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PING to Audio Note Kit One owners

Posted by beto1 (A) on May 5, 2015 at 15:29:53

PING to Audio Note Kit One owners

I was repairing an strange noise on the output transformers of my Kit One, december 2004 version, something like the motorboating noise and then suddenly, I have touched with an screwdriver a component on the filament board on my 300B SET amplifier.
I checked the voltages and were bad. I replaced the regulators and again I get good measures.
But, when I was testing the amp hearing music for a while, the output tubes beggin to run really hot, with the plates in red hot.
I turn OFF the amp and begin to inspect the power supply board and I cant find anything wrong visually, but the voltage to the anode was at least 100 volts above the normal rating. This was the test then:

- *DC voltages anode 300B = pin 1L=583v; pin 1R=583v*
- *Choke output DC voltage (w6)= 583v*
- *Choke input DC voltage (w5)=585v*
- *Filament voltages still are 4.95v both*

I continue to inspect and the R1 resistor that is linked to a ceramic disc high voltage cap begins to smoke. This is the previous stage to the ground lug. I replace several times this resistor and then one electrolytic cap that is linked to the driver board exploded.
I decided to replace all the components on the power supply board, supposing that this way I will attack the problem globally....but I was wrong. The problem is still there. With the new components, R1 again it was showing some smoke when I turn ON the amp. Due to this I can't do any voltage measure with all the tubes inserted, but I suposed that for some reazon, there are too much current passing trough the filter caps or something like that. I done a lot of visual inspection very detailed and I can't find any bad joint or wires contact or something that can produce a short.
Since the R1 beggins to smoke only when the 300B tubes are inserted I decided to run a test with all the tubes, except for the 300B.
There were no problem with the R1 this time.
I performed the voltage checking list that comes on the instruction manual. As I had expected, the H.T voltage on the 300B is too much high.
I'm trying to figure where is the origin of this high voltage andI have an idea reading the amp as stages that begins on the power source and ends on the OP tx. The first stage is the Power transformer, then all the parts that are involved on the voltage rectification and the parts that are sourced directly from the mains tx, like the preamp tubes and the filament board. Then, coming from the PS board, the 300B H.T. voltage.
This is the test:
All DC Voltage taken in respect to ground.On () the nominal voltage:
High Voltage (H.T.) on pin 1 of 300B left = 563V (425V)
High Voltage (H.T.) on pin 1 of 300B right = 563V (425V)
H.T. out of PSU Board (W7 PSB) = 576V (435V)
H.T. out of PSU Board (W8 PSB) = 576V (435V)
H.T. to 5687 (W14 Driver Board) = 526V (395V)
H.T. to 5687 (W15 Driver Board) = 526V (395V)
H.T. to 6SN7 (W18 Driver Board) = 498V (375V)
H.T. to 6SN7 (W18 Driver Board) = 500V (375V)
5687 Pin 9 = 263V (200V)
5687 Pin 6 = 12.75V (8.4V)
6SN7 Pin 2= 233.3V (175V)
6SN7 Pin 5= 238.4V (175V)
6SN7 Pin 3= 6.57V (4.8V)
6SN7 Pin 6= 6.5V (4.8V)
W13 PSU Board = 115.8V (85V)

As I noted on the measures taken yesterday, the 300B filament
voltages still are OK.

On the Kit One instructions, section 8 (version 4.2, dec 2004) there is a checklist with a few voltages
mentioned in order to check without tubes:
AC
nominal 6.3v between pins 7 & 8 (6sn7 tube)= now is 6.88 vac
nominal 6.3v between pins 4 & 8 (5687 tubes)= now is 6.9 vac
nominal 420v between pin 4 (5u4g tube) and negative end of C1(a bit extrange because C1 is not polar, maybe a mistake on the old instruction manual) of driver board => and the same from this point to the chassis metal work (ground point, I guess).= now is, between pin 4 (5u4g tube) and the end of C1= 466 vac.

Then I decided to inspect some components:

The voltage of the 300B filaments board(filaments plate) are good.
The output tubes are good (tested on a Hickok, no shorts also)
PSU board is good (it is new, hardwired version).
3K3 Resitors to pin 3 (300B) are fine
C1 (driver board) is fine
Capacitors and resistors on 300 B are fine.
I can't conclude what is wrong besides the suspicion that the power transformer is damaged by the short circuit. Someone has a clue?
Regards,
Beto

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