SX-780 Rehab

Hey all,

Just a quick sanity check before I jump into my project.

I am planning on a full recap and cleaning. This unit is in near perfect cosmetic condition. Barely even dust inside.

So, here is what I have right now:

Powers up, relay clicks, tuner pulls in some distant staions (although a little off on the tuning scale. For example, 98.5 tunes in at almost exactly 99.0)

Very low output. When the volume is all the way down, there is still audio in the right channel.

When at full volume, it is what I would expect at 10%...

I took some voltage readings. The 40 and -40 volt is fine (39.95).

Q17 reads fine. However, Q18 is odd. 24v on the base, 25.4V on the collector, and 25.4V on the emitter. The service manual states that the readings should mirror Q17, but negative.

Both TA7136's pin one show 25.4 instead of 10.4.

That's a couple of the readings that I have taken so far that have come up as odd.

Am I looking at a bunch of cooked parts, or am I missing something?

It has been many years since component level electronics in the Navy. I want to make sure I am not going to start recapping an un-fixable project.

Anyone want to chime in?

Thanks!

Mountain,

The 780 is a very easy unit to service. If it has been many years since you've performed component level troubleshooting, you're in luck. The 780 is "many years" old in its design, so it should be like straddling an old bike. I expect you'll do just fine with just a meter.

First, if you don't have the original printed schematic, you might find that most of the service manuals available for download have poor copies. It may be helpful to grab this cleaned copy:

http://www.wwdsltd.com/files/SX-780Schematic-Wide.jpg

The voltages at the regulator transistors should be mirrored as you describe. What is the condition of R306? What about the voltage at Q20?

As for the high voltage at the OP amps, it appears that they are powered from the emitter of Q17, but your measurement seems to indicate that the voltage there is correct, at +21V. If they are damaged, replacements are still available, but I'd verify that all the supply voltages are correct first.

Re: q18, check R306 430 ohms 1/4 watt ( yellow orange brown gold ) - it is a flameproof resistor (which means they varnish coated it to suppress spitting embers and just have it smoke - meaning they anticipated possible problems with that circuit position) and could have been damaged - resulting in an increased resistance or just plain open circuited.

Pull q18 and do the 6 way diode test on it...

Nice schematic! And it's fixable!

Thanks! I'll work on it this evening. Had a few million leaves to get out to the edge of the road by this morning, so I didn't make any progress yesterday!

The 780 was one of the most popular receivers PIONEER made in the late 70's. Fortunately it's one of the easier to fix. There are known problems that are well documented and permanently repairable. The packs haven't been made by Sanyo in a couple decades now so everything is a facsimile. Some are good and some are just absolutely Horrible.

Bone up on the NEET's course real quick http://www.audiokarma.org/forums/showthread.php?t=163636
it'll all come back fast.

Then start asking questions.

There will be a TEST at the end of the study period today.

what is a TL-29. NOTE: Do not use one on the 780.

Larry

The TL-29 is the electrician's pocket knife. Maybe I should have clarified 'many years'?

I was and EM and IC in the Navy between 96-99. While that may seem like many years ago, it wasn't long enough to be issued a TL-29. At least, I never got one.

I love this place! Things are starting to come rushing back. I was surprised that I had narrowed it down as far as I did before my first post. Had I felt any level of confidence, I would have kept going. The full sized schem helped immensely. The scans from the repair manuals seemed lacking. Thanks so much for the link!

I did find that both Q17 and Q18 checked fine out of the circuit. R306 was open.

I am ordering some 430 ohm resistors today!

The thing that had me questioning myself, and led to my first post, was that the bottom of R307 was reading -39.9V but the R307 and R308 junction was reading +25. I was trying to convince myself that R307 was the culprit.

Thanks so much for everyone's help! I will let you know if anything else is dead down line.

LOL! Good answer. I was an HM in 73-77. Had to buy mine. Great for cutting off clothing in a trauma situation, as long as the blade was sharp. We had a MM who could hone the edge to razor sharpness. Charged a quarter to sharpen it. IIRC they stopped issueing them in the early 80's. My Brother was an Aviation ET, in the 80's and had to buy his. I've still got mine, somewhere in my stuff.

Good Luck with the 780. Do all the upgrades (mainly heatsinks on the regulators, resolder the pads, finding those sky blue Sanyo caps (2.2uf and smaller) and replacing them) and a couple others, and it'll last a long time .

The link to the NEETS is greatly appreciated! All modules downloaded to my iPad.

I see some late night reading in my future.

I wish that I had all the documentation from my 400Hz power distribution 'C' school. That was fun!

Ok, so I did replace the 430 ohm resistors at R305 and R306. Boy, she is lively now!

I ran her up and checked the sources. Seems that I am missing the left channel on the phono input. I switched the RCAs coming from the turntable and the problem stayed on the left. Mono works fine. I will look at the phono pre circuit next.

I am going to work on the recap after that.

I did notice that it doesn't sound as warm as my SX-980, so I am hoping that after the recap it rounds out the sound. I also need to recap the crossovers in the Nova 7b's.

You were correct! It is like riding a bike... I forgot how much satisfaction I got from locating and solving an issue. I will have a great time with this!

As a side note, I found a great electronics supply house about 15 minutes away. Anyone from NE Ohio, feel free to PM me and I will let you know where. I have no affiliation, other than being a customer. It will be my goto place. As cost effective as Mouser or others online? Probably not. Right down the street and my 430 ohm flameproof resistors were hanging on the wall? Absolutely.

So, thanks again! I will let you all know if I run into a wall. You have helped get me back into a very satisfying hobby/lifestyle! For that, I will always be grateful.

If the AUX inputs are working properly, there isn't much left but the ICs (Q1/2) in the phono section and the connections/switches. Since the RCA inputs were swapped, that means the output of the table is probably O.K. but still leaves a question about the jacks in the rear panel and the wiring from them. If you dare to briefly jumper Pins 68 and 70 on the main board with a signal from the table, that might reveal something. If the channel "wakes up" you might only have a connection problem.

FWIW, if it comes to it, the OP amps are still available at littlediode.com, allcomponents.net, and probably other sources.