SX-1280, weak left channel, some troubleshooting done

[hertzdonut]

I have recapped my 1280 for longevity purposes (there weren't any issues with it). I left the tuner board alone for another time, but replaced appropriate caps and transistors on everything else.

I blew a fuse upon startup (note to self: always always always use the DBT) and thanks to some help from moewillis, corrected the wiring on the big caps. (That "grounding strap" has a negative AND a positive going to it, not just negative. Hence, the blown 12A fuse.)

All is well and good again, except for a weak left channel, I'd say about 15-20% in strength compared to the right.

I have done some troubleshooting, but being of limited experience, don't know where to proceed further.

I have determined that:

- it is only the left channel, regardless of input (phono, tape, FM, etc) or regardless of speaker selection. Swapping speaker connections (so that left channel is connected to right speaker) sends the problem along with it. (meaning, weak sound now coming out of right speaker). Tested all inputs on all three speaker connections, A, B, C.

- Turning the volume knob will increase the volume in the left channel, so it does respond, but at its lower level. There are no sudden jumps in volume (or decreases, for that matter)

- All switches and pots Deoxited, sound is clean regardless of which switch I throw or button I tap. There are no crackles, no cutting in or out.

- I took a close look at the boards I worked on, couldn't find a crack or cold solder joint.

- Power meters appear to be responding as you expect them to with a weak left channel (left channel meter is much lower than right)

- I have checked voltages on the Power Supply (AWR-157), and I think they are good:

Pins
8: +75.4v (spec is +68v)
11: +33.6v (spec is +32v)
12: +33.6v (spec is +32v)
13: +33.7v (spec is +32v)
16: +13.2v (spec is +13v)
17: +13.2v (spec is +13v)
18: +13.2v (spec is +13v)
20: -33.1v (spec is -32v)
21: -33.1v (spec is -32v)
22: -33.1v (spec is -32v)
25: -71.8v (spec is -68v)

Also checked the Protection Assy (AWM-123), seem to be good there, too:

Pins
3: +83.3v (spec is +76v)
4: -83.2v (spec is -76v)
9: -82v (spec is -76v)
10: +81.8v (spec is +76v)
17: +13.2v (spec is +13v)
18: +12.7v (spec is +13v)

I also checked the Power Amp Assy (AWH-074) (left and right)

Pins
10: -85v (left), -82.7v (right) (spec is -76v))
16: +84.4v (left), +83.2v (right) (spec is +76v)

Lastly, I made slight adjustments to the trimmers. Here's what they are now, after adjusting (they weren't off by much to begin with):
DC balance
VR1:0v (left), 0v (right) (spec is 0)

Idle current
VR2: 15.5mV (left), 15.5mV (right) (spec is 15mV, or at least in 10-25mV range)
Note: checked VR2 at pins +17 to -11, and also at pins +18 to -12.

I'm not sure where or what to check next. Or what other boards or pins I should check. And I'm lousy at reading schematics. I'm still hoping that a measurement somewhere will reveal the root of the problem. I appreciate any help I can get!

Thanks,

- Jeff

 

[moewillis]

im not in a place to look through a manual right now but i would also get a set of rca cables and pull your pre out/main in jumpers and flip your channels with the cable (L to R and R to L) to see if its in your pre or in the amp. did you clean all the controls well?

 

[markthefixer]

good moe.

IF moe's left / right swap test at the preamp out and amp in MOVES the weak channel, it's NOT the power amp.

then turn on (up) and off (down) the tone control lever switch between bass and treble .
if roughly the same on and off they are blameless.

turn on and off both the low filter and the high filter push button switches. there are 4 combinations or on and off on high and low filters.
if roughly the same on and off they are blameless.

