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

[Wolfi]

Sorry for my english.

After 16 Years of permament use, my SRM-313 are broken. (The latest SRM-313 version with new black PCB, buy new in 2006)


The left channel is dead.
No plop on the left channel, if switch amp regular off.
As he broke (i not have headphone on the ears) i nearly sure i hear a plop sound at the headphone 50cm away from me (the same typical low noise plop sound if switch amp regular off)


D1 LED not light (but -733mv at diode test with DMM, same as on right channel) (diode in PCB)


Amp Switch on
D1 left channel 0,89V (But LED Complete Dark)
D1 right channel 1,56V


At headphone jack output, Around 1V DC at right channel and 0v at the left channel (In the past and new condition was on both around 1V)


I tested all parts (transistors, resistors, diodes) without removing on the PCB on both channels with DMM in ohm and diode test in every polarity combination. (need much time)
The values same on both channels, exept R2
R2 left channel 2,97K Ohm or -1224mv
R2 right channel 0,99K Ohm or -634mv
If set the main Volume poti to zero the same values i also meassure on R3


Another question is, the Diode D1 have function and is need for sound Amplification or is only a optical indicator for function ?
If Diode D1 is broken, Amp still can work ?


I hope here for help, find the broken part.
I hope kevin gilmour see this thread, i wanted contact him, but he off private messages, and i not know how can contact him.

 

[kevin gilmore]

send me a pm to see if stuff is fixed.

absolutely all led's are necessary for proper function.
and they need to be the right voltage.

if d1 is not lit, then q14/q15 are likely shorted.

 

[Wolfi]

Very thanks for your help and support

On my SRM-313 are no Q15 (Q14 is the highest number)
Maybe you talk about Q13/Q14 ?

On which from my description you think is the Q14/Q15 ?


The different values of Left/Right channel at R2 or R3 sounds for me the preamplifer, but i not know.
D1 also looks in the first stages of amp.


All transistor are be warm, celsius range from 30-60C° with IR-Thermometer, but exactly meassuring are difficult.
Q13/Q14 on left channel around 10-15C° higher.


My idea are, swap 1-3 parts from right to left channel for test which part are broken, later try buy new this part.

But i very very want swap fewest parts as possible, because every part are need 4x solder in/out, it is much heating stress, and Q11/Q12/Q13/Q14 are not easy change.

 

[kevin gilmore]

for this version of the board q11/q12

swapping parts side to side is a very bad idea. more dead parts and damaged circuit board

measure the output voltages with respect to ground with no headphones connected. should be in the range +5 to -5.
-300 indicated shorted q11/q12
+300 indicates shorted q13/q14 or q9/q10

really hard to fix without signal generator and scope.
good luck finding repair parts. virtually all on ebay are fakes.

 

[Wolfi]

The amp is the 100V version, i set as i buy to 240V
232V AC input result in 556V on the main capacitators.

I meassure on the headphone jack.
Left channel +278V and +278V to ground.
On the working right channel around +1,7V and +0,5 to ground. (It starts with 2V and go slowly down with the time)

D1 is more near to Q9/Q10
Maybe the off D1 gives help a help in this case ?

You have a circuit diagram ?

Q9/Q10 left around 40C°
Q9/Q10 right around 60C°

On Q13/Q14 left/right also difference in C°, but not so big.

I can generate with PC Soundcard at amp input any sinus signal from 20-20khz and my DMM can measure the number of this sinus frequenzy.

It is normal both transistors can be damage ?, or more normal only one ?

I not know on ebay are so much fakes from this cheap small parts.
Maybe you know a source ?

 

[kevin gilmore]

this schematic does not match your board, but may be close.
check value of r16
replace d1
replace q15/q14

stay away from dalbini/littlefuse/littlediode (all the same company) guaranteed fakes.
if you do get replacement transistors, check them with a transistor tester first for >800v

 

[Wolfi]

Very thank you for help

R16 left and right channel have on the board 680k ohm or with changed polarity 640k ohm.

Strange, for me looks my D1 is D4 on your circuit diagram. (and D4 is D1)
Because on my board D1 is associated with R16 (same), but one side of D1 is associated with q9/q10 the other side of D1 is associated with q11/q12
My D4 is associated only with q13/q14 and R25 830k ohm

I not know which led diode is used in srm 313.
And which buy ?

I have only transistor test socket on my DMM, for me not possible test it with 800V

What you think about associate the DMM socket to the transistors on srm 313 board for test ?
I know transistors can be bad test in a circuit, but maybe here it works ?

I meassure on the headphone jack left channel +278V to ground, and you say q13/q14 or q9/t10 shorted.
Maybe meassure a pin on q9/q10 to ground ?
It can idanticate q9/q10 or q13/q14 is shorted ?

What you think about the different values of R2 in my first post ?
R2 probably ok, but he associated with q1.

 

[kevin gilmore]

this part may work.
https://www.mouser.com/ProductDetail/onsemi-Fairchild/FJPF2145TU?qs=RECgZY%2BK1xIJS3ThY%2Bx7kw==
not used in the srm-700s. ksc5502 is a compatible part but not isolated.

 

[Wolfi]

The Q1 i not know which type use, because two in a shrink tube.

The shrink tube idanticate this two transistors are matched ?
In this situation probably very difficult find replacement parts.

 

[kevin gilmore]

front end transistors are 2sj109. absolutely impossible to source.
replacements are lsj109 purchasable from diyaudio.com you want the B version.
held together with heatshrink. required for dc stability.

 

[Wolfi]

why 2sj109 absolutely impossible find ? (was exclusive made for stax ?)
two 2sj109 on the circuit called with A and B, this two 2sj109 are absolutely the same ?

 

[kevin gilmore]

20 + years obsolete.
highly popular part.
the linear systems parts are completely functional replacements.

 

[Wolfi]

I have compare your older circuit diagramm (post #6) with my newer srm-313.
R1 on circuit diagramm is R2 on my board.

I meassure on my dead left channel at R2 3k ohm
On the working right channel R2 1k ohm (same value as circuit diagrsmm)

What can be the reason meassure on R1 (circuit diagramm) abnormal 3k ohm instead of 1k ohm ?

 

[kevin gilmore]

flip the leads and try again. open output transistors will influence the feedback to the jfet source.
otherwise pull the resistor and measure out of circuit. if its really open, you are going to want to replace that first.
if the resistor is open likely the jfet is toast.

 

[Wolfi]

swap polarity are same values.
very strange, one side of resistor go to 2sj109, the other side of resistor go to volume poti (poti position have no influence on R2 value), and volume poti go only to cinch input (on this is a resistor with around 40k ohm to ground)
for me the circuit board can have no really influence on R2 value.

i have meassure with 1khz sinus audio and amp on.
i meassure no voltage decrease on R2 at left and right channel (0,0mv), i think R2 are not really damage. (maybe value change to 3k ohm, but it makes no difference in this position)

i have now found good sellers for all transistors, exept 2sc5201
the two on ebay probably fakes.

you have any idea for search a 2sc5201 source ?