SA-7800: another one bites the dust.

Discussion in 'Pioneer Audio' started by USMC Spike, Jan 8, 2017.

  1. USMC Spike

    USMC Spike Active Member

    Tejas, North Central

    Got a SA-7800 from neighbor (15 years ago) as its wood cabinet was 1/2 broken off and
    it had seen better days. That said I used it periodically for TV duty. It kept getting worse and
    worse in the one channel, so it was removed from service placed in the back room for a year.

    Recently, opened her up and took a look inside, figured I'd go through the manual and give
    the amp a tune up. I followed the instructions for adjusting the output transistor bias and that
    is when the fun began.

    Setting the left channel was without incident performed as specified.

    Setting the right channel resulted in the sacred smoke coming out of
    R58, R60, the 150 ohm emitter resistors, that appear to be the drivers
    for Q2,Q4.

    Well shame on me, Operator Error. So I put in a pair of metal films
    and brought her back up...ta dah. More sacred smoke...I felt powerful!


    So I brought her back up with a variac to 40 volts AC on the main line.
    This resulted in a B+ in the amp of 20.6 Vdc measured at pin 1 to chassis ground.
    Total current draw measured 0.1 amp current draw through the Variac.

    Right channel Q2, Q4 readings not good, strange reading on the one emitter
    resistor, other well it is what it is.

    So I then measured the two transistor rows below Q2, Q4, and compared
    them to the two row below Q1, Q3. and they were the same. Okay, then what?

    Did a search and found some pretty good stuff here.

    So, I pulled the outputs, Q2, Q4, and their emmitter resistors R62, R64.
    Tested the pulls, Q2, Q4 are dead, short BCEs on both.
    R62 short, R64 changed value, higher measurement.

    Time to order replacements for all for output device and all four
    emitter resisters. Trying to find what exactly to order though
    and there appears to be some going back and forth about which
    sets to use.

    I can get standard non inductive wirewound in .22 2W...any one source the Nobel resistors?
    Then question what is the failure mode of standard Vs. Nobel? Nobel seems to short out.
    I would think standard wirewound should open? Would that be better in this amp?

    Finally, I have most of the small signal transistors here at the bench along with most of
    the 'lytic caps, except those specific to this amp.

    Once I know which outputs will fit, I'll order them...and emitter resistors.
    The goal is to get the Right side working first, before doing any of the other
    updates to the amp.

    So, just for the record I have the following specs & measurements.
    These were measurement take with the faulty component in and
    then with them removed.

    GRW 112, Measurements (HP34401A) from the following pins:
    Pin 15,16,17: 48.6 / 48.6
    Pin 19,20,21: -47.4 / -47.4
    Pin 29,30: 32.8 / 44.7
    Pin 31,32: na / -44.7
    Pin 22: -1.9 / -1.6

    These AFTER REM values are all within the expected tolerances.

    The following were measured (HP34401A) on the power supply board GWR-112
    Pin Number, expected / measured values:
    Pin 21: - 48 / 13.9....will check again. This is the circuit with the failed components.
    Pin 15: +48 / 48.5
    Pin 29: +46 / 45.4
    Pin 32: - 46 / -45.7




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  2. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Here are pics to start the show.
    Nothing that y'all haven't seen before:

    1.90 degree switch to start things off.

    2.Then a badside of the power board.
    Check out the little 150 ohm resistors
    (the ususal suspects) these were after
    the initial replacement. Nice black center band.

    3. The good side of the board.

    4. The take along power emitter resistors.
    Guess which one is the bad one?
    Hard to tell I underlined it in red.

    5. I like ti keep thing simple and this simple device
    helps guys like me keep things running smoolthy.
    shorts an both outputs.

    Attached Files:

  3. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Just a few more for documentation purposes so anyone who cares
    to follow along can. There's a shot of the Tone Amp raised
    above the other board. GWG-123, with the six orange 'lytics,
    two on right are the BP (Bi Polars)
    I think the center two lytics might be low leakage
    and then two at the lower left are Elna BP caps
    next to the two dual fets.

    The chemi cons are generally very good caps.
    If there are numbers stamped into the tops
    those caps are "qualified". I suspect these are
    no different.

    Then there is the pic of the board with the Transistors removed
    nothing special, after removal, I wick off the excess solder.
    clean with ISO.

    I've got some good before cleaning up the front panel and knobs,
    which I'll post when I take the after pics. It cleaned up nicely though
    there are still a few scratches and nicks, but not bad at all.

    My little girl couldn't figure out why I wouldn't let her wash
    it in her sink!

    Attached Files:

  4. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Las Vegas Nevada
    Good job so far.
    You'll find a DBT will serve you and your set better than the Variac.
    Mouser has what you need.
  5. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Hi Zebulon1,

    Yes, I do have DBT and I find a Variac will give me more control
    and data while the DBT won't. They both have their place and both
    work, albeit in different ways.

    I'll post a pic and you'll see what I mean.

