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Old 9th April 2012, 11:57 AM   #3451
johnm is offline johnm  United Kingdom
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Thanks for the replies guys - much appreciated

K&D: I too would be surprised if it was the JFETS - I have had this board up and running about 2 years back now when it was just laying on a piece of MDF. Worked fine. Resurrected this project a couple weeks back. Had some better quality 10uF caps so desoldered the old ones and installed these. That's the only change I've made. I guess there's a slight possibility I heat damaged one/more of the JFETS from the desoldering/resoldering... Have plenty of spares however so that's not an issue if it turns out to be the case.

Still don't see how a damaged JFET would cause a short from looking at the PCB. Need to get the schematic open and have a close look again and trace out the possibilities for a short, before taking out any JFETs unnecessarily I guess.

Andrew: The PSU tests fine, and has worked fine plugged into other gear, and I do have a lightbulb tester which I rig up when it's the PSU that's suspect.. In this instance it's most certainly the short I am getting on the DC input of the B1 board (with the PSU disconnected).

- John
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Old 9th April 2012, 12:20 PM   #3452
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John,

I agree but there is nothing other than the components that I have already detailed and the JFets between Vcc and 0V.

For Example:-

If D2 the LED was short circuit (unlikely) then you would see R4 = 15K

If D1 were short circuit you might see C2 charging.

R2 and R3 the potential divider for the JFet Gate Bias are unlikely to fail and if they did they would fail open circuit.

C1 has been removed.

That only leaves the JFets or a SHORT across tracks on the PCB.

You have probably created an unintentional short on the 0V plane on the top of the board. Check all solder joints carefully, BOTH sides of the board.
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Old 9th April 2012, 12:34 PM   #3453
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K&D: This is getting weirder and weirder.

Just been running continuity tests. If I put the meters probes across R3 (10K) or R2 (10K) the resistance reading jumps about then reads 'OL'. I get a continuity beep which goes after a few seconds. THEN, if I put the probes across the + and - on the DC input of the PCB, I get NO continuity beeping! However if I leave the probes there the resistance reading drops, when it hits around 50-ohms it starts beeping. What's going on there?! Strange!

Could it be something to do with this that Mr Pass mentioned in the B1 PDF:

"By the way, the time constant of R2, R3, and C2 are long enough that it takes
a minute or two for the circuit to reach normal operating values, so don’t get
excited if there’s no sound for a few seconds when you turn it on."

Have I just been a dufus and not given the B1 enough turn-on time?

- John
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Old 9th April 2012, 12:35 PM   #3454
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If you've inaventently created a short to the 0V plane then one of Q100 or Q200 is probably fried.
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Old 9th April 2012, 12:36 PM   #3455
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Have you tried powering it up with the series resistor.

You might find it works perfectly.

DMMs can indicate strangely with big caps around.

Effectively increase R1 to about 100R.

Last edited by KatieandDad; 9th April 2012 at 12:45 PM.
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Old 9th April 2012, 12:54 PM   #3456
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Quote:
Originally Posted by KatieandDad View Post
Have you tried powering it up with the series resistor.

You might find it works perfectly.

DMMs can indicate strangely with big caps around.

Effectively increase R1 to about 100R.
Left it on for about 20 seconds and nothing happened - didn't want to leave it on longer incase it could damage the PSU. When I turned it off though the LED DID light up for a few seconds though. Had the meters probes in the vacated holes of C1, and I noticed the PSU voltage only started to rise slightly (from approx. 0.235 to 5V) after I'd turned the power off.

Think I'd better do as you suggest and remove Q100 and Q200 and test for shorts rather than try & increase R1?

I don't suppose anyone else could quickly check if they get a continuity/short reading between the + and - on the DC inputs on the B1 PCB?

- John
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Old 9th April 2012, 01:00 PM   #3457
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The resistor will protect against damage, limiting the max current to 24/100 or whatever resistor you are using. 1K = 24mA, nothing will fry at that current.

Q101 and Q201 should be OK. Even if they have full Vcc applied to their Drains they can only pass 7mA or so.

If you have generated a short between C201 or C101 and 0V then the two upperJFets Q200 and/or Q100 could have had full Vcc across them with no current limiting.

With Q200 and Q100 removed check for shorts to 0V at C201 and C101.

Last edited by KatieandDad; 9th April 2012 at 01:03 PM.
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Old 9th April 2012, 01:04 PM   #3458
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Quote:
Originally Posted by KatieandDad View Post
The resistor will protect aginst damage, limiting the max current to 24/100 or whatever resistor you are using. 1K = 24mA, nothing will fry at that current.
Right. Will check the parts bin for a 3W 100R resistor to tac over the top of the stock 1R. Think I might have to make a journey to Maplins tomorrow though.

P.S. checked the board for any visual shorts and it's all clean - I'm a neat worker so didn't suspect it would be that anyway. Will get that resistor installed if I find one (if not will get one tomorrow) and if no joy there will check the JFETS.
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Old 9th April 2012, 01:07 PM   #3459
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It doesnt need to be anything like 3W. I squared R at 2mA = 0.0004W. Anything in the spares bin will do.

No-one has any clue as to why Nelson put a 3W resistor in this position. It might protect against overheating in the event of C1 failing short circuit.

Last edited by KatieandDad; 9th April 2012 at 01:14 PM.
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Old 9th April 2012, 01:29 PM   #3460
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Quote:
Originally Posted by KatieandDad View Post
It doesnt need to be anything like 3W. I squared R at 2mA = 0.0004W. Anything in the spares bin will do.

No-one has any clue as to why Nelson put a 3W resistor in this position. It might protect against overheating in the event of C1 failing short circuit.
OK have found a 100R - will fit that now and leave the power on for a minute or so. if no luck there will check Q100/200 out of circuit.

I guess I could lift a leg of R2/R3/D1 first to check they're all OK before going to the trouble of removing those Jfets?
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