LSR&D 101 Leach Amplifier

[yardiron]

About two years ago I picked up this amp, it was playing when I bought it at a ham radio fest.
The thing has had a rough life physically but it played fine.
The other day I turned it on and all was as usual but after about 20 minutes the left channel started to make some static. Then the right side started as well. I shut it down, checked all my connections and fired it back up.
It played fine for a few minutes then 'POP' and the left channel went out, the right side still plays but it makes some light static.

I pulled the cover off and found a blown fuse, and an obviously burnt 6.25K resistor.
I downloaded some info online for an LSR&D 101, including a schematic and parts list but nothing matches at all?
I took this to a local repair shop and they said its missing a lot of parts and can't be fixed???
I can see where there were two power transistors at one time, but they were never there all this time?
I only found one pic of another one with the top off online, and it doesn't match mine, all of the transistors are different, even the smaller amplifier transistors are diffferent.
From what i can tell this is an early model, from Feb 1979. It still uses the huge brick size e core transformer.

According to the parts list this should have 8) MJ15003, and 8) MJ15004 power transistors per channel, I have a hodge podge of various other part numbers that as far as i can tell do not cross over to the original part numbers.
As this was found, and how its been playing for the better part of two years its only had 14 power transistors installed?
The left channel has mostly Toshiba 2SB554 & 2SD424 power transistors installed, with an ECG87 and an ECG88, while the right channel has a mix of various Motorola MJ15011, MJ15012, MJ15024, and MJ15025 power transistors.
To make it more confusing, not a single other transistor, or 1/4 watt resistor on the board now is an option on the original parts list?
I'm thinking that I may need to start by replacing all the transistors with the correct part numbers or suitable audio grade equivalents? Not having dealt with one of these before I'm not sure how far they strayed from the original parts list when new either. There's no doubt parts have been changed but it did play, and it sounded pretty decent. How on earth it did missing two transistors is beyond me. I see no added jumpers or bridged circuits that would indicate something was modified so it didn't need the two other transistors but both are missing from the left channel circuit, which is also the channel which burnt the resistor. However, the fuse that popped was on the right channel?

 

[Superampman]

I have two of those amps. I also have two pairs of Model 102 monos plus a couple of diy 125wpch stereo versions Yours is the 160wpch The Leach Amp by LSR&D. They are all actual true reference amps you can measure sq of any other amp out there imo. Stereophile adopted the 102s as their reference back in 1980. I have and have owned many highly regarded amps but when I finally hook up the LSR&Ds after other comparisons, it's always a sheer sense of "back to actual linearity" no matter what speakers are used. They just sound dead stark transparent, accurate in every sense of the word as it relates to recording playback. Hence they are awesome and unremarkable, fully dependent on the recording. They are quite rare as Dr. Marshall Leach had a falling out with his student at the time partners and not many were commercially produced. He subsequently offered diy kits which are also now nla. But there is a strong following of this amp over at DIY Audio so you will find the answers you're looking for there.

It's well worth restoring this amp.

 

[dshoaf]

Dude, I'd be all over those amps. I built the original Low TIM amps back in the late 70s and wish I'd never sold them. The other thing is I'd always wanted to take his classes at GA Tech - just didn't live close enough.

Cheers,

David

 

[yardiron]

I'll give DIY a try. I had gone over a few posts listed there but nothing matched what mine was doing and I couldn't find any close up pics.

I started to compare the parts list to what I have and so far only a few capacitors and a couple resistors match the schematic and parts list.
It appears that every power transistor has been changed at one point or another, at least four other smaller amplifier transistors have been replaced, and a half dozen 1/4 watt resistors have been changed.
At this point I can't tell if someone made some sort of modification for a purpose or if it was just repaired with what ever parts were on hand. A few of the power transistors I looked up aren't even considered Audio grade components, a few don't even show up on a cross reference chart.
The only other pic I found of one of these shows a pretty different circuit board layout with different style transistors in at least four positions.
The parts list shows only 8 power transistors, yet I've got 14, and I suppose originally had 16, so I'm assuming the parts list is per channel?

 

[Superampman]

I'll give DIY a try. I had gone over a few posts listed there but nothing matched what mine was doing and I couldn't find any close up pics.

I started to compare the parts list to what I have and so far only a few capacitors and a couple resistors match the schematic and parts list.
It appears that every power transistor has been changed at one point or another, at least four other smaller amplifier transistors have been replaced, and a half dozen 1/4 watt resistors have been changed.
At this point I can't tell if someone made some sort of modification for a purpose or if it was just repaired with what ever parts were on hand. A few of the power transistors I looked up aren't even considered Audio grade components, a few don't even show up on a cross reference chart.
The only other pic I found of one of these shows a pretty different circuit board layout with different style transistors in at least four positions.
The parts list shows only 8 power transistors, yet I've got 14, and I suppose originally had 16, so I'm assuming the parts list is per channel?

Not sure but I think one of mine has MJ15003/4 and the other MJ15024/25, all original. Same with the two pairs of 102s. All have the EI-cores

 

[fred soop]

Leach was way ahead of his time. Many of those design details have been proven over time and are very similar to what is covered today by Bob Cordell, Douglas Self, and Andrew Russell (user name Bonsai). Over a thousand pages of discussion at diy audio in the thread "Bob Cordells Power amplifier book" in the Solid State forum. Link is
https://www.diyaudio.com/forums/showthread.php?t=171159&goto=newpost
though this will take you to the most recent page.

Sounds as if your Leach amp was modified and/or hacked at some point. I'd find the original schematic (on line) and rebuild with the correct parts. The amplifier will function just fine without the 16 output transistors. The number required will determine the power output.

I'd also go with MJ21193 - 96 power transistors as these are newer and greatly improved equivalents to the MJ15xxx series.

Another change I'd make (and already did) was using the single ended VAS rather than push-pull as Leach did. Both Cordell and Self have evaluated this and determined that the slight gains do not justify the added circuitry.

Here is my design, modified from Cordells information. This is the power amplifier section of a Fisher 600-T receiver being redesigned and rebuilt. Sorry, the actual schematic has not been uploaded to Flickr yet.

2001

2002

2003

 

[chazix]

That's some nice work there (and a nice thread revival). Not everyone would have splurged on jumper resistors instead of wires, or used axial caps where fit is better than radial, or thought of arranging for access to the adjustments on the bottom board.

Kudos,

chazix

 

[fred soop]

Thanks for the up vote.

The jumper resistors (called zero ohm jumpers) are not expensive at $2.83 per 100. They are much easier to work with compared to cutting wire to the proper size and stripping. The axial capacitors are 470 µF 63 V and radials would simply be too tall. Also note that the connections are all on one side of the board except the sensitive input and feedback connections. This makes it easy to roll a board up for any necessary work without disconnecting everything. Although, there was still a lot of disconnecting and these boards were actually rebuilt 3 times with minor design changes even after breadboarding the circuit.