That leaves the flat amp awg-060

Since this is a new occurrence, your work bears a fresh eye looking for wrong polarity cap installation -
the cap's negative hole has a line near or connecting to it, since caps usually indicate the NEGATIVE terminal with a stripe
as well as a thoroughly jaundiced checking of transistor lead connections. On whichever board is revealed to be the strong suspect.
That (transistor orientation) is my expectation - there are 5 ways to get it wrong and only one way to get it right. And some signal may get through if things haven't gone up in smoke.
When you find a transistor that went in wrong, CHECK it CAREFULLY for damage, if you have replacements, just use them.

 

[hertzdonut]

Moe, thanks, I will give that a shot! Regarding the controls, I believe I cleaned them well. I didn't attempt to disassemble them, but once I got the boards exposed, hit the switches or pots with D5 (red can), clicked or switched each literally 100 times, then gold deoxit or faderlube as necessary, and repeated the 100. Hopefully, that was enough. Took a long time...

Mark, thanks for the next steps, and for the advice on rechecking. I'm almost OCD when it comes to placing a component, but I realize mistakes can, and will, happen. I will recheck.

One question: You mention that "if roughly the same..." I wasn't quite sure what you meant. Roughly the same as what?

 

[moewillis]

Moe, thanks, I will give that a shot! Regarding the controls, I believe I cleaned them well. I didn't attempt to disassemble them, but once I got the boards exposed, hit the switches or pots with D5 (red can), clicked or switched each literally 100 times, then gold deoxit or faderlube as necessary, and repeated the 100. Hopefully, that was enough. Took a long time...

Mark, thanks for the next steps, and for the advice on rechecking. I'm almost OCD when it comes to placing a component, but I realize mistakes can, and will, happen. I will recheck.

One question: You mention that "if roughly the same..." I wasn't quite sure what you meant. Roughly the same as what?

If the bad channel is roughly the same with the tone controls in or out (on or off) then the tone amp isn't the culprit.

Some of those switches can be a real pain to get at as the openings are on the bottoms next to the board.

Was this thing doing this before the recap? Did you ever listen to it before you started?

 

[markthefixer]

roughly the same in volume. There will be slight differences.

 

[hertzdonut]

Ok, we can eliminate the power amp. I pulled the jumpers, cross-fed the rca's as suggested, and now the weak signal has moved to the right.

I left the rca's in place, and flipped the tone control on and off, pausing and listening a few times. The weak channel volume was unaffected. I then left the tone switch in the off position.

Moving on to the filters, I tapped the high filter and low filter in the different combos, again, no difference in volume.

Time for me to scrutinize the flat amp, I guess.

Moe, to answer your question, this receiver wasn't doing it before the recap. I was playing it for a couple of days, checking things out generally before I decided to set it aside to do the recap.

Thanks,

Jeff

 

[hertzdonut]

I did a component check for everything I replaced on the flat amp, as far as being correctly oriented is concerned. (Only 8 caps and 2 transistors)

All caps checked out correctly installed (C5, C6, C9, C10, C15, C16, C17 and C18), and so are transistors Q3 and Q4. (I believe the list says to leave the 117's alone, that is, Q1 and Q2. So left alone they were.)

Here's a pic of one of the 992's, Q3, I installed. A 992 is ECB, and that orientation looks good to me.

Tomorrow, with fresh eyes, I will scrutinize the foil side. Maybe I'll discover something then.

 

[markthefixer]

That a992 looks good to me, too.

do a straight down pix of the flat amp section of the board and post it.
then from the other side, same thing showing the foils and joints.

I'll double check your cap polarity assumptions, and against the schematic (i.e. error in silk screen printing on the pcb?)

 

[hertzdonut]

OK, Mark, you got it! I believe you can click into these images to make 'em bigger. (They are both about 4000 pixels wide, actual size. Twice as wide as most screens. If you can't access the large images for whatever reason, let me know.)

 

[hertzdonut]

Hm. I clicked into them, but only went as wide as my browser window. That ain't it. :/

Let's try these guys instead, from a different host other than photobucket. Two clicks, and you should be seeing a true full size image.

(Edit: Just tried it. Now they can be viewed full size. Two clicks.)

