SX-1080 protection relay not engaging

[tubmyk2]

Hey guys, I've got a basket-case SX-1080 here that won't come out of protection, and I've already rebuilt the power supply and the protection board with no change at all to the symptom.


All supply voltages are good or within a few volts of what is listed in the schematic. I've replaced all electrolytics, transistors, and the rectifier diodes. Unfortunately, this supply had been cracked in the corner and was repaired at some point / traces bridged. I confirmed this was not an issue, and I even re-did the connections. Entire board had been removed (yeah, THAt sucked) and cleaned thoroughly so I could really inspect the solder joints. All joints have been reflowed.

This is where I'm wondering if I have a problem with Q3 (IC). All highlighted voltages check out fine except in the IC, and once again, completely rebuilt, new capacitors, new diodes, board removed and cleaned and touched up joints.

The issue I have now vs. the issue when I first got this in has not changed, so nothing I replaced caused any additional problem. I even removed the protection relay, pulled the cover off, cleaned the contacts with some paper soaked in deoxit, and threw 48v across it to confirm it engages on its own, so if I'm correct, this means the relay itself is fine.

I've been hesitant to start pulling the amplifier boards because they're all wired direct, just like the power supply and protection boards, and if I'm seeing funky voltages in that chip, I'm kind of focusing around that. But this is where I need some guidance if I'm looking in the right place...

I also tested the preamp section originally and that works fine. Any takers??

 

[mbz]

Pin 4 is the dc offset, should measure on the mV scale but would probably need a few hundred mV to trip protection, you have 0V... If not done, reflow the solder joints on the IC, maybe a reheat will bring it back to life. Otherwise yes the IC looks suspect, will check datasheet to confirm expected voltages.

edit correction, you have a problem with Q2, with Vbe=0.7V the transistor should be on providing a path through the relay to GND. The other possibility is open relay coil, continuity check or measure resistance across D2 then repeat reversing probe orientation.

edit DUH, missed the actual/measured voltages in red, clearly excessive Vdc so amp is in protect.

 

[jaybirdss396]

If you are seeing 3.2V on pin 4 that means the protection circuit is doing its job by not turning the relay on. There should be 0v there, this is the dc offset from the amplifier. Check the input to the relay from each channel at the connections marked #8 and #5 to see which channel has the high dc offset.

 

[tubmyk2]

If you are seeing 3.2V on pin 4 that means the protection circuit is doing its job by not turning the relay on. There should be 0v there, this is the dc offset from the amplifier. Check the input to the relay from each channel at the connections marked #8 and #5 to see which channel has the high dc offset.

Ok maybe I'm not following the measurement flow; So, pins #5 and #8 on the protection board are getting -12mv and -7mv respectively. Is that what you meant? I get roughly the same measurement probing pin 10 on the power amp cards, which is what it says connects to 5 (R) and 8 (L)...

 

[jaybirdss396]

OK, those voltages mean there is no DC coming from the amp board. How do the voltages look at Q1?

 

[tubmyk2]

OK, those voltages mean there is no DC coming from the amp board. How do the voltages look at Q1?

just a tad above 12v on all (EBC)

 

[mbz]

just a tad above 12v on all (EBC)

Guiding value on Q1c is 0V so 12v is unexpected.
Would help if measurements were to 1 decimal place so we can confirm that Q1 is on or if failed short(c-e).
Diode test Q1 c-e incircuit, test for short.
Need to look at what pin 4 is about, need clear sm...

 

[rcs16]

Awm-120 terminal 4 connects to each power amplifier board. Each amplifier has a npn transistor(bjt) having a open collector that is wired together forming a wired-or function. This is the over current protection circuit, so when one or both open collector bjts is turned on (fault condition) it will ground terminal 4, turning on Q1, which should have it's base ~0.6V below it's emitter since it's an pnp. Q1 will be in saturation ( fully on) so the emitter and collector will be very close to be the same V.
An easy way to test awm-120 is to open the connection to terminal 4. If the protection relay energizes, the circuit is operating properly. If you ground terminal 4 the relay should de-energize.

 

[mbz]

Can you access Q8 on each power amp board for b/c/e voltage measurements, need to verify that it's off. Possibly it does over current protection, need a closer look.

edit received notification of rcs16's excellent post while writing/investigating the above. rcs16 clearly has a better understanding of how the amp works and that the transistors in question are for over current protection

 

[tubmyk2]

Yeah, I pulled the heatsink and disconnected each amplifier board and it's one of the boards... relay clicks over with it disconnected, so I'll go from there and report back what I find. I may just rebuild them and then see after what the culprit was.

 

[tubmyk2]

So nothing failed out of circuit when I tested everything, but rebuilding the channel (both) did restore functionality and now it comes out of protection and plays. I did a blanket rebuild, so new transistors, capacitors, diodes, pulled and tested the 2SC979, and measured all the resistors. Something must have been breaking down under load.