leach amp trouble.....HELP!

[Wagener]

I had the following experience with my leach amplifier

The one channel is working fine.

I connected the other one to a 8-ohm sub and I pushed up the volume (not very high though)

when suddenly the amp went dead. I found the negative supply fuse to be blown.

I connected 100 ohm resistors in place of fuses and switched on. After 10 seconds it

started smoking.

I turned the biasing pot all the way up for max resistance and did the same - resistors

immediately started smoking and burned white.

I then disconnected my output transistors and switched it on. This time the resistors did

not smoke but Q16 got very hot.

I checked all my transistors on the pcboard for shorts over CE. I checked my output

transistors for shorts over ce and did a diode check over ce and be.

What else can I do? I don't suppose Q16 should get hot without any output trannies

connected?

[AndrewT]

Hi,
Check your NFB loop is still connected with the outputs missing. With power still connected through your 100r supplies and output tr's still disconnected, try measuring voltage across each base emitter of all transistors. They should all be in the range 600mV to 700mV. Check DC at feedback to -ve input and at the output.
regards Andrew T.

[Jan Dupont]

I always start up my new build amps without the output devices connected
As always when you got to similar modules and one is working, is to place them side by side and make control meassurements between the good and the bad one

[acenovelty]

Hae you replaced Q16 with a new MJE15030?

Prosit

[Byrd]

Hi Wagener.

I think AndrewT might have the answer. Did you follow the Leach Contruction Guide? The following is recomended for testing the boards, which is where you are at the moment. Remember also though that you will need to remove the biasing diodes before undertaking this procedure.(D1 - 4). The most relevent test here for you will I think be 7 (I have not pasted all steps)

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.

4. With a signal of about 1 V peak at 1000 Hz connected to the input and a dc coupled oscilloscope connected to the output, slowly turn up the power supply voltages. The circuit should amplify without drawing more current than about 25 mA when the power supply voltages are about 8 V dc or greater, not to exceed 60 V dc. The output signal will initially be a clipped sine wave until the power supply voltages are increased.

7. 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 hope this helps, it seems you have been having endless problems with this project - wish I was a bit closer & could come around & take a look.

Let us know the results of whatever you try

[dexter]

Greetings,

I'm unable to bias my leach amplifier. the emiter resistor stay at 0 mV.

no excessive current hogging.
all tr stay cool,
power supply rail stay +/-56V

I've double checked everything on the board and couldn't find anything wrong.

any advice is greatly appreciated.

Please help.

many thanks in advance.

best rgds,

[AndrewT]

Hi Dex,
have you inserted 100r resistors into the two supplies to the voltage amp?
Have you inserted 10r resistors across the fuse holders supplying the output stages?
Have you used a mains light bulb in series with the mains live lead?

Is the light bulb glowing or off?
What voltage is across each of the four supply resistors?

[djk]

Did you follow the Leach instructions on how to check out the board?

Did you forget to remove the jumper from C12 before adjusting the bias?

[dexter]

Tks for the reply.

No, i did not test the board as per leach instruction due to lack of equipmentno scope and sine gem)

using 100R, I only got 1.5V and 2.2 DC Volt between q12/13 collector

I've checked the o/p Tr, they all tested ok with my avo.

I tested teh amp with cheap speaker and they sound awful.

any other suggestions?

best rgds,

[acenovelty]

When the bias is adjusted properly, a dc voltmeter will read close to 3.4 V across Q7, i.e. across the collectors of Q12 and Q13. The collector of these transistors is the round metal case.

C6 - Either a single non-polar capacitor or two polar capacitors can be used for C6. I recommend the non-polar capacitor. In this case, use a 220 uF, 16 V bi-polar electrolytic in the holes for C6A (DigiKey/Panasonic P1168-ND, 0.2 inch hole spacing) and solder a short circuit jumper in the holes for C6B. (Failure to install this jumper will result in the loss of all bass response.) For two polar capacitors, use a 330 uF, 16 V radial electrolytic for C6A and C6B (DigiKey P10246-ND).

If you used two 330 uF 16 V caps, the negative poles need to face each other.

Measure the voltage from the tab of Q16 to ground. The positive voltage here should be close to the positive rail voltage.
Measure the voltage from the tab of Q17 to ground. The negative voltage here should be close to the negative rail voltage.

Did you forget to remove the jumper from C12 before adjusting the bias?

Are the bias diodes installed in the proper orientation/ none damaged/solid connection to the board? Is the pot P1 the correct value and not damaged?

If you made your own boards, many other possibilities exist.

A good quality pic of the board would be a big help in identifying possible errors.


HTH