F5 power amplifier

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#2,042
Powered on one of the F5 boards for the first time, unfortunately I heard a noise and saw an orange glow.
On closer inspection the 2SJ74 seems fried, the Drain pin is melted and above that the 100K resistor may be faulty.


Not sure what the problem is of yet, the component orientation seems ok.

Thanks
Thermos
 
#2,049
Babowana said:
What are the source resistors marked xR for? For trimming?

What is the 2u2/250V cap for?
Click to expand...

Yes, all x are for trimming. 2u2/250 is for better supply bypass as I have read here in forums. You really may don't need it.
Many components are there because I just have them in my drawer.
680R paralleled for higher power and lower inductance.
 
#2,050
Little Bang.

thermos said:
Powered on one of the F5 boards for the first time, unfortunately I heard a noise and saw an orange glow.
On closer inspection the 2SJ74 seems fried, the Drain pin is melted and above that the 100K resistor may be faulty.


Not sure what the problem is of yet, the component orientation seems ok.

Thanks
Thermos
Click to expand...

What I am thinking I have done wrong is "matched" the 2SJ74 (P Ch) with the FQA12P20 (P Ch) and the 2SK170 (N Ch) with the FQA19N20C (N Ch) ?

It should be the 2SK170 (N Ch) with the FQA12P20 (P Ch) and the 2SJ74 (N Ch) and the FQA19N20C (P Ch)?

If this is correct, could someone please let me know?!? (still suffering in shame with old Class AB amp)

Thanks (once again)
Thermos.


:smash: :smash:
 
#2,052
umut1001 said:
can i use mpsa42/92 or bc547/557 instead of ztx550/560?ztxs are 1amp transistors and other ones are half watts?.. i want to make it sure..thanks
Click to expand...

IMO you can. I plan to use 2sc1815/2sa1015. In case of limiting the output current of amp they must work against JFET.
Ultimately you can check it out with scope driving low impedance (1 Ohm) load.
 
#2,053
More OT....

All I can say is that I learned more about electronics from Phred and his sockpuppets than from most other members here and at the other side...he's easy in the top ten in this regard.

Sorry for the OT, and back to the regular F5 schedule...

Tino
 
#2,054
One board is working

Swapped the Sj74 and the Sk170 on the other board and it works!

I tried adjusting P1 and P2 and was able to get 0.6v on both R11 and R12, but this resulted in 0.2v when the Output was measured.

If I adjusted P1 and P2, I was able to get the output to 0V but the voltage at R11 and R12 was only at 0.13v.

I performed this with the input open (should the RCA input be shorted? - as I thought I read in a previous post)

Any advice would be appreciated.

Thanks
Thermos
 
#2,055
Hi thermos,
First thing,if you haven't done it yet, remove the 1k resistors you have in series with the trimmers and jumper those positions. You cant trim down to zero if you have a 1k resistor in the circuit.
With the amp off set your trimmers so you read zero ohms across r3 and r4. Now you can power up. If you have two meters put one across r11 and one from output to ground. If not then just move it back and forth. Start by bringing up the voltage across r11 to around .1v, then watching the meter on the output equalize the circuit by adjusting the other trimmer. If this cannot be done you have to find the problem. If all is OK then bring up r11 to .2v, then equalise, and so on. If you can keep your offset around zero then you know the current is identical through r11 and r12.
That's about it if I didn't forget something. Someone else will chime in, I imagine.
Bill
 
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