F5 power amplifier

[acumos]

my welder had a problem so I made some troubleshooting and I found that it had no grounding and the heat point had 40v ac. Now I solved the welder warms again I grounded the point. Can that 50v ac damaged the transistors?

 

[Rush]

Can that 50v ac damaged the transistors?

Yes.

 

[Zen Mod]

nope , unless you're soldering them while circ is ON

I hope you're not doing that ......... from several reasons

 

[acumos]

I'm trying to understand why I have no voltage at r11 r12

 

[Zen Mod]

pull out all semis and test them with simple matching jig

 

[acumos]

pull out all semis and test them with simple matching jig

can you give me a scheme?
my led is not working can this give me an open?

 

[Zen Mod]

http://www.firstwatt.com/pdf/art_mos_test.pdf

small bjts you can test with diode test on your DMM ; http://www.bcae1.com/

you can use 10R for in lower pic

use 9V battery for V

 

[acumos]

thank you zen. Perfect I've broken a pin of ztx550. So now I'm sure it's fault. Do I have to check all the transistors Q1-Q6? What about the led fact?

 

[Zen Mod]

dunno - broken pin of ZTX can be or not

regarding led - is it oriented as needed ?

who cares for LED , if you have proper PSU voltages on pcb ?

check all semis , replace what's needed , try again - following any of published start-up tutorials

 

[acumos]

I've checked ztx450 and the hfe is circa 300 but it increases to 1000. I think it should be 100-300. Is it normal?

 

[KatieandDad]

Going back to biassing, the bias current wil not start to rise until the output FETs start to turn ON.

Once you have replaced the semis just trrying turning up the bias two turns at a time until the output FETs start to conduct. Then you can adjust the bias as necessary.

 

[Zen Mod]

I've checked ztx450 and the hfe is circa 300 but it increases to 1000. I think it should be 100-300. Is it normal?

ztx are "just" current limiters , in case something goes south

besides that - it's either bad battery in your DMM or shitty DMM

that reading is , well , wrong

 

[acumos]

added 2.2k resistor to R3 and R4 but still no voltage in R11 R12.

 

[Zen Mod]

open new thread with plenty of pictures

I can't follow here what you made , what you checked and what not

 

[KatieandDad]

Glad to say that my repair is looking good. The F5 really didn't like being biased at 3A (ish).

 

[juma]

... The F5 really didn't like being biased at 3A (ish).

Obviously. I bet any other amp can take 3A per MOSFET with ease...

 

[jacco vermeulen]

Highest I know of for a MOSFET amplifier is 2 amps per device.
(K.Lang design, 20Vdc rails)

 

[Zen Mod]

Babelfish J , 2A per IRFP150N vertical , 22Vdc rails

hey , but that's only because I'm ignorant

 

[jacco vermeulen]

You win.

The Lang amp lost anyway, the original design uses 2 in parallel, makes it 1A per MOSFET.
(in the early '80s, me lots of cheapo heatsink, but only cash for 1 Hitachi pair per channel)

 

[Zen Mod]

nope

ya win , big Dutchie

coz I love ya more then ya love me