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Old 30th November 2019, 04:49 PM   #21
boywonder is offline boywonder  United States
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I've pulled both boards and have them on the bench...


On the good channel R5 R6 R7 and R8 all have about 18-20R across them


On the bad channel R5 and R7 have zero R and R6 and R8 have 18-20R across them....
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Old 30th November 2019, 05:06 PM   #22
rayma is offline rayma  United States
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Quote:
Originally Posted by boywonder View Post
On the good channel R5 R6 R7 and R8 all have about 18-20R across them.
On the bad channel R5 and R7 have zero R and R6 and R8 have 18-20R across them....
Could you post the schematic that you are using?
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Old 30th November 2019, 06:08 PM   #23
boywonder is offline boywonder  United States
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From Nelson's post #89 in this thread...


F5 power amplifier
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File Type: jpg F5 schematic.jpg (75.9 KB, 119 views)
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Old 30th November 2019, 06:13 PM   #24
rayma is offline rayma  United States
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Ok, then remove (or lift one end) and check R5 and R7 for value.
Also check for ohms R1, R2 and the outputs Q3, Q4 while R5/R7 are disconnected.

Last edited by rayma; 30th November 2019 at 06:16 PM.
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Old 30th November 2019, 10:22 PM   #25
boywonder is offline boywonder  United States
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just lifted one end of R5 and R7.....R5 is a .13R resistor...supposed to be 100R...R7 correct at 100R....


Thank you rayma!!


Will put it back together and see how it behaves....
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Old 30th November 2019, 10:24 PM   #26
rayma is offline rayma  United States
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Was your R5 marked as a 100R and wasn't? If so, it was damaged along with the R1,
probably due to high DC output voltage. Check that the DC offset voltage is small.

Or was R5 actually the wrong value part? I always first measure each part that I install on a board.

Last edited by rayma; 30th November 2019 at 10:29 PM.
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Old 30th November 2019, 10:26 PM   #27
boywonder is offline boywonder  United States
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Quote:
Originally Posted by rayma View Post
Was your R5 supposed to be 100R and wasn't? If so, it was damaged along with the R

No I installed an incorrect R value for R5... .13E blue ohmite looks just like 100R blue ohmite..until you read the fine print. That explains why this channel has never worked...I hope.
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Old 30th November 2019, 10:30 PM   #28
rayma is offline rayma  United States
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Originally Posted by boywonder View Post
No I installed an incorrect R value for R5... .13E blue ohmite looks just like 100R blue ohmite..until you read the fine print. That explains why this channel has never worked...I hope.
Wrongly binned parts happen, always measure first. There's no way the amp could have worked with 0.13R in there.
Be sure to replace R1 also. The R7 will be fine, since it was protected by a short across it.

Last edited by rayma; 30th November 2019 at 10:33 PM.
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Old 1st December 2019, 12:00 AM   #29
boywonder is offline boywonder  United States
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amp is warmed up.....getting .6v bias both channels no problem.....and can't get much lower than about 12ma DC offset...pretty happy!
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Old 8th December 2019, 12:07 AM   #30
boywonder is offline boywonder  United States
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Quote:
Originally Posted by boywonder View Post
amp is warmed up.....getting .6v bias both channels no problem.....and can't get much lower than about 12ma DC offset...pretty happy!



...of course the above should be mV...not ma....


Well....both channels have a fair bit of hum...


On the bench I'm not seeing it with the scope cranked all the way up..but downstairs in the system it's obvious.


I'm measuring 4 ohms from the common ground point on the boards where the PS, RCA and speaker grounds are all connected together ........ to the chassis ground stud.....is this typical or too much R?


I tried clip leads from the PCB ground to the stud..speaker -ve to the stud and RCA -ve to the stud..no change. RCA's are insulated from the chassis..with -ve grounded at the PCB.


I have twisted pair wires running from RCA's to the PCBs...no coax...


any suggestions?
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