• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Pioneer SX-780 - Strange Things Happening! (First Post)

ETEXJNB

New Member
New to site..first post..bear with me.

Ok, so a good friend of mine upgraded his receiver so I upgraded my NAD to a Pioneer SX-780. Now this is probably my best friend, and for him money is not an issue, and he's one of those guys with probably $8K invested in his audio set up, with about 4,000 records. So he's serious and he knows how to clean up and repair vintage receivers.. this was 1 of 3 he SX-780's he had owned. Before I got it from him (not shipped, I picked it up), he says that he: Cleaned all switches and pots with contact cleaner, replaced lighting (I think he put in amber LED's..not sure if that could have any bearing on this issue but just in case), he striped the wood cabinet and sanded and re-stained, replaced some knobs, and tested it before he sent it. One important thing to note is that he tested it with an external pre-amp. Said it worked fine.

Now I get it, I'm stoked for the change although honestly I love my NAD. I plug it in, connect my table straight into the phono input. I turn the knob to phono, and set a record to play. So I heard nothing but a slight buz (I just hadn't grounded it yet..got in a hurry to see it work). So I notice the buzz and ground the record player. The buzz goes away. No sound.. then I keep turning the volume up and I notice at about 75% I start hearing the sound of my record. Very vauge and low..barely audible. So I think..huh lets try speaker input B.. did that, same thing. Same for AM and FM, hardly audible at 75% volume.

So I instantly think..the pre amp isn't working. So what I do is I go ahead and use my external pre-amp and go that way, into the phono.. nothing. Now I have no sound whatsoever.

Then, thinking well what If I go from record player to external pre amp, to TAPE IN. At first it doesn't work, then, I flip up the Tape Recorder knob on, and it works.. sounds GREAT. Then I listen for a bit, play with some of the bass and treble, just seeing what kind of range I was getting. Then I turn it all OFF.

Turn it on again.. nothing works. Its getting power, the lights are on. No sound. I realize..what input did I have it on when it was working..AM,FM,Phono,AUX.. cant remember. I still hear the low sound of the am and FM, but nothing from the record player.

I'm checking fuses tomorrow, in the event that me turning the volume up that much blew a fuse, or some internal mech flipped a switch in some kind of protective maneuver.

How should I go about this? What steps should I take after checking the fuse?

Thanks in advance!!
 
Register to hide this ad
Check the solder joints under the three to-220 power transistors in a group near the corner of the board. T^he have u shaped heat sinks - be careful, raw DC is on each heat sink, a different voltage for each one.

Shipping (and a good jolt) could have shifted a marginal connection.
 
The 4 pictures below are visually what Mark The Fixer alluded to; The Voltage Regulators in this case are the primary suspects. You can check the diode solder at the same time. The Regulators have from the Factory a small solid heatsink on each. DO NOT TOUCH THESE until the unit is unplugged and given a chance to cool off. (about 30 minutes). 1.) They are Electrically HOT. 2.) They are physically HOT!

The 1st two pics are of the Voltage Regulator(V.R.'s) and Relay area. The Voltage regulators are circled in RED on both TOP and BOTTOM. Note that the Heat sinks on the V.R.'s are after market and have removable sections for clearance of surrounding parts. I got these at Radio Shack, but Mouser. Digikey and or PArts Express would all have them. These are the parts that literally get hot enough to De-Solder themselves. I replaced mine with Mark the Fixer's recommended replacements, added larger heatsinks, and installed in 2011.

TOP

780 Relay and Voltage Regulators.JPG

BOTTOM
Regulator leads circled in RED.

Voltage Regulator underneath.JPG

POWER SUPPLY AREA Front Left Area

Regulators in RED. P.S. Diodes in Blue. I had a 780 a couple year ago that would light up the lamps Only. Everything else was dead. Turns out the DIODES in the Power Supply along with the Regulators solder joints heated up so much so as to DeSolder themselves from the board. Installed new P.S. DIODES with complete length legs (didn't shorten any), installing the diodes with the bodies 1/2" above the board; and new regulators(added heatsinks). Problem solved.
780-790  Front Left.JPG

Same Area Underneath.

780-790 bottom.JPG
 

Attachments

  • 780-790 bottom.JPG
    780-790 bottom.JPG
    166.3 KB · Views: 16
Back
Top Bottom