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powe fuse issue on 771

Discussion in 'Exclusively Sansui' started by Campbell, Jan 22, 2018.

  1. LBPete

    LBPete Rolling Along Subscriber

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    Don't use the cap that was installed backwards. It should be replaced. Yes those white ceramic resistors are the emitter resistors for the output transistor.

    - Pete
     
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  2. Campbell

    Campbell New Member

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    Of course. Will use Nichicon 4700uf or anything up to 8600uf 50v as recommended at $7.00 ea. cdn at local electronics store. Thanks :)
     
  3. Campbell

    Campbell New Member

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    Hi Pete. Here are the results of the NPN 2SC1402 transistor and emitter resister test. All were done " in circuit". If they were anything else I would rip them out for recheck but it seems ok to me so no removal was done.....yet, except 1 resistor to double check the meter. ;)
    Base to Emitter tests first then followed by Base to Collector then resistor values. Delta seems appropriate for base to emitter vs base to collector . All other vdrop readings are out of range as necessary.

    TR1 .606; .597; R15: .8 ohm - lead test at .4 ohm = .4 ohm or thereabouts. Some retests got as low as .7 ohms likely due to oxidation on the resistor leads.
    TR3 .619; .614; R17: .8
    TR4 .620; .616; R18: .8
    TR2 .609; .602; R16: .8

    Emitter resistors, although in isolation should be "within range" of .33 ohms in theory if tested out of circuit; my in circuit test revealed .7 to .8 ohms for all 4 . The test lead reading on my cheap MM is .4 so actual reading minus lead resistance is likely closer to .4 ohms but I can't dial in closer than that with the 200 ohms setting. My even cheaper analog meter wouldn't stabalize enough to give me reliable readings. Disconnecting one resistor for a sanity test gives me .4 ohm so I assume all are ok or close. None appear open. So what now Pete? Is there more I need to test or is it time to insert the filter caps and connect to a dim bulb tester? Can I power up slowly with speaker switch to "off" or do I need to connect speakers?
     
  4. LBPete

    LBPete Rolling Along Subscriber

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    First question, why are you testing output transistors and emitter resistors? What is the problem you are having with this receiver now?

    Next question, when testing diodes or transistors with the diode test function of a multimeter, the display is showing the forward bias voltage. It's not a resistance value. The numbers you listed are within a normal range but you need to reverse the probe polarity to ensure there is no voltage in the other direction. You need to check emitter to collector too. Also, I wouldn't test them in circuit.

    If you haven't done so already, it's time to build and use a dim bulb tester. They are simple to build and can prevent accidents that will let the smoke out of components. There are lots of tutorials on line for both building a DBT and checking transistors.

    - Pete
     
  5. Campbell

    Campbell New Member

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    Thanks Pete. I did these tests only based on previous forum suggestions as I have yet to be really sure of why the unit blew a fuse in the first place. I can only assume a double diode failure ( 10DC-1 or 10DC-1R or both as I found 1 failed of each) or a filter cap failure caused the fuse to go, and not an output transistor failure as they all check well within range. I have already built the DBT (properly) and tested it on another unit and have read all the tutorials on diode & transistor testing. I performed the 6 way transistor test ( in circuit however ) with no probe reverse readings whatsoever. With such readings I fail to see why I should rip them out to test them again. Had any 1 reading been out of the norm whatsoever I would have redone them all out of circuit. I will get new filter caps ( hard to find in Montreal ) & I will then proceed with a DBT to see what happens. Thanks for the advice. This is a new area for me and I am not a technician.
     
  6. LBPete

    LBPete Rolling Along Subscriber

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    Just checking. The bad diodes could certainly blow a fuse. So could having a filter cap in backwards. Based on what you posted the transistors are good. If you have replaced the filter cap(s) since the installation issue, and the new diodes are installed correctly, power it up on the DBT and watch the bulb. It should dim down quickly. If it stays bright, shut it off. Use a 60 watt bulb for extra safety.

    - Pete
     
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  7. Campbell

    Campbell New Member

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    Some progress. Ok. I finally installed new 8200uf 50v filter caps. I powered it up with a 60 watt bulb DBT, speakers off, volume off. Everything looks good dim bulb wise. I did hear the speaker relay kick in after a few seconds on first power up but it didn't seem to do it all the time after that. I did this a number of times leaving it on for longer periods (10 - 30 seconds or so) with no issues that I could see but then I did not hear the speaker relay any more. This bothered me. The bulb still went out and did not change. I even put in a 40 watt bulb to see it "glow dimly" once the caps charged and it remained at the same intensity. I put the 60 back in. I tried using headphones to see if any radio sound but nothing. Same with CD player. Then I connected some 8 ohm speakers and switched to "A" and tried again. Small period of a second or two while the new caps charged then the bulb went basically out and then I believe I heard the relay click with the bulb going bright! I heard a small sound from the speakers when this happened and I shut it down asap. I did this 2 or 3 times which was probably dumb. I didn't have the guts to leave it on once the bulb suddenly flashed bright so I don't know if it would have dimmed again or if this was normal or not. I assume not given the reason for this test. Also with the volume off I don't think I should have heard anything so now I think I may have a shorted relay or something shorted in that circuit that is cut in once the relay energizes. On a positive note it appears my power supply issues are behind me and the diode replacements seem to work well as the tuner responds to station changes on the meter. Any suggestions as to what is happening or specifically not happening? No more testing until someone advises.
     
