SX-1980 thermal fuse

Well, im in need of advice. My SX-1980 died awhile back, and I have recently traced it to the thermal fuse on the power supply board. I ordered the correct value thermal fuse, and replaced it. Powered up fine, put it back together, worked for 5 minutes, i cranked it up and it blew again. Obviously there is another root cause. Any advice would be greatly appreciated

Thanks all..

Yikes! I'd take that beast to the best tech I could find before trying that again. That's way too nice of a unit to be popping fuses repeatedly. I'll assume you know there are power modules in there that aren't made any more?

How is the first power supply relay doing..is it working? If it isnt working or has bad contacts..you will be drawing alot of power through the capacitor charging resistor heating that up..(I'm assuming thats where the thermal fuse is close to). The relay is supposed to alow the caps to charge for a split second upon power up then switch through directly to the power supply.

I will check the relays. Thanks

It appears (from the manual) that the thermal limiter is physically located off the edge of the PS board. If the input current to the transformer primary is excessive this will accelerate the opening of the fuse. These fuses typically have a curve (like any fuse) for opening, but these are affected more by the combination of temperature and current as opposed to primarily current as is the case with a more common fuse.

There appears to be a 20W wirewound resistor in the circuit which may be the greatest source of heat on the PS board. Find out of the resistor is dissipating more than 20W (approximately 167 mA @ 120VAC) to help determine if the load is a problem.

well i just blew my last thermal fuse putting it in. Guess ill have to order more before i can proceed. What could cause this wire wound resistor to run hotter than usual? It is in direct contact with the fuse.

Current through the resistor at a sustained 170mA or higher will cause overheating of the resistor.

I'm not saying that is your problem, but it would be good to eliminate that possibility. There are regulator transistors on that board as well, and they can run hot. Look around the physical location on the thermal fuse - What is nearby?

There are quite a few threads here at AK describing the common problems with SX-1980 power supply boards. You are possibly in for a rebuild of that board. Do some searching for threads like these:

http://www.audiokarma.org/forums/showthread.php?t=113381

http://www.audiokarma.org/forums/showthread.php?t=231916

I'm afraid that no one here has quite grasped the function of the circuit.

The 6 ohm 20W resistor is in the primary of the transformer, along with the thermal fuse. They are part of the soft-start circuit, along with the S1 relay. The resistor limits the inrush current on startup...about 1/4 of a second after the power switch is engaged, the S1 relay bypasses the 20W resistor and it is no longer in the circuit (on a properly working unit anyway). The thermal fuse is just that...it is supposed to be capable of 15+ amps, but internally has a spring loaded ball held in place with a wax that is designed to melt and break the connection at a specific temperature. There is no current rating of this fuse, per se. This thermal fuse is in series with the power resistor, and is supposed to be in physical contact with the resistor...its job is to prevent the destruction of the resistor if the resistor gets hot (which should never happen in normal operation). If for some reason the relay does not close, the full primary current is still being routed through this power resistor. At low power, the primary current 'should' be pretty low, and the resistor will stay cool and the unit should operate in a pretty normal fashion. At high power, OR if there's some other problem with the unit where a large primary current is attempting to flow, the resistor may get hot enough to melt the wax in the thermal fuse and prevent operation.

The replacement for the stock thermal fuse is an NTE8108, but if you have replaced the fuse and it opened up again at low power, then there's a real problem somewhere...if the unit seemed to be working, I'd suspect that perhaps the bias current has been turned up to a point where the primary current is high enough to cause the soft-start resistor to heat up. If the unit is not functional, then I'd suspect blown output transistors.

That resistor, 3.3 ohms at 20 watts is SHORTED OUT, ON PURPOSE, to remove it from the circuit, by one of the relays on the board, after the first few seconds of operation....
IF it is NOT removed - it will do exactly as designed - and you are seeing - blow the thermal fuse which is SUPPOSED to be blowing.

Short form, that big resistor should NEVER get hot, ESPECIALLY hot enough to pop thermal fuses.

Either that relay is NOT actuating, which means you have to find out why, or it IS actuating and the contacts are badly burned.

The relay and resistor are supposed to limit the input power that is charging the BIG capacitors - we don't care at turn on if it takes 20 milliseconds or 1200 milliseconds to charge - but the wiring in your wall does... so that 3.3 ohm resistor is in circuit for all of 2 seconds maximum, until the BIG caps are charged.

Reading this thread has been extremely painful. If you don't have the background to research or recognize the most basic of all the relay circuits you have no business fumbling about through the insides of this UNFORGIVING and expensive beast.

There is a MUCH more basic question to be answered, how much current is being drawn, and for how long during turnon. Just charging of the caps? OR are there SHORTED output transistors??

Dim bulb tester time. If you say "what's that" - get it to a competent tech that knows these units...

I sadly no longer have the large amounts of time needed to handhold the individual teaching of basic electronics time after time, repeatedly typing in pages upon pages of the same things over and over again. I tried to, and was able to for a few years, until life came up and bit me.
There is a HUGE amount of information out there, for those willing to read my old posts.

