Scryer ... or how F8 met Mighty SissySIT

[von Ah]

Results from measuring resistors in-circuit (power off) below.
Deviations from the schematic:

• R107 measured 5K (10K in schm)
  • R207 5K (working channel)
• R108 measured 5K (10K)
  • R208 5K
• R111 measured 30R (51R)
  • R211 25R
• R112 measured 25R (33R)
  • R212 23R
• P101 measured 60R to 0.8R (200R)

I’m assuming the resistor deviations are from interaction with the trim pots?

I’m working on identifying all the little bjts…

 

[Zen Mod]

all of these resistors are pretty much irrelevant for Iq

continue with everything right from R114, looking at schm in posts #44 or #188

 

[von Ah]

All resistors after 114 conform to the schematic.

Little bjts are difficult to identify! See attached.

 

[Zen Mod]

one pic all "1-B"
second pic all "3-B"

so, my OCD still functional

try checking all resistors in situ, primarily comparing to same measurement on proper channel pcb
be sure to have probes same oriented on both channels

even if in some cases you can't get valid measurements ( resistor being in circuit), comparison with proper channel can give us some idea

 

[von Ah]

Understood. I’ll check all resistors from the good channel and compare.

To confirm, do you mean measuring resistors with power off? I hope so. It’s tight in there and getting the resistance measurements with the board out was challenging enough.

 

[Vunce]

Just want to sneak a question in-between troubleshooting von Ah's board...

Would any modifications be needed if feeding Scryer +/-28vdc?

 

[Zen Mod]

.......

To confirm, do you mean measuring resistors with power off?........

of course!

measuring resistance always with circuit not energized

 

[Zen Mod]

Just want to sneak a question in-between troubleshooting von Ah's board...

Would any modifications be needed if feeding Scryer +/-28vdc?

dissipation of P channel mosfet is main obstacle

for cozy sleep, nice wide green - 40W

45W even so

50W reserved for all Papa-Wannabies

those, who are crying in solace, boasting in public

 

[Vunce]

Close to 50W it is.....
I don't sleep well at night anyway, this will not be an obstacle

 

[codyt]

I’m running 28Vdc and around 1A7 I think.

 

[Zen Mod]

Keratherm ( torqued to 0.9Nm) or Alumina oxide pad + goop

you'll live and boast



though, if you're using less than 4U/400, enjoy in consequences ...... in solace

 

[Vunce]

Alumina + goop + 5U/360mm

 

[Zen Mod]

 

[von Ah]

Apologies for going off script, but I changed out the SIT in the errant channel.

Biased up fine. Cooking in now with ~ 180mV across R141. DC offset drifting around less than 10mV.

Woo hoo!

 

[Vunce]

Fantastic von Ah!!

 

[von Ah]

Test speaker run is a go! Sounds VERY GOOD. Only the faintest of hum from the SMPS, which is to be expected with my nest of wiring.

Gonna chill with this for a bit, then migrate to the main system to see how it compares to the TDV!

 

[codyt]

Excited to hear what you think after a couple days of listening. Congrats!

 

[von Ah]

Starting now! I’ll let you all know.

(Edit: the hum is gone while connected in the main system)

 

[e_fortier]

Apologies for going off script, but I changed out the SIT in the errant channel.

Biased up fine. Cooking in now with ~ 180mV across R141. DC offset drifting around less than 10mV.

Woo hoo!

Great news !!

So the issue was a defective SIT ?

Was it bought from a reliable source?

Thanks
Eric

 

[von Ah]

Correct, the SIT appears to have been the problem.
I luckily got 10 from a dubious source right when the original thread about the K2087C started here. They were obviously pulls from older equipment.
I didn’t spend a lot, and ended up with two pair that have worked for me. Two other lucky forum members got a pair of K2087Cs from me.