SX-750 Intermittent Relay Shut Off

[aerospac3]

Hi fellas,

Been trying to search the forums for answers, but I'm stuck and forced to make a new thread. Regardless of channel, load, or speaker setting(A vs. B vs. A&B), my SX-750 relay keeps shutting off and turning back on again at random intervals. While the relay is engaged, the amp sounds great. I've downloaded the schematic and taken some voltage readings for the Power Supply & Protection AWR-099 board:

Pin 1: 27 mVDC
2: 27 mVDC
3: 31 mVDC
4: 31 mVDC
5: 7.91 VAC
6: 7.91 VAC
7: 7.91 VAC
8: 43 mVDC
9: 17 mVDC
10: 12 mVDC
11: 45 mVDC
12: 0
13: 0
14: 0
15: 0
16: -50.3 VDC
17: -37.77 VDC
18: 48.8 VDC
19: 0
20: 0
21: 0
22: 12.67 VDC
23: 32.33 VDC
24: 32.33 VDC


Additionally, I have looked into a few threads with kind of similar issues (a generated hum, namely this one and this one) that recommended the following:

- Thoroughly deoxing the power/selection switch (done)
- Modifying or replacing this switch (considering)
- Replacing C17 and C18 on the board due to leakage (mine are definitely in need of replacement)


Does anyone have an idea of where to go from here? Just let me know if there's any more info that could come in handy.

 

[aerospac3]

Bump I'll likely be ordering some replacement C17 and C18 resistors & trying to clean the power switch some more unless I hear any other advice.

 

[zebulon1]

The readings Show an offset of over .032v (32mV's) on both channels. There is no offset adjustment on the 750's. With both channels off that much it might be something in the preamp causing the imbalance. Although not extremely high it might be on the verge of a triggering/not triggering.
Bypass the main power switch with a jumper so the power is on all the time. Check the operation before preceding.
If the problem remains;
First check the amp board for good B+, B- voltages;
Check for the -37v's between diodes D1 and D2 on the amp board.
Check for the 33v's at pin's 25 and 32 of the amp board.
If those are good:
Lift one leg of resistors R29 and R30 (doesn't matter what leg). This will isolate the preamp from the main.
Power up and check the offset at pins 1 and 4 of the Power/protection board.
I think this will help narrow it down.

 

[zebulon1]

Another thought,
After checking the power switch.
You could lift D5 of the protection and verify that the protection is working without the over current.
The reason is, I don't think 32mV's is that high for a non adjustable offset.
Caps C1 and C2 on the protection help decrease the sensitivity of the circuit. Replace those? Are they the Robin egg Sanyo's? (.22/10)
Maybe a different approach is in order.
Sorry for being wishy-washy. Getting over that nasty flu, still a little weak.

Does it come out of protection when the volume is increased?

OK
Check the Power switch per above
Lift D5 of the protection circuit and see if she runs normally
If your going to order caps.
Replace C1 and C2 of the protection circuit.

 

[aerospac3]

Thanks for the reply! Sounds like you'd know better than I; what would be the quickest way to bypass the power switch? I know there's threads with pictures and references to messing with it but I can't seem to find them right now...

I'll be sure to get those caps as well for preventative maintenance. You're correct, C1 & C2 for board AWR-099 are 0.22uf 10V.

Also getting over some sickness but hope to stay active on this over the weekend.

 

[petehall347]

so you tried without speakers ?

 

[aerospac3]

so you tried without speakers ?

Yep. Speakers hooked up and with no speakers, along with all combinations of A, B, and A+B. No noticeable changes in relay behavior, just random shutoff and engagement.

 

[Watthour]

Aero - Regarding the power switch repair, this may be of interest:

http://audiokarma.org/forums/index.php?threads/we-can-rebuild-him-we-have-the-technology.372256/

In the interim, if you PROMISE to be very careful a cheater cord can be constructed with two male 15A plugs to interconnect the "Switched" and "Unswitched" receptacles in the rear of the unit. Once the unit's power cord is plugged in the power switch will be bypassed. As a safety, if you have to disconnect the cheater cord be certain that the power switch is off and the cheater end end connected to the "Unswitched" receptacle is disconnected first and reconnected last so that the end of the plug is not live and dangerous. It can also be jumpered right at the switch but that is relatively inaccessible in an assembled SX-750.

 

[Watthour]

Incidentally, C17/18 may not actually have electrolyte leakage if that's what you expect. If there is electrical leakage I can't address that. These caps were factory installed with glue to help support them, and the glue will look just like 40 year old glue by now. This can easily be mistaken for electrolyte leakage. These caps are usually heated, showing shrunken sleeves and tarnished cans, and should be replaced anyway.

However, the original rating of these was 330µF/50WVDC which is dangerously close to the design 48V operating level. A 63V 330µF cap will do better. I used 470µF caps there but the board holes required enlargement for the larger leads. If you want to avoid that, stick with the 330µF but definitely increase the voltage safety margin.

 

[aerospac3]

Watthour, you're awesome!! That post of yours regarding the switch is the one I recalled seeing but couldn't find earlier.

There isn't leakage on C17/18, just that old glue, but the sleeves are very shrunken. I'll be certain to up the voltage on them when ordering as well.

I'll either be playing some hockey this evening or tackling the switch... only time will tell!

 

[Watthour]

If I were you, I'd skate. You can always find time to toy with the electronics, but ice time is hard to get, at least around here. I did that for about 30 years before I started having difficulty keeping up with the teenagers. Check out old #7:

 

[aerospac3]

No way that's awesome!! I did end up skating and it was a great time. Bit of a late bloomer since I started less than 2 years ago at 21, but better late than never!

Work has had me travelling and I won't be back in town for a while to continue switch work, but I'll be sure to update this when I get back. Watthour & Zeb, i owe you guys a beer if you're ever in central Florida

 

[aerospac3]

Well, life sure got in the way of this project, but I was finally able to resume it. I ended up replacing C1, C2, C17 and C18, and so far the amp is working great. Thanks again to you all for your help.

 

[tsd71]

Nice handlebar mustache Watthour! Had one like that too back in the day.

 

[Watthour]

You like that, eh? It used to be a good trap to keep the head in the stein for those after-game celebrations. Now it comes in gray. IIRC, we won the league that year (big) and they asked us not to come back the following season, so we skated in the Chicago suburban leagues for three seasons after.