo 30 Plus

A friend of a friend, Ian, got in touch ...

"I've got some Leak kit that could do with servicing. Could this be of interest?"

I like Leak kit.

Yeah.. why not?

He's dropped off a couple of amplifiers (this one, and a Stereo 20), and a pre-amp (a Varislope 2, to match the Stereo 20).

First up , the Leak Stereo 30.

This was Leak's upgrade to the Stereo 30, which was an all germanium transistor affair, this is all Silicon. There's a pair of 2N3055's or equivalent in the output stage, developing around 15 watts per channel into 8 Ohms.

Opening it up , and it's obviously had a life! Lots of sad old electrolytics, in desperate need of replacement. Many leaking.

The main smoothing capacitor, has obviously been replaced in times gone by... The replacement was  somewhat smaller than the clamp would take, so was wrapped in tape and stuffed into a cardboard tube! Even then it wasn't tight, and was wobbling about!

Nasty.

The mains switch has failed, and has been bypassed with a bit of choc-block. 

 

The amplifier is divided into a chassis, which contains the bigger capacitors and mains transformer and output transistors, and 4 plug in PCB's... two pre-amps and two "power amp" boards, although the output transistors are mounted externally.

Here are the power amp modules. The left hand board is recapped and ready to go.. on with the right!

 ... and the pre-amp boards, this time the one on the right is done.

As the amplifier is a single-rail design, the output stage sits at "half-rail" when idle, the speakers are coupled to the output stage via a 1000uF coupling capacitor. It's imperative they are in tip top condition to prevent damage to the amplifier or speakers. They are both physically and electrically leaking.

The first one is removed, and has it's gizzards removed! 

Sadly the fibre top broke up, limiting our options a bit for re-stuffing. I scratched by head for a couple of minutes, and then invented a solution!! 

Ladies and gentlemen (and non-binary persons), may I present ... the Cap-cap (TM)

Empty cap, sans lid, ready for re-stuffing.

New capacitor fitted to Cap-cap (TM) ...

... and the Cap-cap (TM) and cap, fitted to the old cap. I hope that's clear !








Get the model from https://www.thingiverse.com/thing:4860779 . It currently only fits 1" capacitor cans, I may make other sizes as I need.

Very smart.

So, back to our "cardboard tube" capacitor. There seems little point in restuffing it, as it doesn't fit anyway. Back to the 3D printer, and an adaptor is made..

... which snugly holds the new capacitor into the clamp. Neat.

With the capacitor replacement complete, all the pots and switches are cleaned up, and it's time to apply some power and see what happens. 

The two red wires to the top transistors are disconnected and multimeters inserted into the circuit to measure the quiescent current. 

The bias controls, which set the quiescent current are located on the two power amp PCBs.

Well, the bias is all over the place, and isn't stable. The two pots are cleaned up, and ... hang on a minute... 

... that bias pot... why is there solder on it? Someone's obviously attempted to make the contact a bit better by soldering the top spring of the wiper. Nasty. Really nasty.

After replacement of the pots with some nice multi-turn jobs, the bias is set and is nice and steady.

It sounds good, but the balance is miles off, the balance pot needs to be set to the right for it to be central. Also as the balance control is rotated more to the right, the left channel starts coming up again, but horribly distorted. 

The balance control is disconnected and measured, and the resistance values horribly non-linear over the movement on one channel. It's a stereo 20K linear pot. Easily available, and duly replaced.

... not so the broken mains switch section of the volume control. After a fruitless search of the internet for a suitable replacement, a few telephone calls are made, and I manage to secure a new-old-stock leak delta control, which will do nicely.

The amp is now done, and performs wonderfully. Should be good for another 20 years :)