EPS Classic No audio out of left side

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EPS Classic No audio out of left side

Postby Hyde » Tue May 16, 2017 2:58 pm

Ok, here is me next project. One of my EPS Classics has no audio from the left output. I've done some diagnostics & seem to have traced it down to this component but, I don't know what the component is? I bridge it with a multimeter & I get audio. So I'd like to replace it, obviously. But I can't seem to figure out if its a capacitor? Doesn't have stripes so it must not be a resistor. Ive done component searches but I can't find a description that tells me what the component is. The ohm reading comes up with no result. The other, similar, components on the board all have a reading when tested. this one acts dead. the item in question in is the one that reads R2A 101(in the attached photo)
https://www.instagram.com/p/BUJ9yuOlPVf ... hydedbaker
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Re: EPS Classic No audio out of left side

Postby Mooger5 » Tue May 16, 2017 9:59 pm

That PCB needs a good clean.
I´ve searched for schematics to no avail. The component looks like a small inductor in which case 101 will stand for 100uH, but without schematics this is nothing but a guess.
What does it say beneath it?
If it´s in the signal path for the left channel then there should be an identical component in the path of the other channel.
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Re: EPS Classic No audio out of left side

Postby Rasputin » Tue May 16, 2017 11:28 pm

Looks like an axial ceramic capacitor with a value of 100 pF to me. But the context in the circuit should give you some big clues, as well as looking at the PCB markings. I'd be surprised if there wasn't something silkscreened on the board somewhere, maybe even concealed by the part itself.

Like Mooger suggested though, symmetry in the channels is almost definitely your definitive answer.

Both channels (left/right) on the EPS classic should have a capacitor and a resistor in parallel with the respective opamp. Is this where they are in the circuit?

There should be a 4053 analog multiplexer in the circuit. Does either pin 1 or 12 connect directly (electrically speaking) to your mystery item?
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Re: EPS Classic No audio out of left side

Postby Hyde » Wed May 17, 2017 12:38 am

It's not the cleanest board but looks dirtier than it is. I can't read that capacitor with my naked eye, it's zoomed in as far as I can get & still have some focus to it. So, it's a lot smaller than it looks ;) I pulled the board. I'll have a look at it tonight & see what I can find. I looked for markings & couldn't see any printed on it, in this area. There certainly are some on many other areas of the board. I'll report back a little later. Thanks
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Re: EPS Classic No audio out of left side

Postby Hyde » Wed May 17, 2017 1:41 pm

Does anyone have the schematic? There are definitely markers under the components. There are many of these "capacitors"(thats what I'm going to call them for the moment) on the board. If you have the schematic, marker C48 & C26 also have the same looking capacitor. C48 has a marking of "RSA 104".
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Re: EPS Classic No audio out of left side

Postby Mooger5 » Wed May 17, 2017 2:22 pm

C stands for capacitor, obviously. It then leaves me puzzled how bypassing this component the audio comes out. It´s a too low capacitance value to be in series; an inductor made more sense. Though the fact it´s a multi-layer capacitor indicates a more important role than a decoupling cap which is usually a ceramic disc.
Lacking a schematic, the best thing to do is to trace the circuit. Just follow Rasputin´s suggestions. Those capacitor leads must, well, lead somewhere.
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Re: EPS Classic No audio out of left side

Postby Hyde » Wed May 17, 2017 3:14 pm

The confusing part is that when I put the ohm meter on it, it acts as if there is nothing there. If I do this to other capacitors on the board, of similar style, I get a reading.
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Re: EPS Classic No audio out of left side

Postby Mooger5 » Wed May 17, 2017 7:16 pm

If your multimeter reads capacitances, use that feature instead of trying to read resistance. Remove the component from the board first, or at least lift one of the leads before doing any reading. What you read with the other capacitors was probably the combined parallel resistance from the nearby components.
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Re: EPS Classic No audio out of left side

Postby Hyde » Thu May 18, 2017 1:22 pm

I wish it did.
I put it all back together last night & tried to make heads or tails of it. Multiplexor? I don't see anything marked 4053. There are a couple motorola chips marked 34085. Half the board is buried under the keys, though. There is certainly a parallel circuit for the right hand side & it reacts differently when the capacitor or the resistor below are bridged. The audio on that side gets quiet. Much like the audio is on the left, already. I'll pull the board again & see if & can find the chip 4053.
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Re: EPS Classic No audio out of left side

Postby Rasputin » Thu May 18, 2017 2:16 pm

Hyde wrote:I don't see anything marked 4053. There are a couple motorola chips marked 34085. Half the board is buried under the keys, though. There is certainly a parallel circuit for the right hand side & it reacts differently when the capacitor or the resistor below are bridged. The audio on that side gets quiet. Much like the audio is on the left, already. I'll pull the board again & see if & can find the chip 4053.


