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Clearaudio basic - IC (probably opamp) name scraped away

Grossel

Well-Known Member
Hi folks.

I just have got asked to try to repair one RIAA amplifier for a friend.
In this particular case, it works for a while - maybe half a minute, and then the outut is just white noise, both channels.

So I decided to triy to look at it. When first opened, the things that first make my attention was:
  • Device have obvious being opened before and tried to being repaired.
  • The four IC have all markings scraped away (This is tha part I need help with) - so I can only speculate. I can always find what is input, output and voltage pins - but maybe not the whole picture.
  • There is nota single label that denotes components ID on the PCB.
When powered up the device - since I have not tried to draw/make up a chematic by hand, the first thing I did try was to feel the heat (well, yes litterally) from all four IC's by holding the fingertip over.
Result : The two IC physically located on "red side" was noticeably hotter than the two IC on "black side" (ref top.jpg).

The two device in TO220 cases are 7818 and 7918 voltage regulators (when posting I noticed that not all markings are visible from pictures).

Also, the naming plate for the device is worn out, so even if I have a decent camera and tried to relocate the light source and camera, it's still not all letters that is visible, even for the eyes.

Power supply is 2x25V and measuring the voltage over time I could not detect voltage dropout.


Basically what I ask for help for is
  • Either if there exist a service manual for this (none found by google)
  • Or have the actual name of the IC (opamps or maybe inhouse amplifier IC ?) used here.

Thanks in advance
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
It's hardly difficult, most opamps have standard pinouts, so you can simply interchange them anyway.

For a start, check the datasheets for TL071 (one amp) and TL072 (two amps), and compare the pin connections to what you have. This will identify if they are single or dual, and allow you to select replacements.

I'd be inclined to fit IC sockets, so you can try different opamps, in case you find one that you like the sound of better than others - but pretty well anything should do.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
It appears to have two single opamps (at the PSU end) and two dual opamps at the audio connector end.

As Nigel says. Fit sockets & try TL071 & TL072 ICs.

Or, NE5532 for the duals and NE5534 for the singles.
 

Grossel

Well-Known Member
Thanks for reply - I'm using Inkscape to redraw the traces, but not finnish yet. See if I have time to continue tomorrow.
 

JimB

Super Moderator
Most Helpful Member
Looking at my annotated version of your picture, the amplifier is obviously built as two handed sections separated by a line of symmetry.

Symmetrical Amplifier.png

Now look at the orientation of the op-amps, one of them is pointing in the opposite direction to the others.
This may be correct, but jumps out at me as quite odd.

JimB
 

Les Jones

Well-Known Member
Hi Jim,
I think the orientation of the op-amps is correct. It looks like in both cases there is a 120k resistor connected to pin 2. Also on the etch side both have smd capacitors connected to pins 4 and 7
AD2.3.1 pcb.jpg
I have added a mirrored version of the etch side picture from post #1
The fact that both channels behave in the same way I would start by monitoring the power supply pins to all 4 op-amps from when it is switched on until the fault appears.

Les.
 

JimB

Super Moderator
Most Helpful Member
Hi Jim,
I think the orientation of the op-amps is correct. It looks like in both cases there is a 120k resistor connected to pin 2. Also on the etch side both have smd capacitors connected to pins 4 and 7
OK, I am convinced that it is correct as built.

Someone obviously spent some time to design the layout symmetrically, but did not quite manage to get the 4th op-amp to line up in the same direction.

JimB
 

Grossel

Well-Known Member
Done some back-tracing tonight using Inkscape. Unfortunately, layers are lost when exporting to png images - but you might get an idea how I intend to use Inkscape with layers (represented by different color/thickness), so it get pretty helpfull if nothing is known, but take some time (like when sitting home boring in winter evenings)

And I'm just hopelessing overdoing stuff like this (e.g. not need both channels etc. . .)

About monitoring voltage supply on opamps - well I may do another run for that tomorrow. Also haven't rechecked that the pinouts are actually correct (seen from below)
 

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JimB

Super Moderator
Most Helpful Member
Also haven't rechecked that the pinouts are actually correct (seen from below)
One of them is the wrong way round.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Done some back-tracing tonight using Inkscape. Unfortunately, layers are lost when exporting to png images - but you might get an idea how I intend to use Inkscape with layers (represented by different color/thickness), so it get pretty helpfull if nothing is known, but take some time (like when sitting home boring in winter evenings)

And I'm just hopelessing overdoing stuff like this (e.g. not need both channels etc. . .)

About monitoring voltage supply on opamps - well I may do another run for that tomorrow. Also haven't rechecked that the pinouts are actually correct (seen from below)
I'm a bit puzzled as to what you're trying to do?, it'd be a LOT easier to draw the schematic out, and it would then be obvious what the pin connections are, and what opamps would fit.
 

Grossel

Well-Known Member
I'm a bit puzzled as to what you're trying to do?, it'd be a LOT easier to draw the schematic out, and it would then be obvious what the pin connections are, and what opamps would fit.
Yes - I'll happily explain that.

Like told before (in other threads) - I've using this drawing software Inkscape for years now. But not so long ago, I figured out how to put the layer structure (just like any other kind of tre structure). So therefore I use this case not only to try to trouble shoot the device, but also to use Inkscape as efficient as possible.

And also - the drawing above doesn't tell the full picture, because when having the full file open in Inkscape, I see all layers including the picture itself at bottom, and I can turn on/off layers as I wish, and there is also an option to toggle visibility for all other layers exept the one selected.

Anyway - big thanks JimB for pointing out one of the IC was actually rotated, I didn't saw that by yesturday :oops: :cool:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes - I'll happily explain that.

Like told before (in other threads) - I've using this drawing software Inkscape for years now. But not so long ago, I figured out how to put the layer structure (just like any other kind of tre structure). So therefore I use this case not only to try to trouble shoot the device, but also to use Inkscape as efficient as possible.
No problem with using it efficently, but I don't see how you imagine it will help trouble shooting? - you need to draw the schematic out.
 

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