Stax SRM-313 Repair Himself (Need some help find the broken part)

[kevin gilmore]

the jfet input stage is not pulling enough current to turn on the second stage.

 

[Wolfi]

1. - If d4 not light (your shematic) it can comes from broken fet q1a/q1b ?

2. - The +278v to ground on headphone jack it can comes from broken fet q1a/q1b ?

3. - the lower temperature on q9/q10 can comes from broken fet q1a/q1b ?

ok, on your explain i understand 3x no.

 

[kevin gilmore]

whats wrong has to be in common with both sides of the circuit.

R15,d4,r26,r25,r7,r8,r22,r21,r16,d1 on my schematic

 

[Wolfi]

Sorry, i not understand.

 

[Wolfi]

I have now notice in shematic all changes i found.
add Trim Poti, add resistors, add capacitators, remove capacitators, transistor types change.
Some resistor values different (maybe from on board meassure)

i have some questions

Both trim potis only for offset ? (is my description correct ?)
For offset correction set volume to 0
Meassuret DC V on Headphone output and set with TVR2 to 0V
Second meassure DC V Headphone output to ground, and set with TVR1 to 0V
It is better make offset corretion with or without headphone connected ?

With this other resistor values i notice in shematic, runs the probably 2sj74 in shrinktube still at 0,2ma, or is it higher ma value ?

 

[kevin gilmore]

tvr1 is offset with respect to ground. should be set for output voltage of zero measure either output
tvr2 is balance between output 1 and output 2. should be zero.
repeat a few times.

if tvr2 is too far from center, adjustment is not possible.
start with tvr1 at the center.

best to have headphone disconnected.

 

[Wolfi]

Very thank you

Can you please look in my new shematic with new resistor values.
On which ma runs the 2sj74 ? (probably inside shrink tube)
I think it is not more 0,2ma you descripted on old shematic.

 

[kevin gilmore]

1/((r15+tvr1)*2)

 

[Wolfi]

Ok, but with this formula my result on your old shematic is different.
1:2400x2=0,0008333333 A
0,8333333ma

With high *2 i have extreme other results.
Who is my error ?

Btw on the old shematic without trim potis and 2400 ohm it must be a big offset

 

[Wolfi]

I have now calculate in other way.
If you say on ONLY r15+tvr1 have influence on which ampere runs q1a/q1b (2sj74)
In this case 2sj74 in new shematic must run at 0,436 ma (1100 ohm)

On the market can buy 2sj74 matched with 0,02ma, 0,1ma, 1,5ma, 6ma, 8ma.
What is best quality if runs at 0,4ma ?

 

[Wolfi]

I have change now q1, q9, q10
But d4 (your diagram) still not light, and on output +280v to ground.

I not understand why d1 not light, q1, q9, q10 can be the only reason ? (Resistors and d4 are ok)

 

[Wolfi]

if d4 on my schematic is not lit, then q1,q9,q10 are a problem. 2sa1413 are easy to get (to252 version required)
OR... input fet is toast.

I have change now to new parts the fet q1a/b 2sj74 and q1, q9, q10, d4 (your diagram)
But d4 still not light, and on output +280v to ground.

After remove fet q1a/b, i also solder out line input to R2, in this case one side of R2 is not connected more to any part.
I still meassure 3,00k instead of 1,00k at right channel.
it looks for me, stax factory put 3k on left channel...

additional i have meassure on all resistors the voltage drop at left and right channel.
I list here only the resistors i found with different values between left and right channel. (Your shematic) (power capacitators running on 280V+280V=560V )
R5 L=4mV R=77mV
R6 L=8mV R=69mV
R11 L=+280V R=-140-240V (only on right channel D2+D3 Brightness go up, and stay with higher brighntess, to the point if switch power off)
R12 L=+280V R=-240V
R15 L=301mV R=813mV
R17 L=0,36V R=1,07V
R18 L=0,36V R=1,08V
R21 L=280V R=269V
R22 L=33mV R=12,46V
R26 L=19,9V R=54,0V
R27 L=9,8mV R=6,1mV (Additional part on my board, i painted in your shematic)
R28 L=9,7mV R4=5,8mV (Additional part on my board, i painted in your shematic)
TVR1 L=57mv R=201mV (Additional part on my board, i painted in your shematic)

 

[Wolfi]

i have change now all transistors to new ones, exept q7/q8 (c5201, not have replacement parts)
But i tested q7/q8 on board as q5/q6/q9/q10/q11/q12 was out of board.
HFE117/125, it looks for me q7/q8 are ok.
all transitors are ok on board...
checked all resistors, are also ok...
for me nothing is damaged.........

 

[Wolfi]

A profesional man have now make the repair for me.
It was R21 broken (High Ohm)

R21 was the only resistor not really possible meassure on board for me, i not have think about a really problem, because the bad meassure behavior on the right channel was the same.