Repairing QRX-7500 Receiver

mdamico

Member
I've been working on making some minor repairs on the QRX-7500 I bought from someone on e-Bay, and I've run into a snag on one of the malfunctioning components: I'm in the process of replacing the "Level Set" control potentiometer just to the right side of the VU meters. To get this control unsoldered, I would think I need to remove the PC Board that controls the balance controls; it's mounted slightly to the left of center, and flush against the bottom inside of the inner front panel (I say that because the outer control panel has to be removed to get at the knob shaft nuts). The knob shafts for some of those balance controls are what secures that board to the inside of the chassis; they use a small hex nut on two of the shafts, and then on the Rear Balance control, they use a big Brass colored threaded spacer. I can not remove the spacer from that control, and therefore, I can't remove that PC Board that's in the way of me working on the Level Set control. When I turn the big spacer counter-clockwise, the outer sleeve of the Rear Balance control is slipping on the front of the potentiometer body. Would anyone know if there is another way to space the inner front panel chassis front away from the rest of the chassis so maintenance can be performed? If I don't get any suggestions, I my have to (very carefully) cut the big brass spacer off of the front of the Rear Balance control potentiometer. Please let me know if you have any suggestions. Thanks!Rear Balance1.JPG

Rear Balance 2.JPG

Rear Balance 3.JPG
 
The threaded portion of the control should be a purely mechanical connection press fit into the front of the control body. Try putting a wrench on the big nut and giving it a few sharp taps with a brass hammer or screwdriver handle ... once you break it loose, it should turn off easily. Start easy and work your way up ... don't want to use any more force than absolutely necessary, and keep your hits square and true.

Poor man's impact wrench ... <G>
 
Thank you very, very much for that poor man's impact wrench idea. I finally got that nut off exactly the way you described! I must admit I first did try to whack my ratchet handle with my fist, but my fist was probably too soft to get the parts to start separating (not enough inertia). I just hope the shock waves I sent through the rest of the receiver didn't ruin something else in the process.
 
Continuing To Gain Access To Level Set Pot

After sKiZo's help with removing that brass spacer-nut, I finally got that Balance PC board and controls loose from the back of the front panel. I will have to unsolder about twelve wires from that board so I can completely swing the board out of the way so I can gain better access to the bad Level Set pot. I must say that the wires in this receiver are still alarmingly pliable. The wires bend so easily that I lost the fear I usually have of a wire breaking off at a solder joint; which is something that happens frequently when tinkering with 40 year old electronic equipment. My guess is that the wire has many strands, which make it better, but more expensive.
 

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Glad it all worked out for ya. I've always been afraid to try it myself ...

(ba dump dump) :D

Common tool of the trade - when all else fails, grab the hammer! I've actually got a six ounce brass hammer that works slick as a force multiplier for that sort of thing.

When you put it back together, just snug em up. There's plenty of mechanical connections to keep things where they belong without getting too carried away with the wrench.

A word of caution on the wires. You may find several black grounds that "float" for different circuits that pass through the board. You don't want to get those switched around, so mark them well.
 
Is there a way to cross reference the screen names to actual names on this forum? For instance, I just know you by Skizo, but what's your real name? FYI, my name is Mike D'Amico and I currently live in Palm Bay, Florida. I was an Electronic Technician in the Air Force, and then got an Electronics job at the Harris Corporation in 1982. I've been working there ever since.
 
Most of us prefer our online anonymity, but what da hey, eh ...

I am Lrrr, ruler of the planet Omicron Persei 8 ... bow and grovel to the majesty that is me!!

Does sound like you have the right background for poking around inside these old relics. I did time as a comm tech (Omicronian Royal Navy), so I could still fix what I broke ... more times than not. And I broke a lot of stuff ... still do! <G>
 
I made the mistake of asking my lovely wife try to remove a wire from one of the POT terminals while I held the soldering iron and the PC board. I should have had her hold the PC board because she pulled the insulation of the wire off with the long nose instead of pulling the whole wire. Now I'll have to put a piece of heat shrink over the damaged insulation. But, the important thing is that she did try to help me! So, so far, that's the only thing we made worse on this Sansui. I'm going to have to resort to the solder - wick and flux trick to remove most of the solder first, and then unwind the wire strands from around the terminals. It's very time consuming that way, but then the wires don't get any shorter after the repair.
 
Head on down to RatShack and get yourself one of these ...
radioshack-desoldering-iron.jpg

http://www.radioshack.com/radioshac...-iron/6402060.html#q=desoldering+tool&start=3

Best $15 you'll spend for restoration work. I picked up a spare a couple weeks back just in case they go under ... Recovery time is a bit slow, so wait a bit between connections for the temp to steady - you'll get the timing down pretty quick. You should be able to drop the iron on (bulb squeezed) and see the old solder pool. A slight twisting motion helps get full contact. Only time I use the braid any more is for the really small stuff.

Rather than trying to unwrap the wires, just desolder the posts from the board. That's the way I do it. The desoldering tool works real well to clean the connection. You'll probably still have a bit of a solder bridge to the board, but just a light push with a hot iron on the side of the post should break that easily, then just pull the post out.

Also, liquid insulation is a whole lot easier than playing around with heat shrink.

liquid_tape.jpg


I paint it with a couple coats. Great for patching up those ooPs! moments when the iron slips too.
 
