Weird Noise When Turning Off E-Piano's Amp

[Rob Strand]

Ok, to clear things up.

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?

If someone explained the behaviour of a working chorus you would pretty much say what you said.   There would be some very fine points that would separate a broken circuit and a good circuit if that's the only behaviour!

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..

That's fair enough.

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.

You might want to try this cap.   The old schematic, which seems to match you board, doesn't have it.  It's very bad practice to leave that cap off.  It should be mounted right on the regulator pins.

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?

Well if it didn't start oscillating it was probably working OK without it.   Always a good practice to have it.

Now let's go to the hum issue

You should do follow the steps I posted in reply #26.   The way hum gets into this circuit can depend if the RCA input socket has a path to ground.  If you just pull the plug and leave the input open you will get different behaviour regarding hum.    That even goes for the green wire passing the transformer.   You can get a scenario where an open RCA doesn't care about the wire being close to the transformer and the shorted input does care about the wire being close to the transformer - you can also get the reverse!   When the keyboard part is plugged in it will act more like the RCA input is shorted to ground - at least at low frequencies.

[anotherjim]

I think audio engineers were glad to see the back of BBD tech for delay jobs - the horror of bringing it all back in the current vintage fad! I had a Logan string synth back in the day, wonderful sound but the volume pedal wasn't a nice optional extra - it was essential to shut the noise off hands-free when you weren't playing it.

Is there a good photo of the heatsink arrangement? Many amplifier IC's used with a single supply will have a "live" heatsink tab that must not get grounded so there may be insulation to check.

[sarakisof]

You might want to try this cap. The old schematic, which seems to match you board, doesn't have it.  It's very bad practice to leave that cap off.  It should be mounted right on the regulator pins.

Already tried but didn't make any difference at all.
Shot a 20sec. video chopsticking so you can hear by yourselves.
https://we.tl/t-Rg2U7FNCwR

Is there a good photo of the heatsink arrangement?

I don't think is such a case, didn't see any insulation when i replaced the IC3.

[Rob Strand]

Already tried but didn't make any difference at all.
Shot a 20sec. video chopsticking so you can hear by yourselves.
https://we.tl/t-Rg2U7FNCwR

From the TDA1111data sheet the tab terminal in connected to the IC substrate/ground.
The ESM532C datasheet is less clear about where the tab connects.

The regulator tabs connect to ground/0V.

Since none of the devices are isolated then that means all the IC's metal tabs are connecting through the heatsink.  Since the aluminium has an oxide coating that connection can be crappy.    If banging the device upsets the ground connections is cause that noise.

There's other possibilities:
- bad solder joints on the power amplifier pins
- bad solder joints on the regulator.
- even a bad solder joint somewhere else on the PCB.
   Mechanical vibrations can propagate throughout the whole structure.

Another thing which isn't a great idea is connecting the tabs together creates multiple earth connections - that's can cause hum problems.  It's an inherently bad set-up.   Fixing stuff like this needs some experimentation. 

As far as the knocking goes I'd check for bad connections but after that you might try isolating the regulator from the heatsink.   While your at is see if the hum changes.

If the PCB grounds aren't done very well sometimes you have to cut tracks and re-route grounds with wires in order to remove hum.

You might want to check if the regulator and amplifiers are wired back to ground via PCB tracks.   You wouldn't want to isolate the regulator if they are picking up the regulator ground via the screw on the tab!!!

[anotherjim]

Was there ever any heatsink compound applied to the back of the ICs? I don't think it should have any in this case as it would make the heatsink ground paths uncertain.
I suppose the best thing might be to fit heatsink insulation then the grounds can only go via the PCB paths. However, I think it's pretty much impossible to insulate without losing some thermal conduction to the heatsink, but I'd expect the ICs to cope with getting a little hotter.

[sarakisof]

There's other possibilities:
- bad solder joints on the power amplifier pins
- bad solder joints on the regulator.
- even a bad solder joint somewhere else on the PCB.
   Mechanical vibrations can propagate throughout the whole structure.

