• 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 need help signal tracing. Powers on no sound. Darlington packs are good.

rcs16 said:
Looks like the old Motorola "PL259" type connector, from the pic in the link, they are common, adapters or cables are available.
the sx has 75 ohm and is a bare wire connection.

with an FM antenna connected (folded dipole or better) tune across the dial, do you get any signal strength deflection on the meter? what number? if you get no deflection you have to check that the the ckts are getting power then have to work from the RF front end thru the IF to the detector PA chip.
Click to expand...

With dipole antenna connected I do get signal strength deflection. I don't know what number there are no numbers but maybe %25 in each direction. Surely not pegging the needle in any direction.
How can I check if the ckts are getting power? What is ckts?
 
ckts = Circuits.

With the dipole antenna connected the FM should be better. The AM is limited to the rear loopstick antenna, and even if an external wire is connected the loopstick must be in place since the AM antenna terminal is wired in series through that factory folding antenna.
 
No stereo - maybe dirty or misadjusted PLL. (Phase Locked Loop). There is a little trimmer VR-1 4.7K on the tuner board near IC- PA1001a. Mark the setting. Use a little de-oxit and clean the pot. (work it a little and return it to the setting point. If still poor or no FM stereo (have to be tuned to a reasonable station) try adjusting it and find the range where the stereo light comes on. If there is a range, set it to the center of the range and stereo should work well. By the book - If you have access to a frequency counter - tune the tuner to no signal spot on dial, then adjust the frequency at test point 23 to 19kHz Either method should wok. If you can't get it at all, try changing the 510 pF cap C36. Usually the are polystyrene caps (clear plastic) but any good quality film cap will work there. If that doesn't work, is probably the IC :( Good luck.
 
I have read the manual and am aware of what it says but what does that mean in the context of your comment. Are you trying to tell me to hook up a coax cable or just giving me options. Or does it mean that when i hook up my RF signal generator that I need to take those numbers into consideration.
Maybe I am over thinking the whole thing and you just need to smack my head. :whip:
 
Last edited:
you have to match your RF sig gen or antenna impedance to the tuner input impedance. 75 ohm coax or 300 ohm balanced twin lead
if you get signal strength deflection on the meter ,you have to look at the ckts between the detector and the stereo decoder to the o/p at the function switch.
with 25% sig mtr deflection you should be getting stereo, so as Steve suggests that has to be looked at.
 
Unfortunately, that series of sig-gen (SG-7 or SG-8) is probably going to be useful on AM only.

Before getting too deep, let's review what you have.
  1. Is the foldable loopstick AM antenna still in place on the back of your 780? (see below)
  2. Can you receive ANY AM signal with the tuner section? Your previous posts seem to indicate you can receive one station weakly.
  3. Does the tuning signal strength meter move at all in AM mode? Post 60 seems to prove it does not.
  4. Is there at least a dipole antenna connected to the 300Ω (balanced) FM antenna input?
  5. If not, is there a coaxial cable connected to an FM antenna connected to the 75Ω FM antenna input? Post 54 indicates an antenna connected, but what type and where is it connected on the receiver?
  6. Can you receive ANY FM signal with the tuner section? - Your previous posts indicate that you can receive FM, but not in stereo.
  7. If so, does the signal strength meter deflect at all in AM or FM? - Still unclear on this one.
  8. Does the FM tuning center meter have any movement? - Post 57 indicates that the meter is centered when tuned to no station, but does it move otherwise? Post 61 indicates the signal strength meter is deflecting, but what about the tuning center meter?
  9. Do you intend to send the unit out for a complete, factory-perfect (or better) alignment, or simply intend to restore operation and live with an average tuner section?
  10. Other than the RF generator, do you have non-metallic trimmer and RF tuning tools available? How about an oscilloscope? Distortion meter? - Post 57 states you have the sig gen, but at the least you will need a multimeter and something like THIS.
That information, summarized here, can help clarify what should happen next.
 
Last edited:
Do you have one of these?

