Aleph 5 help please-32v at output

[george a]

Hi All

My Aleph 5 which has been working faultlessly for a couple of years and recently relocated into a new box (without problems)
has suddenly developed an issue.
In short
supply rails +/- 34v
had 200mv offset at output , found blown fuse and replaced it (it did not blow)
one output terminal now has +32v DC showing up .
The on board led is not lit.

heatsinks staying cold..

Speakers not connected when fault found thankfully


Any ideas where to start looking?

many thanks in advance

George

[george a]

Added info

Across the ZTX450 transistor is 0.6V instead of approx 4-5 Volts

Across the Source resistors (1.5ohm) voltage is 0.1 to 0.2 v instead of 0.6 Volts

Again
thanks in advance
george

[Nelson Pass]

From your description I take it this is a DIY amplifier.

When you talk about the transistors, you probably need to specify which
ones, top or bottom. Also, it would be useful to know what the offset is
with some sort of load.

[george a]

Hi Nelson
Gee I wasn't expecting you to reply....but am extremely flattered.

Ok sorry for the lack of info -electronics a minor hobby as in I know enough to get into trouble.

Yes amp is a DIY, and has been in storage few a few months, and I always check the output terminals prior to re connecting speakers- hence the problem was found with no speaker blow ups..

6 off IRF 240 per channel
all Readings were with no load

I haven't got an Aleph 5 Circuit handy , but on the Aleph 4 schematic the ZTX450 is noted as Q3- this is the one that has 0.6volts across it rather than the 4-5 v noted on the schematic.
(BTW the circuit boards are Mark Finniss's layout for the aleph 4)

When the 200mv offset was noticed there was also no load and it appears there was a blown fuse on the positive rail.

(whenever i move an amp I check the outputs with a multimeter prior to connecting speakers.
if worst comes to worst i may just replace all the semi's on the PCB in hope of some revival- but I am fearful some of the output transistors may be dodgy...


Thanks again
and regards from downunder.

George

[Andy5112405]

It looks as though one of the MOS-FETs isn't turning ON.

Check the gate voltage at each OUTPUT MOS-FET.

[qusp]

try loading the output with power resistors i think is what Papa is saying

[Bill Fuss]

If you're speaking of the ZTX450 in the current source, it should measure .6V or so. The reduced voltage across the source resistor is due to the bias trim circuitry.

I think I would be looking for a simple wiring mistake or loose connection. A dummy load should tell you a lot, and will help others help you.

[george a]

Hi Guys

I've tried to put a load across the output but it started smoking.......32v on the speaker terminal....eek!

Bill
The ztxI speak of is on the positive side of the amp (on the schematic closest to the positive rail) so maybe that is ok
However
I have measured the Source to Drain voltages on the output mosfets.

on the positive rail side there is a reading of some 67 volts... and on th negative side 1 volt.....

I'll look again for loose connections.....

Thanks
george

[Bill Fuss]

To make it slightly easier, the pos half of the output is the current source and the neg half is the signal circuitry.

1 volt across the signal fets means that they are being held fully on by excess gate voltage, or 1 or more fets are toast (shorted). It should have blown the fuse unless the current limiter is working. If it IS working then it is probably excess gate voltage from the front end. The gate voltage should be less than 5v if the front end is working OK.

Isolating the fet gates and shorting them to ground should turn off all the output fets, if you dont have 32v or so across all D-S in this case then you have shorted output fets.

I'm just thinking on my feet, I've never had a problem like this. Hopefully the Master or someone else will be able to tell you more.

Best, Bill

[Andy5112405]

During my build, I inadvertently connected the bottom leg of FETs the wrong way round ie D-S. I had -Vcc at the output.

Luckilly the MOS-FETs are pretty bullet proof and no harm was done. They worked fine when I reversed the D-S.