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F5 troubleshooting
F5 troubleshooting
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Old 29th November 2019, 02:18 AM   #1
boywonder is offline boywonder  United States
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Default F5 troubleshooting

I haven't been on here in quite some time.......and need some assistance....


I have an early F5 (Peter Daniel's PCBs) and one channel is up and running fine and the other channel smoked R1....


Where to begin?


Thanks!
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Old 29th November 2019, 02:41 AM   #2
rayma is offline rayma  United States
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Could mean Q1 is shorted, test it D-S with the DVM ohm function, with the amp power off.

Did R4 and R7 also get hot? If so, then check Q3 like above.

Last edited by rayma; 29th November 2019 at 02:48 AM.
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Old 29th November 2019, 03:21 AM   #3
boywonder is offline boywonder  United States
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..I'm on it....


There was smoke..but R4 and R7 look fine. I can unsolder and check them if needed.


I'm assuming that a good Q1 is not shorted drain to source...


I'm finding it a little interesting that this circuit is so simple I could just replace everything that's not a resistor...but what fun is that?
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Old 29th November 2019, 03:30 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by boywonder View Post
There was smoke..but R4 and R7 look fine. I can unsolder and check them if needed. I'm assuming that a good Q1 is not shorted drain to source...I'm finding it a little interesting that this circuit is so simple I could just replace everything that's not a resistor...but what fun is that?
The R4 and R7 won't smoke even if an output transistor shorts, but they will get hot to the touch if it does.
When testing the transistors, connect the red DVM lead to the upper lead on the schematic; that is,
the lead closer to the positive power supply. Replacing the matched pair of input transistors is not cheap.

Was there something that happened that caused the problem?

Last edited by rayma; 29th November 2019 at 03:32 AM.
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Old 29th November 2019, 04:40 AM   #5
boywonder is offline boywonder  United States
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Quote:
Originally Posted by rayma View Post
The R4 and R7 won't smoke even if an output transistor shorts, but they will get hot to the touch if it does.
When testing the transistors, connect the red DVM lead to the upper lead on the schematic; that is,
the lead closer to the positive power supply. Replacing the matched pair of input transistors is not cheap.

Was there something that happened that caused the problem?

The amp started out as an F4......and I had built the F5 boards years ago, and about a year ago pulled the F4's out of the chassis and replaced them with the F5 boards thinking I needed a little voltage gain.



The problem channel has never worked since I put the F5's in the chassis.


I develop medical devices for a living and used the amp recently to drive a subwoofer for use as a vibration/shaker table...we needed something DC coupled to shake the medical device to simulate G loads like playing tennis and running with a wearable device. We used VHB tape to glue a cheap accelerometer to the subwoofer cone to measure G's...worked great, and I only needed one channel.


I recently brought the amp back home and figured it's about time I actually listen to it....
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Old 29th November 2019, 04:49 PM   #6
rayma is offline rayma  United States
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If it survived a shake table, that channel should be good to go.
Check all the parts on the bad board, there aren't many.
Look for resistors of the wrong value, or shorted fets.
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Old 29th November 2019, 05:02 PM   #7
boywonder is offline boywonder  United States
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I unsoldered Q1 .... I get the following


pin 1 to pin 3 source-drain 39R


..and from gate to either source or drain open circuit


I have some spares of these bought from someone supplying them on the forum years ago; just tested two of those..


Both test about 39R source to drain and about 5.66Meg from gate to either S or D.


I have a packet that has two SK370s and two SJ108s....are these the parts that need to be matched to each other?


The actual parts say K170BL and J74BL.....both on the PCB and my spares.....
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Old 29th November 2019, 05:04 PM   #8
boywonder is offline boywonder  United States
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Quote:
Originally Posted by rayma View Post
If it survived a shake table, that channel should be good to go.
Check all the parts on the bad board, there aren't many.
Look for resistors of the wrong value, or shorted fets.

lol....the amp wasn't shaken...


the small wearable device was taped to a $22 12" subwoofer from parts express....the amp driving the sub through a function gen.
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Old 29th November 2019, 05:05 PM   #9
rayma is offline rayma  United States
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Right, the input fets must be matched for Idss and be in a certain range. Check the F5 write up for details.

Shake tables are an extremely heavy load for an amp, lots of ripple current in the power supply.
I suppose you actually could indeed put the amp on the shake table while it is powering the shaking,
for a double work out.

Last edited by rayma; 29th November 2019 at 05:09 PM.
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Old 29th November 2019, 05:10 PM   #10
boywonder is offline boywonder  United States
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Rayma: did you see my post #7 above? is the Q1 in question fine? from gate to either S or D is open....fresh ones in the pack read 5.66M from G-S or G-D.


I just visually compared components (everything but resistors...all 4 of them..hah!) between boards and nothing installed backwards.


Should I unsolder the output MOSFETs and test those?
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