Zen V4 Troubleshooting

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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
 
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Joined 2002
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.
 
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.
 
Disabled Account
Joined 2002
My ability is reaching to this at best.
Hope you will solve the trouble soon.
 

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R4

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...
 
Shorted R4

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
 
Four R4s

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
 
Q1

Well, I managed to locate a "local" electronics shop - about an hour away each way on city transit. They had a 25K trimmer - the pin layout wasn't right, but I made it fit. However... Once I removed the old one, I tested it, and there was nothing wrong with it! :-( When it was mounted on the board, it was reading maybe 250 ohms. So I checked the source to gate resistance of Q1, and sure enough, it was 30 ohms or so. I checked the same resistance across a different IRFP044, and it was higher than my meter could read. Great! So I had some extra IRFP044s that I bought last time just in case... and I plugged one in, and wow, it worked!

It worked for about 30 seconds - just long enough to call someone over to say, "it works!". :) Now it too has a source to gate resistance of about 30 ohms...

So... It didn't smoke - when it died, it was not spectacular - the speaker just cut out abruptly. I've got one more IRFP044 - any suggestions?
 
Disabled Account
Joined 2002
Sorry to hear that . . .

I had few cases with broken MOSFETs.
It was mainly due to all of a sudden high Vgs.
Therefore, I usually use the protective zener diode (9.1V~18.2V)
between the gate and the source.

Before you use the one-left IFR044 . . .

Did you check any voltage after the replacement of Q1?
 
Q1/Q4

Well I didn't check any voltages, but I remember reading in the manual that if I was careless hooking up the amp, I might ruin Q4. So I checked the resistances of the bad channel vs the good one. R source to drain on the good channel was "high", but on the bad channel it was about 6K. This might cause a Vgs spike on Q1, mightn't it?

Well, either way, Q4 needs to be replaced, so it's Digikey time. (Oddly, there is a R&D branch of International Rectifier here in Halifax, but they didn't get back to me about ordering single transistors, so I may as well get a few spare IRFP044s while I'm at it - some safety Zeners too - good idea jh6you.)
 
Q4 Vsd

This just keeps getting better and better. This time when I turned on the faulty channel, to try to measure Q4 Vsd, the fuse blew! :-( Now it seems Q2 is also dead (Rsg is 25 ohm). Here I was thinking to myself, "at least I haven't had to change Q2 yet". :)

This is really bizarre, because seemingly nothing has changed since last time I turned it on (of course, since I'm a software guy I know that just because I don't think I did anything doesn't mean that nothing has happened).

Well, I can add some IRFP240s to my list - I haven't put in the order yet. This is just a little discouraging, but I think I can get it working again eventually.
 
Success!

Alright! My parts (Q1, Q2, Q4) arrived yesterday, and I hooked them all up last night, and this morning I plug it all in, and it works!

Thanks everyone for your help and suggestions! :)

It's still got a nasty hum, but that's a story for another time (I've used a pretty stupid grounding scheme to avoid long runs of wire, but I think I have an idea how to fix it). When the music starts playing, I don't hear the hum any more - just music.
 
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