The Singing Bush Tips 'n' Tricks

[sontero]

perhaps the 317 is the defendant: on its central out pin I don't detect 12v: I detected with the DMM connected to the GND of the P.Supply and the positive on the central out pin = ZERO.
On the input pin of the 317 I read 9V: this puzzles me.
In AC I detect 16 V before the rectifier bridge

 

[Zen Mod]

LM 317 pinout is - numbers to you, pins down : most left Adjust, mid out, most right input

all ref to GND:

on input you must see close to your Aux AC*1.41; if you have 15Vac there, DC must be close to -20Vdc

of course, worth repeating, that Aux (15Vac) must be independent/floating of main audio PSU secondary

so, confirm that you have proper Aux AC voltage, then if anything is fishy with LM317 voltages, replace it

 

[sontero]

Pinout 317 comes back to me
I confirm 16 V AC input. On the positive of the bridge I detect 8.8v DC !! (reference the negative pin of the bridge itself). this value doesn't add up: it should be 19v

 

[Zen Mod]

remove 317, test again
change bridge if needed

change 317 if needed

 

[sontero]

317 replaced. I am detecting 12v on its output. fet bias retried. I don't detect anything on R8.
Abandonment if there are no other ideas

 

[Zen Mod]

see this

think, try and say where you can see difference to yours

 

[sontero]

the preliminary checks carried out before assembling the components do not always give us back the truth. I discovered the component responsible for the problem after examining it several times and found it suitable, I set it aside. It was the R8 which, checked again today with another tester, did not return any value: it was the faulty one and also interrupted the circuit in its position. I happily replaced it on the fly and of course was able to bias the buffer to 20mA. Finally the first step is done. Obviously if I had sensed that resistance was about to die I would not have tried frantically to shoot into the pile...and despite the shots no one fell...I was swollen with rage that was the engine to control everything again. I hope I don't have any more problems. Everything went smoothly in the previous construction of the Bush.
I thank ZenMod who generously helped me: I'm leaving again also thanks to your support

 

[Zen Mod]

well, even if you didn't had R8 at all, there should be current path through P1, if properly set

not that I'll like 20mA through puny trimpot, but it'll work

anyhow, I'm glad it works now

 

[sontero]

Thanks ZM, I really appreciate your help.
The R8 has been replaced, it was the only quick way to be able to measure and implicitly realize that the circuit was fine. Above all, now I know that I haven't become an idiot yet... I have to think about it at 68 years old

 

[Zen Mod]

well, I did found remedy for that problem many moons ago
realized how big idiot I am, then I could proceed without that burden

 

[sontero]

you will be an idiot ... always less than me and in any case you are a brilliant idiot

 

[Zen Mod]

being an idiot is helpful thing - ppl really nothing expecting from you

but then, what I know, I also know that I could never be a Surgeon , as some are

that is A Craft, and that is A Responsibility

 

[sontero]

biased the power section. Obtaining it was not easy: I had to change the values of two Resistors.
The first on the MU board R3 increased from 100 to 150 R. The other on the SIT board, R11 increased from 1k5 to 3k3.
So I was able to get the design values: 460mV for Iq and the potential of 37V on the drain of the SIT (2SK180). Good progress. Tomorrow I fight with the other channel

 

[sontero]

bias OK for the other channel as well.
In this case I had to increase R11 to 2k7 on the MU board and increase R3 on the SIT board to 220R.
Very good.
Even with an initial block that confused me a bit (the cursed R8 ) everything works according to the directive of Maestro ZM to whom I pay my respects

 

[Zen Mod]

 

[sontero]

Great Zen Mod, the new Bush is playing. Wonderful as always: the magic of this amp repeats itself. Forget the hitches of the initial tests, now the Music and what Music.
A lot of gratitude for the help you have always given to everyone, not only to me, but above all for having given us this incredible amp.

 

[Zen Mod]

 

[danielutzu]

I have a weird situation... I don't see any voltage gain, and in fact I get fewer volts out than at the input. I had it working at first, but then I have changed the output resistors to be 0.3R : 0.2R mu :sit. The bias is 2.8A (0.558mA across R5 on the mu PCB), 38.5V at output node. I assumed the 2SK182ES is too closed, and I measure -4.88V before R13, not -2.5V. All other voltages check out. Any thoughts what I could do to lower the SIT gate voltage? Thanks

 

[Zen Mod]

SIT gate voltage isn't a goal per se; it is governing voltage, by which you're setting apparent internal resistance of SIT itself ..... just take it as variable resistor where resistance is set by Ugs voltage

now, if you have set Iq you wish, and you can get approx. Urail/2 (and few volts above that) at SIT Drain, you're good regarding static values

confirm what you have there, so we can proceed ........ now, gain issue?

 

[danielutzu]

Thanks for the reply.
Vrail is 66V.
Iq is 2A8.
Iq buffer is 20mA.
SIT drain is 38.5V