The Unknown Spring Reverb

[razabri]

Hi! So I got this used reverb thank with it's circuit attached to it's back, to be used as an effect or added to an amp. The thing is, I have no idea on how to decipher the pin out, as it has no marks besides that ELKA 1910 on the back, and I can't find the schematic anywhere. My guess is that the white plug should have pins (there are 8 of them) for power, in & out, control... How would you go about finding out which is which, without burning anything? Thanks!

[willienillie]

If you can't find any documentation online, you'll have to trace it out.  It's single-sided with big traces, so should be fairly easy.  The only IC that I can positively identify in your picture is an LM308CN, the ProCo Rat opamp.  I'm curious about the 16-pin chip with only one side connected.

[tonyharker]

Its a TDA1904 a 4w amplifier.  The side which looks unconnected are all connected to common (ground) and act as a heatsink.

[PRR]

The bigger chip is a well-known small audio power amp (Tony's TDA1904). Find its pinout. Trace power + and - pins to connector. These should also come to the smaller chips, possibly through an R-C smoothing filter.

You can also trace signal input from big chip back to connector. I can see how reverb drive comes from power chip to spring. The other end of the spring must go to the smaller chips, set up as some basic audio amplifier. I do not know what there is a third chip.

[razabri]

Thanks for the reply. I assumed so, that it needs tracing, but I'm not really knowledgeable in the area... I've just made few vero-board effects, following drawings and, as it worked out well several times, I got interested.
Here are the datasheets of the ICs:
https://www.digchip.com/datasheets/parts/datasheet/456/TDA1904-pdf.php?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s
https://www.ti.com/lit/ds/symlink/tl082-n.pdf?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s
https://www.digchip.com/datasheets/parts/datasheet/321/LM3080-pdf.php?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s

[razabri]

Ok, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...

[Rob Strand]

Ok, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...

I came to quite a different conclusion.   

Using your numbering:

1                   DC control for LM3080.   Don't know what this is used for.
2, 3, 4           grounds
5                    Audio Out
6                   ground for audio in
7                   Audio In
8                   +V Power

As far as the circuit goes, it's

Audio IN  -->  Opamp ---> TDA1904 ---> Reverb Tank In ---> Reverb Tank Out --->  Recover Amp (Opamp)

As far as I could see, and I didn't trace the whole thing, there doesn't seem to be any mixing on the board to combine the Reverb signal back with the Dry signal.

There's also an OTA.   The second 8-pin device appears to be an LM3080 OTA not a LM308 opamp.

Connector Pin 1 seems be some sort of control input, it passes through the transistor then to the LM3080 control pin.
The output of the OTA is wired to the Audio -Out.

I don't know what input source is for the LM3080.  It's possible it doesn't have an input.  It could be used for disabling the Reverb signal;  It could be related for Tremolo or Vibrato, but the lack of dry signal mixing means it would an odd implementation.

The fact there's no obvious mixing and the presence of the OTA means the circuit is a little weird and you will really need to trace it out if there's any hope of making sense of it.

[razabri]

Ok, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...

I came to quite a different conclusion.   

Using your numbering:

1                   DC control for LM3080.   Don't know what this is used for.
2, 3, 4           grounds
5                    Audio Out
6                   ground for audio in
7                   Audio In
8                   +V Power

As far as the circuit goes, it's

Audio IN  -->  Opamp ---> TDA1904 ---> Reverb Tank In ---> Reverb Tank Out --->  Recover Amp (Opamp)

As far as I could see, and I didn't trace the whole thing, there doesn't seem to be any mixing on the board to combine the Reverb signal back with the Dry signal.

There's also an OTA.   The second 8-pin device appears to be an LM3080 OTA not a LM308 opamp.

Connector Pin 1 seems be some sort of control input, it passes through the transistor then to the LM3080 control pin.
The output of the OTA is wired to the Audio -Out.

I don't know what input source is for the LM3080.  It's possible it doesn't have an input.  It could be used for disabling the Reverb signal;  It could be related for Tremolo or Vibrato, but the lack of dry signal mixing means it would an odd implementation.

The fact there's no obvious mixing and the presence of the OTA means the circuit is a little weird and you will really need to trace it out if there's any hope of making sense of it.

Thanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense. So I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...
The transistor is BC415B... Don't really know what it does...
Guess I have to learn how to trace and draw a diagram to make some sense out of it all and I hope I'll manage to do so, as I like how this plate and tank look and I'd hate for it to go to waste. If not, perhaps I could just use the tank and make one of the vero-board drivers for it...

[11-90-an]

Actually, since that reverb tank has I/O amplifiers, you can just solder come jacks to the input and output....

But I don't think it would work...

[Rob Strand]

Thanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense.

I only did a quick scan.  Checking stuff like power pins on the datasheet and general flow of the signal.

So I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...