SX-780AMAntennaInstalled.jpg
 
1. Yes the AM antenna is still on the back of my unit.
2. After the cleaning, and as of now I can receive 2 AM stations they very faint.
3. This model pioneer does not have a signal strength meter.
4. Yes I have a dipole antenna connected to the 300 ohm antenna input. (Where is the ohm symbol on my keyboard?)
5. I am not using a coxial antenna.
6. I can receive FM signal and after the cleaning I can receive it quite well. FM seems to be functioning normal except for the fact that stereo is not working. Alignment is also off in other words if I'm listening to 99.9 the needle is on 105.
7. There is no signal strength meter on this model. However the tuning needle does move.
8. Sorry about the confusion with the prior posts I thought the tuning needle and signal meter were the same. Right now the tuning needle will move about 25 percent in each direction and it will center on a strong station.
9.My intention is to get it operational and sell it. I have another one in better condition that I am going to restore. This one however is more of a learning project. With the help of you guys I have already brought it back to life and now I am just working on the radio section. I don't want to send it out for professional alignment because I want to learn how to do it. I don't mind buying test equipment as I have restored receivers in the past and plan on restoring many more in the future. Instead of paying for a professional alignment I would rather take that money and buy more test equipment. ie something that can send a multiplex signal
10. I have the following test gear
Oscilloscope
Function generator
RF signal generator
hand held multimeters (analog and digital)

I also picked up a Elgar Model 121 at a garage sale. It seems to be working good but I don't really now how to use it. It seems to be a very precise and accurate frequency generator that doubles as a variable power supply. It doesn't seem that this has any application in Tuner alignment. However it is something that I can trade or sell to acquire the gear I do need.
20181022_073024.jpg


I also picked up a ONE AC Line Viewer at at sale. Same thing I can sell or trade this one out for other gear I need.
20181022_072947.jpg


I also have a Hicock 6000A Tube Tester that I can sell.
20181022_072912.jpg



Check out all the stuff this guy is selling https://www.ebay.com/itm/PI-Potomac...=item3d7769db01:g:ND0AAOSwQqZbyPQO:rk:34:pf:0
I'm sure there is some stuff here I need. But if I sell my Elgar I should be able to get one of the newer more high tech signal generators like the HP 8657A. I don't know why I need the HP but I think it has to do with matching the dbm and modulation, plus it seconds as a signal generator and frequency counter.
 
Last edited:
Kingbleu said:
3. This model pioneer does not have a signal strength meter.
Click to expand...

DOH! You would think that I might have known that, since I have five of these, plus a 790 version.

Kingbleu said:
4. Yes I have a dipole antenna connected to the 300 ohm antenna input. (Where is the ohm symbol on my keyboard?)
Click to expand...

{ALT-234}

The antennas you have connected should be sufficient.

The FM tracking can probably be corrected using this method:

http://audiokarma.org/forums/index....-new-project-sx780.498294/page-3#post-6610345
 
Once the local oscillator is set an the tuning indication is correct, you can check linearity. If the stations you can receive are in the proper indicated position, chances are that the linearity of the tune is acceptable. That would be very good, since the front end inductors in the 780 are non-adjustable, and any linearity adjustment is made by bending the VC plates. It is cumbersome, and frustrating, but can be done. Still, it is best if it doesn't have to be done.

Familiarize yourself with the FM alignment procedures in the service manual. Locate all the trimmer caps and inductors, as well as the few pots involved. Make a detailed photo documentation of their original positions.

My guess is that with only the broadcast stations available and a good ear, the FM can be improved. Instead of using a signal generator for the 90 MHz and 106MHz operations, use a broadcast station at frequencies near the upper and lower ends of the scale. This sill not be perfect, but can improve reception. Remember, document the position of EVERYTHING before adjusting anything, count the turns and/or degrees of movement of any adjustment, and return the adjustment to the original position if no improvement can be realized.

As for the stereo, if will be best to connect a frequency counter or scope to Pin 23 and adjust VR1 to achieve 76KHz. The theory is described here if you attempt this without the counter or scope:

http://audiokarma.org/forums/index.php?threads/pioneer-fm-tuner-alignment.712855/#post-9583756
 
I have everything on hold right now while I build a workbench. Time to get the electronics off the kitchen table and in the garage. I am very happy with what I have accomplished so far. Thanks for all your help. When I come back I will start a new post on Aligning the stereo. After that I am going to start a restoration on my other SX-780. I purchased a couple other units and some other test gear but I still cant find something that will produce a FM multiplex signal (at least not anything I can afford). Maybe I will pay the $25 and look at barter town. Thanks everyone.
 
You must log in or register to reply here.
Back
Top Bottom