SX-780 Tuning

[Beatnik]

SX-780 Tight Tuning Knob and Distortion

Went and bought a Reciever off of the auction site. I was in a hurry, it's to be my daughters first Pioneer reciever and I planned to give it to her for Christmas.

SX-780. Tuning knob is hard to turn, doesn't free wheel like it should. Also, the FM tuning meter doesn't work. It does when set to AM, but not FM.

Please help, it's a nice clean reciever otherwise and would be a great gift.

 

[Oerets]

Like thoes 780's sure do sound good ....
Have you open up the hood to see if the string is stuck on something ? If you can try it without the string as tight. Sometimes you can loosen up a wheel so that you can remove the tension and slip off the string . If this helps the Knob then look for something along the strings path. If the knob is still tight without any tension then it could be bent or needing adjustment maybe. Or check the tuning cap to see if it will turn easy. This could be dirty and needing some attention.

Hope it just the string off a wheel or caught on something....

On the FM meter, is it receiving stereo ? and lighting the stereo lamp?
It could be just needing a good cleaning to make it work.



Good luck



Barney

 

[markthefixer]

Stupidly bought a Reciever off of the auction site. In a hurry, it's to be my daughters first Pioneer reciever and I planned to give it to her for Christmas.

SX-780. Tuning knob is hard to turn, doesn't free wheel like it should. Also, the FM tuning meter doesn't work. It does when set to AM, but not FM.

Please help, it's a nice clean reciever otherwise and would be a great gift.

It's the knob bearing, the grease may have petrified.
OR
The adjustment lock may have come loose, and the shaft adjustment locked itself tighter.

This thread:
http://www.audiokarma.org/forums/showthread.php?t=104071
should be a big help in removing, cleaning and reassembling the tuning knob / flywheel assembly.

Then deoxit the function switches.

 

[Beatnik]

I went in and checked the string, all was right. Pulled the string off and the knob still turned with friction. Took the whole assembly out and it fiddled with it, gave a couple of light taps to the end of it on the table and it freed up a bit. Thought maybe there was grease inside there somewhere, but I couldn't find anyway to take it apart? So I heated it a bit with my heatgun and it turns much better now, it's passable.

Is there anyway too lube this assembly ?

The meter I noticed is working better here in town, just doesn't move much at all like my others do.

There was shear panic when I picked up the box it was shipped in and felt the contents sliding inside !! Could have definitely been packed better, I was afraid it took a hit on the knob and damaged something for sure. Next I have to actually hook it too some speakers while everyone is gone, hope it makes good clean sounds.

Thanks !

 

[Beatnik]

It's the knob bearing, the grease may have petrified.
OR
The adjustment lock may have come loose, and the shaft adjustment locked itself tighter.

This thread:
http://www.audiokarma.org/forums/showthread.php?t=104071
should be a big help in removing, cleaning and reassembling the tuning knob / flywheel assembly.

Then deoxit the function switches.

I've already got it back together :<), going to read through that thread right now !

 

[Beatnik]

Okay, that picture helped a lot. I have it completely dissassembled and wiped down, should I use some kind of grease on the shaft when I reassemble it?

Mine doesn't have what looks to be a nylon washer between the end of the shaft and the bearing in that picture ?

 

[Fred Longworth]

Regarding the shipping. I am extremely hardnosed about "auction site" items that are shipped to me. When I pay for the item, I always PM the seller and advise to wrap thoroughly in bubble pack. If I receive it loose in the box, and the set shows any signs whatsoever of transport-related damage, I PM the seller again and request compensation.

You should do the same. You are under no obligation to "take the hit" because the seller is ignorant of proper packing techniques.

* * * * *

As far as a sticky tuner, the most common cause is that on the main tuning condenser assembly the bearing on the plastic drum shaft assembly has gone gummy. Often this can be loosened up somewhat using a Teflon lubricant that is allowed to sit and to penetrate, followed by running the dial up and down through the full range about 25-50 times.

Fred

 

[Beatnik]

I put a dab of what grease I had around on the shaft and it's together again. Damn thing only lacks two notches on the dial from going the whole length in a single spin !!!! A little cleaning of the actual function switch did the job on the tuning meter.

As for the packing. This seller looks to do alot of stereo equipment, picture looked to be taken on a bench. I assumed they knew what they were doing, what is it Andy Griffith say's "ass/u/me". He has offered a refund, it's a very clean reciever though.

 

[Oerets]

Nice lookin unit !
Glad it's working better now , never had one with dry bearing myself. Learned something new today .
Hit the Tuning cap with a shot of air before closing it up for good measure. (IMHO)






Barney

 

[Beatnik]

Well, I got crooked to the cleaners. It's distorted pretty bad on the right channel, should have checked that first !! I will only make this mistake once in my lifetime, never bought any of my equipment of the bay before and it went exactly as I expected it would. Just wanted something nice for my daugghter and jumped the gun on this one.

