Weird Noise When Turning Off E-Piano's Amp

[anotherjim]

I found this image...

This seems to be a basic direct output model with a line-matching transformer and the reeds are removed.
You can see the coils and a bus connection strip PCB below. I would expect the coil polarities alternate by the connection order so some hum cancellation occurs - like a Precision Bass split-coil only more so. If it doesn't alternate polarity, they missed a trick! However, with 61 keys, there's one odd coil and spread over a wide area, I think the cancellation cannot be perfect.

[Rob Strand]

When I was looking at this the other day I found some schematics showing one half of the coils connected in opposite phase to the other half of the coils for humbucking purposes along the lines of Jim's post.
[link fix:
http://www.vintageaudioberlin.de/vabgalerien_2/tasteninstrumente_elektronisch/Hohner_Pianet_T/]

I assumed the banks were far apart so the cancelling is limited.   For example, if the magnetic field from the unit's *mains* transformer was a problem it would inject more hum into the end close to the transformer and less into the ones far away and the hum wouldn't cancel.    The Fender Rhodes units were better.  They connect banks of three in parallel then wired pairs of banks of three as a humbucker.   That means the humbucking is in the same physical region and works much better.   (FWIW, I'm not a keyboard guy at all but I have looked at the Fender Rhodes design in the past.)

All the above assumes the hum is getting in because the pickups are picking up stray AC magnetic fields.

Yeap they used to do this
https://youtu.be/84rIQC2Ot18
But as stated here (auto Google Translate) it has more to do with the static than hum.
http://forum.anafrog.com/phpBB/viewtopic.php?t=6060

This scheme is preventing hum and noise getting into the wires themselves.   Another type of interference.  This is capacitively coupled and because of that it sounds buzzy.

Yet another cause, which is very possible on a keyboard is noise in the grounds.    Maybe you should check all the ground wires, ground screws etc on any wires between the pickups and the wires that go to the transformer board.

Often old equipment would make ground connections to the chassis at a place close to where the connections were made.   This often resulted in multiple ground points.   When you have multiple ground points you can get hum problems.   Sometimes you can get away with it but if one of the ground connections goes bad you can get hum.

The way the pickups are wired on those Hohner units seem to have the potential for this type of problem.  If we changed the wiring a bit I suspect it could be improved somewhat.    The input to the circuit uses a transformer but it doesn't take advantage of the transformer to the fullest extent - it just wires the one side to ground  and wires the pickups to ground.   You could rejig the wiring to be sort of differential with respect to the "center" of the pickups.

The last points I'm just mentioning in passing as it's going to be more work than most people are willing to spend on it.    I'm also assuming there's a problem in this area.   Maybe if the grounds are all OK the hum is low enough.

In short there's three ways hum can get in.  It takes some effort to workout what type you are dealing with.   Then you have to find a solution.

[sarakisof]

EDIT: Texted this before you guys posted in the second page.
Telepathy kind of 

**** My model is the Pianet M. (Not T)

All the thing till the amp is the same to the Pianet T. A Pianet M is a T connected to an amp with speakers via an RCA cinch.
So if you want to know about M's electro mechanics, housing, keys, sticky pads, reeds, pickups etc. just search about T and you have the M too.
http://hohner-pianet.com/hohner_pianet/pianet_T.html
https://youtu.be/ypTZHK5UVck

https://go.skimresources.com/?id=24063X829826&isjs=1&jv=15.3.0-stackpath&sref=https%3A%2F%2Fen.audiofanzine.com%2Felectric-piano%2Fhohner%2FPianet-M%2Fmedias%2Fother%2F%23id%3A480265&url=https%3A%2F%2Fmedias.audiofanzine.com%2Ffiles%2Fhohner-pianet-m-notice-technique-480265.pdf&xs=1&xtz=-120&xuuid=845260264d123ceeb1fbe8937147ee22&xjsf=other_click__contextmenu%20%5B-1%5D

Pups are wired in series (in both T and M).
Up to this point they are both the same.




Then, for T, the pickups output (hot/gnd) signal cable goes to an output transformer and then to the out jack and that's it (passive circuitry).





For the M, the pickups cable (green with RCA cinch) goes into a female amp mounted RCA socket (no matching trafo) and from there straight into the input of the amp C9 10uF input cap.



