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SX-780 worked great, now has no stereo FM?

M

MrTube

Member
Fired up the ol SX-780 on Friday and the tuner worked great. Was able to pull most stations in stereo using my finger as an antenna. Receiver had been in storage for around 7 years now.

Ran the receiver for 9 hours with almost no problems other then a bad connection on the FM de-emph switch which I fixed early on.

After 8 or 9 hours the FM STEREO light went out and the tuner plays in mono. I have tried everything I can think of from cleaning switches, to pushing on the tuner board and I even tweaked the VR1 pot in the MPX section, the tuner will not go into stereo.

I have not recapped this receiver yet, I am just concerned that it worked AMAZING in the morning and then, no stereo. Tuner still seems super sensitive.

I also let the receiver cool down completely, no change.

Any ideas? Is this a common problem on these receivers?

Thanks,
Chris J
 
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You didn't say whether you replaced your finger with an antenna. Is the 780 in the basement or above ground level? The 780 doesn't have a tuner that is strong enough to run correctly without an antenna on it unless you are standing within 500 ft of the tower. Also with the Muting/Mono switch in the DOWN or MONO position the Stereo light won't light either. Make sure it is in the UP Position.

Get the Service manual in the AK Database. It has the schematics and board layouts.

Check the voltages on the PA-1001-A. List all the voltages. Pin 6 of the PA-1001A is the Lamp pin. Between Pin 6 of the PA-1001-A and Pin 16 on the tuner board edge is R38. Check the resistor for continuity. List the reading taken.

Do it TWICE. Once for MONO (Mute DOWN) and again for Stereo (Mute UP).



The Indicator light is an led. Check it with your meter set to DIODE. Check the voltage on the Main board 6 pin ribbon connector next to the Stereo/mono switch. Check pin 6 (end closest to board edge) It should be around 6.5V. There is a Zener Diode (D-20) Next to the connector. Check it with the Meter set to DIODE, then turn on the unit and check the voltage on both sides of it. List them also in your reply.



Mark should correct me if I'm wrong, but the way I understand the schematic, when it mono, you should get 6.2-6.5V from pin 5-6 on the ribbon cable to feed the LED's. The PA-1001-A is the switch. As there is no mechanical switch there, it's electrical. So Pin 6 of the PA-1001-A is either putting out 6.5 V(MONO) or 0(stereo) volts. In Mono with both sides putting out 6.5 volts there is a 0 volt potential and no flow, hence no light. When the unit goes to Stereo, the PA-1001-A, turns off Pin 6, shunting to ground, allowing a Voltage potential across the LED lighting it.

That's my understanding of it. I'm still learning, and may be wrong. But check the voltages.

"TWEAKING" the VR1 didn't help the light at all. All it did was throw out the whole Multiplex circuit. It now needs to be looked at and aligned by a tech with the proper tools and scopes.

It's possible that the LED just crapped out or you knocked a wire loose while de-Oxiting it. Check for that possibility too.

Larry
 
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Larry,

I will be aligning the tuner after recapping it at a later date. Tweaking VR1 did no harm in the long run. I decided to tweak VR1 as my assumption was the MPX circuit had drifted from heat. I was of course wrong.

I replaced my finger with an Antenna and the tuner had great reception and was in stereo most of the day. The LED is good as it blinks a few times in between stations but I cannot get the tuner to go into stereo mode on any stations. The tuner is truley playing in mono, I am not simply going by the LED.

I will check the things you listed, your help is greatly appreciated.

Chris J.
 
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Probably the mpx IC PA1001, but could be cap C36. 510pF (probably Styrene type (clear plastic body, metallic appearance)/ Also make sure VR1 is okay. Good luck with it. Steve
 
sregor said:
Probably the mpx IC PA1001, but could be cap C36. 510pF (probably Styrene type (clear plastic body, metallic appearance)/ Also make sure VR1 is okay. Good luck with it. Steve
Click to expand...


I'll have a look at C36. Pot tests good.

I assume PA1001 is a Pioneer chip and impossible to find?
 
On another note,

Does anyone have a list of caps for the SX 680 and 780? I want to recap my two receivers and if someone has a list already made up it would speed things up.
 
wow, how quickly they are spoiled.... :sigh:

what once was an extraordinary luxury, is now taken for granted....

the 780 list exists, after seeing it, you could write the initial 680 list, to which we could add the replacement parts numbers. that list is a multiple component location per line list, the single part per line format adjusts more easily.

it (780) is a word document from February of 2009 - that gives you a target search area. some of the later lists are of the one component per line format, as an example.
 
markthefixer said:
wow, how quickly they are spoiled.... :sigh:

what once was an extraordinary luxury, is now taken for granted....

the 780 list exists, after seeing it, you could write the initial 680 list, to which we could add the replacement parts numbers. that list is a multiple component location per line list, the single part per line format adjusts more easily.

it (780) is a word document from February of 2009 - that gives you a target search area. some of the later lists are of the one component per line format, as an example.
Click to expand...

markthefixer, you are incorrect.

