What happens is that, at random moments during use, the speaker protection relay may open and cut the sound and then close.
I've tried reflowing all solder joints on the power amp board and replaced various electrolytic capacitors, but to no avail. The receiver keeps cutting out at random times.
The receiver seems to amplify normally, even up to the point where the relay opens.
I'm guessing that the relay itself could be at fault, but how? I physically tap the relay during use and it doesn't exhibit any faults.
Bad joints on the relay coil connections. Bad joints in the power supply causing one rail to drop out momentarily. Bad joints in the relay control / fault sense circuit. Resistor on relay delay timing going high / open. Timing cap int o/c. Genuine problem such as defective speaker or wiring. Those are the most common problems I come across.
You can probably look elsewhere for the fault. I would first suspect that the circuitry that controls the relay, i.e., the protection circuitry, is activating.
Some possible reasons that the protection circuitry is opening the relay... (1) DC offset voltage is appearing at the speaker terminals. Causes... Idle current set wrong or component failure in the amplifier chain causing the offset to go haywire. (2) Temperature compensation circuitry faulty. Causes... defective temperature sensing component (thermistor, diode, transistor). (3) Protection circuitry faulty. Causes... component failure in that area. (4) Anything else that would upset the DC balance in the amplifier.
Things to look for: Check for DC voltage present at the speaker terminals. If it's more than a few millivolts, you need to find the reason for it. Is the DC offset only on one channel (more likely) or both (unlikely)? Use the good channel to compare voltage measurements along the amplifier chain and try to isolate the bad stage. Does the protection relay activate with no audio, or only with audio present? Do you have a schematic of the unit?
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Dave M
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Experience: What you get when you don\'t get what you want
Have you sat there watching the meter to see if the offset jumps up when the relay opens? (I'm sure you have better things to do with your time!) Some DMMs have a trending feature that keeps track of the highest voltage.
I managed to find the service manual and performed a test and adjustment of the idling current.
The service manual states that the idling current at open load must be
7.5 mV with a tolerance of 1.5 mV
Left channel was about spot-on at 7.0mV, but the right channel was WAY off at over 30 mV! Seems the trim pot that controlled the idling current has changed in value.
Anyways, I readjusted both trim pots until I got the voltage spot-on to 7.5 mV. Hopefully, that will fix it.
However, I am concerned about the fact that the pot on the right output had drifted so drastically so I am entertaining the notion of finding a replacement set of pots and redoing the idling adjustment.
On Sat, 16 Aug 2008 15:45:37 -0700 (PDT), snipped-for-privacy@aol.com put finger to keyboard and composed:
Assuming that the protection involves some current sensing or DC voltage sensing components in each channel, then why not disconnect the relevant components one channel at a time? I would think that you would not risk any damage by doing this, since you are able to reproduce the fault with the speakers disconnected.
Other replies have focused on looking for a DC voltage on the outputs, but I've just been looking at the protection circuit for a current model Onkyo receiver (TX-8211) that appears to monitor the load current in the 0.22 ohm emitter resistors of the output transistor pair. If either resistor were to intermittently go open, then this would trigger the protection. I have seen this problem in a car amp. In that instance, one of the pins of a ceramic emitter resistor fractured due to vibration.
- Franc Zabkar
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According to the schematics and parts list, if I'm reading it right, Q508 and Q509 (transistor pairs for the left) work with R512 and R513, which are a matched pair of metal plate resistors rated at 2W and .47 ohms. The right channel transistor pair are Q608 and Q609 and their resistors are R612 and R613. The resistors on the right channel are identical with those that are on the left, of course.
So, how would I go about testing the resistors? Would I lightly tap on each one with a plastic probe and see whether or not the protection relay trips?
Right now, I want to figure out what is going on and I have the time at this moment to do that.
I measured the power and, on the left channel, the offset fluctuates between a peak of 11.3 mV and 8.3 mV. Is this significant?
Unfortunately, I missed the meter reading right at the moment the protection relay tripped.
It's hard to say whether or not that is significant. In itself, in terms of absolute voltage, it is not significant. Imbalances up to about 20mV are not uncommon, and are nothing for concern. However, the fact that it is fluctuating may indicate that some component is 'hovering' around its breaking down point, or that one of the rails is not stable. It is not really usual for the voltage, whatever its actual value, to fluctuate. Does your DMM have a peak hold facility as suggested elsewhere? This will tell you if the midpoint shifts substantially at the time of failure. However, note that if the relay circuit is 'quick', it might be *too* quick for the DMM to see the voltage increase, before the relay disconnects it, so really, you need to be taking any measurements actually at the amplifier output, prior to the relay.
Does this amp have a set of discrete component output stages, or are they STK hybrids ?
Monitor the supply rails as well - particularly those to the output stages - and see if they are moving by a similar amount as the output offset. In DC coupled amps, a very tiny shift in conditions back in a preamp stage, can upset the DC conditions so much in the output stages, as to cause a relay trip-out, which can make them very frustrating to troubleshoot. Use the butt end of a reasonable sized screwdriver to have a bit of a bash around, to see if you can provoke the problem at will. If you can, then swap to a biro pen to pin down the actual area, and possibly, component.
You can also try a hairdryer, and a can of freezer, both sides of the board. Bad joints which don't readily show up with disturbance testing, are often surprisingly sensitive to heat and cold. Does the problem seem to get worse or better the longer the amp is on ? Is it worse when the amp is 'doing some work' ? All possibly significant factors in arriving at a diagnosis.
Could the idling current being horridly off be a possible cause?
I managed to obtain the service manual and was able to measure voltage for the idling current.
According to the service manual, voltage should be at 7.5mV with a tolerance of +/- 1.5mV. Left channel measured in at around 7.0 mV, but the right channel was measured at over 30.0 mV.
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