I was thinking it's was from an old keyboard.  Mainly because styling of the separate board.    Nonetheless a guitar amps is just as good a guess.

The transistor is BC415B... Don't really know what it does...

It's pretty common for a transistor to drive the control pin of an OTA.   I'm assuming it's a circuit like that.  It's such an unusual configuration it could be anything.

I think if you had the datasheets in front of you, you could probably do reasonable job of tracing it in about 2hrs.
The TDA1904 circuit will largely follow the example in the datasheet.   The opamp before the TDA1904 is wired
as an inverting amp (many examples on the web).   The other half of the opamp which is used for the recovery
stage is a non-inverting gains stage [Actually it's inverting] ; very common configuration for reverb circuits and it will match circuits on the web.
So most of the circuit you can just copy the shape and fill-in the different values.   The only one which needs careful attention is the LM3080 OTA.    Without tracing some of it, it's hard to give an example to copy from.

[razabri]

Thanks again, and again you're most probably right -

Thanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense.

I only did a quick scan.  Checking stuff like power pins on the datasheet and general flow of the signal.

So I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...

I was thinking it's was from an old keyboard.  Mainly because styling of the separate board.    Nonetheless a guitar amps is just as good a guess.

The transistor is BC415B... Don't really know what it does...

It's pretty common for a transistor to drive the control pin of an OTA.   I'm assuming it's a circuit like that.  It's such an unusual configuration it could be anything.

I think if you had the datasheets in front of you, you could probably do reasonable job of tracing it in about 2hrs.
The TDA1904 circuit will largely follow the example in the datasheet.   The opamp before the TDA1904 is wired
as an inverting amp (many examples on the web).   The other half of the opamp which is used for the recovery
stage is a non-inverting gains stage; very common configuration for reverb circuits and it will match circuits on the web.
So most of the circuit you can just copy the shape and fill-in the different values.   The only one which needs careful attention is the LM3080 OTA.    Without tracing some of it, it's hard to give an example to copy from.

the thing's almost definitely from an old keyboard, as ELKA is the organ brand (it did come up when I first googled it, but I didn't make the connection). There are lots of pictures of the insides of the organs, but I can seem to spot this unit anywhere. Also there's this thingy:



but I don't think it has anything to do with the unit I got...

How would you go about testing this thing, making it usable? I have one small amp that runs on 12v battery and it as loop effect, so I was thinking of adding this reverb to it. Even if I managed to put together a diagram, I'm not sure what would be the next step...

[Rob Strand]

the thing's almost definitely from an old keyboard, as ELKA is the organ brand (it did come up when I first googled it, but I didn't make the connection). There are lots of pictures of the insides of the organs, but I can seem to spot this unit anywhere. Also there's this thingy:

I found a video of an Elka E19 1983 organ.  I had a similar style reverb board and connector to yours but the electronics looked older.   Your IC's are maybe 1988/1990 assuming they are original.  By 1990 the synth models seem to appear.  So maybe yours is mid to late 80's.   No idea what Elka organs fit that era.

but I don't think it has anything to do with the unit I got...

Well, you never know.  I also came across an Elka amp RM 140 which has a reverb.

How would you go about testing this thing, making it usable? I have one small amp that runs on 12v battery and it as loop effect, so I was thinking of adding this reverb to it. Even if I managed to put together a diagram, I'm not sure what would be the next step...

Well the main issues the how and were to  are it into your amp and also mixing in the reverb and dry signal.   The Fender style resistive mixer might work.    You will need a reverb pot somewhere as well.   Another thing is you might need to adjust the signal gain on the reverb board so that the reverb springs are driven at the right level.

The simplest test would be to connect power to the PCB and connect the output to an amp.   Then scratch on the springs and see if you can hear something.   If the test doesn't work it doesn't mean the unit is broken as the LM3080 OTA circuit could be affecting things - we don't know what that does.  If you do hear something then that only checks the reverb spring recovery amp.    So the next step would be to leave it connect to an amp and see if you can get a signal through the unit.   If you do it will be *all* reverb.

[razabri]

It's alive! Yup, you were right again - I've connected it as per your instructions, powering the plate with a 9v battery and I got sound of springs when I poke them, very low, as I turned the amp all the way up, but it's there. I also have connected the audio IN and could hear my guitar as well, but also very quiet and I think a bit distorted as well, but it seems to me that it lacks the "reverbness".

Then I though, ok, it probably needs more power and I just "knowledgeably" hooked it to a 12v power adapter I had laying around. Oh, the smell of burning electronics, and a fair bit of smoke too! It turns out that I only burnt the 10 Ohms resistor that leads to the TDA1904s' power pin. Replaced it and got the same results as before. The adapter I used is rated 1.5A, so it might be it's way too much current... I've read somewhere that these tanks would use a lot of volts, so some circuits need a 18v charge pump. It says in the datasheet of the TDA1904 that the supply should be from 4 to 20v...

