GFA-555 Filter Cap Help - Polarity

PreCd
PreCd Posts: 786
edited December 2011 in DIY, Mods & Tweaks
I have been in the process of replacing the filter caps on my Adcom GFA-555. Took the old ones out. Replaced the old ground bar and set the new caps in. Made the new connections to the new caps. Rechecked everything. Plugged the amp in and blew the main 10amp fuse. I had all rail fuses pulled before the fuse blew so ok there. Hopefully nothing else is toast.

Studied a bit and saw that the old caps seem to be wired in series. Is that correct? If so the only challenge now is for me to figure out what the polarity was on the old caps. The old Adcom caps are not marked with polarity that I can find. If I knew what their polarity was I could backup and follow that pattern.

Pic of the newly wired caps attached. Note: The bottom left is unhooked for troubleshooting reasons.

new_caps.jpg
SDA2BTL
Marantz CD5004
Adcom GFA-545
Bottlehead Quickie Tube Preamp
Post edited by PreCd on
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Comments

  • [Deleted User]
    [Deleted User] Posts: 7,658
    edited December 2011
    Hello,
    According to the schematic the caps are connected in series. If you look at the plastic top cap there should be "+" and "-" molded into the plastic of the cap near the binding post.
    Cheers, Ken
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    Looks to me like the bottom caps(in the pic) are connected incorrectly.Assuming the blue wire is coming from the - terminal of the bridge rectifier the other end should attach to the - terminal on the capacitor(the side with black stripe.)Turning the bottom caps 180 degrees should do it.
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    Ok. I have the caps wired properly now observing proper polarity. The rail voltages read 80v with fuses still pulled which is right on.

    However the offset voltages with the rail fuses still pulled read 320mv on the left channel and 2mv on the right. The left channel does not look good but is that reading high because the rail fuses are pulled?
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    The fuses need to be in to measure offset,without them the entire circuit is not powered.
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    Thank you sir. I just realized that it was -320mv which is not a voltage registering. I will place the fuses in and see what I read then.
    FTGV wrote: »
    The fuses need to be in to measure offset,without them the entire circuit is not powered.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    I got the amp running. Hopefully will see some difference in sound now that the 25 year old filter caps are out. We will see. Thanks for all the suggestions everyone. It really helped.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    Is that a GFA-555 or a GFA-555II, I have a GFA-555 and it looks different the input board is mounted vertically not horizontally and the transformer isn't covered or potted. I'm still looking for someone to go through mine that's not going to cost me an arm and a leg, (well I could give up the legs seeing how I don't use them). I just can't see spending $$$ on this Adcom.
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    gimpod wrote: »
    Is that a GFA-555 or a GFA-555II, I have a GFA-555 and it looks different the input board is mounted vertically not horizontally and the transformer isn't covered or potted. I'm still looking for someone to go through mine that's not going to cost me an arm and a leg, (well I could give up the legs seeing how I don't use them). I just can't see spending $$$ on this Adcom.

    This is a GFA-555 and there are two different configurations for them. The PCB board is a different configuration.

    Yes I see what you are saying about putting out a lot of money for work on the Adcom. I got mine for a great price, sunk money into the filter caps and PCB caps and have about what you would buy one for in nice condition on eBay. I have to say I am pleased with the sound since the rehabilitation. Every little step wrought a different sound. The filter cap replacement has brought clarity to the music. How much at this point I cannot say as I just got her put back together and am listening to it now.

    You could do the cap changes and I have the service manual and schematic for your amp if you want them.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    Gimpod:

    I also wanted to add that the filter cap change dropped my offset voltage from around 33mv per channel to 0 on one and 17mv on the other. Happy camper I is.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    PreCd wrote: »
    The filter cap replacement has brought clarity to the music.
    That would be expected especially with sustained musical plateaus and dependant upon the extent of deterioration of the original filter caps you might hear improvements with bass heavy transients,having a little extra ooomph in the bottom end.
    PreCd wrote: »
    ... per channel to 0 on one and 17mv on the other.
    That indicates very tight matching on the input transistors.
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    FTGV wrote: »
    That would expected especially with sustained musical plateaus and dependant upon the extent of deterioration of the original filter caps you might hear improvements with bass heavy transients,having a little extra ooomph in the bottom end.

    That indicates very tight matching on the input transistors.

    I want to thank you for your help today. Your advice got me going in the right direction and helped me to get my amp back running. I hate not having my music.

    I only had a 15 minutes to listen today and could hear a difference in not only clarity but also the items you mentioned. It is so nice to see that I made improvements in the sound.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    Glad to have helped.
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    PreCd wrote: »
    You could do the cap changes and I have the service manual and schematic for your amp if you want them.
    This would be much appreciated as the ones I have are crappy pdf scans that are almost readable. Another thing I was thinking of doing with mine is replacing those 2 watt 0.47 Ohm resistors (R645 - R660) on the output boards with some 12 watt Mills. Sending a PM.
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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  • george daniel
    george daniel Posts: 12,096
    edited December 2011
    Fred knows his electronics--good source of knowledge,atta'boy Fred.
    JC approves....he told me so. (F-1 nut)
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    Hey can you send me a closeup of your power PCB. It maybe mounted different from mine but your resistor numbers correspond with mine. Normally if you have a PCB mounted flat it carries a different schematic and position numbers. Let me see that and then I can send you the schematic.
    gimpod wrote: »
    This would be much appreciated as the ones I have are crappy pdf scans that are almost readable. Another thing I was thinking of doing with mine is replacing those 2 watt 0.47 Ohm resistors (R645 - R660) on the output boards with some 12 watt Mills. Sending a PM.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    I think that it is important to mention that now that the capacitors have been used for a few days that my dc offset has leveled off at 9mv on both channels. That is awesome............
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    gimpod wrote: »
    another thing I was thinking of doing with mine is replacing those 2 watt 0.47 Ohm resistors (R645 - R660) on the output boards with some 12 watt Mills.
    I would'nt .From your description it sounds like those are the emitter resistors and sometimes changing seemingly basic parts with ones of different contruction can possibly cause instability.IMO even if they did'nt cause stability issues I doubt there would be ay sonic benifit.YMMV
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