F6 Illustrated Build Guide

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#2,001
Hi @ Mooly and all the others,

a short update to my long journey of finding the faulty part in my curcuit.
Finally my new parts arrived and i was really optimistic to fix my amp.

So.... I replaced:
- both 1000uF caps
- 0.5 Ohm 3W resistor
- 0.47 Ohm 3W resistor
- both Mosfets IRFP240

and the hum is still there.

The jFets are still the old ones, because the hum was present even without them in place. (Bad idea?)

Now i tried what @ Mooly suggested. I bridged the jensen transformer on the path to the mosfets (pin 5-7 and 6-8).

But even that didn't get rid of the hum. 🙁

The windings of the jensen measure at ~29Ohm each.

I have no clue what could be the problem... should I just blindly put in a new transformer?

Thank you guys for your help 🙂
 
#2,003
Yes, the second channel is perfectly fine.
I'm so happy that this channel is running.... :/

But you are right... I have to switch the transformers.

I leave the jfets in place for the moment.
In case it's still humming after the transformer swap, I have another set of them.

I will report.

Thank you 🙂
 
#2,005
As another test - have you swapped the PSU between both channels ?
That would rule out the PSU as the possible culprit.

Or, swap the amp board from the faulty channel into the case with PSU of the good channel - that way you could look to case & wiring issues ...


Best regards, hope you get this sorted out,
Claas
 
#2,007
Just something I've been assuming all along so I'd better ask... did this channel work OK before the mishap or was it untested before the incident?

Could you just be looking at a wiring/grounding issue if so.

As to the power supply, you've checked it with a scope and checked it in comparison to the other channel. What you observed on the scope was similar. It also agreed with a simulation of the power supply in terms of ripple shape and size.

Interesting 🙂
 
#2,008
Hi Mooly,

the channel worked OK in the first Version of my chassis.
Then I swapped the hole thing into the second chassis version.
But it was never tested in this chassis before.

The position of the toroidal transformer and the rectifiers are different in this chassis, but the wires are the same.

Also the wiring is identically to the other channel.

I tested a different ground point and re-arranged some wires, but with no effect.

Did I maybe miss something?
 
#2,015
Hums troubleshooting is hard work.

Just my opinion, I would have taken the F6 board on the heatsink out to the bench and hook up a Lab power supply or the perfectly working power supply from the other unit.
This way, you have a minimum ground interfering issues to start with.

I learnt this the hard way with my M2X, tested ok on bench, once installed in chassis, hums so badly.

Just powered up my F6 with SLB, its dead quiet. surprises me coz both channels shared the same power supply.
 
#2,019
KampfKeks3000 said:
Hi Mooly,

the channel worked OK in the first Version of my chassis.
Then I swapped the hole thing into the second chassis version.
But it was never tested in this chassis before.

The position of the toroidal transformer and the rectifiers are different in this chassis, but the wires are the same.

Also the wiring is identically to the other channel.

I tested a different ground point and re-arranged some wires, but with no effect.

Did I maybe miss something?
Click to expand...

I can't see anything obvious in what you describe.

A good test is just to have the power supply connected (so -. + and the psu ground or zero volt line) and to have the amplifier input shorted at the board terminals.

Have no other external wires connected to the board apart from the speaker.

If it still hums then we know it's not wiring related.

A very strange problem 🙁
 
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