The thing is, I have this TX30 Crate Taxi amp that runs on a 12v battery. It's just like a car battery and it can be recharged and the amp is pretty ok for it's purpose - performing at locations with no power, street performance... But it kind of lacks some of the benefit that a reverb can give, less clear, more atmosphere... Thus the reverb tank and all.





I thought I could place it inside, though it's a bit tight and magnetic pull from the speaker could be the problem, then supply it with the same battery and tap into the amps effect loop jack from the inside. I understand that the original plate would need a way to mix dry and wet, but I'd probably just set it at one position and leave it like that, adding just a toggle switch on the outside, to turn the effect's power on.

Adjusting the signal gain on the board... Could it be that trim pot's function?

Anyway, a lot of cool stuff turned out while searching about this - I've seen the Elka E19 video you mentioned

I found a video of an Elka E19 1983 organ. 

- good stuff, I even like the rhythm section. Also, I've read about the guy finding an old Elka organ in a dumpster and it turned out that it only had couple of fuses burnt, he replaced them and it's fully functional and nice sounding too! I also came across another amp that Elka made, a Rig 200 that has a reverb too.

I don't really know what I'll do next with this one - I thought, since the photos I found of some different units suggested that they had a fast/slow switch, I was thinking that maybe that mysterious 3080 is responsible for giving it a sort of short, slap back and a sustaining, long reverb... So I might experiment with connecting it somehow and see what happens. Also, I hope I'll manage somehow to give it more power, as like this it's barely audible.

Thank you for your great insight and for your help! I hope I get it to work fully somehow.

[Rob Strand]

It's alive!

OK cool, that's a start.

but also very quiet and I think a bit distorted as well, but it seems to me that it lacks the "reverbness".

So I think the quietness is due to the LM3080, see below.

Then I though, ok, it probably needs more power and I just "knowledgeably" hooked it to a 12v power adapter I had laying around. Oh, the smell of burning electronics, and a fair bit of smoke too! It turns out that I only burnt the 10 Ohms resistor that leads to the TDA1904s' power pin. Replaced it and got the same results as before. The adapter I used is rated 1.5A, so it might be it's way too much current... I've read somewhere that these tanks would use a lot of volts, so some circuits need a 18v charge pump. It says in the datasheet of the TDA1904 that the supply should be from 4 to 20v...

The smoke isn't normal.   My expectations of handling 12V is exactly what you have written.   At this I'd be thinking the TDA1904 could be fried.   Another explanation is the TDA1904 has been replaced with a chinese fake, these often blow up at higher voltages.   If you have a multimeter you could power-up on 9V and measure some voltages on the TDA1904 pins.

A dodgy TDA1904 might explain the distortion.

The thing is, I have this TX30 Crate Taxi amp that runs on a 12v battery.
hus the reverb tank and all.

If you like reverb it's going be be worthwhile for sure.   You could get an el-cheapo reverb pedal.   They are good enough for a touch of reverb.   It would save a lot of head-aches.

I thought I could place it inside, though it's a bit tight and magnetic pull from the speaker could be the problem, then supply it with the same battery and tap into the amps effect loop jack from the inside. I understand that the original plate would need a way to mix dry and wet, but I'd probably just set it at one position and leave it like that, adding just a toggle switch on the outside, to turn the effect's power o

it's all very doable.   I wouldn't worry about the speaker magnetic pull.

Adjusting the signal gain on the board... Could it be that trim pot's function?

The pot does adjust the level of the reverb.    BUT  I have a very good reason to suspect the LM3080 is reason for the low level, not the trim pot adjustment.

I noticed the trimpot when I eye-balled the circuit.  I had another look and I noticed the reverb recover amp is inverting - I think I said it was non-inverting above.

good stuff, I even like the rhythm section. Also, I've read about the guy finding an old Elka organ in a dumpster and it turned out that it only had couple of fuses burnt, he replaced them and it's fully functional and nice sounding too! I also came across another amp that Elka made, a Rig 200 that has a reverb too.

There's some cool stuff on the web.    Always someone tinkering around with some obscure stuff.   Often it turns out to be a big help to people fixing units.   One small piece of info is enough to get them over the line.

I don't really know what I'll do next with this one -

I think the most important thing at this point is to work out the state of the TDA1904.   I don't have a good feeling about it.    If it's fried you have the option of replacing it, but at the risk of buying some chinese fake which will fail again.   Another option is to build your own reverb on a vero board.   

If the TDA1904 is fried  as it turns out you aren't using many parts on the PCB anyway!   Like you could build a minimal reverb using using a common LM386 amplifier instead of the TDA1904.

