| contaxchen |
Hello...
I'm callling for help here!
When I turned on my EAR last week, it got some flare beneath one of its EL34s. :hot:
I opened up the chassis and started to checked each compenents inside and found out not even a resistor or a capacitor went wrong... Diodes are all good!
Since the accident, the amp simply blows fuse if turn on even without tubes.
Any idea what's went wrong?:confused:
Hope to hear some good idea...
Thanks for any idea.
c.c. |
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| SY |
Start by checking the power supply. Disconnect all the amp circuitry from it and see if it blows fuses on its own. If so, you've probably got a bad rectifier and/or filter cap. If not, start looking for a short to ground somewhere in the amp circuit.
:att'n: High voltages present! Use caution probing around, keep one hand in your pocket at all times, and make sure with a meter that things are discharged before sticking fingers or tools into the circuit. |
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| contaxchen |
Thanks for the suggestion!:nod:
I've isolated the problem after killed all the possible suspects! Now, it still doesn't work. If I change to larger fuse, then everything works fine until the EL34s... I'll try again tomorrow.
Thanks again!
:) :) :) :) :) :) :) |
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| contaxchen |
Hello again,
I isolated the problem to a cathode resistor. That resistor burns when turn on with all the tubes. Otherwise, all parameters seem normal.
Any body has any idea about a burned cathode resistor??
:confused: :hot: |
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| Sch3mat1c |
Burns = lets out magic smoke? Or just burning hot?
Tim |
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| contaxchen |
Hello,
There was smoke... I didn't see any flame.
Phillps smells better than Dale when they burn. Dale resistor smells like ammonia when smokes... I don't feel comfortable!
:ashamed: :ashamed: :ashamed:
The two channels test the same in every respect! I don't understand.
C.C.:confused: |
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| fdegrove |
Hi,
| quote: | | Dale resistor smells like ammonia when smokes... I don't feel comfortable! |
Be glad they weren't Holco resistors...
They smell like a boiling tar pot when they go up in smoke.
Anyway, when the cathode resistor starts acting like a fuse I suspect the output tube has an internal short.
Cheers,;) |
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| SY |
| Or, if it's fixed bias, the negative rail (and/or grid resistor) may be dodgy. |
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| contaxchen |
Hello everybody,
I got it fixed right. At least, for the time being!
Here is my note for the repairing. I hope to discuss it a little bit and make it like a case study.
1. I found out my amp went shorted even without any input or output tubes. I started with checking the power line. Everything was OK before rectifying tubes. I did check the transformer, power capacitors and diodes one by one. It gave normal voltage at 526VDC!
2. I still changed the diodes, because they looked old and diodes cost nothing.
3. It didn't go shorted since I waked up in one morning. Maybe diodes were the problem?
4. I put input tubes back. The amp looked fine! The fuses didn't blow. I put the EL34s back. It still get shorted...
5. I changed all the componets between output cathode and ground. There were 16 pcs of resistor and 8 pcs of eleclytic caps!
6. I put the original power tubes back. It did light up, but the amp always had smoke at certain pot! I know this by putting power tubes back one by one. I know it is the spot.
7. I checked all of the resistors again. They were normal, but that particular spot still burned all the time.
8. I doubed the tube is the problem. Yes! It was!
9. I replaced it with another healthy one. After the some test, amp worked ! But only last for a few hours. It dropped dead again! There were a lot of smokes and sparkle...
10. I changed all over again everything between cathode and ground. Normal!
11. It still burned at that spot! Am I crazy??!!
12.After a few trials, I found out that new EL34 was shorted for some reason! I checked that tube before the test. It just failed! :bawling:
13.Replacing by another EL34, it has been working for over 12 hours...
:D
I hope it won't have any problem this time!
C.C.:dead: :dead: :dead: |
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