Leak Stereo 30 Overhaul... Worth it?

[bleachershane]

Got one of these in faulty condition on eBay for £20 the other day...
I bid on it before I read about what it was... the first transformerless solid state amplifier, etc, etc, and then I saw the words 'germanium transistors'... Obsolete? You betcha, try tracking down any of the transistors used in this 'legend'...!

Anyhow, it is now with me and upon opening it up to look at the innards for any obvious signs of this 'fault' of no description I noticed one of the main PSU reservoir caps was cracking and bulged, more so than the other around it...



One of the tracks for the speaker DC blocking caps is lifting... should be an easy fix.



First off, I've documented all the electrolytics and ordered them from Farnell... A mix of my usual Panasonic FMs, some EPCOS for the larger reservoir caps. Also ordered 6 trimmers to replace the antiquated open track such as these...



And that's all before testing the transistors... prays they are all working! Farnell delivery should turn up tomorrow, will do the electrolytic recap and see what happens...

Anyone had experience of one of these? Are there any worthwhile jobs to carry out? There are some awful hi/lo gain switches that changes the levels for various inputs (which from the service manual seem well out of spec with modern line levels)... what are the chances of modding these for use with modern equipment levels? The switches are awful anyway and need to go...



Definitely a bit more complicated and messy than the more modern solid states amps I tend to work on... well, gotta learn somewhere!

[indianajo]

Wow, Ge transistors in TO3 cans. Those are rare now, you can still get the surplus ones in the flying saucer cans. I have a few of the flying saucers myself, salvaged from the garbage in 1974. The oil exploration company I worked for preferred tubes for power electronics to Ge transistors in 1974, even though tubes were shock sensitive and transistors weren't.
My 1960 ST70 tube amp and the 1966 ST120 transistor amp have way more gain in the input stage than more modern amps. Something about the wimpy output of 12AX7 preamps. I'm thinking of deleting the selected high gain BC108 transistors and fooling with the feedback resistors to cut the gain. The St120 works okay with the PAS2 tube preamp, but with the op amp disco mixer I have to run the mixer gain so low that the residual hum is boosted up by the amp. Whereas the CS800s amp the gain can be turned down and the mixer volume increased, so the hum of the mixer is reduced.
I listen at 1.5 Vpp into 8 ohm speakers, so 30 W/channel would be useful in my living room, if it didn't thermally run away. Ge transistors have a ***** reputation. I wouldn't leave it on very long without installing a fan (which I had to do to the ST120, even though the transistors are Si). Your heatsink appears about average for 1966 - no fins. At least the O.T. bias current pot is in the "failsafe" mode, maximum resistance if the pot wiper drops out. As little as I understand bias circuits. I did something with NTC thermistors (100kohm cold, 14K hot) and 2n7000 FETs to shut off half the idle current to my ST120 O.T.'s when the heat sinks warmed up.

[indianajo]

You know this circuit has the same .5 ohm resistor between the top & bottom output transistors of the Dynakit ST120. I wonder who copied who? (Leak in germanium was probably first). In that case, this could possibly benefit from the O.T. idle current stabilization circuit djoffe arranged for the ST120. See this thread Dynaco Stereo 120...can be beautiful
The circuit has a comparator to keep the current on the .5 ohm output resistor at about 40 ma. (20 mv) That is twice what you need, I'm not quite sure how to reduce it, but I think you remove R2 on Joffe's circuit. I think you would put it in place of the pot P7r and the resistor TH2R but again I'm not expert at this.

[jez]

They are potentially very good but exceptionally variable from one unit to another in my experience. I have one here which last time I listened to it a few years ago sounded simply stunning! Very fast and transparent. In a different league from the later silicon Stereo 30 plus. However I heard another couple of examples a few years previous to that and was most unimpressed... Hence my extreme surprise at the sound of this present sample.
A tip with these amps is that you may well find there is no sound due only to dirty switches! The sample I have made no sound until all the switches had been doused in contact cleaner and then turned back and forth many many times, whilst doing so first one channel then the other eventually appeared. Even after this I would sometimes loose a channel or get a distorted sound until I did a bit more twiddling with the controls so I think this model is exceptionally prone to problems with dirty switch contacts.