I thought, since the photos I found of some different units suggested that they had a fast/slow switch, I was thinking that maybe that mysterious 3080 is responsible for giving it a sort of short, slap back and a sustaining, long reverb... So I might experiment with connecting it somehow and see what happens. Also, I hope I'll manage somehow to give it more power, as like this it's barely audible.

Something along those lines came to me but I'm not a keyboard player or keyboard electronics person so I don't know what to expect.  As an engineer it's something the designers might have done.   The way I see that working is the reverb level would be artificially shaped for long and short.  The envelope would have to come from the key activations.

I scanned over the circuit around the LM3080.   I can't see all the parts or the values but I can make some conclusions.
- the audio passes entirely through the LM3080.   (no signal from the opamp like I originally thought).
- If control signal on pin 1 of the connector is not present then I'm fairly certain the audio with be cut-off.
  That might explain the low audio level.

You want to enable the audio permanently.

One option is to do a mod on the board an completely bypass the  LM3080,  you don't need any of the functions is provides.   

As far as testing the board it is possible to re-enable the audio.   I can't make out the resistor in the top right of you picture.  It looks like red red red which is 2.2k; maybe you could check.   That's a low value.   So to get the audio working you would need to connect pin 1 of the connector to a 10k resistor then the other end of the 10k resistor to positive (+V).   I wouldn't recommend connecting pin 1 directly to +V as could damage the  LM3080.

[razabri]

Rob, I can't thank you enough for taking time and trying to figure this out, as I'm most of the time clueless of what I'm doing with this, but your insights are making things a lot clearer.
I've put together a drawing of the circuit:



As I'm not experienced in tracing this is all I could do - just to put it as is with all the values. Some of the capacitors are marked the way I couldn't make the value precisely and I don't have measuring equipment (thinking of getting some real soon). I hope this helps in making things out...

As far as testing the board it is possible to re-enable the audio.   I can't make out the resistor in the top right of you picture.  It looks like red red red which is 2.2k; maybe you could check.   That's a low value.   So to get the audio working you would need to connect pin 1 of the connector to a 10k resistor then the other end of the 10k resistor to positive (+V).   I wouldn't recommend connecting pin 1 directly to +V as could damage the  LM3080.

I'm not sure if I understand this correctly - that no power should go to the TDA amp, but instead to power the board via first pin, to the 3080 with a 10k resistor? What would be the result of this?

Also, I remembered that when I was making some pedals using vero boards, the schematic often shown that the output of the board should be connected to the first pin of a pot, then another connection would be made from the board and the output would go from the third pin to the out jack. So, I was thinking that out pin and the pin from the 3080 part of the circuit could maybe go together like that - I know that what I'm saying might be silly, but I'm shooting in the dark about this, with that little experience in building that I have...

Oh, and the 1904 visually seems fine, the burning and the smoke seems that have came only from that resistor, which turned all black in the middle. I don't know, maybe I've reversed polarity of the adapter or something...

[11-90-an]

Drawing a schem.. may I ask of the value for this resistor? You didn't write it's value down in your trace...


Also, what's the value of the cap labelled "1047"?

But that's one good first trance you got there...

EDIT: here's the schem...

There are still some components without definite value, (like the orange drop caps, highlighted orange) and there is 1 other cap and 1 resistor that I queried about...

[razabri]

Hey that's so cool! I mean the schematic you pulled out of the drawing and good of you to spot that resistor value.
I also have one more mistake that I found out - there's the link missing and I corrected it in the updated drawing, marekd with red arrow.



I still don't really know what to make of those orange caps, as they suggest the simplest solution - that they are of values .1, .022, .22, but searching the web I found different statements, so I left it like that.

Also that 1047 cap is marked like that since I can't really say what's it's value and I still don't have tools to measure it.

But thanks for the schematic - I've already said that I'm not knowledgeable about all of this (yet), but I'm trying and your input is certainly helpful! I'm ok with graphics, as I do illustration most of the time, but music is my love and I am also very intrigued with all of the gear around playing electric, made some pedals for myself... I believe that there's always something poetic inside of it all, the physics and the behavior of the waves. That's probably why I'm sorry to trow away this circuit and simply build a new driver for the tank. Hope I'll hook it up somehow, with help from you guys! 

[razabri]

Another update - I remembered to mark the connector pins the way Rob suggested earlier, as I did make a first test like this - the +V, IN, OUT, DC control for LM3080

[anotherjim]

1047: The 7 is misleading (is it a Z?). Probably 104 or 0.1uF aka 100nF. Not critical, it's the power supply high-frequency noise bypass/suppressor.
Vref, bias reference voltage for the TL082 opamp - you're missing a ground trace for the 47uF & 4k7 resistor at the bottom (may be outside the image border?).

The orange film cap values 10-90-an gives look right. I would trust them. Any film capacitors can replace those if necessary.

[duck_arse]

if this board is from an organ, it might be a swell pedal output to the LM3080 is the missing control input thingy.