SX-850 phono amp stop working?

[ngcsstodd]

I have a SX 850 that worked fine one day playing my turntable then the next time i turned it on nothing ! i have cleaned the switches and checked connections, cleaned switches again, changed to a different turntable! what would be the next step?

and thanks for your help!

 

[Watthour]

Does this occur with both Phono 1 and Phono 2 inputs? Can you faintly hear the turntable is input to the AUX jacks?

Trace signal from the inputs to and through the preamp, including the switches. There are some 2SA725s and 2SC1313s in that preamp, always suspicious of malignancy.

 

[tarior]

Phono 1 and 2 are the same phono EQ/amplifier.
Is the phono board getting power?
AWF-011 pin 6 should have 28 volts DC on it, pin 2 should have -19 volts DC on it.

 

[ngcsstodd]

i will do these checks later today and report back my findings!
thanks again for the help

 

[ngcsstodd]

Phono 1 and 2 are the same phono EQ/amplifier.
Is the phono board getting power?
AWF-011 pin 6 should have 28 volts DC on it, pin 2 should have -19 volts DC on it.

AWF-011 pin 6 has +27.6 VDC PIN 2 has +21.6 VDC so i do not have -19 VDC and i checked it a few times to make sure i was correct!

 

[ngcsstodd]

Does this occur with both Phono 1 and Phono 2 inputs? Can you faintly hear the turntable is input to the AUX jacks?

Trace signal from the inputs to and through the preamp, including the switches. There are some 2SA725s and 2SC1313s in that preamp, always suspicious of malignancy.

yes it does occur on both 1 and 2 inputs. and yes if i plug the turntable in to the AUX i can faintly hear it.

 

[tarior]

Looks like your -19 volt regulator is out.

 

[petehall347]

awr-101 .
pins voltages .
13
15
14
?

 

[ngcsstodd]

Looks like your -19 volt regulator is out.

so whats the next step?

 

[petehall347]

see above .

 

[ngcsstodd]

awr-101 .
pins voltages .
13
15
14
?

AWR-101
pin 13 -13.9 VDC
pin 15 21.7 VDC
pin 14 -58.8

 

[tarior]

Why, repair it, of course.

Get yourself a service manual first off. Either buy one (recommended), or download it from Hifiengine.

When you have it, look at the schematic. The regulated power supply is AWR-102.
In this order:
You should have:
-13.5V at pin 13
-19V at pin 15
-51V at pin 14.

Check for 13 volts or so across D12, 13 volt zener diode. Replace if bad. Skip if only -19V is missing.
If only the -19 is missing, then:
Replace Q4 and Q6 out of hand. 2SA720 is a part known for recurring failure, and is barely adequate there any way. KSA1013 is a good upgrade for those.
Check R15, 17, 18. Check the whole board for cracked solder joints.
Replace C21, 23.

That should get you back up and running. Afterwards, we can rebuild the whole board.
If other voltages are missing, we'll take that up when you report back.

 

[petehall347]

r18
r19
q6
c23 .
its one or more of them at fault or bad solder .

base collector and emitter voltages of q6 should tell the whole story .

 

[ngcsstodd]

Why, repair it, of course.

Get yourself a service manual first off. Either buy one (recommended), or download it from Hifiengine.

When you have it, look at the schematic. The regulated power supply is AWR-102.
In this order:
You should have:
-13.5V at pin 13
-19V at pin 15
-51V at pin 14.

Check for 13 volts or so across D12, 13 volt zener diode. Replace if bad. Skip if only -19V is missing.
If only the -19 is missing, then:
Replace Q4 and Q6 out of hand. 2SA720 is a part known for recurring failure, and is barely adequate there any way. KSA1013 is a good upgrade for those.
Check R15, 17, 18. Check the whole board for cracked solder joints.
Replace C21, 23.

That should get you back up and running. Afterwards, we can rebuild the whole board.
If other voltages are missing, we'll take that up when you report back.

from the post above.
AWR-101
pin 13 -13.9 VDC
pin 15 21.7 VDC
pin 14 -53.8
now i will check the other s you ask for .

i do have a downloaded service manual.

 

[ngcsstodd]

r18
r19
q6
c23 .
its one or more of them at fault or bad solder .

base collector and emitter voltages of q6 should tell the whole story .

Q6 base -54.6 collector -66.7 emitter -541

i will be checking the rest now

 

[petehall347]

emitter -541 ??

 

[ngcsstodd]

emitter -541 ??

emitter -54.1 VDC sorry about the typo

also note Q6 in the manual shows a 2sa720-Q it has a 507D in it?

 

[petehall347]

r17 must be open or it lost its grounding plus there also must be a break in the board between pin 15 and q6 emitter .

 

[ngcsstodd]

r17 must be open or it lost its grounding plus there also must be a break in the board between pin 15 and q6 emitter .

well if you have not noticed yet i'm a newbie at this! lol
but i can see what you are saying on pin 15 at 21.7 and the q6 emitter should have the same voltage!

i will check r17 and check the ground on it. I will check and see if i have one if needed.

what do you think of the different number on q6 ?

if the board has a break would it be better ti try and fix it or put a jumper on it?

also it will take me longer to check this and get back to you. newbie remember! but i will report back as soon as i know something!

thank you ever so much for you time and knowledge

 

[petehall347]

i am thinking poor solder is the problem . if burnt copper is there we need to look further . q6 voltages say its doing what it is told . but base voltage is way high . q6 might be bad saying that . c to e is most failures but b to e not unheard of . base voltage is way high r17 is best i can see as culprit right now