My first receiver restoration attempt: G-6700.

Discussion in 'Exclusively Sansui' started by ghazzer, Nov 12, 2012.

  1. ghazzer

    ghazzer Sansui addict Subscriber

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    Yep, I'm taking it slow - no rush.

    I have replaced every transistor on the board, all of the caps & resistors in the driver section and most of 'em in the P/S. I have just rechecked all the transistor & diode junctions with the component tester.

    TR08 is wired in because the tracks were mangled during a previous repair.
     
  2. ghazzer

    ghazzer Sansui addict Subscriber

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    Bias Tests

    Tonight I checked out the bias circuits. Some of these points are difficult to get a mini grabber on, but I managed to find another component that was connected, and clipped to that lead. After recent setbacks I am careful to shut the DBT off before repositioning the mini grabbers.

    First I tested per John's description and put a DMM between TR15/B & gnd, and between TR17/B & gnd. I turned VR03 2 turns CW and took readings, turned it 2 more turns CW and took readings, then turned it 4 turns CCW to get it ~ back to where it started. The results are shown in the attached graphic, including readings from the 'good' right channel.

    The lower rows show the relative difference of the readings in mv, so I can conclude that my circuitry is working as John described.

    Then I moved to the emitters of TR 15 &17 and they followed the same pattern when I adjusted VR03, as did readings taken at the base leads of the OP TRs.

    B+ and B- going to the collectors of the OP TRs is a solid ±52vdc.

    The emitter leads of the output TRs do not change when VR03 is adjusted. They both drift around, and read between -12 & -14mv, as does the junction of the two emitter resistors. Voltage readings across the emitter resistors ("Bias Voltage") is solid at 0.00mv.

    Looks like I need to remove the OP TRs and the emitter resistors and check them out of circuit.
     

    Attached Files:

  3. ghazzer

    ghazzer Sansui addict Subscriber

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    Looks like I am back on track!!!

    Removed the Sankens from the left channel. The NPN (2SA1106) was reading ~20MΩ C-E instead of 'open'. The PNP (2SC 2581) reads OK.

    The emitter resistors (0.47Ω) look OK - each pair read ~1.2Ω with my DMMs

    Re-checked the readings on a set of NJW 0302 & 0281, installed them, and now have adjustable bias in both channels. I've been sitting here for about an hour waiting for everything to stabilize when I realized that I still had it on the 135W DBT.

    B+ =50.4vdc, and the OP TRs are getting a little warm, maybe hot.

    Removed the DBT. B+ now at 58.2vdc and bias values are still at ~17mv & ~ 15mv. Think I'll shut 'er down and reinstall the 3058 bd so that the OP TRs are mounted to the heat sink.
     
  4. DCinDC

    DCinDC I'm here to fix it

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    check the OTs with the component tester. That 20 Mohm reading means nothing...
     
  5. ghazzer

    ghazzer Sansui addict Subscriber

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    The 20 MΩ reading must mean something.

    The scope display with the component tester looked good for all junctions of both the Sanken transistors I pulled from the left channel. However, the bias could not be adjusted with them installed, but did work with the replacement OnSemis.
     
  6. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Now that's interesting. If the Sankens test out ok OUT of circuit but not in circuit something else was causing the fault. Without digging into the schematic again might I suggest a voltage reading from the base of each output to ground while on the dbt to ascertain correct voltage rise as you adjust the bias resistor from 0 to the point where you start to see voltage on each output. Compare the two readings (npn/pnp). Most bias circuits use a voltage divider network, too lazy to dig out the schematic tonight, if there was a hairline trace crack and the divider went south there would be a great likelihood the outputs would head north (or bias could not be adjusted).
    Just my .02, might be rambling, been another 12 hour day.
    -Lee
     
  7. Overundr1

    Overundr1 AK Subscriber Subscriber

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    After coffee this morning---
    When you adjust the bias trimmers is there a point where the bias appears on your dmm to quickly fall from some small mv value to ovdc and consversly as you increase the trimmer setting does the bias go from 0.0v suddenly to some other value or is it gradual? (trying to get a visual representation of turn on conditions).
    -Lee
     
  8. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Might be chasing a earth (ground) issue on the mis-behaving channel----
     
  9. ghazzer

    ghazzer Sansui addict Subscriber

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    I have not yet applied the white grease and reinstalled the OP TR bd. When I was checking it out the other nite it seemed like the L/R bias would only go down to ~17/15mv, and then go back up as I keep turning the bias pots. That was across both emitter resistors, so ~16mv across 0.94Ω = 17mA.

    Still need to go back and check the NJWs for the point of conduction as you suggested.
     
  10. ghazzer

    ghazzer Sansui addict Subscriber

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    I decided to check this before I re-installed the OP TRs on the heatsink.

    Right bias came up at 17mv (as before) but the left bias came up at only 1-2mv (was 15mv). TR604/E was -8 to-10mv.

