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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    Im repairing this Fisher RS-2004A for a friend, he has a sentimental attachment to it. Typical symptoms before total loss of sound - one side went then the whole thing.

    The first thing that's apparent is the fuses were blown in the power supply, but Im not sure why. I did a diode test on the bridge rectifier, and I think it's busted:

    AC -> neg DC = .521
    neg DC -> AC = .525
    AC -> pos DC = .521
    pos DC -> AC = .525

    The other caps look heat affected - the whole board has symptoms of heat damage. What's the first step, assume the power supply needs rebuilding before I can check the power amp?
    Fisher RS-2004A power supply.jpg
     
    Last edited: Jun 27, 2018

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    Shorted STK is a possibility. What does the underside of the two large main caps look like ?
     
  3. darkhosis

    darkhosis Active Member

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    Right side, I don't know how you call it but the main capacitor is a Rubycon with 2 6800uf caps in one package. Hard to tell if there's any damage but I see no leakage.
    Left side, the 2 largest caps in the power supply. Lots of heat damage from those 2 resistors next to it.

    re: the STK-0050. I wanted to measure voltages per the service manual but I don't want to create more problems by putting new fuses in the power supply.

    0F4B0C92-4A6A-4F21-B5D0-1E6D9F28843C.jpg
     
  4. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    I'd personally put in new fuses and bring it up on a dim-bulb tester. If it lights the dim-bulb, is it possible to unhook the amp boards on that one? Do they clip in?
     
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  5. elnaldo

    elnaldo Addicted Member

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    You can desolder the power supply pins going to the STK, desoldering just those pins you have the STK out of the circuit to try the unit with a DBT .
     
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  6. darkhosis

    darkhosis Active Member

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    Got parts in to recap the power supply, and soldered in the STK-0050 paks - DBT shows no shorts, so I power it up and the receiver appears to be functioning OK. No fuses blown.

    Before I reinstall the Darlington ICs I want to see if I can improve the audio path. I've been measuring the Power Amp voltages because the DC offset measures around 70 mV DC on each channel. I don't know how to tell which ones the differential transistors are or should I replace them all. Or something else.

    Here's a pic of the Power Amp board and the schematic: any help would be really appreciated


     

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  7. elnaldo

    elnaldo Addicted Member

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    Differential pair transistors are those double transistors 2SA798, q3 q4.

    They use to go bad and noisy. There is a dedicated thread about replacing the 2sa798 , I lately use ksa992, matched pairs.

    I think you need the whole power amp connected to make the feedback network to work and stabilize any DC offset.
     
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  8. darkhosis

    darkhosis Active Member

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    Ah, thank you! Q3 & Q4 are a very strange double transistor that share a leg, if you can see on the board.

    And, yes, everything is hooked up. I meant I haven't reattached to the heatsink.
     
  9. elnaldo

    elnaldo Addicted Member

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    Yes. They share the EMITTER. Search in AK a thread called "2SA798 is there a replacement? " or something similar. The different replacements are explained there. Or, let them alone until they start to make crackling noises.
     
    Last edited: Jul 8, 2018
  10. sregor

    sregor AK Subscriber Subscriber

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    HOLD IT - they share the emitter, not the collector. Bases are on the outside.
     
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  11. elnaldo

    elnaldo Addicted Member

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    You are right! Emitter. Collector are pins 2 and 4. Bases 1 and 5. I'm editing the previous post.
     

     

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  12. darkhosis

    darkhosis Active Member

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    Cheers, thanks for the heads up.
     
  13. darkhosis

    darkhosis Active Member

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    I measured the 2SA798 out of circuit and they weren't bad but I went ahead and swapped them out with KSA992s. Matched HFE within 3-4 points (got lucky, out of 8 four matched!) . Looked at the Power Amp board, Q11 and Q12 (2SB560) had a lot of darkened board around them from heat but measured .600 and no shorts.

    My first experience with the 5 leg mod. Take a look at my work let me know if I missed something, I didn't twist the center leads. Pre-trimmed leads
     

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    Last edited: Jul 11, 2018
  14. elnaldo

    elnaldo Addicted Member

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    Looks fine to me. +- the same I do.
     
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  15. darkhosis

    darkhosis Active Member

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    So I have most of the remaining components, something strange happened when I was measuring voltage on the Power Amp board. There was a spark-pop that sounded like a short, but the resistor which had the burned pad wasnt bad. It was connected to the B+ from the main capacitor. Im worried about it because I cant figure it out B821CEC2-884D-4C3F-9FC6-3E1E539E97C7.jpeg
     
  16. elnaldo

    elnaldo Addicted Member

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    sounds like the main capacitor was still charged and you discharged it shorting something.

    I'd double check the work done, check no wires are cut or desoldered after moving the PCBs, and plug it with the DBT.

    If DBT checks fine, try the unit with headphones.

    Priority is to keep the unit working. Don't worry too much with a small DC offset, check if the unit works, and don't risk to short something to adjust bias or DC offset. Make steady connections and discharge the capacitors before working inside the unit. I use a 150ohm 5W resistor to discharge the caps, or a 1000ohm 1W , for longer time.

    To measure voltages, I always connect the probes with the unit UNPLUGGED, make sure they are firmly connected, and then power ON the unit to read the DMM. I try to avoid moving the probes or poking inside a live unit. I do it only in some cases, when I have an easy access to the test points or when I feel 99% confident I'll not short something while doing it.
     

     

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  17. darkhosis

    darkhosis Active Member

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    6A7255DD-736C-455D-9322-770C9F2A0D6E.jpeg
    that was my thought, I was being careful using a needle probe but the board was loose.. I did as you suggested, DBT showed no signs of a problem.
    Ive been considering if replacing the main caps would be an improvement, they measure an output of around 33V DC (without the DBT). The B+ should be around 45 V, am I missing something here?
    Usually I wouldnt even worry about it and just replace but this is a dual cap and I have never DIY’d one:
     
  18. elnaldo

    elnaldo Addicted Member

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    I'd stop swapping parts until the unit is working OK.

    33V instead of 45V doesn't sound OK, but it could be the wiring of the transformer, perhaps wired for a higher mains voltage. If the capacitor were leaking so bad to the point of dropping 12V from the B+, you should have hum at the output.

    If you can (without risking yourself or the unit), try to measure the AC voltage reaching the diode bridge, to see if it shows that same ratio (lower than the specified voltage)
     
  19. darkhosis

    darkhosis Active Member

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    I did measure the AC voltage last week, I didn't write down the result but it didn't seem off. I should have posted this all when I was taking measurements per the schematic, my memory is crap. I measured test points/posts from the power supply and everything seemed within 10% if not closer, I was measuring the outputs of the new STK-0050 when the pop happened and the voltage dropped 10V DC but no shorts.

    I'm really invested in making this work but I'm kind of on a deadline, so it may seem like I'm being a solder monkey but I'm trying my best to be careful.
     
  20. darkhosis

    darkhosis Active Member

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    Here's where my inexperience shows, I look at the schematic and can see DC output values from the rectifier but there is no AC to check. The transformer drops it to 29VAC (see my first post). I actually pulled the bridge rectifier tested it with my Fluke and it didn't seem defective
     

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