SX-980 for my dad

[Motnick]

When I was a youngster growing up in the 80’s my dad had an SX-980 that he would let me listen to with him from time to time. In fact it was listening to that 980 with him that served as my introduction to much of the music which I still enjoy today! Unfortunately for my dad he sold off his 980 years ago after replacing it with some 1990’s surround sound equipment.

A couple of years ago nostalgia for those days led me to discover vintage audio. Since then I have accumulated several vintage pieces which I have repaired and restored with the help of this forum. SX-737, SX-1010, SX-950, 9090db, 2240, STR-6065, etc.

A couple of weeks back I was cruising craigslist and found an SX-980 for a very fair price which looked to be in very nice shape. With my dads 65th birthday approaching I made contact with the seller and drove out after work to take a closer look. When I arrived I was happy to find the unit to be in very nice shape and everything seemed to be working nicely. The left channel would drop out when I flipped the tone control switch, but I figured that a good cleaning would remedy that easy enough.

When I got the unit home I opened the top and discovered that everything was still original. No one had ever had the top off before I suspect. I cleaned all of the switches and checked the bias and offset. Bias was a bit high in the right channel, and a bit low in the left. I was able to adjust it per the manual but the readings seemed to jump around some while adjusting the trimmers. I cleaned the trimmers with deoxit but they still seem a bit flaky. I plan to replace them with some bourns and I was thinking of also replacing the EC’s on the amp boards while I’m in there. While I have the heat sink out I plan to repaste the output transistors.

I noticed that the preamp board has a couple of 2SA726 transistors that should probabaly be replaced? I can see a blue paint mark on both of them so I guess they should be replaced with a matched pair? Anything else that I should put on the must replace list in this unit?

His birthday is coming up on the 25th and I hope that I can surprise him with this SX-980 then. Last month I cleaned up my father-in-laws old Sansui 661 and returned it to him on his birthday. It turned out to be a really great birthday gift, and I think my dad will enjoy this 980 just as much!

 

[Motnick]

Here is what I was thinking for the amp boards:


Power Amplifier Assembly (AWH-073)

C1: 2.2uf/50v, RFS-50V2R2MF3#5 2.2uf/50v
C2: 2.2uf/50v, RFS-50V2R2MF3#5 2.2uf/50v
C11: .22uf/25v, MKS2D032201C00MSSD .22/63v
VR1: 100ohm, 3386H-1-101LF 100ohm single turn
VR2: 100Kohm, 3296P-1-104LF 100Kohm 25 turn

 

[kevzep]

I like to remove those lever switches and dismantle them for cleaning, the silver plated contacts get an accumulation of silver oxide (the black crap), and unless you actually remove it, the dropout issue will keep coming back to cause issues. The deoxit loosens that stuff and you can have it floating around in there, I clean the contacts with a soft fibreglass brush, you need to be careful not to go too hard and remove the sliver plating.
They are not terribly difficult to take apart and clean.

I'd replace the 726's, and yes, might as well match them if they were matched from the factory.

Definitely replace the trimmers.

Bonus cap replacement is a good idea too.

I would replace the lamps as well.

What a thoughtful gift, I sure it will go down well!!

 

[Motnick]

So far I haven’t dismantled any switches for cleaning in any of my units, but I have read about others doing it on this forum.

I went ahead and pulled the tone control switch from the 980 and dismantled it to have a look inside. It only took me a few minutes to get it off of the board and opened up on the bench.

This is what I found inside. I had already cleaned it once with D100 and once with D5. Filthy!

After a little searching on AK it seems like a pink eraser is the safest thing to start out cleaning with. After just a few passes all of the filth was gone. That worked really well. Makes me wish I had done this in my other restorations.

 

[kevzep]

So far I haven’t dismantled any switches for cleaning in any of my units, but I have read about others doing it on this forum.

I went ahead and pulled the tone control switch from the 980 and dismantled it to have a look inside. It only took me a few minutes to get it off of the board and opened up on the bench.

This is what I found inside. I had already cleaned it once with D100 and once with D5. Filthy!

Yep, there it is....
and you can soak a bit of printing paper in deoxit, then run those sliding contacts along to clean them.
then finish with deoxit SN 5 Shield for protection and lubrication, and its job done....
Be careful putting back together, I put the little sliding contacts on the terminal and then carefully slip it back together....
Whenever I go through a unit and deep clean all the switches like this, I can swear it sounds better, and if you think about it, why wouldn't it? You can see how much non-conductive silver oxide is all over those contacts, it starts acting like a dielectric....no doubt slowly turning into a capacitor as more oxide builds up....

 

[dcgibby]

Let us know if that switch cleaning fixed the dropouts. I've got the same problem on my 980 and it doesn't get any better. I've sprayed it a couple of times but never dismantled it. I had to take it out of service because the dropout became permanent.

 

[Motnick]

I finished up all of the toggle switches and decided to have a go at these tone control switches. They also come apart and were in similar shape inside. They cleaned up nicely as well.

 

[kevzep]

Nice job man!! thats the way to success for sure....

 

[rcs16]

Good job, on the switch cleaning, what a piece of mind, now if you can get the tuner var cap contacts to look that nice
I think the blue paint on the 2sa726 represents the hfe grade, so that is the extent of factory matching. You could match the ksa992f replacement very easy, by matching the Vbe, use the ohmmeter in diode test mode, match B_E junction to under <2mV. However it is not really necessary, since the a992f is also graded, more like process improvements to get the hfe within range every batch.

