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Antique "Acoustic" amp needs attention ASAP

Discussion in 'Audio' started by Pharaday, Feb 21, 2020.

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  1. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    Ok, this is a long one. Antiques are my side hobby. A buddy recently gave me this amp that causes my hobby ven diagram to have overlapping areas.
    20200212_201308.jpg
    It is clearly old. It had obviously spent some time outside. It had leaves and sand in it but still pumps alot of noise. I need help figuring out its proper usage and fixing the issues, if yall wouldn't mind. I know I should hit up a musical forum for this, but I trust yall more and I wanna run it by you 1st.

    AudioGuru, I know you got this.
    1st, is this a regular amp? Or is this just an equalizer and I need to plug an amp into it? Notice it has regular and -10db inputs on the front, "power amp input" and "pre-amp output" on the back. I can plug my phone or PC sound output into either of those back ports and I get great sound, just the volume knobs and equalizer on the front does not affect it.

    When I plug into the front, I have volume and equilizer control. That "BRT" button on the front doesn't seem to do anything. Plugging straight into the front without power amp yields low quality sound and low volume. Plugging into the back via "pre amp output" or "power amp input" yields great sound and volume. It seems odd to me that pre amp output works as an input...

    Anyways here's the issue the amp has: After about 15-30 minutes of music (using front or back as input), the sound slowly turns into static until all you hear is static. I feel like that huge monster cap (7800 uF, 100VDC) is bad since it slowly dies out. Then when I can no longer hear music, I turn off the amp and for a few seconds until power runs out, the sound quality is loud and good again, then the amp powers off.

    So how can I fix this badboy? Not sure I have that same monster cap, but I'll buy one if need be. This is a killer amp. Enclosed, you will find tons of pictures and a youtube vid showing various angles incase you need it. If you need more angles, lemme know. Yalls help keeps me looking like a god to the yokels around my neighborhood!

    Thanks!

    20200221_075202.jpg 20200221_075723.jpg
    20200221_075523.jpg 20200221_075451.jpg
     
  2. Martaine2005

    Martaine2005

    2,731
    736
    May 12, 2015
    That looks like a guitar head unit amp.
    Usually sit on the speakers. Similar to my old H&H set.
    Amp input is to add another amp (daisy chain)
    Or Amp input and Pre Amp out for effects loops. You can use these for a mixing desk too.
    What model is it?. I would imagine it would sell for over £100.
    As to the problem, it could be many things. But a good place to start is replacing the electrolytics and check ALL solder joints and traces. Being left outside is not good for PCBs.

    Martin
     
  3. Martaine2005

    Martaine2005

    2,731
    736
    May 12, 2015
    PS. You still have not answered why we came in 2nd. To what??..
     
  4. Audioguru

    Audioguru

    2,817
    633
    Sep 24, 2016
    It would be useful to have its owner's operating manual, schematic and parts list.
     
  5. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    So it was a race I had last time between yall and another forum. Just a little fun for us. Let me see if I can get the model from the back here. I have no idea what it is aside from an acoustic amp.
     
  6. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
  7. bertus

    bertus Moderator

    680
    236
    Nov 8, 2019
  8. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    ANDIAMO!!! to be honest, I never thought about searching for the model number printed on the device until one of you guys asked for it. This is great info, guys, thanks...
     
  9. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    Here's what I got so far:
    20200221_212250.jpg Every diode on this board allowed electricity to flow in both directions. Can I just replace them with any similar glass diode?
    20200221_212216.jpg
    These three capacitors measured a constant resistance which didn't change. Also, are these inductors in blue? (well, in rust. With blue markings)
    20200221_212139.jpg
    The underside of this circuit board had some kind of Goo, grease Maybe? Stuff looks real funky under here, but I only found one bad capacitor. Although there are some other component under here I cannot identify and don't know how to test.

    I think I'll replace those items, pending yall's advice, then see where we stand.
     
  10. bertus

    bertus Moderator

    680
    236
    Nov 8, 2019
    Hello,

    The brown blocks are the coils in the equalizer.

    Bertus
     

    Attached Files:

  11. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    All on my own I realized that without disconnecting the caps, I cannot test them.... sooooo.... replace them all? My issue crops up No matter which input I use. Does this mean my problem is on a board common to all inputs? Shirley I shouldn't just go replace every replaceable component?
     
