Stuck on F5 Build power-up...

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dh40

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Joined 2008
#1
I'm stuck powering up the F5 (2.0 from cviller)....

I get no heat, and zero volts across R11/R12. Turning the pots has no effect on anything.

If I had to guess, Q1 and Q2 are doing nothing. I see about 30-40 mV across R1/R2.

I have replaced them (Q1/Q2), but from the same stock (yes, I bought crap from China via Ebay).

Any thoughts? Thanks. ~Dave
 
#3
Ebay jfets are most likely fake.

Are you sure turning the pots has no effect? They sometimes take a bunch of turns to do anything.

Are your V+ GND and V- measuring properly at the amplifier boards?

And very important, please post some photos.

Thanks!
 
#4
Make sure your power supply is working properly, and that you have all jumpers installed and soldered in on the boards (I don't know if there are any on the cviller boards)

Then, go over each of your boards and make sure everything is in the right place.
 
#7
Measure the Idss of your current Jfets (as Andrew suggested) if you have a chance to take them out - if they are genuine, they are pretty durable.

Make the changes to your existing board -- save yourself a bit of work. You will probably be able to get this one working.

Edit - One last question, what schematic are you using? The version with P3? If so, is it on one side of it's range or approximately centered?

Check your PMs.
 
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#8
6L6 said:
Measure the Idss of your current Jfets (as Andrew suggested) if you have a chance to take them out - if they are genuine, they are pretty durable.
Click to expand...

I have a couple new ones from the Chinese source... I can measure those.

I see that 2sk170s are still available in quantity for Mouser (and I assume they are the original Toshiba). Is there a source for the 2sj74?

Edit - One last question, what schematic are you using? The version with P3? If so, is it on one side of it's range or approximately centered?

Check your PMs.
Click to expand...

This is the latest version of the PCB that I can find, but it contains components for other versions: http://www.diyaudio.com/forums/blogs/cviller/attachments/x98d1265162390t-gb-f5-guide-pcb-version-2-f5_cascode_forguide.png

Is there a better one? The boards are v2.0 2010-10-23 (standard F5).

Thanks!
 
#9
Off the top of my head I know that you can get 2SJ74s (and K170s) from member " h_a ", and also from the AMB.org site in his store. There are probably other sources as well, I just can't think of any right now.

Your schematic link is really small... and I wonder if the schematic has different part numbers than the original schematic... Do you have a bigger image of it?
 
#10
What voltagge if any at gates of mosfets?
maybe resistor pot combination is running out of steam as it does happens same time and resistor value need to be changed

Is voltagge changing as you turn the pots?

Mosfets need 3.5 to over 4 V to start conducting so ....
 
#11
6L6 said:
Off the top of my head I know that you can get 2SJ74s (and K170s) from member " h_a ", and also from the AMB.org site in his store. There are probably other sources as well, I just can't think of any right now.

Your schematic link is really small... and I wonder if the schematic has different part numbers than the original schematic... Do you have a bigger image of it?
Click to expand...
h_a don't sell only J-fets. but spencer does 🙂
 
#12
Well, it looks like it was a wrong value for R21/22. By putting the correct 22K resistor in there, it is working more like "normal".

I am only getting about 0.35 - 0.38V across R11 and R12, despite cranking the pots to the limit. I think I got them installed correctly, but as long as they are both in correct or reversed, that shouldn't be a problem?

An externally hosted image should be here but it was not working when we last tested it.


Again, I don't know what damage I might have done to other components, and I should probably complete the rest of the boards before spending too much time on this prototype...

Thanks all. Dave
 
#13
Is your .35v measured with zero offset?

If so, you need to increase the value (more ohms) of R3 and R4. Start with 4K or 5K in those positions. Try to bias normally. Needing more series resistance with the pot is a very common problem.
 
#16
Maybe the Jfets are genuine! Cool!

The amount of bias with zero offset in the important number, not the bias itself.

If you can't get enough bias with zero offset, make your R3 R4 bigger. That's all there is to it... easy!
 
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