F5 power amplifier

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#12,302
I have disconnected one leg of R13 and R14, and power up with a signal (1KHz sine). The signal comes from the soundcard (no preamp).

I see:

- Voltage across R12 = 1.3VDC
- Voltage at output = 4.5VDC and rising
- I have measured the signal at R13 and R14 with scope (connected to drain of the JFETS). The signal at R14 has lower amplitude than at R13 (about 1/3 in amplitude). The shape of the signals is a bit of a skewed sine.

Interestingly, when I switch off the power supply to the amp, the signal at the drains of the two JFETS become both a very nice and smooth sine (equal on both JFETS) for some time, until voltage drops below about +/-5V.
 
#12,306
studiostevus said:
I found an incorrectly supplied resistor: R6 was 3K9 and should be 100R

Can somebody please give me the correct values for measuring across R8/R5/R7/R6 in circuit? I get on one channel 40R (bad) and on the other 12R (the good channel!)

Thanks
Click to expand...

GND of output move to GND of board.
check all part if ok-->reduce value of R feedback down to lower 600ohm (50ohm may be more..and increase when it ok).original circuit is simple theory (read again working Jfet,mosfet..)
Q3 open feedback control by Q1 ( Q4,Q2 is the same ).
 
#12,312
fdlsys said:
What does it matter who made the PCBs? They all have the (+)-(0)-(-).
Click to expand...

On the output, yes. But the input to the board is different depending on which particular board.

Charly1 said:
Thanks Yes thats why I have two transformer with each center tap winding. My problem is how can I use this Jims board with two seperate bridges.
Click to expand...

It depends on which Jims board you have and your transformers. Can you give more detail and/or post some photos?
 
#12,316
Ok, here is a question for those if you much smarter than I -- (which is most everybody...)

During the assembly of my new F5, (one channel of) the amp PCB was grounded via the mounting screws, and it took me a bit of time with it powered up to figure out the problem.

Throughout this time the PSU was powered with a 100w light bulb on the mains lead (I.E., a mains lead tester). (Yes, it was fairly bright then...) 🙂 🙂 🙂

Once the metal mounting screws were replaced with nylon, there was no more problem and things were completely normal. Nothing seems burnt, scorched and/or overheated.

Now that both channels are completed and the amp measures and biases normally, the channel that was shorted sounds a bit fuzzy. (it's subtle, but very much there.)

My question is this - Which of the transistors (inputs, outputs, limiters) would most likely have been hurt by this episode? Obviously I could replace the whole set, but if I don't have to go that far, I would rather just replace the suspect ones.

Also, is there any easy test to see if the devices are damaged? (I probably don't have the equipment necessary, as the amp is operative, just a bit wonky...)
 
#12,318
You might use your ohm meter to ohm out conections on the top half vs the bottom 1/2 (swithcing red to black where appropriate) and look for significant differences???
So waht was connected wrong during the episode? Which screw was it?
 
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#12,320
desolder 2 bjts from protection first , and hear is there difference .

besides these , there are just 4 important transistors

you can start from small ones , changing one by one and listening is there positive change

in fact , that demands less energy than typin' one post and reading 3 replies

:clown:
 
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