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vdi_nenna 30th August 2009 01:51 PM

F5 Help Needed
 
Hi All,

Still having issues with my F5 build. Replaced all active components and 4.7 ohm thermistors. Checked all resistor values; all are ok. Check for output shorts to case and none found. No input shorts either.
BTW- I checked the semies out of circuit and they all seemed to check out ok. (Must be a resistor value is incorrect?)

I can't seen to get the bias right on this second build. One channel goes up to about .314v across R12 and the other channel gets to about .51v until I run out of turns on the pots. Could R3 and 4 be the wrong values? I check them and they are correct as far as markings go. Used Bourns pots this time and they are very accurate. They do not have markings for ohm value.
Voltage across R3 & 4 are close to 4.2v as indicated in the schematic, but not perfect. I assume P1 and 2 influence this voltage? Also no DC at output.

Thanks for your input,

Vince

tms0425 30th August 2009 04:45 PM

Quote:

Originally Posted by vdi_nenna (Post 1910309)
Hi All,

Still having issues with my F5 build. Replaced all active components and 4.7 ohm thermistors. Checked all resistor values; all are ok. Check for output shorts to case and none found. No input shorts either.
BTW- I checked the semies out of circuit and they all seemed to check out ok. (Must be a resistor value is incorrect?)

I can't seen to get the bias right on this second build. One channel goes up to about .314v across R12 and the other channel gets to about .51v until I run out of turns on the pots. Could R3 and 4 be the wrong values? I check them and they are correct as far as markings go. Used Bourns pots this time and they are very accurate. They do not have markings for ohm value.
Voltage across R3 & 4 are close to 4.2v as indicated in the schematic, but not perfect. I assume P1 and 2 influence this voltage? Also no DC at output.

Thanks for your input,

Vince

I'm assuming you meant 4.7K not 4.7 ohm thermistors?

vdi_nenna 30th August 2009 05:23 PM

Yes, I went back to the order and checked. It is 4.7k Ohm.

Also something of note- when I would blow air over the circuit boards, the R12 voltage would rise several hundred millivolts, then drop back down to their current voltage. If I put my hand on the heat sink the voltage would drop slightly.

Tea-Bag 30th August 2009 09:58 PM

Quote:

Originally Posted by vdi_nenna (Post 1910344)
Yes, I went back to the order and checked. It is 4.7k Ohm.

Also something of note- when I would blow air over the circuit boards, the R12 voltage would rise several hundred millivolts, then drop back down to their current voltage. If I put my hand on the heat sink the voltage would drop slightly.

Cooling the Jfets and Fets with air seems normal in my book.

I only measure voltage across one of the source resistors, and leave the other DVM across the terminals and measure DC offset. As one pot goes up in turns, the DC offset should move and the source voltage. The idea being that if the source voltage is reasonable, and the DC offset reasonably low, your good. There should always be a value here.

woodturner-fran 30th August 2009 10:00 PM

2 things to do:

check your grounding and take some pics and post them here. That will help hugely.


Yes, if R11 and R12 are different values (eg one is .47 and other is 0.33) then you will get different voltages across them. If they got a good burn at any stage they may well have drifted.

What boards are you using?


Fran

vdi_nenna 31st August 2009 12:25 AM

I have no DC reading/movement at the outputs.

Using cviller boards. i have 2 coming from Peter Daniel soon.

I'll try to post pics but my camera is bad for close-ups.

Nelson Pass 31st August 2009 01:29 AM

I think it's either low Idss on the JFETs or high Vgs on the
Mosfets. I run into the latter often enough. Increase the
value of the resistor in parallel with the pot. I usually go from
1.2K to as high as 2.2K

:)

vdi_nenna 31st August 2009 01:36 AM

Hi Nelson,

I used Jfets from the B1...I remember using Jfets for this amp with a low Idss...let me see...this time I used .29 and .18 on th 2sk170s. I did not measure the 2sj74 jfets.

You think this would explain the lack of DC at the outs?

Thanks!

Vince

vdi_nenna 31st August 2009 01:39 AM

Some pics...not good. BTW- I used IRFP9140 P-ch (100v 21a) and IRFP140 N-ch (100v 33a)

Used 2.2k for R3 and 4 from Fig 6. Should I go higher than 2.2k?

http://i145.photobucket.com/albums/r...s/DSCF0100.jpg

http://i145.photobucket.com/albums/r...s/DSCF0101.jpg

http://i145.photobucket.com/albums/r...s/DSCF0102.jpg

http://i145.photobucket.com/albums/r...s/DSCF0103.jpg

http://i145.photobucket.com/albums/r...s/DSCF0108.jpg

http://i145.photobucket.com/albums/r...s/DSCF0109.jpg

http://i145.photobucket.com/albums/r...s/DSCF0110.jpg

http://i145.photobucket.com/albums/r...s/DSCF0112.jpg

As you can see, I added board connectors this time for quick removal!! :)

vdi_nenna 31st August 2009 06:16 PM

Can I go up to 3.3k on the R3 & R4 resistors?

Thx.


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