QRX7500A Big Job, some faults.....

Discussion in 'Exclusively Sansui' started by kevzep, Oct 9, 2017.

  1. kevzep

    kevzep Its all about the Music Subscriber

    So I have this one on the bench today, parts order finally arrived.

    Faults are Right Front VU meter not working....FM Stereo Light not illuminating....huge DC offset in rear amplifiers causing it to stay in protection.
    Usual intermittent switches and the likes, the Low and High pass filters are causing some issues as well...

    So there's a bit to do...

    So I thought I'd start at the power amps and work back from there...So I fired it up and the rear amps were suddenly fine....
    So I pulled the power-amp module out of the unit (so much easier than anything I encountered on the 9001 I repaired) and to my horror, but relief I could see the problem....
    One of the Main Filter caps has started leaking, and its leaked all over the rear power-amp circuit.
    So there was a lot of cleaning.
    Few pics showing the start of the journey through this mighty quad receiver, I actually quite like the build of this one...
    IMG_5062.JPG IMG_5068.JPG IMG_5073.JPG IMG_5074.JPG
     
    Last edited: Oct 9, 2017
    slimecity, Hyperion and skippy124 like this.
  2. Hipocrates

    Hipocrates Anti-Muppet Subscriber

    Messages:
    1,070
    Location:
    Costa Rica
  3. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,762
    Location:
    Long Beach, CA
    The 7500A was the top of the line quad receiver at that point in time. It's not surprising it's built well. It looks like a very neat package with the output transistors and emitter resistors all combined on the same board. Interesting, the amp board has cartridge fuses built right in. The 9090/8080 are from the same era and the driver board fuses were an afterthought added during production. It's also a pretty rare receiver. We have 20 QRX-7500s in the database but only three 7500As.

    - Pete
     
  4. kevzep

    kevzep Its all about the Music Subscriber

    I was wondering what the difference is between the 7500A and 7500.....definitely from the era of solid construction, they thought about serviceability when they made this, so easy to remove that whole power-amp section...
    I'm looking forward to seeing how this one comes up....
     
  5. kevzep

    kevzep Its all about the Music Subscriber

    This QRX has the GLUE disease, gone all hard crusty and corrosive.
    So it took me a good while to work my way through the power supply board, cleaning as I went....hopefully I can make more progress tomorrow, really need to get this one off the bench....
    I must say though, I am really enjoying the layout of this model. I was thinking of getting a QRX9001, but in all honesty I think the 7500A is a better built and engineered unit....they ALL sound amazing, but from a tech point of view this is the one to have...
    I removed the tuner board and started working on that, taped up the dial pointer chord, I'm not keen on re threading that!! Although I have done them before....
    Couple of shots....
    IMG_5075.JPG IMG_5076.JPG IMG_5077.JPG IMG_5078.JPG IMG_5079.JPG
     
    Hyperion and skippy124 like this.
  6. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,278
    Location:
    Mississippi
    busy tuner board I see :)
     
  7. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,762
    Location:
    Long Beach, CA
    The 7500A is clearly better built but the quad processor in next generation, QRX-x001 series are considered by many to be the best ever produced. The tech in you may like the construction and design of the 7500 but if you really want the best quad reproduction you need a QRX-9001.

    - Pete
     
  8. sKiZo

    sKiZo Hates received: 8641 Subscriber

    Yup - way I understand it, the 7500 used the same decoder setup as the later gen QR receivers. The 7500A (and 5500A) used a hybrid first gen "vario matrix" chipset, similar to that of the x001 models BUT ... the x001's added a third IC that improved definition on the rear channels AND improved separation front to rear.

    One thing I do believe the "A" models have in common with the x001 series is VERY sensitive adjustments for bias and offset, so careful if yall get ta tweakin there ...
     
  9. kevzep

    kevzep Its all about the Music Subscriber

    Thanks man, I make a living outta tweakin, takes a bit to catch me out....But good to know!!:thumbsup:

    Yes that's how I understand it too, with the rarity of these I am unlikely top find one as easily as a 9001....
    I'll have a good listen to it and see how good it is, if I like it, I might wait for one to turn up...
     