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Thanks for recommending the bulb / Iron combination. At work I have a "Solder-pult" soldering iron which has a solder sucker with a trigger built right into the iron. I think I bought it from one of the distributers that sell to my company. Anyhow, I haven't used it in so long that I forgot I had it !! I may as well bring it back home since I don't do that work at my company anymore. I'm strictly a Test Engineer now.

When working on the Potentiometer terminals, I don't see any alternative than sucking or wicking the solder off the terminal and unwrapping the wires. So I'm guessing that when you say "Rather than trying to unwrap the wires, just de-solder the posts from the board", you're referring to de-soldering situations other than working on POT terminals.
 
My bad ... I was talking where wires are direct connect to boards. Now that I think on that, Sui tends to use crimp terminals with posts that are then soldered to the board so it's pretty obvious what needs doing to get those off.
 
I just picked up one of those de-soldering irons you recommended. The one I originally used at work has some sort of blockage in the vacuum path. The Solder wick worked well enough too.
So I have my Sansui taken apart now at the bottom so I can get to the Level Set pot easy enough. I'll put some paper around that four-gang pot so I don't splash solder on anything else. As far as the Fuse Lamps go, until I find someone who tells me how to access them I'm going to leave them as is (two burned out). With as many wires holding the front panel to the rest of the chassis there is a good chance of creating another problem in the process of replacing the bulbs.
 
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Disassembly Update: Level Set Pot Now Accessible

I've removed some of the wiring from the PC Board that holds the controls below the Level Set control and swung the board out of the way (the controls are still attached). I can now get to the wiring on the Level Set potentiometer, although it will be a little like doing open heart surgery! I got a replacement pot from a guy that had a "QRX-7500 Parts Unit" for $20.00. I consider that a steal since the pots are pretty rare. I'm including a photo of the disassembled receiver showing the Level Set pot in case the disassembly information can be used by anyone else that needs to access the same component.
 

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BTW, the 4-gang Level Set pot seen in the photo has some black tape wrapped around each of the circumferences of the individual pot cases. Can anyone let me know if this is done for any other reason than better insulating the cans from the terminals? I could see how it would be possible for a terminal to come in contact with the pot can and possible short out to another terminal. The new pot I received in the mail from another Sansui enthusiast doesn't have the black tap on it. That pot came out of a QRX-5500.
 
Level Set Pot Removed. Rear pot completely fractured.

I finally removed the original Level Set pot from my Sansui QRX-7500 and found that the Left Front pot (the one at the very back) was completed broken up inside. I took a picture of it after I removed the back plate. I also took a photo of the control pot I was sent from hybridrestore.com. I wrapped each pot section with a 3/16 inch piece of electrical tape to protect it from splashes of solder while I make the repair. BTW, A long time ago, I bought a small, Red, plastic suction cup type of vice from Radio Shack. That vice has turned out to be one of my best investments. I used it this evening to hold the replacement control pot while I unsoldered the short wires from it. They guy that sent it to me must have just clipped the wires in the QRX-5500 he removed it from.
While I was in the process of replacing this control, I decided to also replace the fuse style lamps behind the VU meters. Two of mine were burned out, so I bought some on e-Bay that are exact replacements. I got five bulbs for $16.00 including the shipping.
 

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BTW, I forgot to mention that I used the White piece of cotton handkerchief under the control pot to help contain any solder splashes that may happen while using the solder wick and desoldering tool
 
Replacement Level Set Potentiometer Now Installed

This evening I installed the replacement Level Set potentiometer (POT) that I received from Hybrid Restore.com. There were ten wires to solder back onto the POT (two per channel and two grounds toward the back). I have another twelve wires to re-solder to PC Boards that I needed to move out of the way before I can try out the receiver. Since I'm working overtime this week at my job I had to stop here for the night. See attached photograph of the resulting soldering and installation of the POT.
 

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Cleaning and lubricating Sansui push-button switches

These Sansui Quadraphonic QRX receivers all have ten or so push-button switches. While I'm into repairing something else in the chassis, now is a good time to lubricate and clean the push-button switches. Can anyone tell me exactly what opening I need to shoot the control cleaner into so the contacts can be cleaned? I can see the mechanical aspects of the switch to lubricate, but I can't see the electrical contacts. I would guess that the contacts are buried inside the metal housing somewhere. If someone is familiar with these switches, please reply and let me know where the contacts are. See attached picture for a better description of the switch. Thanks!
 

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Here's another example ...

f1363-tone-block-original.jpg


As you mentioned, the electrical contacts are in the back section of the switches. If you operate the switch, you should be able to see the rocker moving there, and that gives you a good idea where to spray the cleaner. There's holes scattered about in the case to allow access. I use short bursts of D5 on low pressure and operate the switch several times, let it sit, and repeat. Careful for overspray and drips, and use no more than needed.

I wouldn't lube the mechanical section if it's working ok. Lube draws dust, and that's what kills em.

Don't know if you've got any of the long type Sui switches ...
daughterboard-off.jpg


With those, you'll see a hole on the switch at each contact point. I use a needle bottle with D5 and just put a drop in each hole and work the switch several times to spread it around. Always let them drip out and dry overnite before applying power.

PS ... you may be tempted to try disassembling the switches for an uber cleaning ... don't ... just don't ...
 
I have Radio Shack Tuner, Control Cleaner and Lubricant in a spray can. I don't know what D5 is, but will the Radio Shack product work well enough?
 
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