AU-717 PSU voltages

Discussion in 'Exclusively Sansui' started by afroaudio, Jun 9, 2018.

  1. afroaudio

    afroaudio Active Member

    Messages:
    157
    Location:
    Cape Town, South Africa
    Went ahead and pulled the 3 pin connector (white grey yellow) next to the transistor that was heating up (TR13) the other 3 pin stayed connected. My understanding here is that I am pulling power from that channel on the EQ board so naturally that transistor will not be heating up any more...

    Voltages I measured in this state were a bit odd:

    On the RIGHT side of the board
    White to Yellow = 32.2Volts
    White to Grey = 1V

    On the LEFT side of the board I still have +/- 24V

    I am pulling some transistors and resistors on the EQ baord around TR13 as recommended by Hyperion.. so far have not found anything off, will pick up again tomorrow, the workshop is freezing, we have a big front pulling through so red wine and a fire seem like the solution!
     

     

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  2. afroaudio

    afroaudio Active Member

    Messages:
    157
    Location:
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    Well interesting day it was... I went at the EQ board again this afternoon pulling and testing some resistors and transistors I had replaced around TR13 then I decided to double check my fuse resistor replacements and..... drum roll..... I had installed 8.2 ohm resistors in all 4 locations! I had these resistors marked up as 82 ohm and somehow had not rechecked them before installing. Feeling very stupid I continued.. replaced with 82 ohms and went on through the circuit checking resistor values and transistor orientations.. all looked good.

    I did find one departure from the schematic with R31/R32 these are listed on schematic as 10ohm but mine are 1.5K definitely factory installed. Would be interested if anyone else has noticed this difference on their unit.

    Anyways, have patched everything back together.. powered up on DBT and amp comes out of protection, no more heat detected at EQ board (suprise!:)

    DC offset and bias adjusted then on to measuring voltages on PSU board (these measurements are on full mains voltage - no DBT)

    Chassis Ground > Yellow = 35.7V
    Chassis Ground > Grey = 34.5V
    Chassis Ground > Blue = 34.5V
    Chassis Ground > Red = 35.5V

    Soooo... some relief to have found a problem (of my own careless creation!)

    Only thing is these voltages all seem a bit on the high side, that and my power switch is arcing!

    Again thanks for the input gents! Your time and interest is much valued

    [​IMG]20180618_164750[1] by Bregtje Cals, on Flickr

    [​IMG]20180618_164621[1] by Bregtje Cals, on Flickr

    [​IMG]20180616_151537[1] by Bregtje Cals, on Flickr
     
  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Well done on finding your error - that's the sort of thing we can't account for when helping you online - unless copiously supplied with pictures good enough to see that kind of stuff. :)
    Yes they are a bit in the high side, not really a problem unless you have 35V rated capacitors C23/24/25/26) on the outputs of the regulators. Most people go up to 50V rating for these positions, to give a bit of leeway.

    Don't forget there are 4 x 82Ω on the F-2720 PCB (tone controls) and another 4 on the F-2723 (phono amplifier). ;)
     
    Last edited: Jun 18, 2018
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  4. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Poor TR13, can just see it whining that resistance is futile :)
    Anyways well done.
    Power supply voltages are fine (and symmetrical), might push 35v caps as John noted.
    Remember taking the Blue train from Johannesburg to Cape Town back in 1972 and that cable car ride to the top of Table Mountain on a breezy afternoon, holidays in Durban and matriculating from Damlin college before returning to the U.S. to start college.
    Still make my own Boerwors too.
    -Lee
     
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  5. afroaudio

    afroaudio Active Member

    Messages:
    157
    Location:
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    Hey Lee.. those sound like some great memories there from the old days.. a little before I was born :) Cape Town and South Africa are still beautiful and special in many ways.. but of course not without problems! Cape Town is very different now to when I was growing up in the 80s thats for sure! And very happy to be talking across continents with a man that makes his own boerewors! Bravo! (thats our local sausage - to be cooked only on the braai aka barbeque)

    Back to the amp.. gents thanks again for your support here, and being so generous with your time and hard earned skills.. its what makes AK a great place!

    I re-assambled the amp and have been enjoying it on and off this past week when I've had time. I am not disappointed.. this is the highest definition sound I have had from any Sansui, and this comes completely without any harshness. This is also the first time I feel the low end of my Tannoys coming alive and the cones are well controlled, bass is still tight but extended.

    Being something of a perfectionist though, and wanting to get the job done very thoroughly, I am still considering trying to balance the voltages on the regulator board so that they are equal. Apparently a good place to start with this is using precision metal film resistor pairs at R27/28 R29/30 R31/32 R33/34 R35/36.. any pointers here welcome

    The other thing I would like to ask is, is it worthwhile matching the "differential pairs" of transistors in the preamp and phono stages? I have done this before for the driver boards in the AU-999 and it did seem to make a difference. (on advice of member stereofun) If this is considered beneficial please could someone point me to which pairs to look at?
     

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  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Firstly I would check the voltage on ZD01 & ZD02 - you may have a 13V +5% zener in there which could be causing voltage issues (I.O.W at the top end of its tolerance range). If that is the case then try a different 13V zener, however, if you are determined to get those voltages to +/-33V it might be simpler to use a 12V zener instead.

    For fine adjustment R31+R33 & R35 are the targets for matching the negative to the positive regulator, and R32+R34 & R36 for the other regulator. Changing to metal film resistors for those resistors alone (+maybe R27&29 & R28&30) could make a significant difference - try that next after checking the zener voltages. (If you want to try adjustment try making small changes to either of the values of the R numbers in bold).

    Matched pairs? - well, you could try TR01 & TR05 and TR02 & TR06 in both the Phono Amplifier and the Tone Control amplifier - (same TR ref numbers in both). But don't be surprised if you can't actually 'hear' any difference. :)
     
    Last edited: Jun 26, 2018
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  7. afroaudio

    afroaudio Active Member

    Messages:
    157
    Location:
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    Well after a bit of a break.. (I started messing with my very dirty AU777) I finally got around to replacing these resistors as per Hyperions suggestion.. all with 1% metal film, this yielded little to no change in the voltages each side, I then changed ZD01 and ZD02 from 13V to 12V Zeners and this brought voltages down to where I am at now:
    Blue/Red side: - 31.6V + 32.6V
    Yellow/Grey side: - 31.5V + 32.4V

    Not wanting to play with this board forever and risk damaging tracks with repeated soldering/unsoldering... do you gents think I am sufficiently in the ballpark with these voltages or would there be some benefit to balancing further? Would be great if I could narrow it down to one resistor on each side that was causing the 1V spread.. be great to get everything to say 32V.

    Amp is sounding fantastic by the way, most satisfying rebuild I have done to date.. working on the AU777 seems quite trivial now!
     
  8. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    For the sake of argument use R31 & R32 - reduce the value (recommend you do one side at a time) to 5.6KΩ or 4.7KΩ and see what you get. But really it isn't necessary to do this now that you have the voltages below 35V and much closer to the 33V on the schematic.
     
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  9. afroaudio

    afroaudio Active Member

    Messages:
    157
    Location:
    Cape Town, South Africa
    Thanks John

    I'll see if I have the appetite for more fiddling with those resistors, thanks again everyone for your help here
     
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