  10. sKiZo

    sKiZo Hates received: 8641 Subscriber

    Confession - I think the QRX7500i is way purtier than the 9001 ... it's them green meters, eh.

    I actually considered swapping out with these from a unit in my inventory, but they were the wrong size.

    [​IMG]

    VU's instead of power, but it's not like I think that'd matter a lot with some minor circuit tweaks, as all you're looking for is average deflection when listening.
     
  11. kevzep

    kevzep Its all about the Music Subscriber

    Yeah its a pretty substantial tuner alright!!
     
    skippy124 likes this.
  12. kevzep

    kevzep Its all about the Music Subscriber

    Yeah I think the 9001 look is quite "LOUD" and garish!!
    The 7500A is more conservative but tasteful....
    I noticed that with the meters, I quite like that...
     
  13. sKiZo

    sKiZo Hates received: 8641 Subscriber

    Still considering pulling the faceplate on the 9001 one of these days and having some fun with the meters. I've got some blue gel sheet leftovers from rebuilding some McIntosh meters that would certainly change the look of those pasty white power meters ... I love blue!! :D

    [​IMG]

    PS ... probably already know this but DO plan to repoint the fuse lamp sockets on any Sansui. They ALWAYS fail due to the hot/cold cycles over time. Just be aware that the light boxes tend to get brittle from the heat as well. If the job is for a client, tell em, either now or later, and it'll cost more if you have to open it up again.
     
  14. kevzep

    kevzep Its all about the Music Subscriber

    Yeah for sure, I'm replacing all the lamps in this one, mostly LED. The ones that just drop straight in....

    I can't wait to find my 7500A!!
     
  15. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

    Messages:
    22,853
    Location:
    San Francisco Peninsula
    I had a QRX 7500, loaned it to a co-worker 4 years ago, I think he thinks I gave it to him:p
     
  16. kevzep

    kevzep Its all about the Music Subscriber

    Hahaha, get him to give it back so I can buy it off you!! Is it the A variant?
     
  17. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

    Messages:
    22,853
    Location:
    San Francisco Peninsula
    No it was the first one...

    IMG_1374_zps5fab466a.jpg IMG_1385_zpsd1518206.jpg IMG_1375_zpsdbc30cf8.jpg
     
  18. kevzep

    kevzep Its all about the Music Subscriber

    Yeah right, its a bit different, I wonder why they used such a small tuning knob, it looks a bit weird don't you think?
     
  19. kevzep

    kevzep Its all about the Music Subscriber

    Getting close to finishing this up now, I am waiting for the main caps to arrive, so I have basically finished everything else so I can slip those in when they turn up on Tuesday.
    There are a LOT of caps in this thing.

    The front right VU meter was down in level, turned out to be a faulty cap and transistor in the meter drive circuit, a real pain to get to as well. The bad cap was a first, it was one of those Orange Sealed Low leakage electro caps, first one I have ever seen go bad, I think I might change my policy on changing those now.

    Usual tuner adjustments were needed to get the selectivity and sensitivity back, almost always dial alignment is needed, so I did all that this afternoon, its a bit if a tricky tuner to deal with, had to go through it twice to get it right, but its pulling in stations loud and clear now.

    I cleaned up the molex which was damaged by the leaking cap and causing a huge offset in the rear amplifiers channel which in turn would shut it down randomly....
    Then dialed in the bias and DC offset which all went swimmingly well.

    Here's a few pictures, I was trying to remember to take a few but I keep forgetting....

    I really do like this Receiver, is so nicely made, and has a great sound about it.
    Love the colour scheme with the meters and everything...
    IMG_5087.JPG IMG_5089.JPG IMG_5090.JPG IMG_5092.JPG IMG_5093.JPG IMG_5095.JPG IMG_5096.JPG
     
  20. tobyj17

    tobyj17 Comfortably Numb Subscriber

    Messages:
    2,562
    Location:
    Wisconsin's west coast
    It's a good thing you live where you do, or you'd be getting all my stuff to restore.. as always, an amazing job! Do these smaller units suffer from the same broken solder joint issues as the rest of them?
     

Share This Page