I have recapped literally hundreds of both vacuum tube and solid state amplifiers and receivers and never had a parts list. I noticed some users were passing them around for other Pioneers so I figured it wouldn't hurt to ask. I have yet to have this, luxury.


I am anything but spoiled.

Chris J.
 
MrTube said:
markthefixer, you are incorrect.

I have recapped literally hundreds of both vacuum tube and solid state amplifiers and receivers and never had a parts list. I noticed some users were passing them around for other Pioneers so I figured it wouldn't hurt to ask. I have yet to have this, luxury.


I am anything but spoiled.

Chris J.
Click to expand...


Okey dokey, my tunnel vision is limited to this forum, and what I thought happened, happens a lot. I kind of rue the day :D I started publishing parts lists years ago. Out of stock and obsoleted replacement parts are the bane of dealing with old lists.

some of the people asking also need to learn how to solder well, and discern polarity. But , like I say, there's no such thing as a dumb question, especially when the answer averts damage.

and at least with the pioneers that this forum routinely deals with, the manual's parts lists really are needed to wring the best performance out of them, as in some cases low leakage electrolytic caps are needed to keep the noise floors down.

that said, once the list exists, it's there "forever" for anyone to use.

back on subject: pa1001 mpx decoder

3 caps in the PLL loop, they used solid aluionum caps - little sky blue sanyos that get leaky enough to be called resistors, but haven't seen many go bad in the mpx circuit.
c37 6.8uf / 6v csza lpf 647-UKL1H6R8KDDANA
c38 1uf 50v ceanl (low leakage) lpf 647-UKL1H010KDDANA
c40 1uf 50v ceanl (low leakage) pilot 647-UKL1H010KDDANA

vr1 is vco freq (free run 76khz)
vr2 is pilot canceler (19?38?khz null adjust)
 
Hi Mark- maybe you're only half-serious, but please don't have any compunctions about posting parts lists. They made the job of very successfully recapping (and more) three receivers ( 1080, 1050 and 880 ) much less onerous for this Pioneer restoration newbie.
Anyone who just jumps in without doing their homework and doesn't learn that these lists need updating deserves whatever misfortune befalls them. Just my 5 cents (which, oddly enough, is what Lucy charges for psychiatric help...)
-Mark
 
Señor Tube,

As long as you are in there with a hot iron, it might be worth considering an enhancement of the voltage regulator heat sinks, or at least the 35V unit. This transistor is overworked in the SX-780 and runs quite warm. I've measured 154°F at idle. That's well below the common 125°C rating of many of the old devices, but it's the hottest item in the unit. I've done two so far with complete success and a reduction of 40°F at least.

I used a donor heat sink recovered from an old switching power supply, drilled/tapped/screwed to the back of the OEM Pioneer heat sink:

RegulatorHeatSinkFront.jpg



RegulatorHeatSinkRear.jpg



Once installed there has been plenty of room. The first one I did I left in the original design with the heat sink electrically live. On the second one, I took Mark the F-er's advice and installed a trimmed mica washer to insulate the collector tab from the heat sink. It still operates at only 111°F after 24 hours of run time instead of 150°F plus.

RegulatorHeatSinkInstalled.jpg


Incidentally, the heat sink isn't anywhere near the wire-wrap terminal adjacent to it. It looks close in the photo, but it's just an optical delusion.

Recap, check offset, check idle, align, and enjoy your ♫.
 
PioneerMike said:
http://www.audiokarma.org/forums/showthread.php?t=278522
http://audiokarma.org/forums/showthread.php?t=213265

That didn't take long too find. Thanks MTF, I'll add it to my list anyway.

Mike
Click to expand...

Mike,

As it happens, I am getting ready to re-cap my 780, and came across both these lists independently - that is, before I stumbled upon this thread.

I printed both out, and did a stare and compare. Be aware that there are some differences between the lists, so you might be best served to compare them with the actual unit that you have.

Watthour or MTF,

Can you expound on the heat-sink modification? Should I do just the rear one, or all three? Should I replace all three transistors as a PM since I'll be in there anyway? I'm a little fuzzy on the concept of an electrically "live" heat sink.....
 
