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New Member. Just picked up an SX-780. Have a few questions.

geebjen

geebjen

AK Subscriber
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Hello all and thanks for contributing to this awesome forum. I've been getting a lot out of it already.

I just picked up this SX-780 for $200 which I think is a pretty good deal. (Side note, also picked up a pair of Bang & Olufsen Bevox 3702 speakers with it for $60). Everything works on the SX-780 with one minor annoyance which I'll get to shortly. As far as I can tell, it doesn't look like it has ever had work done to it and also doesn't appear to my untrained eye to have any immediate issues, but I'll add pictures of the inside in case anyone sees something. I was able to set the DC balance (they were 130mV and 65mV before adjustment) and the meter drive per the service manual.

So here are a few questions.

1) The one issue I have is with the power relay. When powered on cold the relay kicks on in about 5-7 seconds. But sometimes it kicks back off again...and then on again. But it only seems to do this cold. Once it's warmed up, I haven't seen it kick off. Any ides what causes this and what I should look to replace/fix?

2) The solder joints on one of the Power Packs look rough (the one I included a picture of). Do you think I should reflow those?

3) Since everything is working good (other than the power relay issue) I'm tempted to leave well enough alone for now. Or should I go ahead and replace the common failure points? I know it's typically recommended to at least change the power and filter caps even on a working system, and the relay too I think.

If you notice anything else from the pictures, feel free to enlighten me.

Thanks,
Steve

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geebjen said:
Hello all and thanks for contributing to this awesome forum. I've been getting a lot out of it already.

I just picked up this SX-780 for $200 which I think is a pretty good deal. (Side note, also picked up a pair of Bang & Olufsen Bevox 3702 speakers with it for $60). Everything works on the SX-780 with one minor annoyance which I'll get to shortly. As far as I can tell, it doesn't look like it has ever had work done to it and also doesn't appear to my untrained eye to have any immediate issues, but I'll add pictures of the inside in case anyone sees something. I was able to set the DC balance (they were 130mV and 65mV before adjustment) and the meter drive per the service manual.

So here are a few questions.

1) The one issue I have is with the power relay. When powered on cold the relay kicks on in about 5-7 seconds. But sometimes it kicks back off again...and then on again. But it only seems to do this cold. Once it's warmed up, I haven't seen it kick off. Any ides what causes this and what I should look to replace/fix?

2) The solder joints on one of the Power Packs look rough (the one I included a picture of). Do you think I should reflow those?

3) Since everything is working good (other than the power relay issue) I'm tempted to leave well enough alone for now. Or should I go ahead and replace the common failure points? I know it's typically recommended to at least change the power and filter caps even on a working system, and the relay too I think.

If you notice anything else from the pictures, feel free to enlighten me.

Thanks,
Steve


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You need to get in there with a nice soft bristle brush (natural hair paintbrushes work good) and a vacuum cleaner. The amp board is so-so clean but that tuner board sure is messy.

Most any vintage Pioneer receiver needs to have the solder joints of the regulator transistors looked at and usually at least re-flowed. They run very hot and the solder joints degrade. The 780 is no exception. I prefer to remove all the old solder and then apply new, but at the very least re-flow with a touch of new solder. You can see in the picture I enlarged from one of yours the far right regulator has halo joints that will become or already are intermittent. You should do the other two as well. They are the TO-220 transistors on the little aluminum heat sinks.


780reg.jpg

I always like to replace the protection relay but if it is not causing any trouble feel free to leave it alone.

The power pack solder joints look OK from what I can see.

As for the cold start issue with the protection relay it may be the offset (DC Balance) voltage(s) wandering around a bit until the circuit warms up. It may be indicative of some parts that are out of tolerance. It also could be related to the ugly solder joints on that regulator Q25 which just happens to feed the protection circuit which drives the relay. Try it after you re-flow/re-solder the joints on Q25. If it still acts up then watch the DC balance from a cold start. (see below)

Since you know how to set the DC balance, put you meter back on the DC balance test point(s). If you have two meters you can watch both at once otherwise one at a time. From a cold turn on watch the meter(s) and see if the voltage is moving around a bunch, small wobbles (10's of mV) are normal.
 
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Thanks a lot! I'll work on that stuff tonight and get back to you tomorrow with how it went.
 
So I cleaned the dirt and dust out, especially on the tuner board. Then I reflowed all 3 of those transistors and added a touch of solder during the process. Re-did the DC balance (they were about 15mV each, got them near zero). So far so good, 10 for 10 on startup with the relay clicking on in exactly 5 seconds and never clicking back off again.

Thanks again.

If I run in to any more trouble I'll be sure to be back again.
 
Protection Issues:
Check the amp settings during a cold power up.
You can adjust the DC Balance.
No adjustment for the idles on a 780. Maybe monitor the drop across pins 3 and 8 of the STK? I don't know what the normal reading should be. Check and see if they are the same :dunno: ? Go from there.
If your skills are up to it the STK mod from Rick and H. Brown wakes up the receiver like no other.
The orange caps when used in the signal path can cause issues. They all should be replaced if your going to go that route.
The HA1457 op amps will shoot DC voltage through the amp when they go bad. Some history on those. That will show it self when checking the DC Balance.
 
Thanks a lot for the additional information. I'll keep all that in mind if it acts up again.

Steve
 
New question on the same receiver. When I had it apart I cleaned all of the controls with CRC Electronic Cleaner. Everything is quiet and functional, no static or pops on any of the controls. But the Balance Control is kind of rough, not smooth like the others. It kind of feels like its scraping as it goes. It works just fine and I actually don't really have a need to use it, but still don't like that it feels that way.

Any ideas what would help here?
 
The type of cleaner you used washes out any type of lubrication that was in the control, you need to replace it. Folks use many different products, Caig faderlube is a safe bet and you may get other suggestions as well.

I personally like to use DeoxIT D100. it cleans and leaves a lubricating film behind. It comes in a "one shot" metered spray can or as a liquid with a needle tip applicator.
 
Thanks. I was worried maybe that was the issue. Newbie mistake.
Do you mean I need to replace the control, or replaced my cleaner and respray the control?
 
The cleaner you used is OK to flush out gunk but it is really not a DeOxidizing agent and it washes out the lubrication. The metal wiper of the control will eventually damage the resistive track without lubrication. You should be fine once it has some lubrication in it.

Yes replace the lube not the control.
 
If your budget is such that you can't or won't go for the price of DeOxit (Faderlube or Shield), go down to Home depot in the electrical Dept and get a can of CRC 2-26 Electronic lubricant. Give all the controls a light spritz.
 
Thanks for advice. I ordered some Deoxit F5 Faderlube, but good to know about the CRC 2-26, local Lowes has it in stock.
 
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