K8011 - Buzz

Kits Audio Hi Fi Projects
1

Bonjour

J’ai monté ce kit il y a plus d’un an et depuis peu, l’ampli génére un buzz.

Je pense avoir écarté certaines origines de ce problème en suivant les préconisations du manuel :

  • je ne branche que l’ampli et le baffle (rien en entrée ou à proximité)
  • j’ai court-circuité l’entrée et débranché la terre : l’ampli ne buzz plus
  • j’ai remplacé les lampes de puissances : l’ampli buzz
  • j’ai réglé les courants de bias : l’ampli buzz
  • j’ai mesuré les tensions de références, les valeurs sont proches de celles indiquées
  • j’ai mesuré R1 (sans la désouder) à 600ohms alors qu’elle doit faire 470ohms (?)
  • j’ai débranché la terre : le buzz s’est amplifié.

Merci de me donner votre avis.

2

Hi all

Maybe I should have responses and advises if I write in English?

Translation of my former post :

I built that kit a year ago and the amplifier started buzzing few month ago.

What I did :

  • I only plugged the amplifier, nothing around and no signal in the input, I plugged the speakers : I can hear a buzz
  • I mesured the frequency of the signal : about 50Hz, it is certainly a ground problem, but the amplifier is correctly put at the ground (via the R1 470ohms resistor - the value is OK)
  • as the manual says, I shorted the input and disconnected the ground : no more buzz
  • I replaced the power tubes and tuned the bias : it buzzes
  • I mesured the typical voltage, the are near the correct values
  • I diconnected the ground : the signal increases

Help me please, tell me the measurements I have to proceed and the components I have to check, I have a multimeter and (old) knowledge in electronics.

Thanks in advance

3

Could be a humidity problem.

4

Hi

There is no apparent corrosion on the board and the amplifier is rarely exposed to humidity (I use it as a guitar power amplifier).
Maybe you mean that components took humidity and so don’t behave correctly?

I am certainly going to redo all the soldering, it could be a dry one.

Thanks for answering.

5

Your problem sounds like a ground problem. I could be wrong but read this article first :http://www.guitar9.com/studionine7.html

6

Thanks for the link.

I think the problem I have is internal, I mean I think it is the power amp that is generating this buzz.
I did my test with the amp alone and out of its rack, so without other devices influences, without connections (only the jack plug to the speaker) and without multiple ground reference.

7

Any advice or opinion from Velleman Helpful Hands?

8

Such problems are difficult to solve from a distance, but maybe I can give some hints.
Do you have any idea of the magnitude of the buzz ?
You said it occured suddenly. Did something change (speakers, cables; location, power outlet, tubes, bias setting,…) ?
Are you using the dedicated chassis panel for the K8011 ?
Does rotating the output transformer help ?
If you remove the first amplifier stage tube, is the buzz gone ?

9

I know about that, I don’t ask for a miracle :lol:

Sorry I don’t, I can have an oscilloscope to measure it.

I noticed that the LED 1 don’t flash anymore and the LED 2 don’t flash very well.
Do you think that T2 could be involved in that problem?

No, I did it myself, I use an aluminium sheet and screw the PCB via the spacer provided.

I tried to move both the output and the supply transformer, the buzz doesn’t change.

You mean V1 and V4? I’ll try.

10

Do not remove the phase shift tube, only the pre-amp tube.
Check the power-on circuit. Does the relay that switches the HT switch correctly ?

11

You mean V5 and V6.

Correctly, I don’t know. I think it works because the amp stage works normally.
Concerning the power-on circuit, do you have the voltage values I could check on?

12

V6 is the pre-amp stage
Sorry, no voltage values
Maybe one other hint: are the 220 ohm resistors R40 and R43 still OK (they balance the filament voltage to avoid hum)?

13

Thanks for your help, I am going to check all of this and post my results.

Bye