Weird Noise When Turning Off E-Piano's Amp

[sarakisof]

I have a Hohner Pianet e-piano (same as Pianet T but active, with built in tremolo, chorus effect and dedicated amp) in which I've done a full recap except for the two C1,C2 filter caps as I'll source them tomorrow. I've also replaced the ESM532 output IC3 (almost unobtainable) with a TDA1111 (equivalent - hard to get too but locally scored) as the R30 (47R) was slightly burned (replaced that too indeed) and i was getting almost no signal at all in that channel. Problem solved, but i still face a weird robotic/oscillation(?) noise in that channel just 2 sec. after turning the thing off (issue was there from beginning). Like an ic or smthng is near death kind of.
Shot a quick vid so you can hear by yourself.

https://we.tl/t-1BBALB9xPs

What do you think smthng like this could come from?
CD4011, 4558 ...?

I'm also getting pop when turning on and off. Any thoughts on this too?

[Rob Strand]

Is the noise on one or both power amp channels?   That would be something to look at.

My initial guess is it is some sort of power amp motor-boating when the main power rail drops to either 10V or 15V; 3 seconds of idle load through 2x4700uF.

When I look at the power amp schematic then look at the datasheet I can't help but notice that the datasheets use 100ohm for R30 and R39 and don't have diodes D1 or D2.  Is that the cause?  No idea, it's only a difference.  The diode + resistor may help or hinder either reliability or performance.

It would be best to check the noise isn't present on pin 1 or pin 7 of IC 4 before going forward.   If it's in the preamp circuits then that's a whole different ball-game.    IC1 doesn't have a 100nF cap across pins 2 and 3 so maybe regulator is oscillating - I have doubts it's that but it's another crack in the design.

[sarakisof]

Is the noise on one or both power amp channels?   That would be something to look at.

As i mentioned above, it's on the one channel only, the one that i replaced the ESM532, R30 and D2. But was there from beginning.

When I look at the power amp schematic then look at the datasheet I can't help but notice that the datasheets use 100ohm for R30 and R39 and don't have diodes D1 or D2.  Is that the cause?

Where do you see that? R30 is 47ohm in the scheme and on my board too. Diodes exist too. What do you mean by datasheet?
Did you see my video Rob?

[Rob Strand]

As i mentioned above, it's on the one channel only, the one that i replaced the ESM532, R30 and D2. But was there from beginning.

OK, got it.  if the noise is coming from the IC3 channel, notice the preamp of that channel is different to the other channel.  The noise could be coming from anywhere along the preamp IC8, IC5, IC4, or the power amp IC3.   

Where do you see that? R30 is 47ohm in the scheme and on my board too. Diodes exist too. What do you mean by datasheet?

I dug up the ESM532 datasheets and the TDA1111 datasheets.  I didn't keep the links.    I have seen diodes in that type of circuit before however there's a small chance the diodes could cause a problem on those devices.

It's probably worth checking the preamp signals first.

Did you see my video Rob?

Yes.   I've heard plenty of commercial equipment do stuff like that at turn off.  Usually the noise was there since new.

[sarakisof]

Quote
Did you see my video Rob?
Yes.   I've heard plenty of commercial equipment do stuff like that at turn off.  Usually the noise was there since new.

So it could be there since new.
About pop switching on/off noise do you think the fragile CD4011 could have smthng to do with it?
Look, i have in my shelves all of the ics used in this circuit (well except for ESM/TDA1111s). Do you think it is wise to desolder them all (pre's&CD), install appropriate dip sockets(making my life easier for the future) and replace/leave/use accordingly? Or i could damage those "valuable" RC4558s and RCA CD while desoldering in the case of leaving the old component(when this isn't the culprit)?

[Rob Strand]

So it could be there since new.

I wouldn't be surprised at all.

In fact I just realized the .jpg schematic has an extra 4016 IC9.   To me it looks like the circuit had a problem (in the PDF schematic) and they added IC9 to disconnect the preamp from the power amp to hide the noise.    I'll admit I haven't check all signals but it sure looks like a fix.

