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Messages - aruption

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Amplifier Discussion / Another Hohner Electric Piano - Cembalet CF
« on: October 23, 2018, 02:01:16 PM »
Just picked up another Hohner electric piano, this time the Cembalet CF:

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.

There is no voltage at the pickup. The other Hohner EPs typically have 150-200vdc at the pickup. This one also 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. None of the other Hohners behave this way, if they pickup and reeds have continuity, they are shorted and there is no sound. 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.

What's the next thing I should look at here? The transistors seem to have appropriate voltage readings. Thanks!

Amplifier Discussion / SWR 550X Bass Amp Trouble
« on: September 20, 2018, 09:46:35 PM »
Not sure if this is the best forum for this, but some folks on here have helped me out with an electric piano amp, so a bass amp shouldn't be too far fetched, right?

Anyway, a friend of mine asked if I could take a look at his SWR 550X bass amp. It had developed a pretty loud hum, and louder notes from the bass would cause a horribly buzzy clipping effect. This is one of those amps with an 12AX7 tube preamp section. We thought that replacing the tube might solve this, it did not.

What should I be looking at to clear up all this noise? I also had to replace the master volume pot because the shaft had broken off the original pot. The level of the hum increases as the master knob is turned up, as was the case before it was replaced. Any help is appreciated!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: June 17, 2018, 01:54:16 PM »

Yes, the volume pot is activated by a leg bar on the underside.  I leave the bar unhooked because it just seems useless given the volume situation.  There's definitely more volume when I plug the unit into an external amp - but that just means there's more buzzing and noise too, which is acceptable when the rest of the band is playing, but just knowing that it's there...well, you know...

J M:

The original amp was capable of 1W (if that much)  and intended for quiet practice at home, at table radio levels, not more.

You are correct about that.  When I'm just playing it at home, it's perfect for quiet practicing.  I like your suggestion about how to replace the amp.  I'll keep a look out for those TDA2003s.  So far the ones I've seen probably wouldn't fit in the unit (it's incredibly tight in there). 

Now, would this mod increase the volume at the external output, or just the internal speakers?  I think I can live with the level of the internal speakers, it's just that (as I was alluding to earlier) I'd like to get more keyboard volume from the external output, without all the extra noise volume. 

So y'all can hear it for yourselves, here's a video of the Pianet in its current state, played with internal speakers only:

Thanks guys for your help!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 30, 2018, 11:04:18 AM »
I ended up taking it back over to the tech who originally worked on it.  He determined that the transformer was perfectly fine, and that the problem might have been in the transistors of the preamp section.  He eventually found that a wire near the voltage regulator for the was crimped and shorting out the circuit.  Uncrimping the wire brought the Pianet back to life!  Strange thing was that this wire was nowhere near the film cap that caused the spark that stopped it from working for begin with.  We did discuss the possibility of building a replacement preamp for everything past the power section of the amp, to maybe reduce the buzzing and humming.  One step at a time though, I'm just happy that she's running again!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 17, 2018, 01:40:15 PM »
That's what I'm afraid of too.  I contacted the tech who worked on it last year, and I think he's leaning towards that being the problem too.  So I'm going to try to bring it over to him soon.  I'll let you know what happens.  Thanks for your help so far!

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 16, 2018, 12:56:52 PM »
The diode is the white tube on the far left of the board.

The other picture shows the tranny with the lid off and the #6 AD130.  The B30C1000 rectifier is behind the two e-caps.

There's 340VAC from the tranny to the E300/C5.  However, there is no AC going to the B30C1000 - am I right in assuming that would mean the problem is in the transformer?

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 14, 2018, 11:20:43 PM »
So it's not the zener diode it was attached to - the film cap he added was between the E300/C5 diode and the ground.  And that diode is reading OL in both directions.  Still no voltage coming from the transformer to the E300/C5, but I think it's safe to assume that the diode is shot.  Any ideas on locating an equivalent? The zener and the AC107 are measuring as they should.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 12, 2018, 07:32:22 PM »
Of course disaster strikes again...