Makes a nice display piece.

 

[markthefixer]

Well, I got crooked to the cleaners. It's distorted pretty bad on the right channel, should have checked that first !! I will only make this mistake once in my lifetime, never bought any of my equipment of the bay before and it went exactly as I expected it would. Just wanted something nice for my daugghter and jumped the gun on this one.

Makes a nice display piece.

Swap the modules left to right and see if the distortion moves. If so it's $5.39 each plus shipping (do both?) from electronix.com for the stv0050 module.

simple, easy fix.

 

[Beatnik]

Okay, I'm setting here looking sraight at the modules. I can't get the courage so far to mess with them. A capacitor with just two legs is simple, these I ain't sure I can handle. :scratch2:

Mine are marked stk-0050

 

[Oerets]

What tools do you have for de-soldering? STK-0050 is the right one.
They come out easy if you take your time and remove all the solder , just clap a heat sink on each pin as you go.




Barney

 

[Beatnik]

I have heatsinks, but De-soldering tool suggestions are welcome. (Fry's Electronics is right down the road)

I ordered the power packs already, I have a few days before they get here I'm sure. If I'm taking them out, I may as well replace them.

 

[Oerets]

A simple sucker bulb or solder wick will work, you will also need some silicone heat sink compond . RS sells a small tube , get the regular type, not the kind used for computer IC's.
Do a search for SX-780's IC Power Pack replacements threads, there are some solder joints that should be redone. Due to age and heat.


Barney

 

[Beatnik]

Got the power packs today, just put them in. Same thing, right channel is distorted.

 

[markthefixer]

Got the power packs today, just put them in. Same thing, right channel is distorted.

Well, I did say swap them to test, not replace them outright.

So, now we're at the next (first?) step:

Carefully read (and post) the voltages on each pin of both modules, probing with red lead of dmm, black lead of dmm connected to a bare metal chassis ground.

WARNING, the pins if shorted together while there's voltage will instantly destroy the module, so all but the tiniest tip of a sharp probe (no slip) should be insulated.

Plus, you need to see if this distortion occurs with FM, phono or aux.

Is the distorted channel about as loud as the other? Does moving any of the front panel knobs and switches affect it?

 

[Beatnik]

Yeah, I know. Gave me a few days to think it over though.

Okay, here is what I got chassis to pin.

Left:
1) -1.189
2) -41.4
3) -0.034
4) 0.006
5) 0.004
6) 0.007
7) 41.5
8) -0.033
9) 41.5
0) 1.088


Right:
1) -15.41
2) -41.4
3) -14.45
4) 0.001
5) 0.001
6) 0.001
7) 41.5
8) -0.048
9) 41.5
0) -13.10


Same problem Tuner/Aux/Tape1/Tape2/have no Phono here at work, also A/B speakers. First thing I did was swapped speakers and it is the reciever.

Left/Right are same volume and none of the knobs/switches affect it.

 

[markthefixer]

Yeah, I know. Gave me a few days to think it over though.

Okay, here is what I got chassis to pin.

Left:
1) -1.189
2) -41.4
3) -0.034
4) 0.006
5) 0.004
6) 0.007
7) 41.5
8) -0.033
9) 41.5
0) 1.088


Right:
1) -15.41
2) -41.4
3) -14.45 <<<<<< this isn't ok
4) 0.001
5) 0.001
6) 0.001
7) 41.5
8) -0.048 <<<<< this is ok
9) 41.5
0) -13.10


Same problem Tuner/Aux/Tape1/Tape2/have no Phono here at work, also A/B speakers. First thing I did was swapped speakers and it is the reciever.

Left/Right are same volume and none of the knobs/switches affect it.

Looks like an emitter resistor may have let go. R266, 0.22 ohms 2 watts.

I'll BET somebody replaced a blown out module, and afterwards it worked, but sounded like crap. So they sold it on ePrey.

There are four resistors mounted parallel to the module, just below the pins, and just about got in the way of your soldering...
There are two each module, connected to pins 3 and 8. The right module's pin 3 is the one.
Kill the power, and measure directly across the resistor's leads with an ohmmeter, I'll bet that three show less than an ohm or so, and the other has some wild reading.

looking for a replacement part supplier, it's an odd value for pioneer. Usually they're at about 1/2 ohm.

more will be edited into this post and this will disappear when it is done.

 

[Beatnik]

I really appreciate this ! The reciever is at work (can't work on it with my daughter around), I will be back up there tonight with meter in hand.