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Post by serafis on Aug 22, 2024 0:58:54 GMT -5
Just got hold of an Onkyo TX-SR875 for almost nothing, which apparently was working fine and then just stopped. This thing is a beast, 23.1KG (50.1lbs) with a huge transformer and dual output transistor pairs, and I'd really like to get it going again if I can. Basic service manual with schematics is at www.manualslib.com/manual/1064234/Onkyo-Tx-Sr875.htmlThe Standby light is on solid (not flashing), and pressing the power button causes the power relays to click momentarily, but it just goes straight back to standby, no display, nothing. Getting at the insides to measure stuff is tricky as the boards are very tightly packed and stacked, but this is what I've found so far. VMPU (VCC1 & 2) is +3.3V, VOLH is low, PROTECT (VPRO & IPRO) is low, THERMAL 1 & 2 are both around 0.8V which is probably OK. All output and preamp transistors measure OK, no shorts, so it doesn't seem to be a blown channel as I initially thought it might be, and I'm not sure it's even in protect mode - it just doesn't want to power up. The 12V standby rail which powers the MPU board and the main relay, sometimes measures 12V, but more often drops to around 10.5V. Occasionally it goes back up to 12V. I don't know what is causing this. And if I connect the main transformer directly to power (bypassing the power relay, which I have checked is working), it powers up and down as it should when the standby button is pressed. When powered up like this, the speaker relays all click after a few seconds, and there's minimal DC on any of the power amp outputs. All buttons seem to work and I can do a factory reset too. So the problem just seems to be getting it to stay on. I'll continue investigating, but am running out of ideas and would really appreciate any pointers as what might be causing the problem, or areas to investigate further. TIA, Jon. |
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Post by mastertech on Aug 22, 2024 12:19:42 GMT -5
I have not looked at any schematics yet but a couple things do come to mind. Some units monitor certain power supply rails and if one goes out of spec it may just shut it down. Do not know if it would be that 12v line or not. Look at the CPU pins description on the schematic to see if there are any like this. Of course there could be another section somewhere doing all the monitoring of various lines and then just one line to the cpu. Just do some looking.
Are you able to bring up a diagnostic screen?
If I get time later I'll look at schematics.
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Post by ducklite on Aug 22, 2024 12:58:00 GMT -5
Hey serafis
all you have to do is follow the schematic diagram for this receiver to check voltages to see if they are present
if they are then the problem is else where in the receiver
to powerup the amplifier pin98 of the MCU has to go high (3.0V MPOWER) to turn on the power relay to power the main transformer
it will do this when pin 136 (KEY0) of the MCU gets a logic low by pressing the power button(STBY).
the ground probe of your multimeter must be connected to GNDDG (digital ground of the receiver).
hope that helps.
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Post by serafis on Aug 23, 2024 4:42:21 GMT -5
I have not looked at any schematics yet but a couple things do come to mind. Some units monitor certain power supply rails and if one goes out of spec it may just shut it down. Do not know if it would be that 12v line or not. Look at the CPU pins description on the schematic to see if there are any like this. Of course there could be another section somewhere doing all the monitoring of various lines and then just one line to the cpu. Just do some looking. Are you able to bring up a diagnostic screen? If I get time later I'll look at schematics. The service manual is very basic, just block diagram, schematics and parts list, no debug info so I don't know how to get to a diagnostic screen, if there is one, and I'd need to force power it on by bypassing the power relay. And because of the construction, the cpu and regulators are almost impossible to get at while assembled to measure voltages. But this is what I've found so far. Power supply (standby), P36: - with JL901A disconnected (i.e. power supply is unloaded), standby transformer AC is ~9.6V, +SUB is around +12V and POFF is around +9V. Both +SUB and POFF show a clean DC waveform with little ripple. This all seems to be in order. - with JL901A connected, standby transformer AC drops to ~9.5V, +SUB drops to 10.8V and POFF drops to 6.25V. Both +SUB and POFF show an unusual waveform (viewed in AC coupled mode) as per the attached scope screenshots. Microprocessor section (P24-25): - P2010B +SUB and POFF read the same as JL901A from the standby power board - I removed the Microprocessor board and ran 12V into +SUB from my bench supply. Current draw was a few milliamps and no unusual waveform present. Regulators Q7381 and Q7385 measured correctly as +5.6 and +3.3V respectively That's as far as I've got. I am most suspicious of the voltage drop on the 12V +SUB and 9v POFF lines (which are the only lines carrying power in standby) and the unusual waveforms, but haven't yet identified anything that could be causing this. +SUB 12V (measuring 10.8V) 
POFF +9V (measuring 6.25V 
+SUB 12V AC coupled showing unusual waveform 
POFF AC coupled showing unusual waveform  |
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Post by serafis on Aug 23, 2024 4:53:11 GMT -5
Hey serafis all you have to do is follow the schematic diagram for this receiver to check voltages to see if they are present if they are then the problem is else where in the receiver to powerup the amplifier pin98 of the MCU has to go high (3.0V MPOWER) to turn on the power relay to power the main transformer it will do this when pin 136 (KEY0) of the MCU gets a logic low by pressing the power button(STBY). the ground probe of your multimeter must be connected to GNDDG (digital ground of the receiver). hope that helps. Thanks, yes all of that is working. When the standby button is pressed, KEY0 goes low, 3.0V MPOWER goes high, the power relay transistor base gets around 0.9V and switches that side of the relay to ground, the relay closes, the main transformer gets power for an instant, but then 3.0V MPOWER immediately drops to low, the power relay opens and the receiver goes back to standby. I think the issue is somewhere in the standby power circuit (see previous post) but haven't yet been able to identify what. By the way, the other receiver is on hold for the moment - this one is more interesting and hopefully with a greater chance of success! I've got another two or three receivers to look at later as well, but there are only so many hours in the day for my hobby! |
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Post by frankusb on Aug 23, 2024 10:35:01 GMT -5
Your hobby sounds like my hobby. I had an Onkyo that would power off randomly but mostly play for long periods of time. What I found was that in standby the 12V line was at a reasonable voltage. When the main relay turned on, it too would supply 12V to the same line but the voltage would drop. There was a reservoir capacitor supporting the main relay 12V line that I replaced and when I did this, the voltage no longer dropped and the unit stopped turning off. I wonder if you are seeing something similar.
