SX 850 Relay Clicks On then Off

[vwong688]

Hi

I put a new Power Supply board in my unit about a month ago and everything has been great, barring an issue with the left channel being slightly weaker than the right. I did perform a DC offset when I had the cover off and thought I did everything correctly. I had also done a rebuild of the Power Protection board, caps and transistors.

Yesterday while listening to the tuner on very low volume, I head a Thump and the unit relay clicked off. I noticed that after powering down the unit for a few minutes and turning it back on, the relay clicks but then trips. Subsequent attempts to turn on the unit after doesn't cause the relay to click anymore. I had a new relay laying around but hadn't put it in yet and so I did that today. Same exact symptoms.

Here are my readings:

AWR 101	Spec	Test
1	GRND	GRND
2	5.4v DC	5.08v DC
3	5.4v DC	5.1v DC
4	13.9 DC	13.34v DC
5	13.9 DC	13.36v DC
6	54.8v DC	50.9v DC
7	54.3v DC	50v DC
8	37.6v DC	36.34v DC
9	37.6v DC	36.34v DC
10	37.6v DC	36.36v DC
11	28.8v DC	28.1v DC
12	GRND
13	-13.0v DC	-12.96
14	-50.7v DC	-50.2
15	-18.5v DC	-17.89
AWM-062	Spec	Test
1		Fluctuating
2	-13.1v	-12.97
3		Fluctuating
4	GRND	GRND
5		Fluctuating
6		Fluctuating
7		Fluctuating
8		Fluctuating
9	-0.9v (relay off)	36.4v DC
10	-0.9v	36.4v DC
11	-0.9v	36.4v DC
12	7.5vAC	7.5v AC

Apologies, I couldn't find the specs for the blank Protection Board pin outs. I was getting fluctuating read outs for those, am I correctly grounding it while reading to the chassis?

One thing that sticks out is Pin 9 on the protection board. I had was reading it when I turned on the unit after it being off for some time. The reading was coming down to I believe to the spec reading and the relay clicked. When the relay tripped again, the value rose to 36.4 v DC.

Am I correct in thinking that at this point with the relay fully tripped, I can't adjust the Idle and DC Offset?

I know this is a well covered topic, I read some posts but couldn't find diagnoses that were exact to my symptoms.

 

[TomSc]

I'm going to go out on a limb, given my relative limited experience compared to some of the veteran & professional techs here. I hope they chime to correct anything I may get wrong, but here goes:

You can check bias and DC offset in the protect mode. The manual wants no load connected to the speaker outputs. It also says to pull the Pre/Power jumpers and ground the amp RCA inputs with a 5.1K shunt plug. Offset will usually bounce back and forth a little bit, but not more than a few mv. What voltages are you seeing? Does bias bounce quite a bit too?

There is a good description of the protect circuit in the service manual on page 17. But my amateur solder slinger hunch is the protect circuit is not at fault. I think it's doing it's job. Because you heard a thud when this happened, I suspect there's something wrong on the amp board, but without probing around a little more, it's too early to tell.

 

[vwong688]

Hey Tom,

Found instructions on AK about doing the offset and idle in protection mode without the shunts. I was about to adjust everything BUT VR1.

VR1 is at a constant 46.64v DC. Turning either way does not affect the value im getting. It actually fluctuates quite a bit but that is usually where it is hovering around. Everything else I was able to get into spec.

 

[zebulon1]

If the DC Balance is 46v at the center speaker rail then its railed to the plus side meaning the negative side is missing in action.
If these readings are correct then you have a driver stage issue in that channel.
I would of liked to see those fluctuating readings.

 

[vwong688]

Zeb,

The fluctuations are really all over the place. Judging from the behavior of my multimeter (not a fancy one), there will be an intermittent brief period where it would start to auto range but ultimately goes back to reading VR1 as all over the place.

Compared to what I measured for my last reply, it no longer holds a constant 46v DC. It is now all over the place.

With the unit off, it reads -4mv

 

[zebulon1]

Remove the jumpers and check again.
Make the check quickly and power off As soon as you can tell if it’s different or not.

 

[vwong688]

it is no longer constantly and static at 46, now is it fluctuating by a few volts in various ranges, 10-15, 20-25. It really is still very random, i was unable to lock down a pattern

 

[zebulon1]

The amp needs be gone through.
probably not operate it until you get a game plan.

 

[vwong688]

Is it too much to ask how to go about testing the power amp board. Im sorry, i have very limited experience in electronics work.

Before all of this, i would like to add that i had been noticing the left channel was quieter than the right when going back and forth on the balance knob

 

[zebulon1]

You tested it and found out it’s railing Putting excessive DC on the speaker rail.
Visual check of the back of the amp board Is needed. Maybe a poor solder connection?
Also:
A loose or uneven +-51volt prestage supply will cause an imbalance but it would cause an imbalance on the other amp As well.
You did check that. Double check your PS install once again. Look at those Phoenix terminals if you used them.

 

[vwong688]

Unfortunately im not finding anything new on visual inspection. I tried to move around the pins on the PS in case there were any loose ones from my install. I would note that I did find one initially on my very first visual inspection but had fixed that. I tugged on the wires carefully and see if there were any wire wraps that had been broken.

Is there anything else I can measure and check?

The solder joints on the AMP board pins look good. It's hard to get that thing out to have a full look. This may be heading to some local experts here in the Bay Area.

 

[vwong688]

i also wanted to add that I replaced the speaker connections with banana ones off of ebay. They worked fine up until this recent issue. Could there be an issue there? I soldered them in the same way they were originally installed