SemiSouth power Jfets .......... FW J2

[Zen Mod]

what's offset on output now - I see 20V ( yikes ! ) on right pic , but what's on left pic ?and - do you have variac ?

[Tea-Bag]

Yes, I have a variac. But I test up to full voltage for reference. Offset on left pic is ~10mv. I found the biggest influence of this was R9, then R12,R25.Meddling with the pots on the right pic, has no influence, stays about 20v.Not sure what's broke here, although 2SJ74 pairs are a little off, and now R100's a little off Ugs. But I think the problem with Q8 has me most worried.

[Impuls]

It looks like R8 BC556 is short wired between collector and emitter.

[Tea-Bag]

Maybe I should replace Q8 again after Q14 blew.

[Tea-Bag]

no luck replacing Q8 again.

[Zen Mod]

did you fired it up according to post http://www.diyaudio.rs/topic/1765-semisouth-power-jfets-fw-j2/page__view__findpost__p__51840 ?anyway - we can test it easy ;for easy testing , it's best to have outout jfets connected to pcbs with short wiresfirst step - unsolder both output jfets from pcb , and let me know what's Ugs for lower and same for upperthen I'll calc and tell you on which values you need to set every trimpotthen we'll test just front end and few other pieces - without output jfets in circuit

[Tea-Bag]

More or less went by this process, had to get voltage across R3 up, and fiddle with R12 for Source voltage up as high as I wanted to go to in test phase with Channel 1.Channel 2 more or less never behaved or has responded to R12,R25 Pot.I will take the Fet's off bad channel and do measurements as you ask.

[Zen Mod]

is that "proper " channel making sound ?

[Tea-Bag]

is that "proper " channel making sound ?

Yes it is! I am still using test speaker, but sound is promising.

[Zen Mod]

did you test output jfets in bad channel ?

[Tea-Bag]

Yes, before to measure vgs. In circuit vgs, they appear to be quite close to original measurements, of course now the pair on installed is not too well matched.Q13 ~1.3v Q14 ~1.4v vgs measured in circuit at startup. This is of course without Q8 apparently conducting.I will do as you say and remove them from PCB, re-test, and test PCB without R100 Jfets.

[Zen Mod]

ok - now I'll type how to test rest of circ without outputs after that , I must go to sleep ..... early morning is Philip Island ......so - wait a little

[Zen Mod]

ok

everything having on mind that Ugs of outout is in range of 1V3-1V4

output jfets desoldered

both inputs tied to gnd

output also tied to gnd

R12 to zero , same as R25

now - power up just to have all leds lit , and up a little to - say +/-15V to max +/-16V

set R9 to have 9V4 across it

now - if all input jfets are ok , and BC are ok you'll have around 23mV across each of R5 , R6 , R7 and R8

across R35 you must have 10mAx220R= 2V2 ; because you have lowish Ugs comparing to mine Jfet outputs - you can decrease R35 to 180R , so you'll have broader range for setting R12 later ; in that case you'll measure 1V8 across R35

now - if you still have 0V across R23 ( and you'll always have around 0V65 across it , doesn't matter which value of R23 is - it's best to replace both Q7 and Q8

if you have 0V65 across R23 ....... taking in account that you place 60,4R there ( in previous schm maybe there was 68R , but 60,4R certainly wasn't ) , current throughthat CCS will be Vbe/R23= 0V65/60,4=10,7mA , which will make around 4V2 across R26 ; optocoupler is dead now - without current going through 3W source resistors , so LED isn't lit .

it's best to change that optocoupler - who knows what you does ........

OK - let me know what you measure that way ....... and I'll see that in some 8-9 hours

buona notte

[Tea-Bag]

ok

everything having on mind that Ugs of outout is in range of 1V3-1V4

output jfets desoldered

both inputs tied to gnd

output also tied to gnd

R12 to zero , same as R25

now - power up just to have all leds lit , and up a little to - say +/-15V to max +/-16V

set R9 to have 9V4 across it

now - if all input jfets are ok , and BC are ok you'll have around 23mV across each of R5 , R6 , R7 and R8

