K8060 bias too low


#1

Hi,

I've got a K8060 and I've managed to get it working after a fault which blew the transistors. I replaced all of them at the same time and managed to get the system up. However the bias can only be set to a maximum of 1.5mV. 

I’ve re-changed the transistors but still the same thing. I’ve got + and - 40v at the power supply and also 9V at the 9V point. I can’t think of anything else to change. The diodes are the right way round (D1 and D2).

Can anyone help I’m at the end ofthe road here for ideas… Its going in the bin before long because its taking way too much time and money to keep on with it…

Yours hopefully

Jules


#2

By the way, I’m using the recommended transformer and the recommended heatsink. I put some audio through the system and its is very low level and pretty distorted although if the TIP aren’t biased correctly I guess they won’t be pushing and pulling correctly…

I tested the diodes in situ with an ohm-meter and they are open circuit one was and then about 900K the other way. They are soldered in correctly.

Thanks

Jules


#3

Check bias circuit. Check orientation and type of temp. comp. transistor.


#4

Hi, Can you tell me what sort of voltages I should be finding at the base of the T6, T8 and T7 so I can see if they are being biased to the right levels to produce the R19 current?

Many thanks for your quick response anyway!

Jules


#5

Hello,i’m new on this forum and i read the problem of the bias adjustment.
I’ve the same problem and thereabove there is a high voltage over the connectingpoints of the loudspeaker of about 80 volts ac.I’ve checked all thesolderingpoints and i’m sure that every part has mounted in the right way.
If i look to the drawing in the instructionbook , there is made a mistake.
The book shows a connection of the 30 volts ac in the innerside of the pcb, but has to be one on the innerside and one on the outsidewith the discription ac-B.
I’ve replaced the cappacitorsC-12 & C-13 into mF4700 - 63 volts and connected them.in the right way.

What can now still be the problem ( The LED islighting up,so the connections must be correct)
Please help me:
FerdiL :neutral_face:


#6

Hello,

As reference i have used our manual:

vellemanprojects.com/downloa … 0_rev1.pdf

I can’t see any mistake on the drawing ?


#7

Thanks for fast reply.
The drawing is a minor problem; good loking and using yuor brains helps a lot to discover how to connect the transformer and so on.
but now i’m still loking for a solution about the bias adjustment.
And how can there be 80 volts between te two speakerconnectionpoints.What did i wrong???


#8

If there is 80VDC between the speaker outputs than there are a lot of things wrong with your assembly:
Basically, both power supply rails are directly connected to the speaker terminals.
This can either be due to soldering errors, solder bridges, TIP142 & TIP147 transistors that have been interchanged etc… Check your assembly. If there is a problem with one of the transistors, we recommend to replace all of them.


#9

O.K.
I’ll have an other look to yhe pcb.
I Thought i did well, but you never know .
I hope so.
Thanks Again : Ferdil


#10

Hoera,
Thank you for being fair to me.
After desoldering and resoldering and cleaning the pcb everything was working as belongs.
not any problem anymore.
This time i mounted an adjustable resistor on the input contacts and yes no humble or any not wished sound or noise.
The volume is more than enough and sounds to me like what i expected.
Just one advice to everyone : solder proper and clean the pcb after soldering; in my case it seems to help.
Just check three times all the solderpoints. I did and no recrets.
Thank you for being patiently.
FerdiL :smiley:


#11

what did you use to clean the PCB? I would like to clean mine aswell.