Zen V4 Troubleshooting

[Diomedian]

Hi All,

I'm having trouble with my Zen V4 again. :-( It worked for several days after the last time it broke... (that was an interesting seepage of thermal paste :-). This time one channel is out.

Due to the strange way I constructed it, it's not immediately obvious which power switch corresponds to which input/output, so I turned the channel in question on with no speaker connected. I can't imagine this causing a problem, but I may as well disclose, because it was at this time (or soon before/after) that the channel died.

Power supply for this channel seems to work - I get 49.7V unregulated, and 40V even off of the source of Q5. So far so good. It does seem that there's not nearly as much of a power-on hum from... the transformer(?) as there is when I turn the working channel on.

Source to gate voltage of Q2 is 4V as in the schematic. The first problem I found was that I'm dropping no voltage across R0. Well, it was about 0.1mV or something. Due to this, I thought it might be a dead Q3, but I've changed this to no avail.

Does anyone have any suggestions where I can keep looking? Does this sound indicative of anything?

thanks,
Jeremy

[jh6you]

No voltage across R0 means no current through it.
It could happen when Q1 is open. I hope you will measure Vd and Vgs of Q1, and return.

[Diomedian]

thanks for the quick reply - Vd (that's drain to ground, right?) is 6.7V, and Vgs is a few mV.

I'm still quite a rank newbie here - does this mean my Q1 is fried? If so, is it indicitive of some problem further upstream?

[jh6you]

I don't think that Q1 is fried.

If Vgs of Q1 is almost 0V while Vd of it is 6.7V,
Most probably, R4 (trim pot) is shorted (or adjusted low).
You could confirm this by measuring the voltage across R5.
If you get 6.7V across R5, R4 must be shorted.
I recommend you to check this, and return.

[JCM]

To verify whether the current source is working, connect a 10 ohm power resistor from Q2 source to ground, measure the voltage across it, see if it's passing current. It'll get real hot pretty quick if it's working, and will have, of course, about 20 volts or so across it.

[jh6you]

My ability is reaching to this at best.
Hope you will solve the trouble soon.

[Diomedian]

R4? Wow, that would have been maybe the third last thing I checked. :-)

But you're right, I'm dropping 6.7V across R5. I turned R4 fully counter-clockwise (to what should be max resistance?) and now I'm dropping 6.5V across R5 - might there be life in her yet? I can't reach the pins of R4 unless I pull the board off of the heatsink - I guess I'll have to do this...

[Diomedian]

Well, looks like we have our culprit - R4 is 250 ohm when "closed", and 0 ohm when open.

Sigh - time for another Digikey order, unless I happen upon a 25K trimmer at the local Radioshack (I can't seem to find any electronics stores here in Halifax).

Anyway, is there any reason to believe that this won't happen again immediately when I plug the machine in again? That is, should I order four of them? :-)

thanks for your replies!
Jeremy

[jh6you]

Four . . . ? Order only one.
Measure the size before you bring it into the hole . . .

[Diomedian]

The only reason I thought I should order four of them is because there must have been something that caused R4 to short - maybe this condition is still on the board, and R4 will short again as soon as power is applied... probably not, but... the pots cost Can$1.60, but shipping will be $14.50 due to Digikey's policies.

Well, maybe it's time to order parts for the next amp too! It's a blessing in disguise! What a fun hobby. :-)

thanks again - I'll let you know on this thread when the new pot is installed.

- Jeremy