across R35 you must have 10mAx220R= 2V2 ; because you have lowish Ugs comparing to mine Jfet outputs - you can decrease R35 to 180R , so you'll have broader range for setting R12 later ; in that case you'll measure 1V8 across R35

now - if you still have 0V across R23 ( and you'll always have around 0V65 across it , doesn't matter which value of R23 is - it's best to replace both Q7 and Q8

if you have 0V65 across R23 ....... taking in account that you place 60,4R there ( in previous schm maybe there was 68R , but 60,4R certainly wasn't ) , current throughthat CCS will be Vbe/R23= 0V65/60,4=10,7mA , which will make around 4V2 across R26 ; optocoupler is dead now - without current going through 3W source resistors , so LED isn't lit .

it's best to change that optocoupler - who knows what you does ........

OK - let me know what you measure that way ....... and I'll see that in some 8-9 hours

buona notte

Okay,

Outputs removed, input shorted, output shorted.

DC to 15.5v

Changed R35 to 180R, reads 1.8v

Changed R23 to 47R, still reads 0V

R3 to 9.4v

R12 and R25 to 0R

4.7R's all read 23.5mv, all 2SJ74's show 10.4v

R26 is 2.0v

Replaced opto-coupler as well, and Q7.

Thanks a bunch, and rest well. I am no rush here.

One oddity I have noticed is that with R27 installed (33R), the gate to Drain shorts, with R100 installed or not. I don't forsee it causing this issue, but,

I cannot repeat this on the working channel.

[Zen Mod]

Okay,

Outputs removed, input shorted, output shorted.

DC to 15.5v

Changed R35 to 180R, reads 1.8v

Changed R23 to 47R, still reads 0V

R3 to 9.4v

R12 and R25 to 0R

4.7R's all read 23.5mv, all 2SJ74's show 10.4v

.....

well - to have more appropriate voltage for input jfets , one day you'll need to replace all these leds with decent 20mA type , which will gives you few volts up - as in schematic I posted

.....

R26 is 2.0v

Replaced opto-coupler as well, and Q7.

........

??

what's voltage across R23 ?

.........

One oddity I have noticed is that with R27 installed (33R), the gate to Drain shorts, with R100 installed or not. I don't forsee it causing this issue, but,

I cannot repeat this on the working channel.

you made somewhere short ; you must solve that ( without output Jfet installed ) before anything else

[Tea-Bag]

well - to have more appropriate voltage for input jfets , one day you'll need to replace all these leds with decent 20mA type , which will gives you few volts up - as in schematic I posted

??

what's voltage across R23 ?

you made somewhere short ; you must solve that ( without output Jfet installed ) before anything else

I solved the short by drilling out the connection at C3 and one at R27. So some contact was made by the close track there it appears.

Of course, I disassembled that section of the board to see if some component was doing this.

I replaced the LED with 20MA last week. So I will re-assemble the other parts on there, and report back.

The board is made nice, no track has lifted with my de-soldering.

[Zen Mod]

few pics ; I simply forgot to post them ; detail - I made thermal contact between 4 LTP Jfets ( placed in plain row) with piece of thermally conductive silicone rubber ( usually found in old CD ROMS , as conductor between chips and case ) . rubber thickness is around 2,5mm . rubber is cut in strip , width same as Jfet width ; that strip is placed on top of 4 Jfets , then secured with small U shaped Al tin, placed on top of little buggers . voltmeters - left one is across 0R5 source resistor , right one is output offset with full blast temp on heatsink ; offset is starting at ~ 200mV at cold , then gradually decreasing to zero ; that little thermal bridge is really helping in final thermal and offset equilibrium . I wasn't in mood to ....... well - accu's in my cam weren't in mood for further shooting CRO screen ..... , so you'll must believe me ..... that they're flawless , at least in my book .

[Zvu]

Gledam. Čekam. Strpljiv sam

[Zen Mod]

sta cekas ?

[Zvu]

Da neko odavde napravi do kraja, iznese slušne utiske pa da možda krenem i ja u neku izgradnju... Mislim, ako dozvoljavate... ???

[Mikorist]

sta cekas ?

da Bacim Miniku kad ti ovo proradi

[Mikorist]

iznese slušne utiske

slusni utisci za Jfetove u Ulazu i Izlazu......