SA-7500II recap

Discussion in 'Pioneer Audio' started by loweran, Oct 6, 2017.

  1. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Starting on the control board, I am getting the following readings. Should I be worried about Q1 and Q2 Base legs voltages?:

    Q1
    B: 7.6 mv - (0v)
    C: -16.48 v (-16.4v)
    E: .515 v (.6v)

    Q2
    B: 9 mv - (0v)
    C: -16.37 v (-16.4v)
    E: .523 v (.6v)

    Q3
    B: -16.42 v (-16.4v)
    C: 2.12 v (2v)
    E: -17.06 (-17v)

    Q4
    B: -16.41v (-16.4v)
    C: 2.16v (2v)
    E: -17.05v (-17v)
     
    Last edited: Oct 26, 2017
  2. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    And here are some main amp readings:
    Read pin: reading: (spec)
    Outputs:
    Q13
    B: -.54v (-1.2v)
    C: -42.2v (-40v)
    E: 26.9 mv (-.6v)

    Q14
    B: -.513v (-1.2v)
    C: -42.4v (-40v)
    E: 40mv (-.6v)

    Q15
    B: .620 v (1.2v)
    C: 42.2 v (40v)
    E: 51.8 mv (.6v)

    Q16
    B: .6v (1.2v)
    C: 42.2v (40v)
    E: 51.5mv (.6v)
    --------------------
    Q3 (the double transistor)
    B1: 93.1mv (0)
    B2: 82mv (0)
    E: .688v (.6v)
    C1: -46.8v (-47v)
    C2: .691v (-47.6v)

    Q4 (double transistor)
    C1: -46.1v (-47v)
    C2: -46.7v (-47.6v)
    B1: 103mv (0)
    B2: 89.9mv (0)

    E: .695v (.6v)

    Q5 - OK
    B: -46.5 (-47)
    C: 41.8mv
    E: -42.2 (-47.6v)

    Q6 - OK
    B: -46.1v (-47v)
    C: -1.13v (-1.2v)
    E: -46.6v (-47.6v)

    Q7 - OK
    B: 39.45v (39.6v)
    C: 1.217v (1.2v)
    E: 40.12v (41.2v)


    Q8 - OK
    B: 39.43v (39.6v)
    C: 1.219v( 1.2v)
    E: 40.09v (41.2v)

    Q11 - OK

    Q12 - OK
    B: 1.22v (1.2v)

    Q17 - OK
    B: -7.5mv (0)
    C: 20.7v(26v)
    E: -.627v (-.6v)

    Q18
    B: -3.9mv (0)
    C: 20.65v (26v)
    E: -.627v (-.6v)

    Q19
    B: 21.19v (19.5v)
    C: -1.621v (-1v)
    E: 17.90v

    Q20
    B: -1.623v (-1.0v)
    C: 9.65v (9v)
    E: 0

    Q21
    B: 9.65v (9v)
    C: 9,6v (34.5v)
    E: 8.92v

    Q22 - OK

    Q23 - OK

    Q24 - OK
     
    Last edited: Oct 25, 2017
  3. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Going to replace the double transistors and see if that makes a diff. Otherwise I’ll be pulling resistors and checking values next.
     
  4. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Swapped out the double transistors and getting 36.5mv on left speaker and 44.5 on the right speaker terminals.

    Bias are 61mv for left (TP2 TP2) and 16.2v on the right (TP3, TP4).
     
    Last edited: Oct 25, 2017
  5. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Well, everything looks right. I looked it all over again. Get the relay click but got no sound now. May need to clean the switches again.

    I pulled the suspect resistors and all tested spot on. Must be some transistors somewhere. Not sure how to trouble shoot at this point. May shotgun the transistors.

    [EDIT] Looking at my measurements, it seems like most the transistors are correct voltages, or close. The exceptions are:
    The outputs Q13 - Q16
    Q3 - I need to take this one again and verify since I swapped it out
    Q21 - This looks like the relay driver. It's measuring 9.6v on the collector. The relay is engaging.

    Any thoughts on where I should look?
     
    Last edited: Oct 25, 2017
  6. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Put in an order for new transistors and new relay and a few resistors that I think I may need. More testing is required. I may need some help on this one as I am quickly getting over my head. I wish I knew more about how to read the schematics and follow the voltages. Must study up on that.
     
  7. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    163
    Location:
    Maine
    loweran,

    Based on your 1st post in this thread, it seems amp was working fine before you began the recap and replacement of certain transistors. That would seem to me that a your issue must have something to do with a component you have replaced or a suspect solder joint. You mentioned you replaced the dual 798's with 992's. Did you receive your tester and gain match the four 992's prior to replacing? Have you rechecked Q3 pin outs since you say you swapped it out?

    If it were me, I might re-install the original 798's and original Q3 and see if you can get back to original working order. Does the protection relay pull in after the normal 4-5 sec? Hopefully someone more knowledgeable will chime in.
     
  8. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    The amp did operate, but sounded off in the right channel. The relay was and continues to engage slow, like 7 seconds. Maybe a tired relay driver. Also, the sound does not kick in immediately when the relay kicks in, which I have read that it could be breakthrough. I have a replacement on order.