There is NOT a lot of sx-1980 troubleshooting information out there, consider that fact to be the writing on the wall.

edit: original post:

the thermal fuse on the power supply board. I ordered the correct value thermal fuse, and replaced it. Powered up fine, put it back together, worked for 5 minutes, i cranked it up and it blew again

Click to expand...

the +106 volts is tapped off of C204 (220uf/160v) through r102 3.3kohm 2 watts. then from pin 2 on the board it goes to one of the two power switch sections (s26B), and comes BACK on pin 1, to the relay right next to the resistor, thermal fuse assembly.
when the power switch is ON, and the 106v is pretty much charged up, then that relay will be pulled in.
this pulling in will close contacts that short across the resistor. VOILA - cool resistor. the resistor is out of circuit 99.9 % of the time. The relay is energized and contacts closed 99.9 % of the time.

S26A switches the AC power, so don't confuse the two sections.

wrong>>>the relay has no commercially available pinout equivalent. It is only used in the sx-1280 and the sx-1980.<<<wrong
edit - I was mistaken:
653-ly1-0-dc48 if found, otherwise 653-ly1-0-dc24

the sx-1980 pc board pattern ONLY supports a LY1 relay, and mouser no longer carries the 653-ly1-0-dc48, BUT a 653-ly1-0-dc24 can be adapted in quite nicely.
The sx-1280 pattern supports the ly2 relay which IS available from mouser in 48 volts. 653-LY2-0-DC48

You will have to open it and clean up the contacts. Check the resistor, and the power switch section.

Thanks for the tips. A rebuild would be ideal for sure, but thats probably a little out of my league. Guess i should prepare to spend some money and send it to a qualified Tech.
Not fixing it is not an option. had this unit over 20 years, and ive become quite attached.

Thanks

check the rest of my post, the guide to the relay's coil and contacts...

get another thermal fuse, replace it, and then figure out whether or not the relay is pulling in after the unit lights up again. without cranking it...

we'll get the relay squared away, then you can get on one of the expert's waiting lists, and yet still listen to it while waiting.

I've had a run of sx-1980's in.... the more the merrier....

markthefixer said:

That resistor, 3.3 ohms at 20 watts

Click to expand...

I kept looking at the scat, which says on all the versions I could find, that R101 is 6.2 ohms. Now, it has been a while since I worked on a 1980, but I have had to fool with the soft-start many times, and I never recalled that resistor being 6.2 ohms, despite what the manual says. 3.3 ohms sounds more like it.

OK I cleaned both relays. I powered the unit on both relays kicked on. one with power swith, the other 6 or 8 seconds later like usual. After playing the stereo for 10 or 15 minutes the ceramic resistor was still very cool to the touch. After testing this a few times the first relay no longer comes on with the power switch. other one still kicks on 6 or 8 seconds later and stereo plays fine, but ceramic resistor slowly started heating up. So the first relay not coming on is causing my problem. Now Why is it not coming on with the power switch like it should?

Time to see a VERY compentent tech...........

Yeah might be getting to that point. Just kind of hoping someone may have had this problem before, and can say ohh its a zener diode or check this capacitor. before i have to break out the big bucks.

clh69 said:

Yeah might be getting to that point. Just kind of hoping someone may have had this problem before, and can say ohh its a zener diode or check this capacitor. before i have to break out the big bucks.

Click to expand...

You don't say where you are from ---- List your location -- an AK tech might be close by and willing to help you out.

There are two sections to the main power switch...one that allows AC current to flow through the transformer primary, and a second section of the same switch that closes a circuit to allow current to flow through the winding of the soft-start relay. The relay itself gets power from the 100VDC rail, right before the regulator circuitry.

IF the 100V rail is OK, then three main possibilities, in order of likelyhood:
1. The section of the power switch that closes the circuit to allow the relay to function is damaged or dirty.
2. Bad solder joint on the relay itself, the 3.3K 2W resistor, or one of the wirewrap pins (or a busted wirewrap).
3. A bad relay (unlikely).

As EW mentioned, the relay powers through a resistor (R102 - 3.3KΩ, 2W) from the on-board +100VDC bus. If the 100V supply is present check out the few connections between the components. If you study the schematic you'll see the circuit goes off board and back via a jumper on Pins 1 & 2, as if they had planned another device in the circuit for whatever reason. The only thing remaining after that is the relay. If you have a voltmeter handy it should be quick work to find the problem, so long as it happens not to start while you are testing. Intermittents are no fun that way.

Ok i do have 100v at pins 1 and 2 when powered on. R102 checks good. The power switch checks good both sides. Dont really see any questionable solder joints, although you probably cant always see them i guess. Looking like possibly a bad relay? Maybe i'm overlooking something. I'm still looking for a bad connection somewhere.

There should only be 100VDC at Pins 1 & 2 (downstream of the 3.3KΩ resistor) if the relay is out of the circuit. Check the relay connections at the operating coil and the ground path for the relay coil. If the connections are good and the relay coil has not failed the relay should operate.