The Motorola 34085 chips are opamps, and there should be one per channel. Which channel is dead -- left or right?

Pin 2 of both opamps will lead back to the multiplexer -- left channel will connect to pin 1 of the mux and right channel will connect to pin 12 of the mux.

Both opamps should have a separate capacitor connected to pin 6 of the respective opamp. Is this where the capacitor in question is located in the circuit?


One more thing: Instead of probing the "dead" capacitor with the multimeter, try probing it will something similar but non-conductive and check to see if the audio comes back.

Edit: Ignore the strikethrough text and see my next post about the opamps.
Last edited by Rasputin on Thu May 18, 2017 3:35 pm, edited 2 times in total.
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Re: EPS Classic No audio out of left side

Postby Hyde » Thu May 18, 2017 2:30 pm

The 34085 & another under are on top of a raised circuit board. one is removable with a puller & the other, soldered into the board. here is a link

https://www.instagram.com/p/BUPDUn1lT-h ... hydedbaker

The multiplexors must be buried under this board but, I can't tell. Tough to trace, the way it is.
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Re: EPS Classic No audio out of left side

Postby Rasputin » Thu May 18, 2017 2:57 pm

Hyde wrote:The 34085 & another under are on top of a raised circuit board. one is removable with a puller & the other, soldered into the board. here is a link

https://www.instagram.com/p/BUPDUn1lT-h ... hydedbaker

The multiplexors must be buried under this board but, I can't tell. Tough to trace, the way it is.


Nevermind my musing on the 34085. Until I saw your picture I didn't realize it was a quad opamp, but of course the part number itself should have clued me in, so I'm an idiot. I'm not even sure why there are two in that circuit, to be honest. Not having an EPS or schematics to look at means a fair bit of erroneous guesswork.

Try pin 2,6,9 and 13 on both of the 34085 chips to see if that bad capacitor is connected directly.

In one of your pics, there's an IC with the marking CD405x (can't make it out) right next to the A/D offset marking.

Oh God.... I just noticed that your thread is titled with "left channel". I am clearly worthless this week.
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Re: EPS Classic No audio out of left side

Postby Mooger5 » Thu May 18, 2017 7:23 pm

All the small value capacitors are like the suspect one; no ceramic discs to be found. So my theory of a "special purpose multi-layered cap" goes down the drain.

It doesn´t seem to me that cap is in a sample and hold circuit. You find these much closer to the demux and the buffering opamp, so the inductance in the circuit traces doesn´t slow the signals down.
It does make sense though, as I can´t see where else a failing, small value cap to ground would interfere with the audio.

Judging by the caps and the distribution of the components over the PCB, Ensoniqs or at least that particular model don´t follow conventions much. We don´t even know if there´s a demux in there. If there´s a DAC for each channel there´s no need for a sample and hold circuit. Speaking of which, where is/are the DAC(s)?

My best bet is to search for a signal with an audio probe. Easy to build.
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Re: EPS Classic No audio out of left side

Postby Hyde » Thu May 18, 2017 7:32 pm

I bridged the capacitor & the resistor before it & i have both sound in both ears. Still louder on the right but panning a bit evens it out. Not perfect but I can't seem to trace anything with the layout, the way it is, & my lack of knowledge.
Probing the capacitor with a plastic probe did nothing. no difference.

Yes, a schematic would really help but, this is the early version board. The later version, which I also have is completely different. No raised circuit board. I'd really like to take some classes & get some more technical knowledge & hands on learning. I've thought about asking a local technician for some 1 on 1 hands learning. I like resurrecting these machines, when I can.
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Re: EPS Classic No audio out of left side

Postby Mooger5 » Thu May 18, 2017 10:45 pm

Just don´t bridge components around randomly. It´s already too easy to fry semiconductors by mistake.

Use the continuity check function instead. Start with that capacitor. Touch a ground point, or the metal chassis with one probe, and one lead of the capacitor at a time with the other probe. If there´s continuity between one of the leads and ground it means that capacitor is grounded and you only have the other lead to deal with.
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