It was bad cracked solder joint due to mechanical vibrations (importing - exporting the pcb) during the repair job.
Chopstick tap microphonic issue completely solved.
It's great that each all the issues being solved thanks to you guys. Really appreciate.

Now, only one left (if it can be done), is the hum issue.
Will try to record a small sample and upload here later.

[sarakisof]

Back again. After Rob's words at post #26 about my 50/60Hz hum.
(Just to refresh our minds, there's normal slight mains hum by powering the unit up, but gets worse by turning Bass pot (P1) up.
https://www.clavinet.com/shema_PIANET_M.pdf

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.

.
When i pull the keyboard RCA out (disconnect from amp) the hum disappears. This tells us it's not the amp but the pickups themselves. All those had been discussed before at post #16 and below.

Without input signal (pickups disconnected from amp) there's no Bass pot hum.
So maybe it has also to do with the pickups.

But i couldn't find anything culprit related to the pickups.
Pickups resistance (hot signal to gnd windings)/RCA signal to gnd measured at 49ohms. Continuity test beeps but seems right. Little winding turns here, we don't except smthng bigger like guitar/bass pups, more close to spring reverb transducer coils case. Pickups are ok.
Shorting RCA input gives the same hum (maybe tiiiny bit less than the hum we're trying to solve - by A/B test shorting/unshorting signal). This happens in both cases having the RCA connected to amp or disconnecting it.

I start to believe it's one of these cases or a combination of them. (Post #10 and below)

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.

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.
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. 

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.

I don't have any expensive test gear like oscillo here, but have a trusty Uni-T UT139C multimeter μΑ, Hz and many other features, a cheapo chinese "oscilloscope"(never used, mostly used DAW PC scopes when needed), diy signal traces and old common sense troubleshooting techniques 
Have solved more serious issues than this in the past so far but this thing drives me crazy. Maybe, in the end of the day they are all normal, as you guys told actually above, taking as fact how this circuit is designed. I'm more and more into realising it as every demo video i watch on YT they all have in common , pops, hum and hiss, by the time they start playing the instrument.

EDIT: **** Forgot to mention smthng about electros (there's a huge movement against NOS electros, i was always against that for some of them if used in certain places, anyway). So just fir the record, for the C27 and C37, power amp filters, i put four fresh Panasonic FR ones (replacing C27,C28 and C37,C38) and hum was the same (i think i already mentioned that earlier above). The only one i didn't do that (i mean to replace the nos with fresh one) is the C13. I will do that too, but i don't think it will make a difference at all.

[Rob Strand]

What you have described makes 100% sense to me.

Firstly the increase of hum when you advance the bass control is perfectly normal.  You are boosting bass, hum is in the bass frequencies so it gets boosted.  The only thing the test tells you is the hum is getting in before the bass control.   Jim mentioned that a while back.

To summarise:
- pickups alone:         no hum
- amp with open RCA: no hum
- amp + pickups:        hum
- amp + shorted RCA: hum

Because of the last point, the problem is hum on the ground (or Vref) on the amp board.

If you look at the circuit with the RCA open C9 and R6 are just floating.  The amplifier IC5 (pin 2,3,1) sort of has no gain regardless of the volume control setting.

When the RCA is shorted to ground C9 and R6 connect to ground.  Now any hum signal along the ground or Vref will get amplified.   The loop of interest is the (physical) loop from IC 5 pin 3 down through Vref, along the PCB ground tracks, to where the coax ground connects to the PCB, through the coax and back, then to the point where the coax core solders to the PCB then through C9 and R6 back to IC 5 pin 2.

If there are multiple ground points it makes a mess of trying to identify a loop.  And in this case power supply hum can be placed into the ground tracks themselves.

Let assume the noise is on the ground.   The cause will come down to some very fine details regarding where all the ground tracks go.  For example, signal grounds can be passing through tracks carrying power supply current.
Even if the tracks seem OK the multiple ground points of the power amplifier ICs and the LM7815 regulator can cause a hidden connection between the power supply and the audio grounds through the heatsink.

