F5 troubleshooting

..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?
 

rayma

Member
2011-04-29 8:37 pm
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?
 
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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....
 
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.....
 

rayma

Member
2011-04-29 8:37 pm
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.
 
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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?
 

rayma

Member
2011-04-29 8:37 pm
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. Should I unsolder the output MOSFETs and test those?

The gate should be open in either polarity to the D or S. A MOS gate is an electrode that
has no ohmic connection internally, and works by electric field like a capacitor.
The most common fault with the gate is a short to the drain or source due to excessive
voltage from static charge due to handling or a fault in the circuit. Another common fault
is a short from D to S due to excessive current.

For testing any N type fet, the red DVM lead goes to D and the black lead to S, and should be open.
For testing any P type fet, the red DVM lead goes to S and the black lead to D, and should be open.
 
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Well....I thought I had figured it out...but it's still has the issue. I tested the 2SK370 and the IRFP9240 and they both checked out normal.


I thought that one side of R1 was contacting ground since the ground connection is right there and the boards are tiny.......I moved the grounds away from R1 and re-stuffed with a new R1 but it smoked too.


Interesting..with nothing connected to the input the bias and DC offset are quite normal, and R1 is happy....when I plug the function generator into that channel R1 smokes....


I haven't checked the ZTX550 yet.......tomorrow.
 

rayma

Member
2011-04-29 8:37 pm
I'll try that in the morning....

Look very carefully at all the wiring and parts connected to the input RCA connector terminals. If DC conditions
are ok until a connector is inserted, something is wrong related to the socket. Are the two wires reversed?
Maybe the amplifier is oscillating when the input is connected. What happens to the DC voltage at the amplifier's
output when a shorting plug is inserted?
 
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The input wires aren't reversed.....I redid both channel's input wires today.....the other channel still works fine....shorting plug in the morning...


The other channel didn't smoke for months while the amp was driving the sub/shaker table from the good channel...nothing was connected to it.
 

rayma

Member
2011-04-29 8:37 pm
The input wires aren't reversed.....I redid both channel's input wires today.....the other channel still works fine....shorting plug in the morning...The other channel didn't smoke for months while the amp was driving the sub/shaker table from the good channel...nothing was connected to it.

My theory is that this problem is caused by inserting a connector, for some reason that is due to another problem.