Author Topic: Cembalet CF  (Read 275 times)

Offline theseacowexists

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Cembalet CF
« on: October 23, 2018, 07:38:09 PM »
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!

Offline OZDOC

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Re: Cembalet CF
« Reply #1 on: October 23, 2018, 10:00:29 PM »
This sounds obvious - but are you sure that none of the reeds are shorted against the pick-up tabs?

Offline theseacowexists

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Re: Cembalet CF
« Reply #2 on: October 24, 2018, 06:12:15 AM »
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.

Offline OZDOC

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Re: Cembalet CF
« Reply #3 on: October 25, 2018, 05:44:55 PM »
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

Offline theseacowexists

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Re: Cembalet CF
« Reply #4 on: October 26, 2018, 09:54:26 PM »
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?

Offline OZDOC

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Re: Cembalet CF
« Reply #5 on: October 27, 2018, 07:47:43 PM »
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.

Offline theseacowexists

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Re: Cembalet CF
« Reply #6 on: November 28, 2018, 08:07:35 PM »
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.