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Lights flicker on SX 750

Dearslayer

Dearslayer

Super Member
I was thinking about buying a SX 750 from a local ad. Seller is asking $300 in great condition. He says the only issue is that all lights flicker when powering on and it takes a minute. He says he was told it may be the power switch. Is it worth taking a chance? How difficult is it to replace the switch?
 
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I just worked on an SX-3900 with same issue. When ohm across power switch can see resistance bounce between 200-400 ohms then settle down. That's when lights flickering. Jump across the switch confirms lights do not flicker and switch contacts are the culprit. The SX-3900 switch did not look like I wanted to try and open it so I managed to find small openings and gave it a good bath of CRC contact cleaner (non-residual). Switch is now closing properly, and flickering is gone. But I'm suspect this is probably a temporary fix, which owner was OK with. Best if can open and clean the contacts or find a replacement. Replacement is easy (2 wires to solder) if can find a proper switch that fits and can handle the load of your SX-750.

Options:
1. You can try the cleaning like I did and see how long it lasts. (Do not use anything that leaves a residue in a power switch).
2. Disassemble switch housing, clean the contacts, re-assemble. If contacts are severely pitted this probably will not be a long-term solution.
3. Permanently jump the switch by soldering a wire across it which will put it in an always on state. Then use an external power power strip to turn on and off.
4. Search for a replacement on "ePray" and hope it is in better condition.
5. Someone here with more experience may know of a replacement that works.
6. Many tech's recommend modifying the circuit and install a Triac to act as an electronic switch of the mains, activated by the power switch, which removes the load off the original switch. The Triac does the heavy lifting. You can search here on AK for these threads, possibly in DIY Forum. This seems to be the best long-term option but requires some technical abilities.
 
Thanks for that info. I guess I can go look at this unit and see just how bad it really is and decide if I want to buy it and go through the hassle with the switch. I'm not a tech by any stretch and I want to be sure it's within my abilities to clean/ fix/ replace the switch.
 
I just looked at the Service Manual for the SX-750. It utilizes a rotary switch for Power and Speaker Selection. The SX-3900 I mentioned above has only a two-pole flip switch.
The Pwr/Spkr rotary switch is mounted to the front panel but looks like the actual power switch is mounted Edit: to the rear of the rotary switch. [Original entry is wrong: off to the side of that rotary switch with wires going to the rotary switch contacts]. You might get lucky and can try hitting the rotary switch contacts with CRC. If need to get to the actual power switch, may need to do some more disassembly.

Someone with more experience on the 750 may be able to chime in.
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jd3rdcoast said:
I just looked at the Service Manual for the SX-750. It utilizes a rotary switch for Power and Speaker Selection. The SX-3900 I mentioned above has only a two-pole flip switch.
The Pwr/Spkr rotary switch is mounted to the front panel but looks like the actual power switch is mounted off to the side of that rotary switch with wires going to the rotary switch contacts. You might get lucky and can try hitting the rotary switch contacts with CRC. If need to get to the actual power switch, may need to do some more disassembly.

Someone with more experience on the 750 may be able to chime in.
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Well, I picked this unit up on the way to work tonight. Sitting in my car right now. I have to say that the shape of this unit is impressive. It looks brand new. The seller plugged it in for me and it didn't flicker at all at first because I think the switch was already in the on position. I shut it off and on a couple times, and it does flicker very briefly but then stops. Hope to take a closer look at it over the weekend if I get time. Will report back.
 
Nice job with that broken shaft.Hopefully I'll be able to get the switch out to clean.The unit is in great condition so I hope it also reflects the condition of the switch.
 
Just a quick question regarding the removal of the switch. Is it safe to assume that the face plate should come off to make the switch removal easier.
 
Yes, there is a special nut in behind as well. Watch for the screw behind the tuning dial string and carefully slip the tuning string up and over. I'll see if I can find any pics I may have taken.
I like to keep track of where screws go to avoid stripping.
The 2nd last pic is shows the orig broken switch still attached. Hope this helps.
While it's off is a good time to service all pots/switched with better access.
 

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Got the switch apart but it didn't really seem to be arced that bad. The post had a small divot but nothing extreme. I smoothed it a little with 400 paper. I have to wonder if the flickering is caused by something else. I have to solder all the wires back on the switch and re install everything so I guess I'll find out.
 

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Managed to get it all put back together and so far so good. The flickering has stopped or so it seems. i tried the Aux input and there is some "Hum"and the lights do flicker a little but that was just a quick check and I had to run. I'll see if it's still doing it later this evening when i get a minute.
 
Sounds promising. Once that part is good, you may consider replacing filter caps C17, C18 on AWR99 with 330uF 63V caps.
I had buzzing/hum on one of the units I worked on it that fixed it.
 
I just tried the Aux and it seems to be fine for now. No blinking lights. I'll keep an eye on it over the next couple days. I'll be doing a full recap on this unit when time permits but it sure as hell sounds fantastic just the way it is. MegaHurts ...Thanks for your help.
 
So doing some research for a BOM for this SX 750 unit takes me back as far as 2009/10 including MTF's list of parts for a recap. It seems some caps etc from that older list are no longer available. Would there be a more recent list for a total restoration that could be shared?
 
do a search in the forum for the unavailable number and see if a newer part number is listed.

that pans out, use the parts value, like 100uF or whatever.
 
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