    Last edited: Feb 14, 2018
  8. LBPete

    LBPete Rolling Along Subscriber

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    You need to check the DC offset. If the protection relay is not kicking in, you need to measure the DC Offset at a point before the relay but before you do that, just measure the DC voltage at the speaker terminals. Set the volume to zero and make sure the speaker selector matches the speaker terminals you are checking. Put the meter across the terminals and turn the unit on. You should see some change on the meter when the protection relay turns on.

    If the protection relay is not kicking in, you need to measure the DC voltage between chassis ground and R9 and R10 on the power supply board. You need to be on the side of those resistors that connects to the diodes. If you don't have mini-grabbers, be careful not to touch anything else with your meter probes when you are checking that voltage.

    - Pete
     
  9. Campbell

    Campbell New Member

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    I assume this further testing is done with the DBT also?
     
  10. LBPete

    LBPete Rolling Along Subscriber

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    Yes, use the DBT. This isn't a final adjustment, its just seeing what it's putting out.

    - Pete
     
  11. Campbell

    Campbell New Member

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    Test for speaker out left A channnel: (positive lead secured to positive speaker terminal etc.with "0" reading)
    1st turn on is -.2mv with no relay action. Turned off after 10 seconds or so and waited 20 seconds or so.
    2nd on gives -.2mv but with relay action which immediately drops the voltage to zero. Turned off and waited 1 minute while taking notes.
    3rd turn on again gives -.2 mv with no relay action and 4th is same as third.

    Test for speaker out right A channnel:
    1st turn on is -.2mv with no relay action. Turned off after 10 seconds or so and waited 3 minutes taking notes.
    2nd turn on is -.2mv with no relay action.

    Stopped testing. Relay action inconsistent and once engaged there is "0" mv reading.

    Testing of R09 & R10: with speaker switch off and no volume & on D09 & D10 side.
    At turn on R09 shows 1.3 volts but no relay action. Turned off unit. Voltage bled off to zero over about 2 minutes.
    At turn on R10: shows 0 volts. Reset probe & tested a second time. This also gave 0 volts but relay cut in on the second test.
    Next?
     
    Last edited: Feb 16, 2018

     

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  12. LBPete

    LBPete Rolling Along Subscriber

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    If you are getting 1.3 volts at R09, that could be your problem. Also, you should have some voltage reading at R10. See if you can adjust the voltage at R09 down using the trimmers on the driver board. It will be one of the inner trimmers. Just tweak each of the two inner trimmers and see if the voltage changes. If it adjust, bring it down to near zero. Also check R10 and see if you get a voltage reading other than zero.

    - Pete
     
  13. Campbell

    Campbell New Member

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    Tweaking VR01 (left chan) affects no reading change for either R09 or R10. ( R09 remains at 1.30v and R10 remains at "0"v.)
    Tweaking VR02 (right chan) affects a temporary change of plus or minus .03 volts for R09 ( goes to 1.33 or 1.27 but settles back at 1.30 v. at any setting) but R10 remains at "0"v.
     
    Last edited: Feb 17, 2018
  14. Campbell

    Campbell New Member

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    See in circuit test results for driver board F1499 transistors and diodes. Transistors TR03 & TR04 results for cathode to emitter made no sense to me as they rose continuously on one polarity test. Doesn't look good to me IMHO. All the transistors TR05 thru TR10 showed strange results also, however they were somewhat consistent in readings regardless as some sort of a pattern appears. See the attached pic of the results. Diodes seemed ok to me. Also could not find any fault in VR01 or VR02.
    Gut feeling tells me I may need to do more testing on the power board.:dunno:
     

    Attached Files:

    Last edited: Feb 17, 2018
  15. LBPete

    LBPete Rolling Along Subscriber

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    We are heading down the rabbit hole here. The readings you found at R09 and R10 don't make sense and I'm concerned about the accuracy of that test. There are just too many variables to give you good advice you really need to know how to read a schematic to go further. As far as your transistor and diode tests, you can't get good results checking transistors in circuit with diode check function on a multimeter. That's why some of your readings don't make sense.

    I'm very concerned that more damage will occur if we continue this interstate diagnosis. It's time to get it to a pro.

    - Pete
     
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  16. Campbell

    Campbell New Member

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    Understood. Thanks for the help to date. Much appreciated.
     

     

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  17. Zekeman

    Zekeman Super Member

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    I just looked at this thread and was going to suggest those rectifiers...I have had two or three 771's where this was a problem. Glad he found it!
     
  18. Campbell

    Campbell New Member

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    However I still have a speaker relay not coming on line reliably and a channel showing no DC offset adjustability I think I have a simple power issue somewhere so I will go back to the power board for a closer look :).
     
  19. Campbell

    Campbell New Member

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    Had to think out the situation regarding the power board and after testing everything I could find on it, there was no obvious issue other than maybe a reluctant relay. Then I played with the driver board pots. Playing with the DC offset pots after flooding them with a drop of alcohol (and let them dry overnight) and tapping the relay a few times with a screwdriver handle today has changed everything! Both offsets now respond and stabilize at approx. +/-5 mv or less on both channels after warm up. Spec calls for zero +/-10mv :) (I guess new multi turn pots and a new relay wouldn't hurt). Now to try setting the bias again. Finally, 19 +/- 1 ma on both channels with 0 vdc +/_ 5 mv approx. Next a cold beer as a persistence reward and a speaker test to see if it actually works.

    Update. We have music again :) Thanks to LB Pete and a few others who guided me in the right direction. :beerchug:
     
    Last edited: Feb 21, 2018
  20. LBPete

    LBPete Rolling Along Subscriber

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    Glad to hear it cleared up. Give yourself a well deserved pat on the back.

    - Pete
     

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