EICO ST40 help please? Only 1 channel operating

Discussion in 'Eico Place' started by Rickyo, Nov 20, 2018.

  1. Dandy

    Dandy Super Member

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    Have you given all the pots a good cleaning with Deoxit?
     

     

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  2. Rickyo

    Rickyo New Member

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    Yes, but it was a few months ago. I will reapply. I noticed when turning the bass pot, I’ll get a little “pop” out of the speaker from one channel @ about 80% turned. After that the bass is intense the from 80-100%. Any thoughts?

    My parts for the loudness switch modification are on the way. Can’t wait to restore it.

    Thanks again for any help or suggestions.
     
  3. Dandy

    Dandy Super Member

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    4,640
    Location:
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    Any update on your restoration work?
     
  4. Rickyo

    Rickyo New Member

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    Thank you so much for asking! It’s a good news bad news situation. I did the loudness switch mod to no avail. There was little to no change in sound. That’s the bad news. Luckily I had only spent less than $5 on the parts. So I re-soldered the originals back in place.

    There was a green wire coming from one of the output tubes that was deliberately cut from a resistor on one of the speaker terminals. I don’t know why someone would have done that. I soldered that green wire back to the resistor. The good news is the sound is so much more balanced. The balance function actually operates normally now. I need to keep the level (volume) knob above the 4 position. If I drop it below that, one channel cuts out and a pop here or there, but no more hum at all.

    I can live with that because my music source is supplied by my iPhone or iPad through the aux inputs and volume can be lowered via those devices. I usually just keep the EICO level at 5 and it sounds REALLY good there and above.

    Any thoughts on what the issue may be? I’m starting to lean toward a bad volume pot.
     
  5. Dandy

    Dandy Super Member

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    Reviewing your thread, I notice that you still have old coupling caps in your unit. That leads me to wonder what overall servicing you've done on the amp. Forget the loudness mid for now.

    Because this Eico section is rather quiet, I would suggest starting a new thread in the tubes section. Describe where you've got to so far, and include some nice clear shots from below. Others in the tube section might recognise symptoms you describe.
     
  6. Dandy

    Dandy Super Member

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    Hi again Rickyo, I popped the base off my ST40 just to check what green wires you are talking about. I believe the wires are the feedback circuit made up of a 15k resistor paralleled with a 100mmf cap. The is a connection for each channel from there to pin 8 of the 7247 tubes. The previous owner would have cut the wire to see what the amp sounded like without feedback.

    But as I said in my earlier post, I would definitely replace the 0.1uF coupling caps between pins 1 and 3 of the 7247 tubes and pin 1 of the 7591 tubes. The connection goes on to pin 6 via a 10k resistor. Do that asap!

    The manuals linked to in an earlier post have useful pictorials as well as the schematic.
     

     

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  7. Rickyo

    Rickyo New Member

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    Thank you. I am so appreciative of that info. The instructions you laid out seem a little far over my head. However, I do have the full manual and schematics to help along the way. Pictures really help me the most. Once I see the coupling caps? in a picture, I can go ahead and replace them. I should be able to identify them from the schematic also. BTW, I’m not getting any feedback now that I’ve reattached the green wire to the resistor. It sounds great! The only issue is when the level knob is turned below position 4.

    How many 0.1uF coupling caps will I need to order?
     
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  8. Dandy

    Dandy Super Member

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    4,640
    Location:
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    You would need four 0.1uF polyester/polypropylene film caps, usually rated at 630v. Something like 'orange drops' would be fine or the yellow ones illustrated in the link I've added. These are not polarised, unlike electrolytic ones. The yellow ones look like the ones in this link, but are widely available, so no need to order these exact ones: https://www.aliexpress.com/item/Who...ctronics-0-1uF-630V-630V-104J/2047908132.html

    The ones to replace are the black tubular ones in your first picture above showing the cut green wire, post #7 and are the big tubular shaped ones in the middle of the sketch attached to post #5.

    Sorry that I can't post pics of my unit, as I modified mine to use 6L6 and 12AU7 tubes and the connections are slightly different.

    But, it does sound as if you need to replace your loudness pot. You can get a similar one without the switch quite easily.
     

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