Leach Amp: help troubleshooting

Hi there!

Well, I finally got one channel up and running on my Leach Amp, and I did all my testing at the workbench. So I decided to take her out for a ride and hooked the darn thing on to my Yamaha DSP A2, that would do as a pre-amp.

It started out really well and it sounded great!

BUT

In my joy, having her up and running, I got a bit trigger happy and powercycled the amp, and this is where my troubles start. (I cycled it really quick. The amp was off for only one second or so)

All by a sudden, there was this really squealing noise from the loudspeaker and after a second, the fuse blew. When I replaced the fuse and turned on the amp again, C22 blew up (and there was this smell, you know)...

http://users.ece.gatech.edu/~mleach/lowtim/graphics/ckt.pdf

I tried and replaced the transistor Q17, but I blew another set of fuses.

Where should I start looking for errors/broken stuff?
How do I see that a transistor is broken?
Which transistors/diodes/caps are most likely to be broken?

As I mentioned before, the amp worked and sounded really good until I powercycled.

Hmmmm... I'll try and change q12, q13, q14 and q15.

Any ideas?

Have a good one,
//magnus
 
Bummer. The way to start is to check all of your semiconductor devices in the last two stages. It is best to remove each from the circuit and at least check the diode junctions and also collector to emitter for transistors. Put your meter on the diode test range. Each diode junction should give a reading of at least about .4V for a power transistor and around .7V for most other devices containing diodes. Then reverse the probes on each diode junction and you should get an infinite reading. For collector to emitter checks, the reading should be open circuit in both directions. Basically, the main thing is that no reading between any two pins on a semiconductor should read a short, or close to 0.
 
Counterfeit parts?

Sounds like what happened to my Leach Amp when I used (unknowingly) counterfeit MJ15003/4 output devices! Check the output and driver (Mj15030/31) stages.

After I've replaced my output stage to real MJ15003/4 devices, I can powercycle as many times as I like without problems...... except the dimming of the lights when the transformer powers up and charges the input caps......
 
Oops...... too large....here's the image, Magnus...
 

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I would like to sugest Better Transistors to the Ones mentioned. The MJ-15003 and MJ-15004 TO-3 Power types witth Type MJ-2193 and MJ-2194 TO-3 Types if available in your area. Also the MJE-15030 and MJE-15031 with MJE-15032 and MJE-15033. All these transistors have Better Performance and Higher Ft's than the Old types showen.
 
The circuit was apparently oscillating; I hope you have an
oscilloscope. It may do fine on a resistive load but misbehave
on a reactive speaker load. I had that happen when I didn't
install the Zobel network and hooked it up to my speaker system,
which is four ohms and uses Kimber 4TC cable. It wasn't
audible, but the output stage ran very warm, the oscillation was
very high frequency, and was bias setting dependent. I even
missed it the first time I put a scope on it because the frequency
was so high. The recommended Zobel fixed it instantly.

Recheck for wiring errors and that your grounding scheme
conforms with the recommended practice. Given that a bypass
cap on the circuit board blew, you'll have to replace that one
and be sure it's the recommended value. If you can find them,
try using low-impedance caps.
 
Geoff: thanks for the Part # Correction.
Sweede: These transistors i mentioned might just blow as yours have. i mentioned them Because In my Variation of the Leach amp the Big one 250 Watt. i used these to replace the ones you mentioned and it sounded alot better. The Breakdown voltages are better, so this just might Help in your situation. I have Driven my version very hard and not had any problems Except when one rail fuse blew it also took out one of the Input Diff amp Transistors. This only happend once and was Because i had By mastake installed a 1 amp fuse in one rail rather that the 10 amp fuse. I have had this Amp since the Mid 1980's and it has been a great Performer. I must say though that my version is almost a complete redesign and only similar to the Origenal Leach. The Basic Topology is the Same but just Refined.
 
Hi again,

I got this weird feeling after the discussion about the Zobel network and checked it out again, and there it was. The 33 ohm resistors I had in parallel was not really what I expected... I accidently used 0.33 ohms in parallel...

So I'll replace them and get back to you if it still doesn't work (after replacing the transistors as well). Though my supplier of transistors were out of stock, so I have to wait for a couple of weeks.

Regards,
//magnus