 

[JHS2RT]

My eye spots a questionable solder job on one of the connections ----but lets wait for Mark

 

[zebulon1]

Several need a good check and reflow, four that secure the two large caps at pins 23 and 24.
And a few others.
They don't look good at all.
Those bigger caps have a lower gauge leads, need more heat, more time on the leg lightly touching the solder pad.

 

[hertzdonut]

Thanks for taking a look, guys, I appreciate it. I didn't realize they looked that bad.

I will fire up my 808 and desolder my work, and then attempt to do a better job at putting them back in.

 

[markthefixer]

Sounds like a good path to take, use solder with flux and be SURE that you get melting heat on BOTH the pad and the component lead

You see, the component has sat around and oxidized in a parts bin, as well as having various unknown coatings on the leads, like adhesive from machine tape feed mounted components. When manufacturing does a wave solder of through hole components, they use a LOT of flux to clean the surfaces as it hits the solder wave. Usually IIRC the entire back of the board is sprayed.

I routinely have to watch for and reflux doughnut solder joints on replacement components. Looks like you have quite a few.

I started a list, then read your desolder resolution, and stopped, so I expect you to redo ALL 16 capacitor joints and all 6 transistor joints.

Your capacitor and transistor component placements look faultless in the pictures. Thus it HAS to be the solder joints.

Just as an aside, when I remove the soldering iron from a joint I am soldering, I don't pull it from the bead, but rather draw the iron and any excess solder in an overfilled bead UP the component lead. IF it doesn't "wet" with the solder on the way up, you will see the difference and KNOW the lead was too dirty to take solder.

 

[hertzdonut]

Mark, thanks for your thoughtful reply. And I like your tip about drawing the iron up the lead, that will help also. I will redo all 16 cap joints and all 6 transistor joints.

By the way, there is one other bit of info regarding this board. Maybe it's meaningless, but when I originally pulled C17 and C18, I was surprised when I saw that they not 220uf/35v, but were 100uf/35v. A couple of recap lists I had indicated the originals were supposed to be 220uf, and my schematic showed that as well. To the best of my knowledge, I don't believe this unit has ever had work done, and it was working fine before the recap. Regardless, that's what came out of there. However, the replacements I put in were 220uf/50v as recommended.

Like I said, maybe means nothing.

 

[hertzdonut]

Looking at the solder points from all angles using two pair of 3 power reading glasses (yeah, I probably looked like something out of a science fiction movie) I think there were 3 or 4 that indicated the pad wasn't heated enough. Doesn't matter, I redid all 22. Thank goodness it wasn't a hundred and 22!

Fired it up, gradually increased the volume and what to my wondering eyes and ears appear?
<

I checked out FM, Aux, turntable and reel to reel. All awesome.

You guys nailed it. And I am extremely grateful. I wish I was close by to shake your hands and buy you a beer. I'm going to put this baby back together with a big sigh of relief. And...
THANKS.

- Jeff

ps For anyone that's curious, that's a band called Dorothy. Video shot on my iPhone, so that's how the room audio was picked up. Bare bones.

 

[moewillis]

That's great man! Glad to hear you got it sorted! Bet it feels good to crank some tunes for real now.

 

[markthefixer]

You wouldn't believe how well my bench is lit, nor how I tilt the joints to inspect them from many angles. AFTER I clean them of all flux etc...

It's probably as bright as the lights they use in the ER and OR.

 

[hertzdonut]

That's great man! Glad to hear you got it sorted! Bet it feels good to crank some tunes for real now.

Thanks, moe, I wouldn't have been able to get where I did without your help and your thread on the 1280!

You wouldn't believe how well my bench is lit, nor how I tilt the joints to inspect them from many angles. AFTER I clean them of all flux etc...

It's probably as bright as the lights they use in the ER and OR.

Lol, I believe it Mark! People know when you're at your bench when half the state goes dim.

And I am going to be oh-so-careful when it comes to inspecting those solder joints in the future.

I am so looking forward to some tunes tomorrow...