That's being the case I'm suspecting the preamp more now.

The fact you only have the issue on the IC3 channel means the noise is likely to originate from that chain of preamp ICs I mentioned.

I would just listen to the outputs of those points and see if you can workout at what point the noise starts in the audio chain.

About pop switching on/off noise do you think the fragile CD4011 could have smthng to do with it?

I've got my doubts but we can't assume anything.

Look, i have in my shelves all of the ics used in this circuit (well except for ESM/TDA1111s). Do you think it is wise to desolder them all (pre's&CD), install appropriate dip sockets(making my life easier for the future) and replace/leave/use accordingly? Or i could damage those "valuable" RC4558s and RCA CD while desoldering in the case of leaving the old component(when this isn't the culprit)?

I would leave it as is.   Find where the problem is first.   If you have an oscilloscope the shape of the waveform can be used to deduce what's causing it.

Hmm, that fix looks more like a muting during power ON rather than power OFF.

[ElectricDruid]

I agree with Rob that the 4016 switches do look like an attempt to prevent this sort of thing. Disconnect the power amp from the preceding stages to save your ears and speakers.

It's a bit tangled to work out, but I'd get the oscilloscope on the 4016 switch control pins (pins 6 and 12) and see if you can see anything happening there.

I also agree with Rob that testing the output of the various amp stages going through to see where the noise first appears is a good idea. It'd be nice to know how far back it starts, and that might give us a clue.

[sarakisof]

OK, seems there are two schematics out there 

1. https://www.clavinet.com/shema_PIANET_M.pdf

2.https://www.musiker-board.de/threads/hohner-pianet-m-an-verstaerker-angeschlossen-nun-stumm.654929/
Post #9


Mine is mixed but mostly the first one.

On my board there is no CD4016. Only CD4011.

[sarakisof]

Just replaced the C1,C2 filter caps and the noise is gone. Well you can still slightly hear a tiny pop like but certainly not that loud constant faded robotic noise of the video i uploaded.
So filter recap seems it did the job.


Now one last thing that was there from the beginning too and never gone (maybe it's a bit normal like they say in all those e piano forums, but heh i think no any hum/noise is acceptable, right?), is a 50/60Hz hum i get from both channels. It's there with all pots down - not so loud though and that could be normal - but what it's not normal for sure is that it gets increased then as you turn up the Bass Pot P1, so this hum is related to P1 bass pot. Unfortunately no scope yet.
Any thoughts?

[DIY Bass]

I have an Eminar Bass amp that will get a tone that starts high and reduces in frequency quickly a few seconds after turning it off.  It's always done it.  I once had somebody trip over the power cable, pull it out of the power socket, and bend down and plug it back in without losing any volume.  I just assumed the nice big power supply caps, and a toroid transformer with a heap of magnetic field can probably hold some charge for a bit, and the tone is what happens when they finally discharge.

[anotherjim]

I'm not surprised you get hum with the volume turned down. The only volume control I see is a gain control in the preamp. Any hum present in the circuit is given free rein and boosting the bass is sure to increase it. There is a 7.5v reference supply filter C13, a 15v filter C3 and power amp 30v filters C27 and C37. Probably change those caps at a minimum.

While you're at it, you might as well replace electro caps at the power amps. As you know these amps failed sometime previously.

[sarakisof]

There is a 7.5v reference supply filter C13, a 15v filter C3 and power amp 30v filters C27 and C37. Probably change those caps at a minimum.
While you're at it, you might as well replace electro caps at the power amps. As you know these amps failed sometime previously.

Have replaced every electro except for the blue radials which I'll do today.


