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

[kevin gilmore]

this is in stock
https://www.mouser.com/ProductDetail/Taiwan-Semiconductor/TSC873CW-RPG?qs=dD%2BUdDqNEJYsVIh/xTqJIA==
or this
https://www.mouser.com/ProductDetail/STMicroelectronics/STN0214?qs=pmcUWs1B1IKOCzUPe9wAgg==

you probably should use a to223-4 adapter

 

[Wolfi]

very thanks, this will help in a emergency situation.
for me no problem solder this smd transistor to board.

but i search and prefer the original 2sc5201.
it seems very difficult find...
maybe any spezial trader, like gerwert, he have some old types.
he can not be the only one.

 

[Wolfi]

i have meassure now before the output transistors, on this is +280V to ground on the dead left channel, on the working right channel 1,1-2,5V
there is also a additional 100 ohm resistor before on my board.

between 2sc5201/2sc2705 i meassure -278V on left channel, and -268V on right channel

also interresting, some times the dark D4 light very low short time up, if i meassure on this transistor point R27/R28 or 2sc5201/2sc2705 to ground with my multimeter (it has 10 m ohm resistor)
D4 on circuit diagram is the dark D1 on my board.

between 2sa1091/2sc2705 i meassure -281V on left and right channel.
2sa1091 on circuit diagram is 2sa1156 on my board.

for me looks one or both 2sa1413 are damage ?
please look on circuit.
what you think ?

 

[kevin gilmore]

the bottom of the output stage is a current source and because the led is not lit, it is not working. you need to fix this first.

 

[Wolfi]

the bottom of the output stage is a current source and because the led is not lit, it is not working. you need to fix this first.

On which part you talk on circuit diagramm ?

 

[kevin gilmore]

remove q14/q15 and the heatsinks.
power on unit, led should light.
if not replace led and resistor.
test q14/q15

 

[Wolfi]

Very thanks for help.
You shure about q14/q15 ?
As i say before, D1 on your circuit diagram, is D4 on my srm-313.
On your circuit diagramm is it D4 are not light, and D1 are light.
I know not much about circuit diagramms.
But for me the dark D4 (your circuit diagram) have not much influence on q14/q15.
Sorry for question.

 

[kevin gilmore]

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 apologize for the misunderstanding on which led is out. very different problems.
you should probably do a schematic that matches your board.

the blown input fet is much more common than output transistor problems

 

[kevin gilmore]

real 2sj74 replacements available here
https://diyaudiostore.com/collections/frontpage/products/matched-jfets?variant=39335246954569

 

[Wolfi]

i am happy have your help.

make shematic himself is very very much work.
i have check some points, and mainly are identical to your shematic, but many numbers on board are different.

yes, for me also logical, and i will swap first q1, q9, q10 (your shematic)
q9/q10 left around 40C°
q9/q10 right around 60C°

i have order 2SA1413ZK-E1
smd solder with new wires no problem for me


have you look ?
https://cdn.head-fi.org/a/11873497.png
i have meassure some points on transistors.
maybe it makes possible find the broken part more exactly.


i have buy 2sj74 from high-amp, he matched to 1,5ma.
but he say on your shematic it runs at 0,2ma.

 

[kevin gilmore]

i would probably swap the fets first since you can't test them with a voltmeter.
yes they run at 200 microamps each. part numbers are BL grade on most amps i have seen.
measure to ground the connection between r5 and r6.
check value of r26

 

[Wolfi]

R26 value of both channels are 68K ohm
I have meassure in the past all resistors on board, on both channels are same values.

between r5 and r6 to ground (R26(2) descripted on your shematic)
Left -0,259V
Right -0,369V

 

[kevin gilmore]

no idea. change q1 first.

 

[Wolfi]

I have change D4 and Q1 to new parts (your shematic)
Nothing change.
i have test outside of board, diode and transistor are ok.
I put the old original parts back on board.

I tested a1156 on multimeter transistor test socket.
Original a1156 have hfe value 59
New a1156 have 84

Next i will swap q9/q10 when i receive new parts.

 

[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 ?