What caused these sloblo fuses to go. Fisher RS-2004A

Discussion in 'DIY' started by darkhosis, Jun 27, 2018.

  1. darkhosis

    darkhosis Active Member

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    OK, I reassembled the amp and tested voltages with the DBT in line. Is that useful information? Im measuring everything OK except for the DC output from the mains - 33 V DC (9V under spec). It's affecting the Power Amp board, anywhere where it's supposed to be +/- 40 V DC it's measuring the lower by.. about 8-9 V.

    Oops, nevermind. Took the DBT out of line, it's up to 42 V DC.. offset DC is 50mV DC :(
     

     

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  2. elnaldo

    elnaldo Addicted Member

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    Are the 20.7 and -20.9 volts OK? If so, I'd think the AC is OK at that rectifier.

    You can always desolder the +V and -V pins at the STK. Sometimes desoldering the IC lead from the PCB hole is enough. Sometimes you can desolder some wire bridge at the PCB traces. Doing this you could check if one stk is dropping the supply voltage.

    Also, if the unit works OK, and nothing is overheating, perhaps you can use it for a while and see if it deveals some fail.
     
  3. elnaldo

    elnaldo Addicted Member

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    Ok. Posted while you was writing. With the DBT voltages are lower. I didn't know you was measuring with the DBT.

    50mV DC OFFSET is aceptable for an amp without DC OFFSET adjustment. You can try swapping some transistors at the differential pair, or leave it alone.
     
  4. darkhosis

    darkhosis Active Member

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    For your first question: yes, I think so. they measure -19VDC and 18VDC with the DBT, so Im guessing they are ok on full power. I will check again tomorrow when I install the new lamps.

    For DC offset, is it a direct relationship to the differentials. Lower HFE rating equals lower offset?

    The good news is that I ran it for 15 minutes on radio/aux and no suprises. Feeling optimistic.
     
  5. elnaldo

    elnaldo Addicted Member

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    No need to check the 19V, I was trying to see why you had low V+ and V-rails, but it was the DBT.

    The offset is handled by the differential pair, correcting it on the fly until stabilize, as a "servo" circuit: if DC goes up, one transistor starts to conducts more and DC goes down. If it's negative, the other transistor starts to conduct more and raises the voltage. In theory a perfect match in hFE should have better performance. But sometimes swapping transistors can correct the offset (perhaps one transistor conducting a bit more has a beneficial effect). I had good experiences sometimes just swapping the pair, reversing the position. Sometimes the offset increases, sometimes it goes lower.
    In some amplifiers, the offset was part of a poor design , and I had to modify one resistor to make it work better, that's why I tell you to leave it alone if it doesn't get better swapping a couple of transistors. 50mV is fine (I've seen service manuals asking for "100mV or less")

    I think you got it fixed, so don't worry too much about the offset, I wouldn't be too curious, you can break a working amplifier in a wrong movement.
     
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  6. darkhosis

    darkhosis Active Member

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    But that’s the only way to learn! I have a microcontroller that does transistor testing, when I measured them the HFE difference was in single digits. Maybe I handled them wrong?
    Matching differentials is important for me to understand because I have more receivers after this, 2 with blown channels.
     

     

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  7. elnaldo

    elnaldo Addicted Member

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    perhaps the transistors are matched, but some resistors at the same network are not so closely matched, that's why swapping transistors can adjust that little offset. Another option is to modify some resistor in a small percentage. We are talking about just a few mA flowing trough that circuit, a small difference can move the DC at the output. As said, 50mV is an acceptable value, within factory specs in many amplifiers without DC offset adjustment.
     
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  8. darkhosis

    darkhosis Active Member

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    I admit that my understanding of how electric sound reproduction occurs is very simple. It makes sense that over time components will change in value and/or be affected by environment and things like the IC going bad! Maybe the other possibility is the STK-0050 aren't operating in the way the original were. And as you said, the value of the things downstream are working differently.

    The service manual only has instructions to complete a harmonic distortion test and tuner alignment. Im missing a distortion analyzer, too bad.

    I have a good Fluke meter and some needle test leads so I will be taking a closer look. I doubt I will be changing much, this project needs to end and it doesn't need to be perfect. My friend will be happy to have it back, he bought it new. I'll post anything interesting, thank you elnaldo!
     
  9. darkhosis

    darkhosis Active Member

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    Almost done, finished new bulbs on Friday night and just have a new antenna to arrive.
    I keep thinking about matching differentials. I measured both original 2SA798 comparing the left 3 leg to the right 3 leg and I was getting results such as:
    R / L
    322 / 320
    342 / 321

    When I chose the new transistors I matched the Left legs and the Right legs. Was I supposed to match the L and R side, in other words matching 4 transistors to each other?
     
  10. elnaldo

    elnaldo Addicted Member

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    Just each pair of transistors must be matched. One single 2sa798 is a pair in a single body. That transistor is supposed to be a perfect matched pair since it's manufactured in the same package, also with perfect thermal tracking being in the same case.

    You don't need one pair matched to the other channel.
     
  11. darkhosis

    darkhosis Active Member

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    I see, clearly I have much more to learn about amplifier differential circuits..
     

     

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  12. elnaldo

    elnaldo Addicted Member

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  13. darkhosis

    darkhosis Active Member

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    I had an interesting experience today. Powered up the receiver on DBT to check my work with the bulbs and last cap replacements. Lights work but no FM signal. Suddenly, small surge of power (light glowed ) and then FM started working. Checked connections, it was stable for 10 minutes before I unplugged it from the DBT and began testing.

    I'm very interested in transistors now. I noticed the EQ section was definitely quieter and less bassy (even with level control at max) than when bypassed.

    I began checking all the transistor values in the Preamp section and found the base and emitter of Q3/4 & Q5/6 (2SC1570 that controlled 60Hz and 250Hz) were off the schematic more than 20%. -.2V DC (spec -.8V DC) & -1.4V DC (spec -.8V DC). Collector measured right on the money

    I went further back to Q1 and Q2 and found the 2SC732 pair were even worse. The base was 1.5V DC (spec -.8V DC) and emitter .8V DC (spec -1.4V DC). That's a swing of over 2 volts. The collector is again, right on the voltage spec.

    I have the replacement for 2SC732 but it's strange to me that it's both transistors, I'm worried I might have a difference from the schematic and the build.
     

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    Last edited: Jul 17, 2018
  14. darkhosis

    darkhosis Active Member

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    All done with this one, put the original transistors back in the graphic eq circuit and it’s been running fine. My friend came to pick up his receiver and was very pleased to have it back!! This thread is closed!!
    0FE85F7E-0040-4FA8-AB90-D965C3A73BA4.jpeg
     
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  15. elnaldo

    elnaldo Addicted Member

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    Good news!

    So, finally, "What caused these sloblo fuses to go"? And what happened with the eq that was quieter?
     
  16. darkhosis

    darkhosis Active Member

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    Thanks!

    The question i can answer with more confidence is the EQ section. It is going to continue like that. Replacing the transistors did nothing. I think only a total rebuild of the Pre-Amp would change much. Not for this one, and there is still enough control to make the sound pleasant.

    My best guess (I wish I wasnt guessing) is the fuses blew because one channel IC died and voltage level ran away. this screwed up the power amp - transistors ran very hot, capacitors were burnt. after enough surges it burnt out the first fuse, and later the 2nd fuse suffered the same fate.
     
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