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Old 29th December 2011, 11:50 AM   #11
AndrewT is offline AndrewT  Scotland
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Where have you located the rectifiers of the PSU?
Post2 does not show.
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Old 29th December 2011, 12:26 PM   #12
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Andrew T, this is only a test circuit to prove the Pumpkins.

As they are they are RUBBISH.

Both are acting like VHF oscillators.

Running perfect sine waves at 200MHz
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Old 29th December 2011, 12:27 PM   #13
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Quote:
Originally Posted by AndrewT View Post
Where have you located the rectifiers of the PSU?
Post2 does not show.
The bridge is behind the transformer.

The layout is not brilliant but I am trying to find the source of the "Whooshing" sound.
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Last edited by KatieandDad; 29th December 2011 at 12:31 PM.
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Old 29th December 2011, 12:35 PM   #14
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Quote:
Originally Posted by AndrewT View Post
Replace the IN to OUT diode with a 30V Zener.
It still acts as reverse current protection but when the IN to OUT voltage exceeds 29V, the Zener starts to conduct. The in out voltage should never exceed the 317's voltage limit.

The Zener junction temperature is going to go astronomic during start up.
I have no idea how many restarts will cause it to fail.

You can add a CCS pre-reg to limit the temperature rise of the Zener.
WHY.

Even if the full 42V is applied at the output of the LM317, the zener has nearly 4K in series with it.

If R2 was set to minimum, which it isn't. its set to nearly maximum, then I would agree.
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Old 29th December 2011, 01:12 PM   #15
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Inestigations so far, show fault is in the Pumpkins.

I have found so many copper whiskers on these boards - I can't even start to list them.

FIRST was when LEDs would lite up in randon pattern.

Problem exists even when INPUTs are SHORT CIRCUIT.
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Old 29th December 2011, 01:18 PM   #16
Salas is offline Salas  Greece
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Moved to line level forum, analogue source is about carts, TTs, arms, tape machines, tuners, mics, and their boost circuits.
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Old 29th December 2011, 01:54 PM   #17
AndrewT is offline AndrewT  Scotland
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The limiting IN to OUT voltage for the 317 is 40V absolute maximum.

At start up the OUT is at Zero Volts.
The IN pin goes to +Vdc of the PSU.

What is the maximum PSU output at start up?
If you have a nominal 42Vdc when drawing continuous operatimg current and when mains is at nominal UK voltage then I would suggest that you allow for 42Vdc+10% as the worst case start up IN voltage.

If you replace the IN to OUT diode with a 30V Zener, then the Zener will pass current when the IN to OUT voltage exceeds ~29V and the higher the applied Zener voltage the higher the current.
The Zener acts as a temporary bypass to help prevent the 317 failing due to excessive IN to OUT voltage at start up.
a 500mW 30V Zener can safely pass 17mA continuously. In temporary (transient) bypass mode it will pass morem but I don't know how many transients it can pass until the Zener starts deteriorating.
That's why I suggested adding on a CCS pre-reg. This will limiting the Zener peak current and thus extend the life of the Zener.

You could use a 1W3 20V Zener for 65mA continuous duty current capability.

That is "WHY."
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Old 30th December 2011, 06:31 AM   #18
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So you were talking about your xener then, not the one in my circuit.

There is NO WAY that the zener in my circuit can be stressed with R2 set to approx 4K.
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Old 30th December 2011, 06:34 AM   #19
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OK.

Lets get back to the problem of oscillation.

Initially I'm going to reduce the gain of the Pumpkin from 10X to a more realistic (in my application) 3X.

As components on these double sided boards are difficult ro move I intend to simply solder a 47K across R5 and R6.
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Old 30th December 2011, 09:17 AM   #20
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OK. Gain has been dropped to 2.7X I managed to replace R5 and R6 with 27K.

The Pumpkin still operates like a perfect 20MHz oscillator with over 20mV pk-pk of pure 20MHz sine wave.

I've checked all the components, my soldering and for obvious copper bridges. I can't see anything amiss.
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