Some pics of the bottom top and bottom of the PCB would help spot any obvious problems with the tracks.

However a very easy test would be to isolate the LM7815 regulator from the heatsink.   Without looking at PCB tracks it stands out as a possible problem area.   

[sarakisof]

Some pics of the bottom top and bottom of the PCB would help spot any obvious problems with the tracks.

However a very easy test would be to isolate the LM7815 regulator from the heatsink.   Without looking at PCB tracks it stands out as a possible problem area.

LM7815 isolated from heatsink, no change at all.

I shot some pics of bottom and top of the board. Also made a Photoshop blend mix from both so with a little effort you could see top and bottom in one pic. It's not 100% accurate as the two pics combined need to be 1:1 and same dimensions but i did my best to align them properly.
Hope it can help.

PS. Ignore the flux mess, I'll clean the bottom when full job is done.

[anotherjim]

That is interesting because it looks like the power ground has to go to the regulator 0v before is daisy chains via the power amps toward the preamp side. If you are prepared to cut a few tracks + lift links, it should be possible to change it to a star ground scheme with some added wires to a star point at the power caps negative common.

It would also be worth investigating the AC input/transformer ground arrangement to get the whole picture. This was all often done in a manner to suit economy of production, especially in the past and you wouldn't do it (or be allowed) that way now.

[sarakisof]

Yes as long as I'm into it i realise that it's simply how it was designed. AC input/trafo gnd is indeed built in an economic manner. On the other hand I'm romantic enough with this kind of old stuff, specially this one, as i know I'm one of the few people owning this unit in my area at least.
As mentioned above, i would like to do my best but in terms of not altering the originality of this piece.

For now i think I'm in a good level with it so far, maybe I'll install dip sockets for all IC's just to make my life easier in a future repair and that's it.
Rob, Jim and other people contributed here, thank you very much, without you things would be more complicated for me. Always open on learning from you guys.

One last thing only (and the very first one too, as i never managed to solve it 100%) is those pops in powering on and off the machine, even with all pots turned down. There are not so loud now (specially this robotic faded noise which has almost completely gone after having replaced C1,C2 filters), but it isn't a noiseless/popless shut on/off either. It's a small "bang/puff" when you release on/off switch, each time the same. Maybe there's an easy fix for this, like an old trick putting a safety x/y cap across mains/to gnd or smthng similar?

[Rob Strand]

I'm still trying to follow where the grounds go.

When the ground enters the lower left side (as orientated in the pic) of the first power amplifier it seems to disappear.   I was thinking they passed all the ground current *through* that IC.  Haven't finished looking.

The other weird thing they have definitely done is created a very broken ground trace.   The grounds connect to the solder tabs of the pots then continues through the pot body and then comes out on another solder tab.  That continues for all the pots.

I was wondering if the pot are mounted to a metal panel or a wood panel?

If it is metal, does that metal connected through screws and metal chassis back to the aluminum heatsink?

Given what what they have done to the pots I'm suspicious they are running the ground through the amp heatsink tabs.

While I haven't identified any cause, such a connect is going to be a headache to mod.

As for the turn-on pop.  There's not much you can do directly as it normally occurs because the chips lose control when the power goes off.   Also, the amps have output caps so that type of amplifier tends to bang at turn-on.    Some of the modern amplifier IC's had extra circuitry inside to have "bangless" power on and off - the reason they put it in there is because of this same objectionable behaviour on the old ICs!     Having said that if you could disconnect the two preamps from the power amp and they band goes away then the bang could be from the preamp.   You can sometimes do simple mods to fix the bang from preamps.   It's going to take some form of investigation, then some of mods! You have to change something and that means cutting tracks etc.    A way some stereos side stepped fixing the electronics was to have relays on the speakers which delayed connecting the speakers at power on and quickly disconnected the speakers at power off!

[sarakisof]

I was wondering if the pot are mounted to a metal panel or a wood panel?

they're mounted to a wood panel.
Tried every possible way (except for modding anyway) and it's all about how it's built Rob. Did my best eliminating any noise, I'm done and happy of how it is now.