My pleasure Jim (hope it's not boring for you though), me too didn't have any clue at all untill i got the instrument in my hands. I then started to complete the puzzle.
Actually I'm waiting for the T's equivalent replacement transformer, Calrad 45-703 (as suggested by Hohner itself). It can be sourced only in US, i have my ex co-producer there, so he bought and will import in EU for me in a few days.
Doing this, i will mod the M into a T when i want to amp it and be able to return back to M  playing through speakers / trrmolo chorus fx and its cheapo amp when i want to.

***** UPDATE: when i disconnect amp and plug the pickups (green rca cable) into a common phono preamp i get no hum at all. Ultra clean signal.

[sarakisof]

Thanks for the reply Rob, note that my model is the Pianet M. Read my post above.
So no passive, no output transformer. Mine is an active instrument, the pickups go straight into the amp's input (no trafo in between) via the green RCA cinch cable.

Also, as i say above, noticed when i disconnect amp and plug the pickups into a phono preamp i get no hum at all. So it couldn't be the pickups alone but probably a combi of common regular pickup's noise in interference with the cheaply made amp unit, its Power trafo placement, wires path etc..
Actually that green pickup's rca cable clearly touches the PT and gets attached to a cable tie adhesive mount which is stuck on filter cap's body! Is this wise?

***Edit: about pickups wiring, there were two submodels in T's. In the one they were wired in series (like in M too) and in the second they were wired like in the link you posted Rob

[Rob Strand]

Wow so much info to process!

So far I've got this picture:
- the ground of the green wire (coax) connects to the chassis near where the
  center conductor of the green wire (coax) connects to the last pickup.
- I would assume the pickup at the other end connects to ground?

So having those two separate ground points so far apart is a *potential* for noise to get into the ground.
It might be OK.

From that the green wire goes to the RCA jack, then connects to the RCA socket the amp.

On the amp side a second green wire (coax) connects to the RCA socket.

Then we get to:

Actually that green pickup's rca cable clearly touches the PT and gets attached to a cable tie adhesive mount which is
stuck on filter cap's body! Is this wise?

And the coax terminates on the amp board.

So now a big oh yeah!   Passing the coax past the transformer like that is just asking for problems.

We can ignore ground issues for now.

If you can re-route the wire so that it is at least 100mm away from mains wiring and the transformer it would be a very good experiment.  Perhaps even a solution.

[sarakisof]

I already tried it but had no luck Rob. Well, as i get deeper into it i realise more and more things about it. Seems the M model was popular enough in France, that's why you can find many things on french forums like audiofanzine, anafrog and german musiker-board if you use auto Google Translate. In the first one, after 10 pages of reading, i see dozens of postings from guys complaining about speaker "breath air" (translated from Google), all kind of noises, clicks, pips etc. Seems to be very common for this model and no fix even after visiting pro techs (it's a forum mostly based on musicians, users of gear, not repairmen  )
I think one guy only was saying most of the issues were solved by replacing PT with an equivalent toroidal and anti-noise dedicated one. The bottom line told by most of their techs (from Paris to Berlin) was "leave it as is, it's just the way it was designed and it was designed with cheap in mind - Pianets were the poor man's Rhodes" and that "things were noisy back in the days" (I don't say the opposite on that lol).
But many folks have solved enough of the issues by interchanging IC's. It's said that the IC's responsible for the chorus effect are most of the times the culprits for all kind of noises and weird sounds.
And about this, only today, i realised smthng else 
Having turned up the Chorus pot while the tremolo pot is down you get a weird clock-y , periodically wind/waves noise. When you turn up the tremolo pot up too (so both of them are turned up) the wind noise gets away.
Wind repeatable noise only when Chorus pot is turned up alone. When tremolo pot gets in the game no wind noise.

So that it is clock-ey repeated and it's the same reminded me my old days troubleshooting on my various bbd's . Definitely related. I insist on my first thought: i believe i could see changes if i install dip sockets and play around with different IC's in there. So far, i have replaced the CD4011 and found that the tremolo effect suddenly wake up, became more present and hearable. Don't know how a 4011 can be related with a tremolo effect (I'm not expert in IC's, I'm mostly in old big transistorized and valve gear), i searched about 1' and found that can be related, but in my case changing the 4011 fixed the dead tremolo (was expecting from it to do something about the pop speaker noise, but this was mostly fixed by filter recap).
So why not try another TDA1022, 1458, 4558's?

[Rob Strand]

Well, it's pretty normal for chorus effects to have some degree of noise.   However, if there's circuit faults or circuit design issues these can obviously cause more problems.    The design issues would have to be designed out and mods put in.   It's not possible for me to workout what types of bugs you have since it can take quite a deep look at what's going on.

