Babelfish M25, SissySIT - general building tips and tricks

Official Court Jester
Joined 2003
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I'm always saying - I don't care for CCW or CW or whatever , regarding orientation of trimpots ......... sole thing I believe in is - reading of DMM

so , if I wrote max value , I believe you did right thing - measuring across parallel resistor

point is , praxis acquired from years of service work , in majority of situations even without any service documentation , even if I remember/chose what is "normal orientation" of trimpot , I'm already confused first time when I rotate anything for 180deg - either entire apparatus , or just my head

:rofl:

so , yes , solder trimpot whichever way you want ... even on other side of pcb ..... electrically that's irrelevant :)
 
Official Court Jester
Joined 2003
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recheck everything , complete what you didn't made and wait for me

you made me slightly hysteric , so I need few hours to check some things in vivo

even if I just few days ago assembled complete Babelfish M25 , too much snakes did bite me so I'm slightly nervous even when I see a Lizard :rofl:

and - of course that missing R3 is stupid thing ..... cross your fingers that you didn't scorched them

but relax - that's just electronic , not life support .....even if broken , it can be repaired
 
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Official Court Jester
Joined 2003
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just checked , had second coffee of the day , and conclusion is that your problem , logically and obviously to properly caffein-ed mind , was exactly because gates of lower group of JFets are floating , due to absence of gate resistor (R3)

point is - gate being floating , D-S of JFet is nothing else than silly resistor of some low value ....... there is nothing to establish any limiting effect

so , now , presuming that you trice checked all parts and positions , and that you soldered R3 in place (or you inserted them in 1-pin sockets, whatever) , try entire procedure again.

as I didn't get (in my edits of post #1) to part of output critters mounting and necessary insertion of 0R1 in negative rail (Iq monitoring), it would be clever to do whatever you're doing - without output critters even mounted on pcb

keep me informed .... my fingers are crossed that tiny bugs survived
 
half good news,
i can´t get lower than 21.5mV aross R8 and R6 pot has very little influence on Iq. Several turns do just about 0.2 mV difference on R8, but vary offset for about 3.5 mV for example.
offset pot reacts supergood, no problem to adjust 0mV.

i´ll send you coffee : )
 
Official Court Jester
Joined 2003
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some musings and some questions :

looking at schematic , if you had 0R for source resistor of upper group , you could get only Idss through entire group , even if lower group of JFets you had them acting as silly resistor ...... so current through entire chain (from pos to negative rail) couldn't be 200+mA

so , I'm still puzzled how you get , supposedly , more than 200mV across R8?

check (powered off) - what you can get as min and as max value of source resistance for both groups , just to be sure that trimpots are still in good shape

also , I hope you didn't had both input xformers in place and JP closed ..... as clearly written on schematic , JP closing only after you finish entire procedure with input buffers

generally - you can set JFets Iq anywhere between , say 18mA to 25mA , without any change in performance ...... I chose 20mA as convenient value from several reasons ,, but it isn't carved in stone

we just need to find is everything in working condition , then just blame JFets Idss span for not being able to get less mA with 100R ll 27R as source resistor

no big deal
 
they seem to be ok, can trim both for 21.3 R across the 27R Resistors. While Iq-pot is in endposition, offset around 9.2R across the 27R resistor.

..puhh, sweat (your thinking, my fear : )

Have to leave workbench for today, i´m the cook for Kässpätzle :) you are heavily invited!
 
Official Court Jester
Joined 2003
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if you're eager to get strictly 20mA though buffer , replace lower 27R with , say , 39R

hope they didn't Berserked with that episode

btw. in all these posts , you didn't tell - you had same issues with both channels , or you stopped after first one ?


anyway , I intend tonight to further edit post #1 , so you can proceed with playing ...... mounting outputs and everything , and preparing for first important powering up :)
 
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I just hooked up left channel (frontend). The other one has still some drilling to do.. so it will come in few days.

Maybe i change R4 (27R) to 39R. And letz hope for da buffer to have survived.
i owe you a day : ), mnytnx! maybe you´ll hear a hurayy behind the Alps tomorrow, but i´ll give ok anyway,
best
st.
 
Official Court Jester
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oh yes

yesterday , after checking things for you in my workshop , I needed to go out of town and came back late , so I didn't made that promised editing of post #1 , nor I posted list of already sent kit packages

will do that today , after ........ everything :)

I believe everything is clear from notes on schematics and few pictures bellow , but still better to wait for SuperSlow ZM , to type few sentences more

procedure is slow but safe and easy ........
 
Official Court Jester
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:) quite sure you had to give yourself a drink somewhere, because of me numb guy, triggering your adrenaline : )


its up an running (left channel), a cheap speaker, good sound : )

now listening and coffee,
soon,
st

no porn , no glory!

Iq , offset ?

after being in temp equilibrium for 10 mins at least , with cover closed , and everything set - then you can connect the speaker and enjoy .....
 
pulling out the (bias) 0.1R Resistor rised my offset to around 130mV. I than turned offset down via pot to under 10mV. But don´t know what that did to bias. Will check the next days.
Heatsinks, (double mono) are easy warm, ready for hot summers.

Gain is of course not so much. But driven directly from i-pod (don´t have others in studio) on 90 dB papers it´s ok listening level, laptop is even louder : )