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TU-9900 Upgrades

Discussion in 'Exclusively Sansui' started by Rotoflex4, Jul 26, 2018.

  1. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Hi Everyone,

    Doing a bit of work on my TU-9900 and thought I'd post a couple pics/comments.

    The Power Supply was pretty straightforward. Used Panasonic FC caps. Followed advice on the T.I.C. for higher capacitor values. Replaced TR01 and TR02 with TIP41C transistors. I ended up not touching D01-D04 as they are working fine and I wasn't really confident about what I would use in their place.

    Next, I started in on the F-2526 board. The thing that surprised me here is how far out of whack are the original carbon comp resistors. I don't like the idea of replacing them all (would take forever), but I've pulled about 20 so far and not one is within 5%. Most are between 20 and 30% over their stated value. So....I'm replacing them with .4watt thin film resistors. They are really tiny, which is a plus on this crowded board (see R21, R23, R25 in top left corner). This could take all summer.
     

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  2. kevzep

    kevzep Its all about the Music

    What meter and probes are you using, you say "most", seems to be your meter could possibly be reading 20-30% over?
    Changing the resistors will require the unit to be aligned afterwards...
    I recently did some work on one of these tuners and found it was an excellent performer....
     
  3. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Thanks for your thoughts. I've got a high quality DMM and the new resistors are all within 1%. Since my original post I've pulled a couple more - they were a little closer to spec. My plan is to have it aligned after I finish swapping parts.
     
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  4. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    I'm replacing just a few parts at a time, then hooking the tuner up to make sure things still work and I haven't messed anything up. After my last work session the stereo no longer kicks in. I figure that's likely just a consequence of using resistors with values that differ from the old crappy ones.

    The 22K resistors were pretty bad. They all measured 27 to 29K. The 10K's were mostly around 12K and the 47Ks were at around 51-53K.

    Just pulled two 100K resistors (R17 and R18). Measured 132K and 137K.
     
    Last edited: Jul 26, 2018
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  5. kevzep

    kevzep Its all about the Music

    Wow, thats quite extraordinary they should be so far out...
    Well if that's the case, its going to be impressive once its finished.
    Always good to confirm the test gear is good, would not be the first time I have seen an issue, no offence.....
     
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  6. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Good Advice. Can't take anything for granted. Just pulled two 15K resistors - they were pretty much spot on. Then I pulled a 150K, and it tested at 204K. Yikes.
     
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  7. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Been a long week replacing resistors. Only a few left to do on the Switch Circuit Board. Replaced 206 quarter watt composites so far. Less than 10 were within 5%. Most were 10-30% over. The few half-watts and one watt resistors were all pretty good. The most suprising thing about all this is that the tuner still works and I haven't messed anything up.
     

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  8. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Found a modification on the F-2528 (Switch Circuit Board). I don't know if this is a factory mod or a previous owner's handiwork. It's not documented in the service manual. Two traces have been cut, a jumper wire added and a 220ohm resistor installed. It is directly behind the Bandwidth pushbutton switch. I don't suppose anyone else has their lid off and can comment on what theirs looks like? (easily viewed from the top).
     

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  9. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    Here's a pic of the completed F-2526 board. I wanted to use polypropylene film caps on C625 (1.0uf) and C626 (.47uf) but the space is too tight. So....I placed a .47uf on the front of the board, and a big 1.0uf on the backside. Seems to work pretty good.
     

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  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
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    Location:
    Hertfordshire, UK
    You could ask about this in the AK 'Tuners' forum - the moderator Mike may be able to help. ;)
     
  11. Rotoflex4

    Rotoflex4 Active Member

    Messages:
    131
    That's a good suggestion. Thanks, I'll do that.
     

     

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  12. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Location:
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    I know you know this but I just thought I would post it for those that aren't familiar with the variation expected with carbon comp resistors. From resistorguide.com

    "Fifty years ago, carbon composition resistors were widely used in consumer electronics. Because of the low stability of the resistance value, this type of resistor is not suitable for any modern high precision application. For example, the resistance value can change up to 5% over a shelf life of one year. With heavy use the value changes even more: up to 15% for a 2000h test at full rating with 70°C. Soldering can cause a 2% change."

    If all those changes add up a resistor will be way out of spec. Understandable about the high resistance (low amount of fine carbon particles in the mix) could end up much higher in resistance over the years. They are just due for replacement.
     
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  13. kevzep

    kevzep Its all about the Music

    I work on electronics, every day, and a lot of it from the 70's, I do not see many resistors at all, to be out of spec like you have found in this unit, its not that common, trust me......When I have a fault in something, I always check the resistors as a matter of course, I have never found as many out of spec as you have in this one unit...
    Not trying to turn this into a pissing contest, but electronics is not just in the weekend for me, its every day, what you are finding there is a bad batch of resistors because as I said, its not normal to find that many so far out of spec....
     
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  14. Blue Shadow

    Blue Shadow I gotta get me a new title

    Messages:
    21,017
    Location:
    SE PA
    You are probably right. I haven't measured lot of cc resistors but I know some folks, weekend warriors wielding soldering irons wanting to get intimate with some gear are wholesale replacing cc resistors. I know when it comes time for a rebuild on my SP-3a preamp, I will be measuring and replacing, but do I use original carbon comp style to keep the sound quality the same? IDK. Not there, yet.
     

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