For the controls I would call the first knob the Chorus Depth and the second knob the Chorus Speed.   The design of the circuit looks a bit weird/simple.  It does have the potential to do weird stuff when the Depth is set below about 1/3.   I'm not sure if this is deliberate to cut-out the Chorus effect all together when it the Depth is set to zero.  I would expect not as it would kill the audio in one of the speakers.   The only way to fix it would be to wire the counter-clockwise terminal of the Depth pot to a voltage divider instead of ground.    If the design has potential issues then it's possible it might do weird stuff when the chorus is on as well (as the sweep can go into the bad zone).  The voltage divider mod should fix that too.

As for your hum issue  I have a suspicion you have a noisy ground.   What happens when you pull the keyboard RCA out, does the hum also disappear from the Pianet or does it change and go buzzy?   What happens if you put a short across the RCA input jack?  Does that make the hum better or worse.  Don't forget to set the Bass consistently to 12 O clock note the volume control positions.  Advance the volume control if you can't hear hum to see if there's any hum there at all.   The idea of all this is to see how much hum is from the amp part.

Pianets were the poor man's Rhodes"

It's a cool sound just the same, and it's kind of unique in its own right.

[anotherjim]

Did the original 4011 have an "A" suffix? The A series didn't age well, but perhaps more importantly, they were unbuffered, unlike a "modern" CD4011B part. This can make a difference in a timing circuit as the switching speed of the internal logic changed.

Thanks to the new information, the keyboard design seems to have done its best with continuous coil reversal and a single-point ground. It just needed something like a phosphor-bronze insert in the plastic key fulcrums to keep the hammer bars grounded.

No, hum & hiss wasn't considered too a big problem in the last century. The slight "chiff" from the static in the Pianet when a key is pressed and released does, I think, lend it something in the same way that the contact click and leakage of a tonewheel Hammond does.

[sarakisof]

I'll reply to Jim first as Rob's suggestions are correct but didn't find time to test, I'll be in amp's troubleshooting when find time and will be back with results. Rob, as i play the Pianet i find Bass's pot hum not to be so loud, so maybe it's a bit normal acting like that. It's just a bit louder than volume's pot hum. Yes vol pot hums too but very slightly, you really have to stick your ear to speaker to hear it and if so, certainly less than tube amps common humming. My biggest concern on amp is now the wind noise / hiss ("breath") of the IC3 / Left channel.

@ Jim, yes the original 4011 was a CD4011AE RCA 010 one, so if that counts as an A series (i think so) it may explains my case, according to your valuable input. Thanks. Now i have installed a NXP CD4011BP one.

[Rob Strand]

Without having the unit in front of me it's hard to know if we are dealing with a fault or normal behaviour.

What's the value of C8 on your PCB?   The schematic is unclear:  looks like 830pF but that's not a standard value so is it 330pF, 820pF, or other.

If I ignore trying to read the schematic a value of C8-820pF produces a nice filter response.  If the part is 330pF changing it to 820pF might reduce the noise.

Going beyond that you could tweak the filter to reduce the noise.  That needs some care because you don't want to change the tone.

@ Jim, yes the original 4011 was a CD4011AE RCA 010 one, so if that counts as an A series (i think so) it may explains my case, according to your valuable input. Thanks. Now i have installed a NXP CD4011BP one.

I had a quick look at CD4011A vs CD4011B and the 4011B increases the clock frequency.   That could reduce the hiss *but* if that's not the intended part we are just stuffing up the chorus circuit with the side effect of less noise.  The chorus circuit won't be the same.   The change to CD4011B doesn't fix any weird behaviour when the Depth pot is reduce.   You could also put in a CD4011B then change the part values to operate like the CD4011A but that's a redesign.

[sarakisof]

You have the eagle eye Rob, couldn't be 830p, indeed C8 is a 630p styrene on my board, although scheme calls for a weird 830p. I have some nice nos 820pF HSG (Siemens) styrenes in my shelves, BUT being 630p (and not 330/390/560p), would such a small pico capacitance increase reduce hiss in IC3 path channel even a bit? I doubt. On the other hand, I'll salvage and stock a 630p one which I don't have in my shelves 😂.
Could a 630 to 820p increase change the tone and originality of the unit on the other hand? 



