Leach Superamp problems

[still4given]

Hi Guys,

As some of you know I have been building a Leach Superamp. I made my PCBs from the foil pattern on Dr, Leach's site.

It's been a bit of a ride, sourcing all of the parts. I have finally gotten around to testing the boards. I had several projects going on at the same time as I have been building this amp. I wired up all of the MOSFETs and mounted them on their heatsinks. I never tested the boards without being hooked up to the MOSFETs, which is what Dr. Leach suggests in his instruction.

Like I said, I have several projects in different stages of completion and I must have missed this until now.

At any rate, in the instructions Dr Leach says this;

Testing the Circuit Boards
After you solder the parts to the circuit board, it is tested using the same procedure specified for the Low TIM circuit board. First, you must solder the short circuit jumper across Q7 and you must solder the 100 ohm 1/4 W resistors from the loudspeaker output to the emitters of Q16 and Q17. If you don't have a bench power supply that puts out plus and minus 85 to 93 V dc, you can test the circuit board at a lower voltage. I would prefer test voltages of at least plus and minus 50 V dc. An option is to connect bench power supplies in series to obtain the plus and minus 85 to 93 V dc. I have routinely connected two 40 V Hewlett Packard power supplies in series with the positive and negative outputs of a Hewlett Packard 50 V dual power supply, and I have never had any problems. To protect the circuit boards, you might want to put a 100 ohm 1/4 W resistor in series with the plus and minus power supply leads for the tests. The current drawn by the circuit should be low enough so that the voltage drop across these resistors is less than 1 V if nothing is wrong on the circuit board. There are 2 ground wires from the circuit board. Both must be connected when testing the boards.

OK, here's my problem. I didn't read this before I hooked up the MOSFETs so I made an assumption that I didn't need to ad the jumper across Q7 nor the resistors from the output. Also, since I wasn't planning to apply any signal I didn't hook up the ground at the input. I installed 100 ohm 5W resistors between the PSU and the both rail connections. I hooked up a voltage meter to measure across these resistors. I started out by bringing up the rail voltages to 15VDC and read about 70mV across the 100 ohm resistors. I increased the rails to about 30VDC and read about 100mV across the 100ohm resistors. I checked voltages on the MOSFETs by comparing the PNP vs NPN and they were all very close. I measured 5mV at the output. It seemed as though things were looking good so I slowly increased the variac. When I reached about 60V rails both R32 & R33 released some of their magic smoke. I shut the variac off and started looking for a problem but couldn't find any bad solder joints or bridges and no other parts looked damaged so I'm at a loss as to how to proceed from here.

I would appreciate any help you kind folks can provide. I don't have a working scope, so I am limited to the two multimeters.

You can see the schematic here. It is in PDF form.

Thanks so much, Terry

[JensRasmussen]

Terry,

There are no mosfets in the Leach amp

\Jens

[still4given]

Doh, you're right of course. They are not MOSFETs. They are the TO-3,s that are called out in the BOM, MJ15003 and MJ15004.
I have been working on a Hafler P500 and a Soundcraftsmen PCR800 and they both have TO-3 MOSFETs. I just spaced out. Sorry.

So with that out of the way, do you have any suggestions as to why R32 & R33 burned?

Thanks, Terry

[audiofan]

First thing to check after solder bridges wiring....ect... is 4 zener diodes D13-D14-D15-D16 if reversed you will have heavy current in R32-R33-R13 and R14

Then check C13-C14-C5-C4 if reversed

[still4given]

Originally posted by audiofan
First thing to check after solder bridges wiring....ect... is 4 zener diodes D13-D14-D15-D16 if reversed you will have heavy current in R32-R33-R13 and R14

Then check C13-C14-C5-C4 if reversed

Hi audiofan,

I will check those when I get home. I know that C13 and C14 are right because I checked them last night. I'm not sure about C5 and C4 or the diodes though I really did try to be careful when I assempled these boards. What's weird is that at 30v rails everythig was OK. I wold think if I had the diodes reversed that the resistors would have given up way before 30 volts.

Thanks for helping me.

Blessings, Terry

[jacco vermeulen]

Originally posted by still4given
I measured 5mV at the output.

Mind if i ask which position the trimpot P1 was in ?

[audiofan]

To check zener you may replace R32 & R33 and connect the amp to a low voltage supply ( plus 15V minus 15V ) then with a voltmeter measure across each zener if voltage is around 0.7 volt the zener is reversed if voltage is a few volts they are not reversed.
DONT forget to use current limiting resistor in series with the power supply....as indicated in Leach text

[still4given]

Hi jacco,

It was centered. I did try moving it at the lower voltage setting and there was almost no change.

I have checked the orientation of the diodes and electro caps and they are all situated correctly.

I will sit down with my ohm meter and check all of the resistors just to make sure I didn't make any mistakes. I will also try and do continuity checks at all locations to make sure I'm not missing any solder bridges but it looks pertty good to the eye.

Thanks fo rthe suggestions so far.

Blessings, Terry

[still4given]

Hi guys,

OK, I wasn't able to replace R32 & R33 because out of the 50 different values I have, of course, none of them are 82 ohm. I will have to find them. I did notice one thing though. the zeners on the two boards are not the same. I remember getting my order and finding out that I only bought 4 when I needed 8 so I ordered more and when they came they looked different than the first order. I just figured that they were from different manufactures. Now I measure them with the diode test and the ones in the board that burned the resistors measure 00.0 no matter which way I measure them. the others measure 6.4 and only in one direction.

I hooked up the power to the good board and at 15VDC rail I get 188mV across R32&33.
At 40VDC rails I get .498V across R33
At 42VDC rails I get 1V across R33
At 46VDC rails I get 2V across R33

I was afraid to go any higher without consulting you guys. Any idea how much drop I should see across R33 at full rail voltage?

Thanks, Terry

[sek]

Hi Terry,

Originally posted by still4given
Now I measure them with the diode test and the ones in the board that burned the resistors measure 00.0 no matter which way I measure them. the others measure 6.4 and only in one direction.

This could be an indication that they were mounted the wrong way. The diodes appear to be shorted. This is how silicon diodes react on over-current. It also explains the burnt resistors.

I'm sorry, but I can't help you to determine wether bad or reverse-mounted zeners were the actual cause of the damage. But a diode with 0Ohms in both directions is no longer a diode.

Ciao,
Sebastian.