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Old 5th November 2013, 12:18 PM   #13431
Zen Mod is offline Zen Mod  Serbia
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pull semis from circ and test them separately

search here for mosfet matching , jfet matching
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Old 5th November 2013, 12:35 PM   #13432
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Quote:
Originally Posted by Zen Mod View Post
pull semis from circ and test them separately

search here for mosfet matching , jfet matching
----------------------------------------

Thanks for the reply - isnīt there an easier way :-)

... It is so nicely assembled, and it is not easy to disassemble.

I have read Passīs documents on FET matching.

In a tube circuit I would have measured the voltages on all the tube pins, and probably from that been able to diagnose what is wrong. But I can not do that in a FET circuit.

I did search for a schematic with voltages, but didnt find one.


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Arthur.
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Old 5th November 2013, 12:39 PM   #13433
cviller is offline cviller  Denmark
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Not ideal, but you can do that. The most interesting voltages will be the R3 and R4 - they need to be around VGS voltage (~4V for typical IRFP240). If you cannot get any voltage across those, your input fets are probably not working.
Be prepared for more smoke though...
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Old 5th November 2013, 03:05 PM   #13434
sangram is offline sangram  India
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You can use a light bulb in series, set pots correctly (fully shorted gate to source, putting output in cutoff - easy to measure). At this time F5 pulls practically no current, not even as much as a Class D amplifier.

Use a variac to bring up the voltage and if the bulb lights, the output is smoked.

Note that as per Mr. Pass the Drain resistors are somewhat optional. If you do not want to smoke your new set and do not want to buy new equipment, I suppose you could try gambling again, but I'm not sure a public forum should discuss such things.

Another easy way to test is with a low-voltage, current-limited power supply to see if the outputs are indeed toast. I think 12V, 1A regulated and current limited supplies should be easily available, or you could simply hook up two computer PSUs, cheap ones, in series - ensure they have working current limiters, set bias to zero and see if the supplies trip at startup. I would think even a single supply should do it is all you're testing is output failure. Just don't put a big honking transformer on it.

I would suggest fuses in the DC line, but I don't know if they will offer protection from fire and smoke.
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Old 5th November 2013, 03:52 PM   #13435
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Cviller and Sangram,
thank you for your replies. I will check the voltages.

I have no access to a variac, but the other suggestion with the low voltage testing is a very good idea - I have 2 small variable power supplies with current limiting on the way home. 2 because I wanted to be able to make an easy +/- supply.

About the gambling you could be right, but on the other hand I guess most of us are here to learn from other peoples successes and failures, and I am not the first, and I am sure also not the last, who will turn the bias pots the wrong way.

I did read the entire thread once, but never saw a schematic with voltages, which I think is kind of strange.

Best regards
Arthur.
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Old 5th November 2013, 04:16 PM   #13436
tinitus is online now tinitus  Europe
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I don't think the variac is needed when using a lightbulb

but do not adjust to full classA with lightbulb
if considered ok and working, remove lightbulb, and adjust

if I remember correctly you should also measure DC when adjusting bias
slowly adjust both bias pots, bit by bit, and keep DC in grip

(hope this is correct)
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Old 6th November 2013, 06:13 AM   #13437
Corpius is offline Corpius  Netherlands
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Yes, that's correct. It is also very well documented in the downloadable build guide
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Old 6th November 2013, 06:47 AM   #13438
tinitus is online now tinitus  Europe
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never seen that pdf file, with detailded build guide ... looks great, thanks
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Old 6th November 2013, 11:53 AM   #13439
sangram is offline sangram  India
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Arthur, the voltages of interest are all available on the original schematic. The build guide has some typos (it omitted one decimal point in the schematic), look at the original schematic from the website - the owner and service manual for the F5.

All voltages indicated start at zero for the starting setting, and ramp up as you begin to supply gate bias. The three voltages of interest, as has already been stated, are the voltage across R11, R12 and the output. The former two have to be close to 0.6V, and the last to be as close to zero as possible.

Good Luck with your troubles and the gambling

Last edited by sangram; 6th November 2013 at 11:56 AM.
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Old 6th November 2013, 12:39 PM   #13440
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[IMG]H:\1-Personnel\F5 - In.jpg[/IMG]
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