As for the turn-on pop.....

great infos here, read and kept everything in mind but as i said above I'm not going into cutting tracks and heavy mods. I see this instrument as a piece of history in a romantic way.

Now, a very last thing I'd like to do from the beginning with it (mentioned in second page too). It's the only mod I want to do with it, even in this case I wouldn't like to open a new hole, or alter the wood case / chassis at all.
I'll explain. I want to mod it to a Pianet T (completely bypass the amp) so i can use it like a passive instrument, but being able to return to Pianet M and make use of its amp/speakers when i want to.
To do so, i had to get the Pianet T's impedance matching transformer. I sourced the appropriate equivalent replacement one (Calrad 45-703 as suggested by Hohner). It's just a mini audio 15:1 trafo 2.26K : 43 ohms wired backwards. Pickups to 43ohms primary, 2.2K sec to jack out (across a 2nF bypass cap-most people remove that) ready for external amp.









How could i do this simple mod without opening hole for jack out? My first idea was to just solder an extra coax/shield cable from RCA input socket  to one of the two DIN outputs (canceling output for hifi speakers or headphones) which probably I'm never going to use anyway , but thought again why altering the function of the unit and who knows if someday in the future I'll need to use that headphones out?
In that case the idea was then to make a DIY cable from DIN to a box with the trafo inside and a jack out, so from that i would connect a guitar cable to external amp.

Then i realised that i could do the job from underneath (wood base of the instrument) and the trafo could be simply mounted inside there exactly like in the T (see the pic above) on the upper right corner.
Just soldering an extra coax (like and where the already green starts) wire from pickups end solder points. (see the pic)

going to the trafo.
Then there are many unused holes in the M's base (intended for T - actually the keyboard bases are the same, just in M they got veneered and T black tolexed), which i could even pass through a hardwired output guitar cable, just like they were doing in Farfisa Compact organs.
BUT, in that case (that goes for the first case too), are there any don'ts in soldering an extra cables/wires coexisting with the old green one? Any losses? I guess no, have seen many signal coax cables starting from same turret lug in every organ I've repaired.

The whole idea is to avoid opening the instrument (unscrewing) disconnecting that green M's RCA and connecting another T's RCA and vice versa every time, or using a toggle switch or whatever.
Simply, when i want T, i will just plug the hardwired cable to external amp WITHOUT powering up M. And when i want M, do what i did before turning on M, just "T's guitar cable" being unused floating around. Is this bad, to have an "extra" cable not affecting circuit, floating?



Any other recommendations and ideas welcome.

[Rob Strand]

they're mounted to a wood panel.
Tried every possible way (except for modding anyway) and it's all about how it's built Rob. Did my best eliminating any noise, I'm done and happy of how it is now.

Well if it's wood then at least you don't have a whole lot of hum loops connecting back to the chassis.   It really narrows things down to perhaps the fact the amp IC's are connecting to the heatsink.  (Of course no guarantees that's the cause, but it's something to pursue.)

i said above I'm not going into cutting tracks and heavy mods. I see this instrument as a piece of history in a romantic way.

I understand 100%.   I can tell you now I've fixed all sorts of hum problems in equipment and sometimes you end-up with a lot of cut tracks, a big mess really.   Neater efforts a result of using isolated connections so there less audio grounds through the chassis.

How could i do this simple mod without opening hole for jack out?

Is the RCA connect accessible from the outside?   If so just build an external box with the transformer in it then plug the RCA into that.

[sarakisof]

Is the RCA connect accessible from the outside?   If so just build an external box with the transformer in it then plug the RCA into that

Nope, that's why i want to find a solution so to avoid unscrewing opening the instrument each time.
The RCA is located underneath amp cabinet. You have to unscrew about 8 screws from the narrow sides to lift up and unplug the RCA cable.





Is soldering (in the same place with the old green) like that an extra coax cable (which will go to trafo then to an output) wrong?

