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Old 3rd May 2014, 08:42 PM   #3251
sts9fan is offline sts9fan  United States
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Quote:
Originally Posted by 6L6 View Post
If you have a new pair from Spencer just replace the blown ones with the new pair.

L to R channel Jfet Idss doesn't need to be matched, the bias pots take care of that. What does matter is a good match between the N and P channel Jfet, on that channel's board.
Ok. Jfets replaced and cascode parts added. The channel that was good still works great. The channel that I had to replace the Jfets is now acting wierd. The bias pots seem to only affect one side each. Plus the DC offset does not do anything and just reads 1.2mv.
So I can bring each side up to .3vdc but it's not right.
With music fed in it sounds very distorted and farty.
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Old 4th May 2014, 01:59 PM   #3252
BigE is offline BigE  Canada
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Did you lose one polarity of the output transistors in that channel?
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Old 4th May 2014, 02:41 PM   #3253
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Quote:
Originally Posted by sts9fan View Post
Ok. Jfets replaced and cascode parts added. The channel that was good still works great. The channel that I had to replace the Jfets is now acting wierd. The bias pots seem to only affect one side each. Plus the DC offset does not do anything and just reads 1.2mv.
So I can bring each side up to .3vdc but it's not right.
With music fed in it sounds very distorted and farty.
Print the f5T schemtic and write down the important voltages. Make sure that you have adjustability with both pots without the circuit turned on. It should be roughly zero to sub 1K
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Old 4th May 2014, 02:57 PM   #3254
sts9fan is offline sts9fan  United States
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Did you lose one polarity of the output transistors in that channel?
Not sure how to know that.

Is there a schematic with voltages on it?

The pots adjust fine with no power.
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Old 4th May 2014, 03:02 PM   #3255
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Give me measurements across the following:

r27
r28
r3
r4
r17
r21
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File Type: jpg F5Tv3_schematic.jpg (146.0 KB, 276 views)
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Old 4th May 2014, 08:12 PM   #3256
sts9fan is offline sts9fan  United States
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Originally Posted by buzzforb View Post
Give me measurements across the following:

r27
r28
r3
r4
r17
r21
R27 and R28 are 18VDC
R3 and R4 are 47ish mVDC

R17 and R21, are these the test points on the Output boards?
These are the resistors that you bias across, correct?
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Old 4th May 2014, 08:47 PM   #3257
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r17 and r21 are easily measured across the mosfet source resistors or an inner and outer diode pin. Everything else look ok. JFet bias is a little low, but not a problem.
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Old 5th May 2014, 02:23 AM   #3258
BigE is offline BigE  Canada
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Quote:
Originally Posted by sts9fan View Post
Not sure how to know that.

Is there a schematic with voltages on it?

The pots adjust fine with no power.
Perhaps measuring Idss would tell you if the mosfet was fried or not?
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Old 12th May 2014, 11:27 PM   #3259
sts9fan is offline sts9fan  United States
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And I am back...
So I re-biased the good channel just to prove I remember how to do it. The other now can only get up to ~6mV before the pots run out or the DC offset is greater then a few mV. Does this mean I need to increase the resistance or that one resistor right? I need to read back on which one. R11 and R12?
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Old 13th May 2014, 12:01 AM   #3260
6L6 is offline 6L6  United States
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The resistors in parallel with the bias pots need to be increased . More ohms. Start with double the stock value.

Are you sure you're turning the pots the right way? 6mv is not very much for full turns on the bias pot...

Last edited by 6L6; 13th May 2014 at 12:04 AM.
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