    The Variac on the left lets me see both the
    AC Volts reading from the main line and
    AC Ampers show's me the current draw.

    I've also got some DC power supples that will
    help me also.

    My original plan was to fire up the SA-7800 and
    take berore and after repair and upgrade distortion
    measurments. That didn't seem to go as planned.



    Attached Files:

    Last edited: Jan 9, 2017
  6. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    How can you not appreciate a guy who posts something like this.
    Just a good reference so we are all on the same page thanks
    Zebulon1, and thanks for the source too.

  7. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Here is a before and after pic of the faceplate.

    Nothing but a little soaking in water and a bit of
    dishwashing liquid, then a bit of spit shining the
    old school way. washing liquid?

    She never had it so good.

    01.12 Before After.jpg

    Postscript, now that it's all cleaned off, it is wapped in
    stretch "saran" wrap so it doesn't get ruined before I finish
    and reinstall or "other issues" turn up.
    Last edited: Jan 10, 2017
  8. tarior

    tarior Dirty pool, old man? Subscriber

    Kelso, Washington
    I guess you found out to never, ever touch the bias trimmers in an old Pioneer NSA amplifier. If they glitch even in the slightest, poof! go the outputs, drivers, emitter resistors and a few other odds and ends.
    You're gonna need some solid chops to make that one work again, or have Mark (thefixer) walk you through the repair.
    There are like a handful of guys in the whole country who can properly fix one of those, and Mark is probably the only one who can (and is also willing) to do it over the internet.
  9. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    @ Tarior, Thank you my brother for being a week late and C-note short!

    I guess I did, shame on me! They should have put it in the F-ing manual...
    Don't touch the f-ing trim pots....They should have just glued the fricking
    things...actually they should do it the method we all love and revere,
    I'll identify here and forever after it will be known through out the

    "The RonCo Method"

    How's that people?

    People: "You Set It And Forget It."

    So when I get close set it, I'll pull the trimmers and put in resistors.
    Now I've got to go look and see if they are rheostats or not?
    or if I've got to use two resistors each pot, to make a voltage divider.

    I just need to know currently the folowing questions:

    1. What are the optimal output transistors for this amp?
    2. Does just any old non-inductive wire wound work
    or are they Nobel Specific or similar ceramic resistors.?

    Any one who knows, please stop by and share.
    I"ve spent the last few days reading through threads
    finding information, etc. And as I compile my notes
    and post them here for others who will follow after,
    if you see an error or mistake, please speak up.

    Know that it isn't just helping me, but the whole community
    here and I'll try to document and make it easy for the next
    guy or gal down the line.

    Also know that I'm the lucky guy who's back at my community
    college and I'll be starting my third electronics course next week.
    I've completed DC Circuits, AC Circuits, and will be starting
    Solid State Devices using the electron flow method.

    FuBAR? That is ok, but please don't invoke Murphey.




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  10. tarior

    tarior Dirty pool, old man? Subscriber

    Kelso, Washington
    It's one of my pet peaves: every time a member posts up having a problem with a receiver or amplifier, the first 10 responses go something like "didja twiddle the bias and DC offset?"
    Bias and DC offset have fuckall to do with channels dropping out, odd behaviour, noise, distorted sound, lamp failures, peeling veneer, scratch controls and so on.

    The problem with the trimmers in old gear is the same as the volume pots, they get scratchy. Only when a pot that is in the bias string gets scratchy, particularly in this amp design (some Sansuis do it too), it instantly turns on the output transistors to full power, thus releasing the magic smoke. Replace them with Bourns 3299 or 3386 with the correct form factor and value.

    I've repaired a couple of amps of that design (SX-3900, SX D-5000), and have refurbed a couple more that weren't smoked. But, I am pretty bad about taking notes. IIRC, any ceramic wirewound resistor will work just fine on the emitters there. Go with non-inductive if it makes you feel better.
    I'm going to refurbish a working SA-9800 soon and do a writeup. I'll cover all of the measures that would have prevented this tragedy. I also have SA-7800 that I'm going to repair at some point in the future. I suppose I should document it here just for posterity.
  11. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    @Zebular1, Tarior, others,

    Tell me about DBT usful ness. please. I also suppose I'll
    need an assortment of bulbs as well. 25W, 50W 75W, (100W installed).
    Or is it the 3-6-9 bulb and socket ? AKA three way.

    I'm sure y'all are blue in the face already,
    I am not, yet, but I'm sure will be at some point.

    So if you don't mind sharing... with an older guy
    approaching sixty with a three and one-half year old
    little girl around his neck and calling out his profs
    when they're "incorrect" and competing with the
    young whipper snappers and using math he forgot
    45 years ago...

    ...I'm all ears.

