Cornet 3 Low B+

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ttan98

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Cornet 3 Low B+
« on: 7 Nov 2014, 06:38 am »
Hi,
 Just finished powering up the Cornet, forgot to insert the tube first before powering up the amp. After inserting the tubes This is what I obtain:
The B+ is not  210v DC but all the LEDs lit up, I got about 90V DC instead. What could be wrong?

1. Faulty IRF fet?
2.  555 not working?
3. Dry joint?

Can you suggest the voltages I should get at certain pins on 555 or any particular components?
Thanks.

hagtech

Re: Cornet 3 Low B+
« Reply #1 on: 8 Nov 2014, 08:06 am »
To debug the 555 circuit you need an oscilloscope.  There could be all kinds of things wrong.  I guess check the 9V dc rail first...

jh

ttan98

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Re: Cornet 3 Low B+
« Reply #2 on: 8 Nov 2014, 10:49 am »
Hi,

Replaced the 555 with a new one, the output remains around 93Vdc, slightly higher than 90Vdc with previous 555. The supply voltage is 8.9Vdc, resolder some components around 555 and the IRF740. To help me diagnose the problem(I have a CRO):

1. what the is expected output at 555, it should a square wave but what peak voltage?
2. what is expected output at the IRF, peak voltage, around 210V?

hagtech

Re: Cornet 3 Low B+
« Reply #3 on: 9 Nov 2014, 05:09 am »
Yes, pin 3 is an output pulse but not 50% duty cycle.  Probably more like 70%.  Frequency about 50kHz.  Output from MOSFET is a narrow pulse up to 210V. 

Can you post a photo?

jh

ttan98

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Re: Cornet 3 Low B+
« Reply #4 on: 9 Nov 2014, 12:20 pm »
Hi,
 
I know what's wrong with it. I will  confirm it first myself, once done, I will report back.

ttan98

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Re: Cornet 3 Low B+
« Reply #5 on: 10 Nov 2014, 05:42 am »
Hi,

Fixed the problem, I soldered in a cap in reverse polarity, hence PCB was drawing too much current. When I remove this cap., the supply comes up to 220 Vdc.

Question:

can I reuse this cap which was in reversed in polarity again, or do I have to change to another similar cap?

G Georgopoulos

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Re: Cornet 3 Low B+
« Reply #6 on: 10 Nov 2014, 05:46 am »
don't reuse it (if electrolytic),use a new one same type...
if non polarised reuse it..

hagtech

Re: Cornet 3 Low B+
« Reply #7 on: 11 Nov 2014, 03:12 am »
I would not re-use it either. 

jh

ttan98

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Re: Cornet 3 Low B+
« Reply #8 on: 11 Nov 2014, 10:47 pm »
Thanks, this thread is closed.