Eico ST40 Balance Control

Discussion in 'Eico Place' started by cmcguire, Apr 26, 2016.

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  1. cmcguire

    cmcguire AK Subscriber Subscriber

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    I've recently acquired one of these quirky little amps and am becoming familiar with its idiosyncrasies. I'm wondering if someone with more experience than I can enlighten me on whether it's normal for both channels to be relatively quiet in the middle position of the balance control and quite noticably louder when it is adjusted side to side. I have it driving two Tannoy Chatsworth HPD315's and its fairly quiet.
     

     

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  2. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    No, not normal.
    Check the position of that "Balance Check" switch on top right of the front panel -- even it if is in the "normal" (unmarked) position (as opposed to "ON"), -- you might still want to try it in the other position (i.e., "ON"); or just 'fiddle' with it a bit.
    Any and all switches on a five-plus decade old amplifier may be dirty and/or have very worn contacts at this late date.
    Also...
    as my late father would always remind me when I was trying, frustrated, to troubleshoot an old kit-built hifi component that had found its way to me, "That's the problem with a kit. You don't know if it ever worked right!" :- P

    In other words, when issues present themselves, sometimes one just has to check every single connection, point to point wire, and solder joint -- against the original assembly manual.

    Yes, they are a little quirky -- not to mention ugly (well, at least homely). They have some well-documented idiosyncracies that can, and should, be mitigated.
    Pretty good iron in 'em, though.

    [​IMG]DSC_0198 by Mark Hardy, on Flickr

    EDIT: If you don't have an ST-40 manual, it is readily available on teh webs. I think it can even be found on AK's own "Digital Doc Database".
     
    Last edited: Apr 26, 2016
  3. gadget73

    gadget73 junk junkie Subscriber

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    Excellent point about not knowing if it ever worked. I've had my hands on a kit or two that was downright questionable with the build quality.
     
  4. cmcguire

    cmcguire AK Subscriber Subscriber

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    Thanks! Its funny because the amp sounds so much better when the balance pot is engaged either way, its just, um, off balance that way. The first thing I'm going to troubleshoot is the Loudness switch which seems to have been modified at some point. Im fairly sure the balance check switch is off and functioning properly so Ive ruled this out so far.

    Strangely enough the balance pot seems to work in the fashion described above (makes one channel MUCH louder than both at centre) UNTIL the Level reaches half way. Then both channels even out and the Balance pot starts adding bass. When the Loudness switch is in the up position it doesn't seem to be affected in the same way.
     
  5. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    Kitbuilder on acid, perhaps?

    Mind you, I'm just spitballin' here...

    There is, as I reckon the OP knows, one of the canonical modifications is"required" for the loudness switch to perform anything akin to logically. Whether that is the mod that was actually performed on his (or hers), and whether it was performed correctly, are different matters entirely.

    EDIT: The canonical mods (and more): http://www.tronola.com/ST-70A_mods_package.pdf
    Yeah, these were for the ST-70, which is in many ways a very different beastie, but still helpful guidance, if memory serves, for the ST-40 as well.
     
  6. cmcguire

    cmcguire AK Subscriber Subscriber

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    Does anyone happen to have an actual picture of the back of the loudness switch that has a working loudness mod? It would really help if I could actually see the wiring. Ive read a couple of different descriptions but have yet to see a pic.
     

     

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  7. Kim G

    Kim G Super Member

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    I think there is a website that covers this with photos. You might try the eico yahoo group.
     
  8. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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  9. cmcguire

    cmcguire AK Subscriber Subscriber

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    Amazing! Thanks.
     
  10. Dandy

    Dandy Super Member

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    With amps of this age, I recommend replacing all the slide switches. A simple like for like replacement rules out problems with contacts. I needed to really clean the tube sockets in my ST40 too.
     
  11. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    There's a loudness switch mod, which IIRC, covers ST40, 70, and 84 pre.
    It wouldn't account for your balance issue. I'll see if I can dig it up for you later
     

     

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  12. s-petersen

    s-petersen Scott Subscriber

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    With the balance switch on you are supposed to "null" the bass treble and balance for the lowest output, then switch it to normal and the channels will have the the same volume and EQ assuming it works like the ST-70. The switch phase inverts one channel against the other.
     
  13. cmcguire

    cmcguire AK Subscriber Subscriber

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    I spent the day getting to know the little guy and man was this thing filthy. I gave it a good dusting and cleaning and the problem seems to have magically dissappeared! I rewired the loudness switch but I think the main culprit is the switch itself. Its pretty finicky with how you switch it on and I'm hoping to replace it with a standard DPDT switch. If anyone knows how I'd do it I'd appreciate the guidance but in tge mean time I'll try to draw something up and use a little trial and error/brute force and ignorance until I get it right.
     
  14. Dandy

    Dandy Super Member

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    The replacement switch should fit in place of the old one exactly. Just replicate the wiring. (Or have I misunderstood your question?)
     
  15. cmcguire

    cmcguire AK Subscriber Subscriber

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    Thanks. The old switch looks so impossibly large I haven't seen anything like it before. Another issue I'm having is that anything I put into the line level inputs is very low volume. Like I have to crank the amp to almost full to get a decent listening level. The Phono A section is plenty loud. Is this normal for a 20 watt tube amp?
     
  16. Dandy

    Dandy Super Member

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    No, that's not normal. Again it sounds like dirty contacts somewhere to me. Have you sprayed all the contacts, tube sockets, switches and pots with a switch cleaner like Deoxit, and worked them free?
     

     

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  17. cmcguire

    cmcguire AK Subscriber Subscriber

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    So I should remove the tubes and spray the sockets as well? Thats one thing I still havent done. Could this problem also be caused by old tubes? I see there's also a 5U4-GB that isn't on the schematic. I'm wondeing if this is caising my power problems.
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    The stock rectifier is a 5AR4. Those output more voltage, and draw less current than the 5U4GB. Might not be the problem, but its probably a problem.
     
  19. cmcguire

    cmcguire AK Subscriber Subscriber

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    Thanks! I swapped out the 5U4GB for a new Sovtek 5AR4 but the problem still remains. Argh. The last thing I can think of is the fact that two of the 7591's are old ROGERS type and two are new Electro Harmonix. There's a pretty big size difference between the two types.
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    If its low in both channels, that should not be it. They don't run a common cathode resistor, so even if the two sides performed very differently it wouldn't get too awfully ugly.

    The size difference is just how they are made. The EH tubes are built in the 6L6GC bottle, which is bigger. They now make a Tung Sol 7591 that is the same size as the original. The EH ones do perform fairly well though as long as you can fit them in the amp. Not everything will accept the fat bottle though.

    I suspect its time to be looking at other things. If you can dig up a schematic with voltages, or a copy of the assembly manual with voltages, confirm they are what they should be. Would also be worth looking at component conditions to see what might be bad. Problems in both channels are generally power supply though, or at any point both come together. On this that involves that balance switch thing and perhaps the selector switch. The tape/monitor switch is also worth looking at.
     

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