Don't judge, i just replaced them with NOS electros. I know i know. It's my first time doing this. Scored a big bag full of NOS components for almost nothing (10bucks) from an old small shop's clearance stock, including about 200 electros, 250 film foil caps (mustards, fish's, styros, PIOs etc), 100 germaniums (ac122/132s mostly), other interesting Lockfit/TO105 transistors i need for my old organ/keys repairs, old ceramic caps and military USSR and Beyschlag resistors. The only bad thing (if you can call bad such a deal) is the lack of the common values. I just took whatever had been left there, so 1K,10K,100K resistors were gone. Same for film caps, 22 and 100nF.









So it's the first time I'm using nos electros. Tested for capac. & leakage, used wisely and accordingly. It's for my own gear anyway, time will tell.

About hum, I'm talking about the hum i get as i turn up the Bass pot P1 in scheme.

but what it's not normal for sure is that it gets increased then as you turn up the Bass Pot P1, so this hum is related to P1 bass pot.

Any thoughts?

[anotherjim]

With the volume control right at the start of the amp board, you may never get rid of the hum. It would ideally have a stereo volume control at the inputs to the power amps then the preamp circuit hum & noise can be turned to zero at the same time as any signal. The only noise left then is from the power amps and their supply.

NOS electro caps might well be below their best now, but if you can leave it powered up for a good while they can reform to a degree and hum may, if you're lucky, reduce.

[sarakisof]

NOS electro caps might well be below their best now, but if you can leave it powered up for a good while they can reform to a degree and hum may, if you're lucky, reduce.

I know, thanks for the advice Jim.

With the volume control right at the start of the amp board, you may never get rid of the hum.

So as it is now there are no many things left to do i guess. Like playing with different opamps (ic5/6)/replacing 2458DC with 4558..  Or a culprit component in between and before the P1 pot ..

[anotherjim]

Considering that the bass control happens soon after the input gain stage, I think it's likely that hum is present on the opamp Vref supply that comes from the 10k resistor divider off the 15v supply. Try adding capacitance across the Vref cap to see if it improves things. Remember, opamps have good supply noise rejection so the most important place for ripple reduction is the opamp reference supply.

The Volume pot ideally goes to zero ohm at minimum. Try to measure this. Not all pots manage to zero and it's as a variable resistor that this is most troublesome. Then any hum from the pickups will still get some amplification.

I seem to remember reading that Pianets couldn't be used sat atop a Hammond without terrible interference being picked up while the Clavinets were fine in that placement, so I expect there is some hum coming in at the preamp and it might not be directly amplifier power supply related.

[anotherjim]

I found a clear schematic with no 4016 here...
https://www.clavinet.com/shema_PIANET_M.pdf
And the Vref divider values are 15k with a 47uF filter cap. That ought to be pretty good at removing power hum.

[sarakisof]

I found a clear schematic with no 4016 here...
https://www.clavinet.com/shema_PIANET_M.pdf
And the Vref divider values are 15k with a 47uF filter cap. That ought to be pretty good at removing power hum

You mean with 4011, right?
The 47uF vref filter cap you're talking about is the C13 right? If so, instrad of 47uF it's a 100uF on my board. So i guess that ought to be really good.
Yeap i was also reading about Pianet placing right now. No room left, it sits just above a Philicorda and a Farfisa Compact  maybe it could bd only from this. Without input signal (pickups disconnected from amp) there's no Bass pot hum.
So maybe it has also to do with the pickups too.

[anotherjim]

Without input signal (pickups disconnected from amp) there's no Bass pot hum

That pretty much proves the hum is from the pickups.
I've never seen the inside of a Pianet. Most keyboards with wooden bodies had a sheet of foil stuck to the inside bottom face under the keyboard frame to screen it. I don't even know how the pickups are arranged and if they are supposed to have any humbucking arrangements.

[sarakisof]

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

[anotherjim]

That "static" noise seems like guitar string noise, usually reduced (but not completely) by grounding the strings via the bridge to the jack screen. The cure shown for the Pianet is a similar thing. That does nothing for hum pickup though, it isn't really screening anything.
It's the actual disposition of the pickup system and wiring I'm interested in knowing about. I'm not finding much about it.