Help troubleshooting Zen V4

[ophilip]

This is turning into a blog, but it is fine, I am sure some will find it interesting.

Here is the damage report from last night's tests.

Right channel:
Q3 junction still good (0.66V)
looks like Q1 is dead (gate-source junction is conductive)
Q2 probably dead

Left channel: Q3 still good (holds 0.66V under power)
Q1 good: I get 4V between source and gate when I bring power up
Q2 probably dead: voltage between R0/R1 rises above 0.66V during power up.

I tested all values of all resistors on both boards (all fine).
All caps seem fine.
Wiring seems fine.
No shorts found.

The only explanation I can come up with is that Q2 transistors got damaged (electrostatic discharge?) before they were installed or that they were blown when I tested the amp without sorting the inputs (oscillations?)

I will get my replacement parts Friday and continue my "detective work".

O.

[Netlist]

Don't be afraid of powering up with a variac.
As far as I know there is no way to damage components that way.
Keep us posted.

/Hugo

[ophilip]

I finally replaced all faulty transsitors and I get the follwing behavior:

I can monitor the voltage through R0/R1 and it reaches 0.66V when the voltage on the transformer primary is about 90V. The regulated voltage is then about 42V.
If I go any higher the voltage across R0/R1 increases and Q2, Q1 and the fuse all blow.

This behavior is similar on both channels.
- the base-emiter voltage across Q3 and it is fine at 0.66V.
- the source-gate voltage on Q2 and Q1 is 4V

Where could this power be going?

Do you have any suggestion?

Thanks,

Olivier

[Blues]

Check (condition, pin-out, etc.) all semiconductors -transistors, zeners, diodes...they're usually the first to go. Since the Zen is a simple circuit and you have all the time in the world, might as well check all components plus solders. Concentrate on one channel. Once you nail the first the second will be a breeze.

Don't get frustrated. Keep the dectective work fun.

[Blues]

BTW, usually a transistor that went bad in a circuit will have a shorted C-E or D-S junctions.

[ophilip]

As far as I understand, the problem with my amplifier is that Q2 is not regulated.
Either Q3 is not doing its job or there is a wiring or part problem in the regulation loop.

Or it may be Q1 not functionning properly, conducting too much and drawing too much current from Q2.

The problem seems to be consistent on both channels and I haven't found it yet.

O.

[Netlist]

1) You still might have the wrong pinout for Q3. Double check again.
2) You have a wrong resistor value somewhere and made the mistake twice because you took both R's out of the same lot.

/Hugo

[Nelson Pass]

Originally posted by ophilip
I can monitor the voltage through R0/R1 and it reaches 0.66V when the voltage on the transformer primary is about 90V. The regulated voltage is then about 42V.
If I go any higher the voltage across R0/R1 increases and Q2, Q1 and the fuse all blow.

I would definitely check the actual values of R7, R11, R12.

I would also remove R16 (you don't need it right away) and see
if that changes anything.

[ophilip]

Hi all,

I think I found my problem.

It was a tricky one. I brought one of my channel to work and we tested it using a large aluminum plate as heat sink and monitoring several voltages.
Everything went perfectly.

The problem I had was with my heat transfer (I still have to get it to work in my chassis, but I have less doubts now).
Eventhough I have 3 massive heat sinks per channel, the thermal coupling between the transistors and the sinks (aluminum plate) was not as good as I thought. As a consequence Q2 was probably heating up very fast and Q3 couldn't regulate.

I will mount each transistor directly on its own sink and everything should work just as well as in the lab.

Thank you all for your advice, I will keep you updated and I will post some pictures when it is finished.

Olivier