Clearaudio basic - IC (probably opamp) name scraped away

[Grossel]

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

 

[Nigel Goodwin]

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]

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]

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

 

[JimB]

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

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]

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

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]

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]

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)

 

[JimB]

Also haven't rechecked that the pinouts are actually correct (seen from below)

One of them is the wrong way round.

JimB

 

[Nigel Goodwin]

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]

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

 

[Nigel Goodwin]

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.

 

[Grossel]

Just figured out what's wrong - the DPDT switch fails to make contact between two poles. May be more failures, but that is what I know about for now

 

[Grossel]

Figured out id had to be either TLC071 for the single opamp, and for the dual opams, both TLC072 and NE5532 (same input). Guess it make sense when TLC071 is used then TLC072 is the probable choice for the other dual opamps.
Then the Offset inputs are not used.

Now it's time to put some ID on the components and draw the schematic up.

 

[schmitt trigger]

A dead giveaway for dual opamps is that V+ is alway located on pin 8 and V- on pin 4.
For single opamps, V+ is located on pin 7 and V- on pin 4.

So if your red trace is V+ and the blue one is V-, then the board has a dual and a single opamp package per channel. As others have mentioned, very likely a TL072 and TL071, very popular devices in the audio world.

But since this is a RIAA preamp, most likely a NE5532 and a NE5534. They were considered lower noise, if I recall correctly.

 

[Nigel Goodwin]

A dead giveaway for dual opamps is that V+ is alway located on pin 8 and V- on pin 4.
For single opamps, V+ is located on pin 7 and V- on pin 4.

So if your red trace is V+ and the blue one is V-, then the board has a dual and a single opamp package per channel. As others have mentioned, very likely a TL072 and TL071, very popular devices in the audio world.

But since this is a RIAA preamp, most likely a NE5532 and a NE5534. They were considered lower noise, if I recall correctly.

They were, and still are!.

 

[Grossel]

Well, I'd drawn up the schematic (have not taken the time to see if it make sense yet) based on the IC being TLC071 and '72. Here is my result so far:

Got som time to look into this now since the corona situation prevent me from going to work as normal

So far, there is a couple of things that remain:
* Want to reallocate some components for better reading.
* Compare it to known filter circuits and see if it make sense (it get totally wrong if there is actually another IC that being used on the pcb)
* Numbering on resistors and capacitors should be renumbered so it make sense reading the schematic from left to right.
* Drawn the Left channel as well (just a duplicated version of the right channel when I'm satisfied with that).
* And maybe a better layot drawing with all component labels.
* Most important (I guess) : Actually try to locate the fault here, not just play aroud to make the drawing look so damn nice

Then I think I'd make a usable schematic for this thing.

[edit]
And of course, the switch being defect, shouldn't make the complete circuit not working.

 

[rjenkinsgb]

The schematic makes sense.

The way it appear to me, IC1 generates the bias voltage for IC3A input & stabilises it by comparing the IC3A output voltage to 0V.
IC3A has a gain around 29.

The resistors and caps in the coupling circuit are possibly intended for a low frequency & high frequency roll off, but possibly that is not quite right, the way it is connected? With an extremely high input impedance on the opamp pin 5, I do not see it doing much as show, other than some high cut.

IC3B is a pretty straightforward gain stage, with the feedback / gain selected by the switch.

If the switch was completely open, the opamp would likely drift so the output was near one supply & not pass any audio.

 

[Grossel]

Having a ton of time, I've reworked this one.

Notice: I beleive the inductors may be a part of the feedback loop - the voltage supply to the opamps goes through those.

By this, it should serve as Clearaudio basic schematic.


[edit]
What I forgot was to ask if the schematic make even more sense now, or would some suggest any more rearrangement?

Also, the S1 button settings are labeled "MC" and "MM", but I haven't worked out what pins on the schematic that are supposed to be. For those of you having worked more with this kind of amplifiers, I was thinking since it's source are mostly mono, could it be that the S1 missing a leg is done by purpose? That thing is so weird (among scraping off opamp ID, etc) - to make a kind of odd stereo effect? Silly idea, huh?

Also - every single labeling is done myself, I have no access to it's original schematic.

 

[KeepItSimpleStupid]

MM is moving magnet and MC is moving coil. MC is a nice cartridge, but really low output.