Cembalet CF

[theseacowexists]

Hey gang, just got a Hohner Cembalet CF! Not a C, but a CF. Of course, I'm having some issues with the electronics right out of the box (as expected).

http://hohner-cembalet.com/hohner_cembalet/Cembalet_CF.html

It powers up and does nothing but hum - no keyboard signal. Replaced the e-caps and resistors, same deal. The original volume pot had been removed and jumped together. Installing a replacement 250K pot did not make a difference, and does not affect the hum level when turned.

The vibrato bulb is burned out. I have a Cembalet N which does not have a working vibrato bulb, this does not stop the keyboard from working, it just means that I don't have a vibrato option, so I am assuming this one must work the same, and that the dead bulb is not my problem.

There is no voltage at the pickup. My DMM shows continuity between the pickup and the reeds. Even with the preamp unhooked from the pickup and reeds, their hookups on the board still show continuity, so it's not anything with the harp itself. I've never seen any of the other non-Pianet T Hohners behave this way. But this model has a strange coil thing in parallel with the reeds and pickup, which if I'm understanding the schematic correctly, would explain the continuity.

Anyone have experience with these? The Pianet CH is apparently the same design. What's the next thing I should look at here? I'll look closer at them, but the transistors seem to have appropriate voltage readings.

Thanks!

[OZDOC]

This sounds obvious - but are you sure that none of the reeds are shorted against the pick-up tabs?

[theseacowexists]

That's what I originally thought, and made sure that no reeds were touching the tabs, but the same thing happened even when the preamp was removed from the keyboard. There's no visually obvious shorts on the preamp board.

[OZDOC]

The CF version of the Cembalet is one of the versions where the reed/pickup voltage differential is derived through a transformer from the 12V DC supply (rather than being derived directly from the mains transformer). I think the first transistor in the circuit (the AF101) is forming an oscillator in conjunction with the primary winding of that "unexpected" coil you are seeing.
The secondary winding provides the stepped up voltage to the reed bar.
With the reed bar disconnected from the circuit you should be able to read an AC potential of something around 100V (I think).
If that oscillator stage is not functioning then the reed bar does not have the FM carrier wave that it requires to function.
Unless someone has badly treated the circuit board you'd first suspect the AF101 transistor has failed.
But make sure all four connections from the transformer coil are in place.
David

[theseacowexists]

Ah, that makes sense. I actually brought it over to my trusty amp tech who knows more about this stuff than I do. I passed along your tip to him in case it helps (he, like me, is more familiar with the other Pianet models). I will let you know how he fares with it. Is there a possibility that the transformer coil could have gone bad?

[OZDOC]

I think that "gone bad" is improbable. For this transformer to burn out the mains voltage would need to get to it - in which case lots of other things would have fried.
The most likely fail for this transformer would be a broken wire near the connection point to the PCB.

[theseacowexists]

Once he finally got around to it looking at it, he found that the bridge rectifier was shot. Replacing this brought the rest of the board back to what he thought was appropriate power, including at the AF101 transistor. But at the connections to the pickup and reed bar - nothing. He doesn't think there is anything wrong with the transformer either, so he is stumped at this point, as am I.

[theseacowexists]

I had to make a trip to NYC last month, so I decided I would bring it to a place that serviced my Hohner organ when I lived down there many years ago. Turns out there were three things wrong: first, I had the capacitors reversed   Fortunately I had taken a picture of the board in it's original state to confirm this. My bad! Second, the bridge rectifier symbol on the schematic was reversed, so the last tech installed the new one backwards - not like he would have known better. Third, it needs a working vibrato lightbulb to get any sound at all, even with the vibrato off.

So that's left me to put the mechanical part of the instrument back together. Nearly half of the reeds needed to be tuned to some degree. The lowest G G# and A seem to have corroded beyond repair. So I've set those aside for now and focused on the other 58 notes that are functioning.

The five highest keys are what is giving me the most trouble now. They have an excessive amount of static and popping when activated and released. Before, it was some keys in the middle of the keyboard that when pressed would sound like a loose connection would get looser and hum and crackle - with and without the harp cover on. Same thing happened by tapping on the treble end of the chassis, near the mains switch. This seemed to be solved by (of all things) tightening the screwed-on wire connections to the mains indicator lamp. Where the mains wires came into the chassis was also a screwed connection, I tightened these as well. So now it's just the crazy static on those five notes, which still kinda sounds like something coming loose. I wonder if I should ground the copper damper holders like I did on the N, since this has a different pickup system. Stefan (plucker-maker) says he's never had to ground those on his instruments. My only other thought at this point is to replace the original mains cord with a three-pronger and eliminate that screwed connection for a more stable solder joint.

You can hear some of the noise in this video. It's usually worse though:

https://www.youtube.com/watch?v=o66NOglsirY

It already has a nice sound to it - brighter than the N. I just gotta get rid of those bad sounds and I think I'll have a real neat sounding instrument!

[OZDOC]

Crackling can obviously be tiny particles caught in the gap between a reed and its pickup.
I'm assuming you're confident that the gaps are all dust free?
I sometimes use a piece of clay coated 80gsm paper slid through these gaps as a way of making sure they are clear of invisible particles.

[theseacowexists]

Yeh, I even went in and cleaned them again just to be sure.

This is a vid of it at it's worst:

https://youtu.be/whTme0fxNRA

A friend that was over last night, and he wonders if maybe a wire inside the right cheek block is pinched or crossing or something, since it sounds like a loose connection. I wonder if the mains switch itself is bad. When I got the instrument, the switch wouldn't always power the instrument up, like you'd have to switch it a few times to get it to "catch" - although there doesn't feel like there's anything physically wrong with it. After cleaning it with DeOxit, it now works properly 100% of the time. I should have a 3-prong cord coming in the mail today, so if I have time tonight I'll pop that on there.

[theseacowexists]

I think I got it the noise nipped. I ended up taking apart the actual switch and loaded it up with some DeOxit. This tamed most of the noise from tapping around that area of the instrument. Since it seemed less that something was loose electronically and more that something was loose physically, the only other thing I could think of were the couple dozen screws on the underside of the case that hold the harp and key rail. Sure enough, every single one of them was super loose, and tightening after tightening them, I could tap anywhere on the instrument without any awful sounds!

So we're 90% there. That last 10% are several reeds in the first octave that are rusted and cracked beyond repair. I've gotten in touch with a pump organ repair guy locally who will supply me with old pump organ reeds to try out, it we can get the bad ones out. There's so much corrosion in that area though, it is really gonna be a challenge. It looks like someone stored the instrument on it's backside for a long time, while water dripped right down into the middle of the lowest octave, since there's water damage to the wood in that spot too.  Otherwise, this Cembalet has cleaned up brilliantly!

[OZDOC]

While I have no proof of this, the reeds in a Cembalet may well be from one of the larger piano accordions that Hohner was making at the time.
In the bass they are lighter than the reed set that was used in the Pianet. Can you locate a piano accordion repairer who might be able to comment on this?