Author Topic: troubleshooting my new to me Mingda  (Read 11140 times)

Offline stevenvalve

  • Administrator
  • Hero Member
  • *****
  • Posts: 1693
  • Liked: 358
Re: troubleshooting my new to me Mingda
« Reply #20 on: April 15, 2011, 07:30:57 PM »
Measure all the voltages on one side then the other, and see if there are differences, Do the same with filament voltages. 

Offline donberry

  • Newbie
  • *
  • Posts: 32
  • Liked: 0
Re: troubleshooting my new to me Mingda
« Reply #21 on: April 15, 2011, 11:32:53 PM »
will do. Thanks...

Offline donberry

  • Newbie
  • *
  • Posts: 32
  • Liked: 0
Re: troubleshooting my new to me Mingda
« Reply #22 on: April 17, 2011, 01:10:28 AM »
here is where I am at -
started measuring the 12AT7s first because of the way the one flares up.
The one that flares is V2 the other one is obviously V3
Seems the problem may actually be in the circuit for V3.
tests here were done to chassis ground -

V2 - I read pins 2 and 4, where the jinvina film caps are, and it reads a steady 80v on pin 2 and  a steady 241.3 on pin 4

V3 - the voltage readings on this one fluctuate real fast. I go to take the reading on Pin 2 and a loud popping noise starts coming thru my speakers (preamp is not on), to the beat of the fluctuating voltage. It continues until I take a reading on pin 4. Pin 4 also fluctuates just as fast, between 242-245 volts - as long as I am taking the reading, the amp is dead silent - no popping noise at all. When I stop, it goes back to the light popping noise I had before.
So reading pin 2 aggravates it and it pops real loud
Reading pin 4 quiets it until I stop, and then it goes back to normal

I would guess it is stopping the popping when I read pin 4 because it is grounding ?

was wondering if anyone had any ideas of what it could be before I have to start snipping components one by one.
Could a bad EL34 cause this ? (just gave me an idea to switch the EL34 and see if it switches to V2)

Any ideas or tips appreciated.
I guess after switching EL34 I just need to start snipping out components and testing them ?

Edit - I swapped the EL34 - both sides now fluctuate, popping has switched to V2. I took the reading on pin 2, started the loud popping. Stops when I touch a probe to pin 4 (does not even have to be touching ground, so apparently just the extra resistance ?)
was going to check bias of tubes since I swapped them, couldn't because voltage fluctuates so much...

sure looks like a bad tube or tubes to me.
Am going to bite the dust and order some now. Just won;t get winged c yet.

Edit again -
ok, I ordered some tubes. I want the Winged C, but can't afford them right now. So after trying to find reviews, opinions etc on cheap tubes - it came down to the Ruby EL34B or the KT77 - thought I would be different and ordered the jj KT77.
So now just basically wait until they get here to see if that is/was the problem.
My fingers are crossed (pretty verbose ain't I  ;D )
« Last Edit: April 17, 2011, 02:53:11 AM by donberry »