EDIT: I put again the original CD4011(AE) to see if the hiss could be reduced (just saying) and it indeed reduced it BUT the depth Chorus pot doesn't do anything at all. And that's why i had it removed. The old 4011 is faulty. (discolouration in its body was the first one i noticed as faulty). So it has to be changed. I could find an A series one, but definitely cannot use the old one. With that, the chorus effect doesn't work, only the tremolo (speed) works. Just to mention.

[Rob Strand]

You have the eagle eye Rob, couldn't be 830p, indeed C8 is a 630p styrene on my board, although scheme calls for a weird 830p. I have some nice nos 820pF HSG (Siemens) styrenes in my shelves, BUT being 630p (and not 330/390/560p), would such a small pico capacitance increase reduce hiss in IC3 path channel even a bit? I doubt. On the other hand, I'll salvage and stock a 630p one which I don't have in my shelves 😂.
Could a 630 to 820p increase change the tone and originality of the unit on the other hand?

630pF makes perfect sense.  I'd say the 630pF is the correct value and 830pF is a typo.  The response looks fine with either.

That doesn't mean you can't mod/tweak the response to see if you can remove some noise.
The filter cut-off is fairly low already, -3dB @ 3.4kHz, so we don't want to push that down
too far.

It should be obvious that adding more filtering will remove noise!  However you can over do it
and start filtering the top end off the notes.   The trick is to find the best balance. 

If I had to take the best stab at a simple mod to try I reckon changing C8 to 1nF or 1n2 might help.
If you want you could just add a parallel cap, value around 470pF to 680pF, in parallel with the
existing 630pF.    You can make changes to the other caps as well but there's good and bad ways
to go about it.   You have to test these things by ear.  That's were adding a parallel cap helps, you
can add it and remove it to do a quick A/B test and see if the noise drops and the notes are OK.

[anotherjim]

I hope this helps, although the different versions of the board and component values may mean some references are wrong for yours.

It might be worth checking the BBD clock pin 1 and 4 voltages. These should be square wave pulses but on DC, your meter will average at half the supply voltage at around 7.5v. Any big difference from that or between them at various speed and depth settings could give a clue. The same goes for the 4011 input pins 8 and 13. The pair of timing caps C17 and C18 need to have closely similar values to get closer to matched square wave clocks for the BBD rather than different pulses. While you're at it check R19 and R20 resistance. Diodes D4 and D5 shouldn't give much trouble but the quality of these wasn't so certain back in the 1970's so a leaky or intermittent one could happen.

The BBD clock modulation from the MC1458 pin 7 is a triangle wave that gets filtered some by R18 and C16 to make it smoother and more like a sine wave. An intended side effect will be the amplitude of the modulation automatically reduces at faster modulation speeds. R16 and R17 determine the amplitude of the triangle wave.
For the range of modulation to suit the 4011 clock generator...
C14 and R15 values determine the fastest clock speed. Too fast and the Intensity control may have no effect.
C16 and R18 determine the minium-maximum intensity available.

Without more than a DMM to test with, you can only check resistance and voltages, but if you understand what they do, you can make reasonable assumptions about the capacitors.

[sarakisof]

What a circuit analysis Jim, so much info here!
Seems i found smthng weird.

It might be worth checking the BBD clock pin 1 and 4 voltages. These should be square wave pulses but on DC, your meter will average at half the supply voltage at around 7.5v. Any big difference from that or between them at various speed and depth settings could give a clue.

TDA1022 voltages at various speed & depth settings for Pin1 and 4 were between 7.48 - 7.54 VDC. Seems right.

BUT

The same goes for the 4011 input pins 8 and 13.

CD4011BE pin8 measured 15.04 VDC and pin13 7.52 VDC.
Are you sure they should be the same and at half of 15V (~7.5V)?
If so, smthng wrong must be here.

R19 and R20 measured 151K both.
Diodes seem fine too.

[Rob Strand]

CD4011BE pin8 measured 15.04 VDC and pin13 7.52 VDC.
Are you sure they should be the same and at half of 15V (~7.5V)?
If so, smthng wrong must be about Pin8.

Can you try the measurement with the Depth/Intensity pot in a few different positions: min, center, full.

It's weird that pin 1 and pin 4 on the TDA1022 are OK but pin 8 on the CD4011 isn't.
Pin 8 must be doing something.
The multimeter could be loading down the circuit and stuffing something up.

Do you have a frequency meter?  Perhaps on your DMM?