    I replaced all the e-caps. I walked back through the board and verified placement and values. They are all right. The only transistors I replaced were the 2 2sa798s. I did very carefully install them per the silkscreen. I did match them and they did not seem to make much of a difference. Before replacing them it was 23VDC on the left (TP1, TP2) and 60VDC on the right channel (TP3, TP4). After replacing them they are 61mv for left (TP1 TP2) and 16.2v on the right (TP3, TP4). Seems they flipped. I could replace them, but they are on the known suspect transistor list. I am studying the schematic and think there are some other resistors that need to be checked. Need to verify R33, R27, R37, R36, R38, R34, R28, R30... basically all the resistors that connect into those transistor's pins.

    I am going to take more reading to verify, and also test some resistors. Most the transistors voltages look acceptable. There are several that should be at 0v on pins, but read in the low mv, Not sure that is a concern.

    I am likely going to need some help from someone to help me diagnose this if it goes much farther.
     
    Last edited: Oct 26, 2017
  9. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    I have been studying the schematic and something does not make sense. Q3 and Q5 are tied, Q5 emitter to Q3 collector, Q5 base to Q3 collector. My Q5 readings were good, but one of the collecors on Q3 was off. I need to take another pass at the readings. Is it even possible that Q3 reading would be different than Q5 given they are connected?

    Screen Shot 2017-10-27 at 9.19.34 AM.png
     
  10. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    I want to add that the set passes the DBT test and again, the relay is engaging. Should troubleshoot while on DBT or am I OK to work on line power?
     
  11. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Took some measurements again:
    Q3, Q5, Q4, Q6 look OK
    All outputs at the collectors are at +-40.6. The other legs of outputs are low. Not sure why. Q21 collector at 9v. Going to swap the relay and see what happens.
     
  12. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Swapped the relay and relay driver. Sweet music! Going to button it up and put it on real speakers for a spin.
     
  13. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Wow, sounds fantastic! Good punchy bass, smooth highs. Playing through my Epi 110s. Good match! Might have to A/B it with my 737.
     
    Oldsansui441 likes this.
  14. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,200
    Location:
    Victoria, Australia
    Nice work, well done :). Bet you are pleased to have it buttoned up.
     
  15. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Thanks man. I am happy to get it going. And as a bonus I have all these extra transistors I didn’t need. Will add them to my stock for the next project! I think it was the relay all along. Went ahead and replaced the driver for good measure.
     
    Oldsansui441 likes this.
  16. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Noticed some odd behavior with it. It seems to take a while for audio to get going after it's been off for a while. Origionally thought it was the volume pot. I scrubbed it several times and its still the same. Once it warms up it sounds fantastic. None of the knobs and switches have any static, and everything works as expected otherwise. The phone section is strong. Need some more time with it to sort it out I guess. Seems like when I fidget with the volume knob, the right channel starts playing, then eventually the left. Think it may be the volume pot?

    Any ideas?
     
  17. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    Messages:
    20,730
    Location:
    Bensenville,Illinois
    Did you remove the volume control from the board it's mounted to, to work on it?
    How are the volume control's solder joints?

    Have you have deoxited and then faderlubed the volume pot?

    I ask this because the word deoxit only appears in this thread in MY post.
    Cleaner ISN'T deoxit. Different chemistries. Cleaner gets you clean corrosion...
    The volume control still has the commutator for the movable tap, to accumulate corrosion.

    AND you deoxited ALL the other switches, as well? Most of the switches on the bottom row can kill the sound when they are dirty.
    Stereo/mono fails to stereo, loudness won't compromise the sound level, just the tonal balance (not your symptoms).
    The rest - yep... YOUR symptoms....

    Can you "fidget" the other switches without affecting the symptoms?

    This unit only has two pc boards, and internally they are wire wrapped together.
     
  18. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Hi Mark,
    I did not remove the volume control from the little board it is mounted on. I did pull it away from the chassis to get access to spray deoxit into it. When I clean switches and pots, I alway use Deoxit D5 and finish it up with Deoxit faderlube. I cleaned it 4 times and worked it really good.

    I cleaned all the switches and pots, but only twice. Working any of the other controls does not seem to have any impact on the symptoms. That is why I suspected the volume pot.

    Yes, it has two board. A control board, and a main board that has the power, protection and main amp section on it.

    I am going to try and look at it again today. I may just go ahead and clean all the controls one more time to see if that clears it up. However, when I was looking at it yesterday. I was able to get the audio going by moving the volume control back and forth rapidly. The odd thing is, once it gets going, the volume control works perfectly. There is not static, or odd behavior with any of the controls once I get sound. It's almost like the voltage has to increase in the caps before there is sound, but I am not sure that makes sense.
     
  19. redk9258

    redk9258 Super Member

    Messages:
    1,161
    Location:
    Illinois, close to St. Louis.
    It sounds like a bad protection relay but you said you replaced it, right? Like Mark mentioned above... Check the solder connections.
     
  20. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    163
    Location:
    Maine
    loweran,

    Just another thought. Have you tried listening through headphones when you experienced the no sound issue through the speakers? If you have sound through headphones and not speakers, that could indicate a faulty/dirty speaker selector switch since the headphone jack signal is prior to the selector switch.
     
    redk9258 likes this.

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