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Post by serafis on Aug 23, 2024 14:33:12 GMT -5
Your hobby sounds like my hobby. I had an Onkyo that would power off randomly but mostly play for long periods of time. What I found was that in standby the 12V line was at a reasonable voltage. When the main relay turned on, it too would supply 12V to the same line but the voltage would drop. There was a reservoir capacitor supporting the main relay 12V line that I replaced and when I did this, the voltage no longer dropped and the unit stopped turning off. I wonder if you are seeing something similar. I checked all the capacitors on the power board and around the Q7381 and Q7385 regulators, they all read OK on the ESR meter but I replaced them anyway as I have plenty in the spares bin. Unfortunately didn't make any difference. However, I did notice a very bulgy capacitor C2508 (2200uF) on the Video board (P21) so I replaced that too - that area is driven from the main transformer. I haven't checked the voltages on that board yet, that's the next job. |
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Post by serafis on Aug 23, 2024 18:18:31 GMT -5
I'm looking at the 12V +SUB line. From the schematic, it runs from the connector JL901A on the power board (P36) to JL901B on the Video Section board (P20) then to connector P2021A on the same board, then to P2010B on the Microprocessor board (P25). from there it runs on the same board(P24) to diode D7381, then to regulator Q7385 (+3.3V), also to diode D7383 to regulator Q7381 (+5VST). So simply speaking +SUB only powers two regulators for +3.3V and +5V standby. I can't see it going anywhere else, and both these regulators are working fine when I power +SUB on the microprocessor board from the 12V bench supply. When I bypass the power relay (to provide power to the main transformer), +SUB goes straight back to a steady 12V without the sawtooth-like 'ripple', and the receiver powers up out of standby just fine. Then I tried providing +SUB (12V )from one bench supply, and POFF (9V) from another bench supply (luckily I have two!) to JL901B, with the other pins connected to the power board as usual, and a common GNDSUB between the two bench supplies and the receiver (see picture below). And the receiver powers up out of standby just fine again.  So the issue definitely seems to be something to do with the +SUB and POFF lines, but for the life of me I can't work out what the problem is! Question 1: Why does +SUB drop from 12V to 10.8V and POFF from 9V to 6.25V when powered from the Power board, and where is the ugly sawtooth-like "ripple" coming from? Why do they stay at a clean 12V (13mA) and 9V (negligible current draw) when powered from the bench supplies? Question 2: Why do +SUB and POFF go back up to a clean +12V (no 'ripple') when the main transformer is powered up by bypassing the power relay? Since +SUB only powers the two regulators, and there's no direct connection with the main transformer, I'm just not seeing the logic here. Help!! |
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Post by ducklite on Aug 23, 2024 23:42:11 GMT -5
I think the capacitors are not good
so I suggest changing C9001,C9002 and C9008 and maybe D9007
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Post by serafis on Aug 24, 2024 0:58:10 GMT -5
Yes I have replaced all of those, no difference.
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Post by ducklite on Aug 24, 2024 13:29:20 GMT -5
to me it seems like the standby powersupply does not have enough current to run the amp
let me check my onkyo stuff
is your manual 269 pages?
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Post by serafis on Aug 25, 2024 2:15:13 GMT -5
Agree, but why? It's a very simple linear power supply, just a transformer, a couple of diodes and a smoothing capacitor for +SUB and similar with a 5.1 Zener for POFF. I've replaced the diodes, capacitors and Zener, so difficult to see what else could be wrong with that, given the miniscule current draw when powered from the bench supply.
The one I have is only 166 pages - do you have one with more info?
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Post by ducklite on Aug 25, 2024 3:39:15 GMT -5
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Post by serafis on Aug 25, 2024 15:36:53 GMT -5
Thanks, that's a much more useful version of the manual. While the bulletin doesn't apply to this specific model (it has no board soldered to the main transformer and no fusible resistor - the wires come out of the transformer and go straight to the connectors), I'll check the secondary AC lines and their DC outputs. May be a few days though as I have other (non-related) things I need to spend time on.
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