F5 Help Needed

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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
 
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?
 
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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.
 
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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.
 
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
 
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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
 
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When you measured those JFETs, what do you mean by
.29 and .18 ?

The .29 and .18 values are from the Idss matching when I built the B1.
I have jfets with higher Idss values.

I'll throw in a few 3.3k and see what happens. Just wanted to be sure I wouldn't make things worse by changing the relationship between the 5k pot and R3 and 4. Then again...how bad can it get? :D

Thanks!

V~
 

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Well, that might mean that you haven;t matched them, but dosen't mean that they are not working! i had mine in backwards but no harm done to them.

The Q5 and Q6 - yours look really flat like ztx parts. but IIRC, cviller boards were made for BC550/560. If thats the case, maybe the pinout is different?

Just clutching at straws here.


Fran
 
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