    I gotta get it in now or I'll forget.
  12. tarior

    tarior Dirty pool, old man? Subscriber

    Kelso, Washington
    The DBT is just a thermally variable current limiting device (light bulb). It prevents the unit from drawing enough current to destroy the ouputs in the case of a serious fault in the amplifier.
  13. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Tarior, yeah I see already how you are. You waited till post # 8 to tell
    me I should change my name to FuBAR. Well you know close enough. :)

    Kind of like how we'd alway's heed our mothers words
    especially when we knew beyound a shadow of a doubt
    they were 110% correct, for example:
    When I was a young man I received this
    warning from my mom ( long departed now)
    but her words are etched into my brain,
    like groves in vinyl. Stay away from
    the divorced women, she warned.
    Why? my disinterested reply.
    "Because they know all the tricks", she said.

    Did these words just come out of a burning bush somewhere?
    Were they written on stone tablets?
    An alcoholic looking for the next drink?
    A junky needing the next hit?

    It was as if the heavens opened up to me for the first time.
    I can't recall exactly the situation I was in at that
    moment, I just knew that I had to freeze, not move,
    not turn or look or do anything like the Cubs winning
    the World Series, even though it was a Revelation.

    So I did nothing,
    I didn't move,
    I looked where ever I was looking
    and I uttered these life changing words...


    then I kept on keeping on.
    While in my head I heard that guy screaming out

    ...YYYYEeeeeaaaaaaooooooohhhhh, I feel goooooooood,
    with the saxes blaring out....
    With four or more decades of hits, singing the blues, the rhythm
    and countles hits, but I can't remember the guys name. though.

  14. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    While that thermally varies the current, I don't see the difference between that and watching the
    AC Amperes meter on the variac and turning down the knob when it starts moving full scale.
    Unless these amps pop like unloaded HEXFreds?


  15. tarior

    tarior Dirty pool, old man? Subscriber

    Kelso, Washington
    The DBT will still deliver 120 volts. Not to mention it costs like $5 to build, you only need one bulb, 100 watt. Variacs are spendy. I have two of 'em, and I've never used them in 15 years. BTW, I didn't even see this thread until post #8.
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Glen Burnie Md.
    James Brown!! My neighbor sang backup on one of his songs way back in the mid 50's( I cant' remember which one now) before he went into the Army, Made SGTMAJ, had 3 daughters (2 of whom retired as Col's) and is now 80 and running circles around me at 61. I call him SgtMaj and he calls me Doc. I Swear it's that MILSPEC Gene !......I still slip up and call my Dad CHIEF, although I append Geezer to the front now. He's 83.
  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Glen Burnie Md.
    DBT's are a poor man's amp meter. It will ramp up immediately when a short happens and faster than a meter does in most cases.

    Kind of like changing barometric pressure on a diesel electric sub that is snorkeling. When the head valve closes the engine's take the air from the hull inside, and the barometric pressure dropped. They used an altimeter for this. But with 50 or more gauges to watch the enginemen didn't always have an eye on it plus it took a few seconds for the altimeter to wind up to altitude. In the mean time the engines were creating a vacuum in the boat and wiping compressor lobes on the engines. So some enterprising EM took a milk bottle and put a condom on the top. When the head valve closed the condom stood straight up, with extreme VIGOR (it was painted RED) and the EM's knew the head valve had shut usually before the altimeter had a chance to wind up to whatever altitude the pressure related to and they shut the engines down. The story has it when the Admiral came on board to inspect the boat, it was on display in Maneuvering and he laughed his ass off when they explained it to him. Typical sailors thinking with their D***'s ..
    TRUE STORY! It was on Grampus (SS-523) in 1950 when they crossed the Pacific from Hong Kong to Pearl, submerged.


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  18. markthefixer

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

    DBT's perverted use is to make different voltages by balancing it's wattage versus the unit's wattage.

    DBT's INTENT is to save yer ass when something cuts loose - and it's damn faster than human reactions watching an ammeter - I know, I use BOTH.

    Normally it will sit there (with a high wattage bulb) duin nufffin... But when the unit tries to do a big gulp of power, the see saw tilts and the bulb strangles the unit.

    The closest thing I can offer personally to military service is the unrelenting laws of physics, and the ownership of a double stack .45 along with a 92.
    I was of that narrow generation that the selective service didn't want to know about.
    USMC Spike likes this.
  19. USMC Spike

    USMC Spike Active Member

    Tejas, North Central

    I was just joking around... I figured as much. I've got both it should be in one of my pics, I think after
    the Babe by the van post.
  20. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Gentlemen, it's all good.
    Here is the first transistor list that I compiled. On one page and in PDF format so no one
    can screw it up. I hope I changed the problem 2SA1914 to proper 2SC1914 and moved
    it down to the 512 KSC1845EBU.

    Anyone know the lastest final Output transistors it use in these?
    I don't have a problem affixing them at 90 degrees.

    Has any one made the MT-200 to fit without redrilling or trimming the board?
    Anyone ID the correct MT-200 device to use?
    I will include both in the list.
    I also don't have a problem making more tables for the other boards in that amp
    and another one for capacitors. Please let me know ASAP because my classes
    start back next week.



    Attached Files:

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