Sony STR-DH740 no sound output (possibly transformer)

s65amg

Member
2016-05-26 5:57 am
i posted this in someone else's thread but think i should start another for this issue.

I have a DH740 that did not power on properly (fixed now) but now has no sound output at all.I fixed the not powering on (relay clicking and green light flickering) by reworking the transistor joints as some had bonded with legs of nearby capacitors and resoldering the clicking relay.I've checked all of the emitter resistors and the transistor and they are all ok.The receiver has absolutely no sound but interface/powers up fine.The hdmi out/ins are also not working, like the hdmi board has no power.Im almost certain it has no power because the front usb has no power (no 5V) and the usb lines are coming from the hdmi board. Someone mentioned that these receivers have a seperate PSU for the display interface and one for main board/output.I can hear the big transformer humming or buzzing, its noticeable when you're standing near it.Combined with the fact that usb and hdmi board is not receiving power leads me belive the main transformer (not display PSU ) is at fault.I've also tried the headphone jack which actually did work but only in HP direct mode not in 2ch.I used the rear audio jack to my phone to test this.Also i noticed the 2 big 71v filter caps were not even charged after having been on for a while.they received no power whatsoever.So the transformer has to be faulty or have a blown internal fuse, correct?

I can't come to any other conclusion. Any help would be greatly appreciated.
 

Welcome

Member
2013-03-11 10:52 am
Measure the voltage on the wires coming out of the transformer. If you get a zero volt reading (or close to zero) the transformer is toast and the whole amp becomes a parts unit. In this situation some folks may suggest digging into the transformer to repair it - don't do this.
 

s65amg

Member
2016-05-26 5:57 am
i took the transformer out of circuit and found its working on all lines.I got the following voltages:

black-0
red and brown-41v
blue and yellow-15v

low voltage ( 2 lines)

green and yellow-9v
brown and orange -9v

What else could be at fault? should i test the transformer in circuit to see if its a relay? when i power the unit i hear one click immediately and one after 5-10 seconds.
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
The first immediate click "should" be RY951, that relay energizes the main power xfmr. The second click could be that same relay de-energizing as you say you have no voltages on the the main filter capacitors. You need to monitor the line voltage to the main xfmr while all of the clicking is going on to see if that is what's happening. You can probably try monitoring the secondaries if you think the xfmr is OK.

Craig
 

s65amg

Member
2016-05-26 5:57 am
just tested the transformer in circuit and it is providing voltage on all lines.i looked for that buzzing sound and its coming from the small PSB with the mains.It's got a small transformer, fuse, relay(definitely not making the sound) and capacitor on it.There is also a 12v relay near the speaker output and right by where the main board gets high voltage output that might be making that buzzing.It could also be the small transformer.I tried uploading some pics but got a token error.

update- just tested the filter caps again immediately after reassembly and test, completely empty.might be the relay as JonSnell Electronic stated?
 
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s65amg

Member
2016-05-26 5:57 am
Unless the relay, RY920, is completely fried and open it has to be connecting the secondaries to the rectifier, D921. Can you get to the rectifier to measure voltages? Are fuses F912 and F913 good?

Craig


so there are two D5SBA bridge rectifiers.The one on the DCDC PWB seems ok reading around 425 on the doide test range and no reading when alternating leads but the one on the main board is getting a reading of around 425 with negitive lead connected to + and Positive lead to the closest AC pin but it also reads over 1000 and continues to go to 1900 and fluctuates when is conect positive lead to positive pin and negative lead to the closest AC pin.Same thing happens on the other 2 pins.So i'm getting a reading when i shouldn't be at all.I did test both of these in circuit, should i take them out of circuit and retest? Or is the main board bridge rectifier toast?

F912 and F913 fuses are good.Is there a schematic for this receiver? I couldn't find any.
 
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s65amg

Member
2016-05-26 5:57 am
tested the DC out of the rectifier and the filter Capacitors voltage while powered on and was getting 110VDC on the Rectifier DC out and 55VDC on the 71V filter caps. i disconnected the power and the caps dropped from 55V to 0.2V instantly.The filter caps are bad?
 
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s65amg

Member
2016-05-26 5:57 am
so i tested the outputs with music from my phone via rca.I got one of the speakers working (Front Left) none of the other ones work.Im still getting AC around 3VAC from all outs and in's.Also the only jack that audio plays from FL is the video jack and not even the right side input works for that one.I checked continuity of the output posts and get full continuity from + and - for all posts except For Front Right (no continuity at all).I also get continuity between all + red terminals except for Front Right again.Is this normal ? i Know the Negative line is connected toghether but not the + line? I get partial continuity through the Working Front Left post because i have audio playing.I hear a bit of static through the only working speaker. I rechecked all transistors and emitter while ON and playing music through RCA and they all showed around 55VDC ( only transistors not emitter resistors).So the outputs near the rear are shorted somehow?
 
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