Re: Progress Report

[ FSAudio Forum ] [ Help ]

Posted by matth [ 66.27.119.228 ] on March 21, 2005 at 22:54:51:

In Reply to: Re: Progress Report posted by matth on March 21, 2005 at 22:09:29:

Hmmm. Everything is now hooked up but I am having a problem.

The initial measurements without the tubes installed looked good. Everything was well withing tolerance.

After installing the tubes, everything looked good except for the + pole of C4B. All other tubes measured 18.5V on this pin, but 0V C4B. I checked the other pins on the tube and here is what I found.


      {Bad)         (Good)
     Channel B     Channel A
Pin   Valve2        Valve2
 1      40V           41V
 2       0V          18.5V
 3     .65V            0V
 4    6.24V         6.24V
 5    6.24V         6.24V
 6     1.3V          239V
 7     1.3V          239V
 8      42V           40V
 9     177V          177V

So the Triode portion of the tube (Pins 1, 8, 9) appears normal. However, the Pentode (Pins 2, 3, 6, 7) is all wrong.

After quite a bit of checking around I decided to ohm out the windings on the output transformer. What I found is that it looks like thereis an open between the U2 and center taps on the suspect channel.


     (to speakers)
       -        -
  ---------------------
 |                     |
 |                     |
 |                     |
 |                     |
 |                     |
  ---------------------
   -   -    -    -    -
    \/   \/   \/   \/
    50  open  50   50  (measurement in ohms of suspect transformer)
    50   50   50   50  (measurement in ohms of good transformer)

So, it would appear that I have an issue with the output transformer.

I am also suspecting that I have a dirty volume pot. There is a grinding noise and for the brief few minutes that I had speakers hooked up, there was a very audible noise when adjusting the volume.

If anyone has any suggestions/comments on what I am reporting then please let me know. I am sure the output transformer has an issue, but is there something else I should be checking or concerned about?

Oh yea, I do have some pictures of the completed assembly but have been too busy troubleshooting to get them posted. And yes, I will get them posted soon.

Replies:


[ FSAudio Forum ] [ Help ]