  12. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    Full disclosure: I noticed that the LEDs installed were ugly red. I swapped them for blue LEDs, which helped the light coming out to be blue. Matches much better but I still have the same sound issue, If you can imagine!
     
  13. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    Look at this old cap I replaced:
    20200222_063220.jpg
    Look, it's prolapsed. See how that section is poking out from the middle? Is this just how they were made back then? All the 1uF caps are like this.

    I'm not even 100% sure anything we say is right, without AudioGuru's input...
     
  14. bertus

    bertus Moderator

    680
    236
    Nov 8, 2019
    Hello,

    When those capacitors go bad, the electrolyte will likely pour out at the side of the rim, now close to your fingers.
    A lot of information on capacitors can be found on the CAPSITE:
    http://www.iequalscdvdt.com/

    Bertus
     
  15. Audioguru

    Audioguru

    2,817
    633
    Sep 24, 2016
    Instead of measuring those glass diodes, you are measuring the transformer or other circuit parts in parallel with them.
    I still use the HH Scott stereo receiver I bought in 1964 (it is 56 years old!) and its parts are much newer looking than in your newer amplifier.
    Only its selector switches have failed but I souped up its circuit over the years for lower distortion.

     
    davenn likes this.
  16. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    Oh, man, that's killer, you got a sixties amp?! Yeah, completely ignore that. I realized the same thing about the capacitors. I need to disconnect them before I can test them. I'll do 25 Hail Marys for even thinking it.

    In this amps defense, it spent 50 years precariously not being owned by me, which will be the downfall of man. It did well, all things considered.

    So I'm trying to narrow down which component has the issue and narrow down which board it's on. And I'll keep you fellas apprised.
     
  17. bertus

    bertus Moderator

    680
    236
    Nov 8, 2019
    Hello,

    There are 6 diodes on the powerboard.
    I do not think the diodes are bad.

    What is happening with the amplifier?

    Bertus
     

    Attached Files:

    Last edited: Feb 22, 2020
  18. Pharaday

    Pharaday

    101
    3
    Jan 18, 2016
    AudioGuru has given input. I'm more confident.

    In an attempt to locate my issue, I played music until the sound became static, then attempted to plug my "pre amp output" into a seperate amp to test if my pre-amp is ok. The only other amp I have with me is a pair of decent PC speakers with built in amp. That works IF I switch my music input cable from a back input (I usually use "preamp out" or "power amp in" as input for better sound) to the front "0db" input. Then I can hear the music but at a very low volume. Almost like I'm just setting a pair of headphones on the table as speakers.

    I'd say that fails the test and the pre-amp doesn't work well at this point because I can get much more sound from those speakers by plugging them into my phone or PC. I guess the question is do I get more sound with a rested, "fresh" amp.

    Incidentally, when I plug my large speakers into "pre-amp out", obviously I get no sound cuz its just a pre-amp.

    FYI, anytime I ever use those front "0db" and "-10db" inputs, the sound is very quiet. I have to turn up every knob to hear anything. This may be because it's expecting an input from another amp (ppl were made of amps in the 70s) with a certain amount of db to begin with.

    Then I let the amp rest a bit to let the caps charge or to let the gremlin take a nap, whatever the case, so I can try again at full.... cap charge/gremlin nap. It takes a good hour min for the amp to be fully "rejuvinated", as it were. So, please join me in an hour of silence *looks at watch*

    .....ok, time well spent. I got the same results with the rested amp. I get almost no sound using the front "0db" input and the rear "pre-amp output".

    So.... we.... cannot rule out the pre-amp board? Anyways, I'll get it, guys. I Feel like I'm well enough versed on this to hammer it out, thanks, you guys correcting my mistakes. Continue to offer advice if you like. However, I have released you from your bond of mentoring so that I don't hog up your knowledge.. I hold your oaths fulfilled. Rest now.
     
  19. bertus

    bertus Moderator

    680
    236
    Nov 8, 2019
    Hello,

    What happens when you plugin a sound source into "power input" ?
    The sound might be a low level, but you can see if the power amp is working.

    Bertus
     

    Attached Files:

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