Help troubleshooting Zen V4

[ophilip]

Hi,

I finished wiring my Zen V4 and I ran into trouble when starting it up.
I tried one channel at a time with a variac and got the same problem.

I fused the transformer at 1.5A and set the bias potentiometer to the maximum value.
I first checked the supply voltage which was correct at 50V.

I then connected one channel to the supply and monitored the voltage drop through R0. This voltage is not supposed to go beyond 0.7V. My problem is that I reach this value only about half way on the variac (corresponding to a power supply voltage around 25V).
This indicates that too much current is being drawn, but I need to find out where.
The PCBs were ordered directly from Passdiy and I don't suspect wiring problems.

I did forget to short the input before doing the test , but I redid the power up with shorted inputs and reached the same result.

I am wondering if this current draw is caused by a damaged component.

Would you have any suggestion as to which component would be most suspect and how I can test it (them)?
Any suggestion is welcome.

Thanks,

Olivier

[Netlist]

It might be possible that the voltage drop of 0.7V is already reached at 25V. Slowly increase the voltage and monitor the current. I bet all will be well.

/Hugo

[ophilip]

Hi,

Thanks for the reply.

I tried increasing the supply voltage yesterday, but the R0 voltage drop kept increasing and blew the fuse (1.5A).

In a past thread someone had a similar problem when Q3 was installed wrong. I don't think I did the same mistake but I will check to be sure.

Could a damaged Q3 cause this problem?

Thanks,

O.

[Netlist]

Yes, make sure Q3 is ok, many people use BC550 which has a different pinout.
Also, Q1 and Q2 need a check.
Is the 1.5A fuse at the primary side?

/Hugo

[ophilip]

Thanks Hugo,

How do I check Q1 and Q2?
Do I check the gate-source voltage with power on?

O.

[Netlist]

It takes more time to type than to lead you to this nice link:
http://www.uoguelph.ca/~antoon/gadgets/mostest.htm

/Hugo

[ophilip]

Hugo,

I just looked at the schematics and the board layout and I think I found the problem.
The outline of Q3 is printed on the board, but I think there is a
mistake and the collector and emitter pins are reversed.
I looked at the manufacturer datasheet and the order of collector-emitter-base relative to the transistor outline (the flat face) does not seem to correspond to the outline on the board.

I will double check tonight, but it would explain the symptomes that I had on my amplifier. I just hope that I didn't damage any part in the process.

Thanks for the help,

Olivier

[ophilip]

Well, I checked again and there is no error in the schematics or the layout.
I checked all my wiring and board stuffing without finding any problem.
The only thing I can imagine is that some transistors are blown.
I will order spare parts and change all of them.
I cannot find anything else right now.

O.

[Nelson Pass]

Here's your chance to play Sherlock Holmes, as it almost always
turns out to be something simple.

[ophilip]

I bet it is simple, but the wierd thing is that it happened on both channels and since the PCBs are from PassDIY, the problem is not from the PCB.

It means that I probably made the same mistake twice, but I cannot find it out yet (kind of weird making the same mistake on two PCBs).
I will check the values of all resistors and check the base-emiter voltage of Q3 with a multimeter tonight.

I am pretty sure Q1 and Q2 are dead on one channel and they might have killed R10 in the way.

I have ordered replacement transistors, but I would like to understand why the regulation of Q2 doesn't work when I bring the power up.

A friend was wondering if bringing the power voltage too slow might
damage Q1 when Q2 is not yet regulated. Q1 being dead could cause the death of Q2 and R10.

O.