 

[Motnick]

I’ve got all of the switches back together now. Everything is working nicely. I’m going to hook up a TT and CD player just to make sure they are still working correctly. I think I’ll go after those two 2SA726 transistors next.

I also lost the phono indicator bulb in the shuffle. I’m going to go ahead and order new bulbs and replace them all while I’m at it.

 

[Motnick]

New problem. The protection kicks in way to easy.

If I increase the volume until the VU meters are getting up between 1 and 10 the protection circuit kicks in. If I adjust the balance all the way to the right this does not happen. If i have the balance centered or all the way to the left the protection kicks in early.

I used some interconnect cables to cross over the channels at the preamp/amp jumpers and the problem moved from the left channel to the right. If I move the balance pot to the right with the channels crossed over then the protection kicks in early, and if I move the balance to the left the protection does not kick in.

I think this suggests that something is wrong with the left amp board or with the left speaker? Maybe the protection circuit as well?

 

[Motnick]

I swapped the speakers left to right and the issue stayed with the left channel, so I guess it must be something up with the left amp board?

I sprayed all of the transistors on the amp board with freeze spray, but that did not cause the protection to kick in.

I also measure a bunch of voltages and they all seem good.

Power supply awr-154
16 - 13v (12.9v)
18 - 7.5v (7.0v)
20 - 33v (34.3v)
22 - 50v (51.2v)
23 - 23v (23.5v)
25 - -23v (-23.7v)
27 - -50v (-52.2v)
28 - -13v (-12.9v)

Protection awm-124
1 - 21vAC (20.4vAC)
2 - -50v (-52.4v)
10 - 65v (71.3v)

Amp awh-073
1 - -50v (-52.5v)(-52.4v)
4 - 23v (24.1v)(24.1v)
5 - -23v (-24.2v)(-24.3v)
8 - 50v (52v)(52v)
12 - 52v (53.4v)(53.4v)
22 - -52v (-53.3v)(-53.3v)

Any tips on what else to check? I was planning on replacing the trimmers and caps on the amp boards. I’m not sure if I should go ahead with that and check for bad solder joints while I’m at it as a next step?

 

[tsd71]

New problem. The protection kicks in way to easy.

If I increase the volume until the VU meters are getting up between 1 and 10 the protection circuit kicks in. If I adjust the balance all the way to the right this does not happen. If i have the balance centered or all the way to the left the protection kicks in early.

I used some interconnect cables to cross over the channels at the preamp/amp jumpers and the problem moved from the left channel to the right. If I move the balance pot to the right with the channels crossed over then the protection kicks in early, and if I move the balance to the left the protection does not kick in.

I think this suggests that something is wrong with the left amp board or with the left speaker? Maybe the protection circuit as well?

Voltages look spot on. If it jumps to the other channel with the crossed jumpers that could indicate something in the flat amp or tone amp at the front of the receiver, there are a few problematic transistors (2SA726) on the flat amp that could be causing the issue.

 

[Motnick]

I did some more testing last night by plugging in only one amp board input at a time.

If I plug the left or the right preamp output into the left amp input the unit will go into protection once I turn the volume up some.

If I plug the left or right preamp output into the right amp input the unit stays out of protection.

I was planning on replacing the 2SA726 in the preamp, but I wasn’t sure if I should do that just yet or not. I can do that next if you guys think that I should.

 

[Watthour]

It sounds like you've isolated the issue to the left channel power amp.

It would be reasonable to replace all the 2SA725 and 726 devices eventually, but may be better to focus on the power amp repair/adjustment first.

 

[Motnick]

I was always planning on replacing the trimmers and caps on both power amps. Should I do that next?

I did spray all of the transistors on the amp boards with freeze spray, but it had no effect that I could notice.

I’ve also cleaned the output transistor pins on both amp boards with a pencil eraser.

 

[Watthour]

Check/recheck the bias voltage. After about 20 minutes of operation at a level below what trips the protection circuit, check the idle current, and compare the channels.

 

[Motnick]

When I first turn on the unit I have ~22mv L / ~26mv R.

After 20 minutes of running ~34mv L / ~38mv R.

Bias readings are stable.

I was playing FM at a level that barely gets the VU meters moving for twenty minutes. Nothing seems warm at all after 20 minutes. Outputs and Driver’s are still cool to the touch.

 

[rcs16]

I did spray all of the transistors on the amp boards with freeze spray, but it had no effect that I could notice.

That stuff gets expense I could only detect bad ecaps using freeze spray.
iirc there is a 0.22uF ecap in the protection ckt (part of the over current bridge) that should be replaced with a equivalent value film cap using 5mm lead spacing.
It is C11 on AWH-073
this design is sensitive to DC offset since it has no ecap in the feedback loop (high DC gain) where as earlier designs like the 950 used one. you want to use a multi-turn offset pot if you change it
Newer amps use a DC servo, since opamps are cheap
One nice thing I see is the power amp pcb comes out easy since it is connectors, another thing, clean those connectors so they shine again.

 

[merlynski]

On the left channel power amp pcb (AWH-073) replace D6, D7, Q8 and C11 (see Post 19 above). If that does not resolve it lift one end and verify the resistances of R18, R19, R23, R24, R28, R29, R30 and R31. These are all parts of the overload detection bridge on the power amp pcb.