Accident - Shorted Cathode to grid!! - diyAudio
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Old 17th April 2013, 12:46 PM   #1
surfstu is offline surfstu  United Kingdom
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Default Accident - Shorted Cathode to grid!!

Feel like a fool,

Was probing voltages in my amp and I accidently swiped the probe across pin 2 and pin 3 causing a short and a serious amount of hum.

The hum remains! That channel is now just giving me a little music and a lot of hum, I hope i have not damaged the transformer.

I am aware that I have shorted the screen of the pentode to the cathode of the triode which is fed by ccs....

Can anyone shine some light on what I have done?

S
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Old 17th April 2013, 12:56 PM   #2
Mooly is offline Mooly  United Kingdom
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Check the DC voltages on the current sink. Is the LED lit ? You should have around 1.2 volts across the 1K. The semiconductors are the parts most likely to have been damaged.

Edit... voltage across the lower 1K that goes to the emitter
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Old 17th April 2013, 12:57 PM   #3
surfstu is offline surfstu  United Kingdom
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So, I have just measured the voltage at pin 2 (with a crocodile clip!!) where the CCS meets the cathode and it is 9V on the side that i shorted.

It is 4 V on the otherside of the pair (same channel)

This was always 1.5 V on each side of the diff amp pair...

Could I have blown a transistor in the CCS supply?

S
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Old 17th April 2013, 01:00 PM   #4
surfstu is offline surfstu  United Kingdom
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Thanks Mooly, the other channel appears unharmed

I will swap the CCS over and see if it works, then I guess I will know that it is the CCS supply that has blown!!

S
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Old 17th April 2013, 01:04 PM   #5
Mooly is offline Mooly  United Kingdom
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All the evidence will be there in the voltage readings but yes, you have probably zapped one of both transistors.
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Old 17th April 2013, 01:26 PM   #6
surfstu is offline surfstu  United Kingdom
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Yep, channel workes fine now that i have swapped the CCS supply over from the other channel.... yes both LED's are lit.... I will go buy another pair of transistors!h
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Old 17th April 2013, 11:09 PM   #7
surfstu is offline surfstu  United Kingdom
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Unbelievable!! So I ran up my electronics store and bought 2 new bc547b's for the CCS. Spent a good deal of time undoing my work and finally replaced them, only to find that I still had a problem... it turned out the short had wiped out my 220R pot that I used to adjust the CCS delivery to the two tridoes...

Am learning never to assume, I should have checked the voltage all the way back to the CCS!

Getting there slowly...

S
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Old 17th April 2013, 11:27 PM   #8
GoatGuy is offline GoatGuy  United States
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I HIGHLY recommend checking all the resistors ... the 470Rs, etc.

Dumping the HV into the cathode leg can short 'em all out.
They're probably film, and I've found that unlike "old fashioned" bulk-body carbon composition types (which with overvoltage would rapidly heat, then bubble'n'stink), films just quietly (and invisibly) go "pop".

There very likely isn't any damage to transformers. They're hunks of iron, not whispers of silicon and hair-thin wires. Hunks, I tell you. Hunks.

GoatGuy
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Old 18th April 2013, 07:01 AM   #9
tomchr is offline tomchr  United States
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Quote:
Originally Posted by GoatGuy View Post
They're hunks of iron, not whispers of silicon and hair-thin wires. Hunks, I tell you. Hunks.
Yeah... The silicon will die before the iron.

~Tom
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