[amptramp]

Have you considered a bluetooth transmitter inside the box?  No need for holes and you can probably power it from the rest of the circuitry.  Failing that, just an RF link.  Even a baby monitor could get your signal to the outside world with no holes and they usually run from a 9 VDC 200 mA adapter and they have a selection of two frequencies to avoid jamming.  And they pass all the necessary FCC certifications.

[sarakisof]

Have you considered a bluetooth transmitter inside the box?  No need for holes and you can probably power it from the rest of the circuitry.  Failing that, just an RF link.  Even a baby monitor could get your signal to the outside world with no holes and they usually run from a 9 VDC 200 mA adapter and they have a selection of two frequencies to avoid jamming.  And they pass all the necessary FCC certifications.

Thought of that for a while but soon decided it's too modernized for my taste. 

About the query i was wondering above, could soldering another extra coax cable like the one which already exists (in the same place) be wrong ? See the pic below, i have drawn the extra one coax green cable.

[Rob Strand]

About the query i was wondering above, could soldering another extra coax cable like the one which already exists (in the same place) be wrong ? See the pic below, i have drawn the extra one coax green cable.

Soldering on a second wire is OK provided you don't start getting noise problems.  A good coax can help but also keeping the center leads as short as possible at the solder points.

The problem with the transformer is it is permanently connected across the pickup so it will add a permanent load.     The loading works the other way too.   When the pickup is in use the existing electronics is loading the pickup (R6 1.5k).  If you wanted to run the keyboard unpowered via the transformer output the 1.5k is feeding into unpowered electronics.  You will get sound out for sure but loading something fundamental like the pickups could change the sound. 
You could do a test but sometimes the changes in sound are subtle.

One way around these issues is to use a jack with switch contacts.  When you insert the jack it connects the new circuit and disconnects the existing circuit.  Instead of using jack switches you can use a small signal relay so when the power is off the signal is disconnects from the existing ckt and sends the signal to the transformer and added jack.

[Rob Strand]

To do so, i had to get the Pianet T's impedance matching transformer. I sourced the appropriate equivalent replacement one (Calrad 45-703 as suggested by Hohner). It's just a mini audio 15:1 trafo 2.26K : 43 ohms wired backwards. Pickups to 43ohms primary, 2.2K sec to jack out (across a 2nF bypass cap-most people remove that) ready for external amp.

The impedance values you quoted are the DC resistances of the coil.

The true impedance levels for the transformer are much higher than the DC resistance.
I'd guess somewhere around primary 43ohm * 20 = 1k ohm and secondary 2.26k*20 = 45k ohm

Here's the datasheet,
https://datasheet.octopart.com/45-775-Calrad-datasheet-37165076.pdf

45-703 Mic Input 200k ohm  (DCR 2.26k),  Output 1k ohm (DCR 43 ohm).

(Someone posted the real Pianet T transformer DC resistances at 38 ohm and 2.57k ohm.)

So the transformer will end-up putting 1k to 2k of loading on the pickup (very rough).   Which just happens
to line-up with the R6 value.

The pickup *DC* resistance is 43 ohm, so you expect to load it with 1k to 2k.  So that adds up as well.
(I'm not sure if that's the entire pickup or just one coil for a single key.)

FWIW, the purpose of the 2nF loading cap is get the maximum bandwidth out of the transformer.   You often have to tune it for each specific transformer and it's not a clear-cut process.

All I'm saying here is the transformer looks like it is correct for the job but the problem is the affect on the tone of two lots of loading can only be done by experiment.

[sarakisof]

The impedance values you quoted are the DC resistances of the coil

sure indeed.

I'm not sure if that's the entire pickup or just one coil for a single key.

it's the entire pickups.
In mine entire pickups DC resistance is 48ohms.
All you mentioned are correct, amazed at how much homework you did on this thread Rob, you've discovered almost every info i have also found searching the net, like it was your case. Really honoured.

One way around these issues is to use a jack with switch contacts.  When you insert the jack it connects the new circuit and disconnects the existing circuit.

I will do that. I'd really appreciate if you could provide a simple diagram and give ideas on how i could make the wiring inside the instrument so i can finally close the case and start playing it. Even via a simple hand sketch diagram.