DC offset problems with leach amp

[wrl]

Hi everyone. First off, I'm going to apoligize for my lazyness and for not matching my diodes and transistors from the start. But here is the scenario. I finished assembling my leach amp boards. The first one, I tested and it worked great. However the second board I tested had a bad DC offset problem (300mA). I checked the two boards and they are identical. (yes, I took a multimeter to each and every resistor, + the caps all are the same values).

My question is... can not matching produce this bad of a DC offset, or should I be looking for something else? If it is due to not matching, should I start with the diodes or the transistors?

Thanks in advance,

Wes

[wrl]

This is with no load and +-12 V applied to the board.

[sylin]

Leach amp is 100% feedback for DC and in no way can the unmatching of the devices make such a bad offset.

[wrl]

I assume diodes and transistors. Should I just make sure all are correct, or do I need to check and make sure all are working properly. If the latter is the case, how can I do this simply?scratch:

Thanks,
Wes:

[pooge]

Raise the PS voltage applied to the board. 12v is probably not high enough to bring everything into sufficient operating range.

[wrl]

This is interesting because 12 V seemed to be fine for the other channel. This is just to test the boards (they aren't connected to the output transistors yet.) Is this still not high enough?

However, I can't seem to find anything different about the two boards. To my knowledge they are identical. Every component on the boards is brand new, so I can't imagine anything would be burnt out or broken.

I'll try your suggestion the next time I can get access to the scope, but in the mean time, is there any other way I can check for what might be wrong? Are there suggestions about which parts I should start checking first?

Thanks Guys,

Wes

[peranders]

wrl, if you just take two transistors (the input transistors) out of a bag and solder them in you will expect 50-70 mV offset (times the DC-gain).

+- 12 V seems little (for a Leach original). Have you done any calculations so you know the working conditions of the amp?

[wrl]

Could you expand on what you mean by this?

[Damon Hill]

If the output transistors are not connected, there should be
no voltage present at the output terminal. You'll have to bite the bullet and connect at least one pair of output transistors and a load. The temperature sensing diodes will have to be connected properly as well.

The conditions under which you are attempting to test the board are not the normal operating conditions, at all.

It would be helpful to have a variac to slowly bring up the power supply voltage, or to put light bulbs in series with the power supply, in place of the fuses. These will limit current if a serious
fault condition exists, and may limit damage.

--Damon

[wrl]

I was just following the test procedures as per Prof. Leach's web sight. He says:

"The circuit boards should be tested with a bench power supply before they are installed in the chassis...

Several additional tests should be performed on each circuit board before it is disconnected from the power supply. These are as follows: ...

With the input signal generator disconnected, the dc voltage at the output should not exceed 100 mV, preferably less than 50 mV. Mismatched + and - power supply voltages can affect this."

I was wrong before, as the output signal is loaded with two 100 ohm resistors. An observable DC voltage was present at the output of both boards. From what I understand this is "normal" so long as it doesn't exceed 100 mV. I understand that these conditions are far different from the operating conditions of the amp, but they are the test procedures Prof Leach suggests. I am hesitant to connect the rest of the board if I can't be sure the boards are working correctly.

BTW, the amp will be used to drive 2 electrostatic panels. I assume that DC offset will affect these in much the same was as it does conventional drivers.