    Started SLOWLY increasing the bias trimmer. Got the bias up to ~5mv and it jumped to 16mv. TR604/E went to -20mv. This is the first time I have noticed that jump. All other observations have seen gradual increases in bias, and I have never monitored the emmiter of the OP TR to see when it starts conducting.

    The left ch OP TRs are already getting WARM - hot after only 10 minutes , so I shut it down. What could be causing tem to get that hot so quickly?

    These OnSemi replacements have slightly different specs than the original Sankens. Does that mean that the bias voltage might need to be set at a different level???
     
    Last edited: Aug 9, 2014
  11. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Its the whole chain of transistors that have changed. Different transistors wil have different current requiremnts before they will conduct across their junctions. I have seen mje pre drivers and drivers when paired exhibit the same higher initial turn on state results. As long as you stay below 20mv you should be fine. Might try 0.33 ohm emitters if you still have them lying around on dbt and note any differences.
    Lee
     
  12. ghazzer

    ghazzer Sansui addict Subscriber

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    I'm gonna be out of town for a week+, so can accomplish very little now except a good plan.

    First I'd like to resolve the heat issue, or at least find out how hot these OP TRs should get in 10 or 15 minutes. I will try to get a temperature probe on them so I can quantify how hot it actually is.

    Then I want to take a better set of readings at the emitters of the OP TRs and document the voltage readings so I can determine what the bias reading is when the TRs first start to conduct. Perhaps someone can clue me in as to what the desired condition should be: Just before conduction; Right at conduction; or some percentage increase in voltage beyond the point where conduction begins.

    Having Sankens in L ch and OnSemis in the R ch should give me a range of readings.

    Is there anything else I should check on, or measure at this point?
     
  13. ghazzer

    ghazzer Sansui addict Subscriber

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    (very late Tues nite)

    Right bias was 16mv after SO relay kicked in.
    Left bias started off at ~0.5mv, and stayed there for a few minutes. By the time I had found the right leads for DMM #2 and hooked it up to the emitter of an OP TR, the left bias had jumped to 17 mv and OP TR/E was 19mv (conducting?). I adjusted VR03 and got bias down to 14.8mv and then it started back up.

    I put a temperature probe on the rear housing (collector) of the OP TRs. All four started at ~28°C. Both of the left ch ones started slowly climbing and reached 60°C after 19 minutes. Both of the R ch ones are still mounted to the heat sink and only climbed to 30°C.

    L ch OP TRs climbed to 67°C at 21 minutes, so I shut it down.

    Could the heat sink have made that much difference???
     
  14. vigman

    vigman SUBSCRIBER

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    So wait a sec... are you running the left w/o a heatsink ??
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yeah - kind of an expected result - no thermal circuit. :scratch2:
     
  16. petehall347

    petehall347 the brandy coffee man

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    its never a good idea to run transistors without the intended heat-sinks .. you can and will get what's called thermal runaway .hotter they get more they conduct so they get even hotter and conduct more .etc etc etc bang ...
     
  17. Grainger49

    Grainger49 Old Fart

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    Pete, I like the Avatar.
     
  18. ghazzer

    ghazzer Sansui addict Subscriber

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    Another lesson learned. I did not realize it would get that hot that quickly.

    I have some heat sinks that I can attach to the OP TRs while the F-3058 bd is external. That will make it much easier to run these trials and take measurements, and eventually replace the Sankens with a set of OnSemis that match the ones in the right channel. I'll take readings on those as well.

    The 3058 bd was completely installed when the L ch OP TRs blew about 6 weeks ago.
     
  19. vigman

    vigman SUBSCRIBER

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    If you MUST run externally.... you need to fab a temp heatsink and maybe some smallish fans...... Wrench N Learn..GOOD JOB~
     
  20. ghazzer

    ghazzer Sansui addict Subscriber

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    Test set up

    Here is what I have lashed together to take voltage readings of the OP TRs. My goal is to test several sets of these transistors, verify that they are working, and determine what bias level causes them to start conducting. Function switch will be at AUX, no input, and volume control at minimum (max attenuation).

    I'll remove the OP TRs from the left ch F-3058 bd and mount them direct to the heat sinks with an absolute minimum of thermal grease, then solder jumpers from the 3058 bd to the transistors. Measurements will be recorded for E/C/B of each transistor and the bias voltage across the emitter resistors as I adjust VR03.

    I was very careful to keep the heatsinks isolated after it sank into my feeble brain that each collector is tied to B+ or B- and the only thing separating them when installed in the chassis is the thin mica washer. I have omitted the washer to get maximum heat transfer to the heatsinks, and can vary the thermal conditions by moving the fan.

    Any suggestions/comments will be appreciated, and I'll wait for them before starting the test.

    [​IMG]
     
    Last edited: Apr 15, 2017

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