Bad relay, protection board or something else?

[bzenic]

Thank you for your fast response, starting a new thread as advised.
My issue with SX-737 is that the with relay is switching randomly cutting the sound on and off .
According to some advices found here I measured the voltages on the protection board WM 025:

Pin 1: 6 mV
Pin 2: -13.3 V
Pin 3: 13 mV
Pin 4: 0 V
Pin 5: -1 mV
Pin 6: 3 mV
Pin 7: 26 mV
Pin 8: -6 mV
Pin 9: 37.1 V <--> 8.9 V (the relay keeps switching)
Pin 10: 37.1 V
Pin 11: 37.1 V
Pin 12: 7.1V AC

These measurements don't tell me much.
How can I know whether it is a bad relay issue or something else?
How to know if the relay is actually doing the right thing protecting output?
I would appreciate any advice what to try next.

 

[merlynski]

Since the relay is cycling in and out, and pin 9 voltage shows that the protection circuit is driving it, it is probably not the relay itself (rarely is for these symptoms).
Set the volume to minimum, tone controls centered or off, input to Aux, with no input signal connected.
Is the relay still cycling?

 

[mdexter51]

Is it rude for me to jump in here and try to learn also. I have two sx-828 with the same problem. Both may not energise at power up. Else, may work fine for 5 minutes, 5 hours, or several days or even months. I replaced one od the protection boards to no change. My conclusion is problem is in the power section and the protection circuit is doing what it is supposed to do???

 

[merlynski]

Is it rude for me to jump in here and try to learn also. I have two sx-828 with the same problem. Both may not energise at power up. Else, may work fine for 5 minutes, 5 hours, or several days or even months. I replaced one od the protection boards to no change. My conclusion is problem is in the power section and the protection circuit is doing what it is supposed to do???

You are most welcome to follow along and learn, but I suggest you start your own thread if you want to post your symptoms and get help troubleshooting your unit.. Multiple units in one thread tends to be confusing for the original poster and the helper.

 

[bzenic]

Since the relay is cycling in and out, and pin 9 voltage shows that the protection circuit is driving it, it is probably not the relay itself (rarely is for these symptoms).
Set the volume to minimum, tone controls centered or off, input to Aux, with no input signal connected.
Is the relay still cycling?

With these settings the relay would switch just a few times in the start and then stop...
but then again after a few minutes...not frequently, only 2 times.

 

[merlynski]

Monitor the cathodes of D5, D6 and D7. With the controls set as before they should stay at about +17 to +20vdc. If any of them drop in voltage when the relay cycles that indicates a possible source. The likeliest possibilities, if the problem (as I suspect from the posted voltages) is on the protection board are: Q7, 2SC1384, is old and tired, or Q1, Q2, 2SC869, are getting leaky. They are not the only possibilities. Rebuilding the whole AWM-025 board (transistors and electrolytic caps) with modern components is a good option.

 

[markthefixer]

Rule of thumb: IF the relay stays off for the time delay period, then the protect circuit is commanding the relay.
If the relay stays off for LESS than the time delay period, then look to the relay driver circuit (transistor, capacitor sometimes, etc)