[jez]

Oh yes... You will probably find all those capacitors work fine in-spite of appearance. And the transistors are specially selected ones supplied to Leak under a deal with Mullard to give them only the selected cream of the crop.

[indianajo]

I wonder if the Ge transistors were faster than the later earlySi ones. No datasheet on ad140, they probably didn't have Ft specs then anyway. Here is the thread that has Lineup's simulation of a Leak 70, that gets a lot better with more modern transistors in sim. www.diyaudio.com/forums/solid-state/154441-leak-delta-70-attempt-diy-overhaul-buy-another-amp.html Also in that thread is Lineup's suggestion for cutting the gain of the input of the Leak 70 from full volume at 0.5V input to < half that.

[Bone]

AD140 - Vceo = 40, Vcbo= 55, Vebo=10, Ic (max)=3.5A, Ptot(Tcase 25C)=43W, Tjunct (max)=90C, Hfe 30-100 @1A Ic, Ft=430kHz, Leakage Icbo <100uA @5V Vcb.
Details from my Data Book.

[bleachershane]

Thanks for the replies... quick update on the repair...

Capacitors/trimmers turned up from Farnell.

Replaced the three reservoir capacitors (2500uf/50Vand 2500uf+1000uf/50V) for two 4700uf/63V EPCOS capacitors. The difference in size is quite amusing, the amount of space left on the board is ridiculous, but makes it a bit easier to move around inside! Anyway, they'll need to be hot melt glued into place, but for now they are wired in for testing.

It powered up now, good stuff. Put a line level signal through the input that I believe bypasses the tone/preamp section (tape monitor) and measured the outputs at the loudspeaker terminals, one channel had a healthy AC voltage swing in relation to the input, the other was miniscule, but there nevertheless. Plugged in sacrificial speakers... John Martyn's "Solid Air" from the one channel with the healthy output, nothing from the other... Hmm.

Originally Posted by jez

A tip with these amps is that you may well find there is no sound due only to dirty switches! The sample I have made no sound until all the switches had been doused in contact cleaner and then turned back and forth many many times, whilst doing so first one channel then the other eventually appeared.

At that point I had a little tap and wiggle on the "Tape Monitor" switch at the front, crackle, pop, bang. Yeah, these switches are a nightmare. But it didn't bring the other channel back. But my tech friend noticed that one of the speaker output capacitors seemed to be lifting slightly from the bottom, and that was on the channel that wasn't working...

Couple of days later me and said techy went in for the speaker output capacitors. Replaced the two 1000uF/30V for 1200uF/100V EPCOS caps... VOILA, audio on both channels! One of the originals looks like it had completely dried up, the other was on it's way out by the looks of it. When faced with electrolytics nearing 50 years old, don't even think about it, replace them! That's why I ordered the whole lot before I'd even turned the thing on...

You know, it doesn't sound awful despite only going through the long messed up bookshelves I use for testing. Far more treble detail than I expected, and I was eternally grateful that the AD140 (and the other germanium drivers in the power section) were all working!

Few other things to work on now:

Those switches, they've got to go. Will be working out how to bypass them and whether or not there has to be any sort of resistors on the input lines.

Seems to be some loose wiring connections, whilst one channel was on, if the wiring loom running around the top inner ring of the case was tapped, you would intermittently lose the audio with a nice loud pop! Seems the ground connection to one speaker terminal is a bit iffy. Loving the power/audio output lines running intertwined and bundled up together! Will figure a better way to route the cables.

Is it worth trying to cut out the pre-amp tone section? If I was keeping it for myself I would, less pots/switches/components to run the signal through. But I'm planning on selling it, and perhaps someone will want it 'unaltered' as such, even if I know that passing it through that section will degrade the audio signal?!

[ingenieus]

If you intend to sell it - keeping it in original working condition will fetch a better price AFAIK.

[Mooly]

If you have tried a CD using the tape monitor inputs with tape monitor set to on what do you think of it ?

Have you replaced all the smaller electros. Some of the old values belon to yesteryear, just replace with next size up.