I opened it up this morning so I could draw a diagram of the preamp.  The amp tech who initially replaced all the e-caps when I bought the Pianet added a small film cap between the zener diode and ground (I think), since he thought it might help with the noise. It didn't really, so at one point I disconnected it by just disconnecting one of the leads from the board. 

Well that came back to bite me today when I was poking around trying to see something, the clipped lead of the film cap came into contact with where it had been clipped off, made a tiny spark, and no more sound.  Now, in the past when I've done work on the preamp, sometimes it doesn't work when I turn it back on.  The solution to this has been clipping a lead from the pickup itself to the tip of the output jack.  This was a suggestion from the amp tech who worked on it, and it worked every time.  But not this time.  Tried it several times, and no sound ever came back to either the speakers or the output.  The pickup is getting 140 volts as usual, but there doesn't seem to be any voltage going to the preamp section.  Hope I haven't screwed it up too much this time...

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 12, 2018, 08:13:10 AM »
What I'd actually like to do is attempt to build a replacement preamp.  Just as a project, and to have as a backup.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 05, 2018, 09:54:53 PM »
How would I find equivalents for the bridge rectifier and rectifier based on the schematic markings of B30 C1000 and E300/C5?  Should those numbers tell me what I need?  I'm looking to eventually rebuild the entire amp.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 27, 2018, 08:34:26 AM »
I would say it actually finally sounds pretty great.  Still noisier than my other Hohners, but since the L is AFAIK their first solid-state model, it wouldn't surprise me if the other models were designed as improvements on the L.  If I ever come across replacements for the other transistors, maybe I'll pick those up and see if they help improve the sound any.  Thank you all for the help!

My only concern left with this thing is that the transformer itself produces an audible buzz and hum.  It doesn't seem to cause any problems, I just worry that it could be a sign of eventual failure.  My other Hohners don't make that noise, and I've never played another L, so I have no way of knowing if it's an issue unique to mine or something shared by other Ls.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 24, 2018, 01:47:23 PM »
BTW I did adjust the bias pot so that the collector of #5 is putting out 7V.  I had written down all of those readings in my notebook, but when I typed them in here, I accidentally typed mV instead of V for the #5 collector.  So the initial reading was 5.8V - NOT 5.8mV.  Anyway, I was able to adjust up to 7V no problem, and did get a noticeable increase in volume without any additional distortion.  The internal speakers have a bit of a crunch to them, but I would put that down to them being so small.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 24, 2018, 11:10:28 AM »
I don't know what power the speakers are.  They are 3X5 ovals that I installed because the speakers that came with it (which I don't think were even the originals) were dead.  They aren't necessarily that loud, nor do I expect them to be for their size.  Anyway, I will check the bias pot and see if that changes anything.  If I'm understanding correctly, that won't affect the amp output, just the speakers?  Or should it affect both?
What I'm more concerned about is the sound when I plug the Pianet into an amplifier.  That said, I took it along to a rehearsal last night and the max Pianet volume didn't really seem so distorted anymore.  Sure, there was still some buzz and hum in the background, but it was hardly noticeable while we were playing.  Maybe I'm just expecting too much from this old thing? 

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 24, 2018, 07:08:19 AM »
Is the supply voltage what is coming from the emitter of #6?

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 23, 2018, 06:33:44 PM »
Well I got the new AD130s in the mail today and installed both of them in the Pianet.  It's hard to tell, but it seems that the output is not quite as distorted at max volume.  However, the same buzz/hum is still present across the board, and the volume pot still behaves the same way it always does (and affects the buzz/hum level as well).

Here are the voltage readings I am getting on all the transistors now (left to right E B C):
#2 AC107: 1mV, 95mV, 195mV
#3 AC125: 1mV, 98mV, 249mV
#4 OC308: 1mV, 157mV, 335mV
#5 AD130: 2.4mV, 456mV, 5.8mV
#6 AD130: 9.9V, 10.3V, 11.5V

Also I measured the voltage on (what I believe is) the Zener diode: anode 1mV, cathode 94mV.

How do those measurements look?

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