My EAR834 dropped dead! Help!

[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.
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?
Hope to hear some good idea...
Thanks for any idea.
c.c.

 

[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.

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.

 

[contaxchen]

Thanks for the suggestion!
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!

 

[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??

 

[Sch3mat1c]

Burns = lets out magic smoke? Or just burning hot?

Tim

 

[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!



The two channels test the same in every respect! I don't understand.

C.C.

 

[fdegrove]

Hi,

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,

 

[SY]

Or, if it's fixed bias, the negative rail (and/or grid resistor) may be dodgy.

 

[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!
13.Replacing by another EL34, it has been working for over 12 hours...

I hope it won't have any problem this time!
C.C.