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

[Zen Mod]

Gents ,please stay on topic

[Magura]

Gents ,please stay on topic

Says who?????????Magura

[generg]

Correct....!

[Zen Mod]

sorry , typo

[generg]

Hi Zen Mod, Meditating over the the circuit I wonder why we need such a big cap C3 when we have parallel R25 and R26 with only 600Ohm, in sum. In my case, I use the 0,25 variant for source and drain, I had to replace R26 by 100 Ohm. That means I have a parallel resistor of 220 + 100 means 320 Ohm. That could be nearly the value for a gatestopper and I can omit the cap.Where is my fault?

[Tea-Bag]

Tea - what voltage you have across R23 ? (just in case - what's value of R23 on your pcbs ? somehow I lost the track with you all )

R23 measures .585 and confirmed 47R :blum3:The other channel measures the same.If no other concerns here, I am going to try a different pair of 100's - see what they do.

[Zen Mod]

sorry , typo

Hi Zen Mod, Meditating over the the circuit I wonder why we need such a big cap C3 when we have parallel R25 and R26 with only 600Ohm, in sum. In my case, I use the 0,25 variant for source and drain, I had to replace R26 by 100 Ohm. That means I have a parallel resistor of 220 + 100 means 320 Ohm. That could be nearly the value for a gatestopper and I can omit the cap.Where is my fault?

nope - you can't ; you must "bootstrap" (block for AC) exactly that "nearly value of gatestopper" ; that means - if you want to go low , cap must be huge :sarcastic_hand:remember F=1/(2 x Pi x R x C)

R23 measures .585 and confirmed 47R :blum3:The other channel measures the same.If no other concerns here, I am going to try a different pair of 100's - see what they do.

that sounds right ; double check : R2 , R21 and R24 - all are 4K7 ?

[Magura]

Tea,There is little chance that the JFET's makes for that big a difference, so as Choky says, check that all resistors are of the right value, and even measure that none of them has been harmed one way or another.Magura

[Tea-Bag]

I will check the resistors cold and voltages again. What's got me is why it biased okay with 'normal' source resistors.And this is the channel that didn't have the original issues with things blowing up

[Zen Mod]

you certainly have that schematic I made with voltages in red measure everything , write down on that schm , write parts value changes ( those resistors on output ) and send to my e-mail

[Tea-Bag]

Zen,I will. I still might try to swap JFETs just to see if it's any better. But I think other peoples hunches are likely true, something else if off here.

[Tea-Bag]

you certainly have that schematic I made with voltages in red measure everything , write down on that schm , write parts value changes ( those resistors on output ) and send to my e-mail

ZM,I sent the measurements to your sc)))) gmail account. No hurries. I'm gonna try to listen to it.

[Zen Mod]

okidokias soon I meet with more than 3 gray cells , I'll respondsometime in 2015 ?

[Zen Mod]

just to paste here what I wrote already in e-maldidn't want to wait for all 3 gray cells ..................... just measure voltage across R28 - to determine which current is going through optocoupler LED it seems that optocopupler's transistor is eating much of Q7+Q8 CCS current , and that's reason why you must increase so much R26 I resume ( but first - measure and write to me that voltage across R28 ) that optocoupler gone south from some reason ; what's value of R26 on other channel ? ......................

[Tea-Bag]

R28 is 200R,Voltage was 1.01vI swapped Optocoupler out, no it's .7v, but I put a 390R resistor back into R26.R26 is 390R in other channel.Q7 and Q8 seem to have close voltages on both channels.I noticed (even before optocoupler change), after listening session last night, while on bench, that signal went real low on output, as in could hear very little music.Also when I hooked up speaker, to this channel, the tranny started buzzing. So this channel has gotten more ill. I suppose a transistor or something here is failing or dead. I may have crossed something taking DVM measurements.

[Zen Mod]

oh boyis that channel functional now , or not ?

[Zen Mod]

Tea - whatever is reason - we'll do everything to solve it ;Generg just PM-ed me , saying that you are still in troubles with health ....just take your time , and go in rhythm which suits you ...

[Tea-Bag]

oh boyis that channel functional now , or not ?

Well, I will remeasure voltages later, but you get a tiny bit of output. The offset is a bit off with new opto. The power resistors still measure the same.I've been feeling pretty good since I went to see an acupuncturist. The nausea will occasionally take over though, and well most people know how that feels. Not being a carnivore for 6 months or so is going to be real tough. Always lessons every year on how fragile the body and mind are.

[Tea-Bag]

Zen Mod, Did you get my last set of measurements? And more importantly, did they make sense? Despite it's penchant for acting bad, it's playing music quite well.

[Zen Mod]

yes - I got them ;obviously - I need to take a look again - because I'm lost in changes in parts valuescan you - please - list exact parts which are different from our last official schm and write their values ?

[Tea-Bag]

ChangesR2, R21, R24 4.7kR25 500R PotR26 390R22 .47RR29 1RR36 1RR30 2RR37 .47RR31 1RThe new values are below the orange higlights if they changed. The last swap was only on R25,R26.current offset in the channel .5v runs steady at least. Other channel is .01v and is stready.Listening right now. The above channel runs at ~ .6v bias, the good offset channel .520v. The higher bias and offset channel sounding a touch warmer sonically.Let me know if you need anything else.

[generg]

Hi Zen Mod, Did you implement the 5 ohm resistors of the SJ74 to compensate not properly matched parts?Or is there still another advantage?Curious generg