If the frequency on pins 1 and 4 of the TDA1022 is too low it can cause a lot of noise.   It is normal for a lot of noise when the frequency is low however, maybe the problem is the fact the frequency is low!

[sarakisof]

Just triple checked. BBD pin1 and 4 are about 7.48/7.50/7.54V at min/center/full.
For the same positions for 4011 pin13 is 7.47/7.50/7.55V
and pin8 stable 15.04/15.04/15.04V. 

It's weird that pin 1 and pin 4 on the TDA1022 are OK but pin 8 on the CD4011 isn't.

Sure it is.
And more weird, that 4011's pin4 (connected to pin8) measures 7.52V !!

[Rob Strand]

Sure it is.
And more weird, that 4011's pin4 (connected to pin8) measures 7.52V !!

I have a very strong feeling pin 8 has a bad connection or a break in the PCB to pin 4.   You should at least check the continuity between pin 4 and pin 8.

When you don't have the DMM connected to pin 8 the sharp edges of the signals couple from pin 4 to pin 8, just enough to trip pin 10, pin 11.

Another option is the PCB doesn't match the schematic.   The basic circuit might be the same but they have used different gates in different positions.   You would have to check the PCB against the schematic.  If all OK go back to the pin 4 to pin 8 broken theory.

[sarakisof]

Damn i made a kid's mistake. Working on it alongside other things since many hours and was too tired. My apologies, was measuring at pin14 instead of 8 so obviously all i was getting was the Vdd 15.04V  Pin8 is 7.48/7.50/7.52V so everything is fine there too.
I'm really sorry for the trouble Rob.

EDIT: About smthng i noticed. By chop stick tapping on IC1, IC2, IC3 's bodies (voltage regulator and power transistors) and their common heat sink i get the tap tap pops through speakers. They are microphonic.
Is this normal?

[Rob Strand]

I'm really sorry for the trouble Rob.

No problem, easy to do.

It looks like it's working OK.

What we don't know is the frequencies, as mentioned a couple of posts back, it can be a problem.

In addition to that, the way the depth pot works is a bit weird.   When the depth is set to full the
output of the LFO (pin 7 IC6) swing about 3.2V to 11.8V.   That's more or less in a range that works
for the VCO (IC 7).  However when you dial back the Depth the range of voltage moves toward 0V.
When we get to LFO voltages (pot wiper voltage) around 5V the frequency of the VCO starts to take off and at some point it seems like it could even stop oscillating (because the speed is beyond what the CD4011 can do).
I can see what they were trying to do, as you back off the depth the delay (from the TDA1022)
gets smaller and has little effect on the sound.

What position of the Depth control would you say are noisier: min, center, max, other?

[sarakisof]

Ok, to clear things up.
Hiss: There's always a slight windy hiss in the L channel (IC3) only by just powering the unit. It has certainly to do with the Chorus effect as you can "hear the wind changing as you turns the chorus pots" . It's like you hear the BBD but you don't have signal, so you hear the hiss making that effect you're controling. (Have experienced in the past smthng similar, common bbd behaviour so maybe it's normal).
Depth pot at min has no effect at all, at max you get "chorus" to maximum.
So when you have the depth pot to min you just hear the hiss.
When you turn depth pot to max you can hear the hiss "come and go" periodically/ clockey.
Then, if depth pot is up  and you turn up the speed pot you just hear this hissy effect coming and going more quickly / slowly..
That's all. All this in the left channel (IC3) only, the "chorus" channel. (Makes sense).

Hum: Hum exists only if you start turning up the Bass pot. In both channels, but mostly in the Left.

Note: Have a look at the EDIT in my last post. Volt regulator and / or power output transistors are microphonic . You get really loud pops in some cases by chop stick poking. And you can microphonically listen to the tap being produced through speakers.

Also: the other day i found that during recapping i had left C27 unsoldered (really tired these days , recovering from H1N1 flu). Quickly fixed, but could something went wrong during all this time i was troubleshooting/ functioning the amp 8-12hrs/day?

EDIT: About hiss maybe it's normal. https://www.diystompboxes.com/smfforum/index.php?topic=122825.0
Just seems weird to most people in Pianet M because you have to deal with stereo here and you get the hiss in one channel only (the bbd channel ) so it makes people noticing more than in a mono stompbox
I guess I answered by myself the hiss issue, did i?
Now let's go to the hum issue, as long as I don't want to change originality by mod /LPFiltering the circuit etc. in a try to eliminate hiss. I'm already used to it, a velvet windy old hiss..

🙂