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Old 9th April 2012, 01:35 PM   #3461
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Just remember you are not jumping R1 but replacing it. ie fit it in series with Vcc.
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Old 9th April 2012, 01:38 PM   #3462
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If you are going to remove the JFets, mark and remove all 4.

It's a simple job to test them for IDs and IDS matching. At least then you will know that they are all OK.

With all four removed you can confirm that there are no PSU issues.

If you have got some tweezers or narrow nosed pliers you can heat shunt the legs as you remove them and short them together at the same time. They're not fairy dust so they will take a little bit of abuse as you remove them.

Last edited by KatieandDad; 9th April 2012 at 01:41 PM.
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Old 9th April 2012, 01:47 PM   #3463
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Replaced R1 with 100R. Left it powered up for just over a minute - no dice alas. Voltage reading at the C1 pads hovered around 0.223V.

What next - check R2/R3/D1 out of circuit, or remove Q100/200 instead and check?

(as I said before, the board has worked in the past. The only change I made was to replace the two 10uF caps. I also desoldered the temporary hook-up wires from the 'test rig' of a couple years back. I can only assume this desoldering/resoldering must have caused heat damage to a component (or two).)
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Old 9th April 2012, 01:51 PM   #3464
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R2/R3/D1 are unlikely to cause this sort of problem.

Unless in a complex fault.

If you can just remove Q200 then Q100 and see what you get at each stage. Leave them out once removed.

Label them though as they are matched pairs.
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Old 9th April 2012, 01:54 PM   #3465
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Just to make sure that we are not squashing a snail with a steam roller.

Can you connect the original C1 across the PSU and see that it works properly.

Even better also load the PSU up to about 10 - 100mA.

Last edited by KatieandDad; 9th April 2012 at 01:57 PM.
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Old 9th April 2012, 01:56 PM   #3466
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OK will do - have to do some cooking shortly so will hopefully get around to that later this evening. Fingers crossed they'll be the cause. That PCB is an utter pig to desolder components from though, and I've tried a solder sucker and desolder braid in the past - must be those rather tight fitting through-plated holes. Have mounted the JFETS as close to the PCB as I could manage, so I think they'll be toast anyways one removed there's only about 3mm of leg visible on each.

Thanks for all the help thus far - very much appreciated indeed

- John
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Old 9th April 2012, 01:58 PM   #3467
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Quote:
Originally Posted by KatieandDad View Post
Just to make sure that we are not squashing a snail with a steam roller.

Can you connect the original C1 across the PSU and see that it works properly.

Even better also load the PSU up to about 10 - 100mA.
I'll put a 1000uF cap there with the legs intact so it's easy to remove again if need-be. Think I have a Panny FC in the parts box somewhere.

Sorry how do I 'load the PSU'?
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Old 9th April 2012, 02:01 PM   #3468
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Just put a few resistors across it.

5mA should prove the point so at 24V

4K7 at silly low value. (1/4 Watt)

Try a few just to load it up 5mA at a time.
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Old 9th April 2012, 02:05 PM   #3469
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Sorry, so when you say across it, you mean across the + and - ?

Leaving the 100R in place too? (R1).
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Old 9th April 2012, 02:06 PM   #3470
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I would test the PSU away from the B1 just in case it has a fault. IE disconnect it from the B1 and place the C1 across it and also the load resistor(s)

I am just trying to ascertain that the PSU is indeed OK.

Another way of approaching this is to simply fit a PP3/MN1400(I think) across Vcc and 0V - obviously with the PSU disconnected.

Last edited by KatieandDad; 9th April 2012 at 02:09 PM.
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