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It means the heat sink has voltage on it, so it can be short circuited and in these cases, the "live" means directly connected to a poweer storage capacitor - this is where chunks disappear from errant screwdrivers.

the metal tab on the transistor is connected to the collector, actually it IS the collector, the transistor is mounted to it inside the epoxy.

it is possible to mount the heat sink to the transistor using a mounting kit of a mica insulator (conducts heat, not electrons) and a shoulder washer to keep the screw head or shaft from touching the tab face or the inside of the hole in the tab.

Replace all three transistors as a PM, yes...

re: larger heat sinks, I would have to calculate the heat load of each transistor (volts collector to emiter times current collector to emitter for wattage) and I don't have the time to look that up. A quick way would be to run it and see how hot they get (IR thermometer) and the "OUCH" hot ones 120 degree F plus get more heat sink. Discolored board is also a good indicator.
 
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markthefixer said:
It means the heat sink has voltage on it, so it can be short circuited and in these cases, the "live" means directly connected to a poweer storage capacitor - this is where chunks disappear from errant screwdrivers.

the metal tab on the transistor is connected to the collector, actually it IS the collector, the transistor is mounted to it inside the epoxy.

it is possible to mount the heat sink to the transistor using a mounting kit of a mica insulator (conducts heat, not electrons) and a shoulder washer to keep the screw head or shaft from touching the tab face or the inside of the hole in the tab.

Replace all three transistors as a PM, yes...

re: larger heat sinks, I would have to calculate the heat load of each transistor (volts collector to emiter times current collector to emitter for wattage) and I don't have the time to look that up. A quick way would be to run it and see how hot they get (IR thermometer) and the "OUCH" hot ones 120 degree F plus get more heat sink. Discolored board is also a good indicator.
Click to expand...

Ok - off to find a mounting kit to accomplish this.

Thanks for the elucidation!

Search turns up these replacement transistors for the three originals - can you confirm? I actually have these on-hand, left over from the 838 repair/rebuild - you advised me to order extras, and I did :) :

Q25 2sb712 - replace with 512-ksc2073tu
Q19 2sd325 - replace with 512-ksc2073tu
Q20 2sb536 - replace with 512-ksa940tu
 
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yup

edit -- NOPE!! How the *(*&^&%^& did THAT get past me?????

Probably edited it in as I was digging up the mouser numbers... timing is right...

Q25 2sD712 - replace with 512-ksc2073tu
Q19 2sd325 - replace with 512-ksc2073tu
Q20 2sb536 - replace with 512-ksa940tu

..... 532-7721-7PPS to-220 mounting shoulder washers, 4-40 $0.29 ea
..... 534-4672 to-220 mica insulators $0.12 each...........
..... 532-4880SG @ $0.98 with sil-pad insulator, screw, washer, lock washer, insulating shoulder washer & nut.
..... 532-4880G @ $0.77 with thermal film insulator, screw, washer, lock washer, insulating shoulder washer & nut.
..... 532-4880MG @ $1.15 with mica insulator, screw, washer, lock washer, insulating shoulder washer & nut.
 
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markthefixer said:
yup

..... 532-7721-7PPS to-220 mounting shoulder washers, 4-40 $0.29 ea
..... 534-4672 to-220 mica insulators $0.12 each...........
..... 532-4880SG @ $0.98 with sil-pad insulator, screw, washer, lock washer, insulating shoulder washer & nut.
..... 532-4880G @ $0.77 with thermal film insulator, screw, washer, lock washer, insulating shoulder washer & nut.
..... 532-4880MG @ $1.15 with mica insulator, screw, washer, lock washer, insulating shoulder washer & nut.
Click to expand...

wow - thanks again!

The 532-4880SG is the only kit Mouser has in stock, so that's what I'll order!
 
Uncle Bambi said:
wow - thanks again!

The 532-4880SG is the only kit Mouser has in stock, so that's what I'll order!
Click to expand...

THAT'S why I listed the mica as a separate.

mica is more forgiving of overtightening, and the sil pads might be junk, i grabbed a text block....

get the (extra) mica and dump the pad, it's cheap insurance.
 
markthefixer said:
THAT'S why I listed the mica as a separate.

mica is more forgiving of overtightening, and the sil pads might be junk, i grabbed a text block....

get the (extra) mica and dump the pad, it's cheap insurance.
Click to expand...

Already on the BOM - thanks again!

Will I need a tube if thermal grease, also?
 
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