Is this why my Leach amp output transistors blew?

[earjo951]

Hi

I powered up my Leach amp for the first time - rapidly smoked R36 and two output transistors. I've checked the bias diodes and they're fine.

The P1 pot was set to least resistance when I fired it up. Would having this on minimum resistance have been enough to smoke R36 and two output transistors?

Thanks for your help.
Joe

[rolandong]

There is excessive current flowing which caused this to burn.
Strictly follow Leach instruction carefully on testing circuit board.
This include soldering 100R resistor on both PS rails.
If you make mistake these resistors will smoke and save yourself expensive transistors. You should at least have variac to slowly increase PS voltage then slowly observe voltage drop on these 100R resistors.
Check all connections and transistor pins correct.

[AndrewT]

You can manage without the expense of a Variac.
Assemble a bulb tester and read up on how to use it.
Read up on how to test mains powered equipment safely.
Read up what Leach has to say.

[Dave S]

The pot should be at the highest resistance (since it in the Vbe circuit of the bias tr) and wound down to adjust bias up.

[BobEllis]

There is probably some other issue than the setting of the bias pot. When using my Leach amps as a greenhouse heater I found that the highest bias setting I could generate was about 360 mA per device (version 4.5 built to spec with MJ15003/4 outputs). R27 prevents the bias setting from going too high.

Check that you've connected the power supply correctly positive supply connected to positive rail, etc. Reversed rails can cause all sorts of trouble. (been there, done that). Use the light bulb tester Andrew suggests to verify that your PSU is operating correctly and then use Professor Leach's power up plan with 100R/.25W resistors in place of the rail fuses with P1 set to maximum resistance. If there is a problem the resistors will smoke quickly and save the rest of your circuit.

[nigelwright7557]

Originally Posted by Dave S

The pot should be at the highest resistance (since it in the Vbe circuit of the bias tr) and wound down to adjust bias up.

The min resistance would have had a large bias voltage and switched on both output mosfets shorting out the supply.

[BobEllis]

There is a resistor in series with the pot that limits the minimum resistance and therefore the maximum bias. At 360 mA per device (the max I could generate with the standard bias setup) the output devices should not blow quickly unless they had poor contact with the heat sink.

[earjo951]

Well I powered everything up with the 100R on the PS rails and the bias pots set to maximum resistance. Again, one channel worked fine but the other channel smoked up the 100 ohm PS rail resistors (though the output transistors were saved!)

I've triple checked the output transistors, bias diodes (including polarity), PS rails, potentiometer and R25/R26/R27. I still can't find the reason for why the one channel is failing.

I know the boards are assembled correctly because they worked fine when I had them in a different chassis. I shifted them to their new chassis, upgraded the heatsinks and transformer to the specifications of Prof Leach.

Are there any other components I should be testing.

Thanks for any suggestions.

Cheers,
Joe.

[AndrewT]

Power up just the faulty channel from a smaller PSU supplied via a bulb tester in the mains supply.
This will allow the majority of the heat to be dissipated in the bulb and only just enough power/heat to pass through the faulty channel to let you measure voltages.
You may be able to identify the short circuit by Zero Voltage measurement.

[pooge]

Make sure your output transistors are not shorting to the heat sink through the insulators. If using TO3 outputs, check continuity between metal case and heat sink. If plastic outputs, check between collector lead and sink.