Leach amp problem

[Curioprop]

I am currently constructing a Leach amp Ver 4.5. This is my first electronics project and for the most part, things have gone rather smoothly. Until now. I do not have access to a scope or bench power supply for testing the boards so I had to wait until everything was put together. I tested the power supply, +59V and –58V measuring from the transformer side of F2 – F5 with no fuses in place. I installed 100-ohm resisters in place of F5 and F4 and again measured the above voltages across F2 and F3. No signs of damage to this point. I decided to install F5 and measure the current at F3. When I switched on the power I immediately burned R33 on the right channel. “Well” I thought, “there is definitely a problem with that channel”. I decided to try this test on the left channel and immediately proceeded to burn up R33 on that side as well.

As you can imagine, this is driving me crazy as I sit at work many miles away from the project. I will be going over all wiring and testing all components when I get home this evening. It would seem I have one big mistake or I have committed the same error on both channels. Does anyone have any ideas of what I should check first that would cause this problem on both channels? To put it another way, if this had happened to you, where would you start your search for the culprit?

Any assistance would be greatly appreciated.

Curioprop

[acenovelty]

1. Temporarily tack solder two 100 ohm resistors to the back of each circuit board, one from the loudspeaker output to one side of R36 and the other from the loudspeaker output to the other side of R36.

2.Tack solder a short circuit jumper in parallel with C12.

3. Connect the positive, negative, and common outputs from a dual power supply to the board. Note that there are two circuit board grounds to which the power supply common must be connected, one is near the input and the other is near the output. The power supply output voltages should be set to zero and the current limits should be set to about 50 mA. If the power supply does not have a current limit function, connect a 100 ohm 1/4 watt resistor in series with the positive and negative power supply leads.

1. Test the power supply before installing fuses F2 - F5. The dc voltages should correspond to the values given in the power supply parts list for the transformer used. If the power supply is wired wrong, fuse F1 will blow. If a variac (variable autotransformer) is available, use it between the amplifier and the AC line to slowly increase the ac voltage from 0 V to 120 V AC for this test.

2. The filter caps must be discharged before installing F2 - F5. Do not short circuit the capacitors to discharge them! You will get a loud pop and a big spark. A 100 ohm 2 W resistor is recommended to discharge the capacitors, but it can get hot! Use pliers to hold the resistor across the terminals of each capacitor for at least 30 seconds to discharge the caps.

3. Adjust P1 on each circuit board for maximum resistance and install the power supply fuses. If the previous tests have been successful, you can proceed to the next step where you power up the amplifier. Again, I recommend the use of a variac to slowly increase the AC input voltage from 0 V to 120 V for this test. If you are unsure of anything, remove F2 - F5. Substitute a 100 ohm 1/4 W resistor for each of these fuses. You may wish to power up only one channel at a time.

4. With no load connected to the amp, it can be powered up. If anything is wrong, the 100 ohm resistors in place of the power supply fuses will limit the current. They will also smoke! The dc voltage drop across the 100 ohm resistors should be less than about 2.5 V (25 mA or less) if nothing is wrong. If this test is successful, turn the amp off and wait for the power supply to discharge. Remove the 100 ohm resistors and install F2 through F5.


HTH

[AndrewT]

Hi,
I think you must have fitted too high wattage of bulb to the mains tester.

But, seriously, use a light bulb tester for every new or maintenance project. Will save a lot of grief.

[jaycee]

Have you used the original 2N3439/2N5416 transistors?

Be aware - there are fakes of these around, branded ON (and On Semiconductor do not make these devices). You will know because the 2N5416 will be NPN instead of PNP when you test it.

It seems Mr Leach is aware of this. He has a warning on his page about it, and a picture of the offending articles:

[Tony]

OT, mods, can we have this thread sticky? seems like a thread like this pinned is of great help to diyer's....thanks.

[Curioprop]

Thanks for the replies. I have checked most of the items you all mentioned. P1 was turned counter-clockwise for maximum resistance. I am using 2N5415 and 2N439 transistors from STMicroelectronics. They passed the NPN /PNP test from Prof. Leach’s web site. AndrewT – for a light bulb tester do you mean a light bulb in series with the hot wire before the power cord? If so I can make one easy enough. What wattage bulb do you use?

I have removed the circuit board from the amp and removed all the wires. I have replaced the blown resistor. Here is a pic if anyone wants to look and see if anything jumps out as wrong.

Once again, any help and assistance is greatly appreciated.

Steve

[lineup]

Originally posted by Curioprop

Thanks for the replies. I have checked most of the items you all mentioned. P1 was turned counter-clockwise for maximum resistance. I am using 2N5415 and 2N439 transistors from STMicroelectronics.

They passed the NPN /PNP test from Prof. Leach’s web site. AndrewT – for a light bulb tester do you mean a light bulb in series with the hot wire before the power cord? If so I can make one easy enough. What wattage bulb do you use?
.

Hello
What you should try to test, if you have a transistor tester in your multimeter
is the pin configuration
If I remember, somewhere in Leach website,
was a report of same transistors having different pinning.


Yes, if bulb is made for 120 Volt mains or 220 VAC
this means it can be use before the transformer.
In series with you transformer, of course.

You may only need one bulb, in one Mains wire.

Lower Bulb Wattage = higher resistance
Higher Bulb Wattage = lower resistance

So for example 100 Watt Bulb will reduce the voltage across transformer less
while for example 10-20 Bulb will reduce more.


Regards
lineup

[AndrewT]

Hi,
wire up a plug top with socket outlet to supply your new equipment. Wire in a bulb holder with both terminals in series with the live wire.
The neutral and earth go straight from plug top to socket outlet.
Use a lower wattage bulb to start with. It does no harm if it is too low and if there is a wiring fault less curent flows and so damage is less. Amps with a voltage detector on a soft start circuit may burn out the soft start resistor, for these you must temporarily short out the soft start resistor (in a safe manner) to allow the bulb tester to get the amp up to voltage. A correctly sized bulb will allow about 90 to 95% of mains voltage through to the equipment.

An amplifier set up with zero output stage bias draws very little current once started. It draws a lot of current at start up. Initially to magnetise the transformer core and once this is established then to charge up the smoothing to near final voltage.
You will see this initial current requirment as a brief bright flash becoming a dimming as the current falls over a period of between half a second to 4 or 5 seconds for a big PSU. If the bulb continues to glow or stays bright, then investigate BEFORE changing to a higher wattage bulb. Remember the amp is live, so take care while measuring.

Once you have proved there are no wiring or serious component faults then disconnect the bulb tester, and make sure you have a close rated fuse in the primary side of the transformer. A switch in parallel to the bulb would allow full mains through. A disadvantage of this tweak is leaving the switch in the wrong position and losing the protection you need for first switch on.

[KISS]

Light bulb tester

The light bulb is actually a current dependent resistor in this application. The higher the current the brighter the lamp. If the amp was a short then the most it could draw is the wattage of the bulb. The cold resistance of a tungsten bulb is about 10X less when cold than when the bulb is hot.


I lost all my hair with my Leach constructed nearly 30 years ago

I had made a mirror image of the circuit board. With some adjustments, I was able to salvage the board.

Temporarily short C12

[acenovelty]

Nothing obvious from the pic.