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Old 18th December 2011, 01:01 PM   #1261
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Quote:
Originally Posted by richluvsound View Post
I have 0 @ the bottom of R10
Excellent news. The right hand side of the stage (Q11 et al) is biased correctly.

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Originally Posted by richluvsound View Post
R6 = 17.50 OSC
R13 = 2.0 OSC
Gate of Q4 not being close to 0 volt is strange. Can you check the joints of R13? or perhaps Q4 is toast. Re-test it.

What is the voltage -across- R6 ? Between 1 and 2 volts? Can you adjust it to be in that range with P1?
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Old 18th December 2011, 01:24 PM   #1262
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Originally Posted by PierreQuiRoule View Post
Excellent news. The right hand side of the stage (Q11 et al) is biased correctly.

Gate of Q4 not being close to 0 volt is strange. Can you check the joints of R13? or perhaps Q4 is toast. Re-test it.




What is the voltage -across- R6 ? Between 1 and 2 volts? Can you adjust it to be in that range with P1?
Changed Q4 Gate = 4.9 mV OSC

Across R6 = Approx 3.0 V .... No change when adjusting P1

Last edited by richluvsound; 18th December 2011 at 01:50 PM.
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Old 18th December 2011, 02:48 PM   #1263
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Changed Q4 Gate = 4.9 mV OSC

Across R6 = Approx 3.0 V .... No change when adjusting P1
Q4 is good now. But your reading across R6 (and ineffectiveness of P1) shows that there is no current through Q5 meaning that both Q4 and Q5 were toast. Replace Q5, and remeasure across R6. Hope you read 1.5 volt. If you do, recheck and note voltages. Replace 2.2k by actual Q2. Compare voltages to those with 2.2k.

Once you have a running board, you will want matched Q4 and Q5. IMO matching is more important for these two than for the quad of jfets in the first stage. This can be taken care of later.

Pierre
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Old 18th December 2011, 03:19 PM   #1264
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Changed Q5 and still 3.7 across R6 and P1 still flakey !
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Old 18th December 2011, 03:56 PM   #1265
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Changed Q5 and still 3.7 across R6 and P1 still flakey !
Rich - When you adjust P1, does it change the voltage across R6?

You want the voltage across R6 to be in the order of 1.5 times the voltage across R28. This will indicate that Q4 and Q5 are both drawing enough current.
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Old 18th December 2011, 06:19 PM   #1266
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approx 1.5 v across R 28


I am suprised by the amount of heat at the 2.2k (Q2 )

Last edited by richluvsound; 18th December 2011 at 06:22 PM.
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Old 18th December 2011, 08:46 PM   #1267
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Changed Q5 and still 3.7 across R6
Seems in the ball park from my remembering, assuming 2,46 mA runs in the diff pair so slightly more than 1mA per se, seems fine


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approx 1.5 v across R 28

that means 3 mA flowing. Slightly more than the measure on R6 but in an order of magnitude.
But if Vref of the led is 1,65V, you should find about 1V on this resistor ( 1?65 minus 0,65V drop between B and E )
So I suspect your led to have slightly greater Vref than 1,65. Mostly a 2V one can you measure the direct voltage on led ?

I am suprised by the amount of heat at the 2.2k (Q2 )
Which is the reading on R29 ? That would tell us what current is flowing in here. It should be around 10 mA. Flowing through the 2K2 it means 0,22W. So if you use a 1/4W standard it means near max dissipation, or even on a 0,6W model it can be hot. if you use a Dale 1/10W, it will burn one day or another

In any case you should find very similar measures on both R28 & R29. About a volt.
If you can't get it, you can always keep the led you use actually and change R28 and R29 values. In your case, you could change R 28 for 750R and R 29 for 150R and it would be also fine

Best,

nAr
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Old 18th December 2011, 09:02 PM   #1268
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Seems in the ball park from my remembering, assuming 2,46 mA runs in the diff pair so slightly more than 1mA per se, seems fine
Not quite. R6 feeds Q5 only, so about 1 mA in 1.5 kohm should give about 1.5 volt, as Wayne shows in his schematic. Rich measures twice that.
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Old 18th December 2011, 09:09 PM   #1269
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Originally Posted by richluvsound View Post
approx 1.5 v across R 28
This means the differential pair is biased with 3 mA total. There are 2.5 mA flowing through Q4, leaving .5 mA for Q5. The split should be more even. Again, does the voltage across R6 changes when you adjust P1?


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I am suprised by the amount of heat at the 2.2k (Q2 )
As nAr kindly pointed out, this is normal. It is dissipating about a quarter watt . No worries.
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Old 18th December 2011, 09:13 PM   #1270
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Not quite. R6 feeds Q5 only, so about 1 mA in 1.5 kohm should give about 1.5 volt, as Wayne shows in his schematic. Rich measures twice that.
Oopps, my bad, should have looked closer. Seems the diff. pair doesn't share the bottom current evenly
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