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Messages - aruption

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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?

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 15, 2018, 04:01:13 PM »
I was starting to figure that the problem might be in the transistors. I have two AD130s on order right now - those are the two labeled 5 and 6 in the diagram. 

By "bottom power transistor" you mean the one at the bottom of the schematic that is labeled 6 in the diagram, correct?  And that should read 15V on the emitter?  I got 8.5V on both the emitter and collector of that one with my DMM.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 15, 2018, 02:11:48 PM »
All three volume controls I've tried do the exact same thing.  Seems unlikely to me that they would all have a break in the wafer at the same spot.  That's why I'm wondering if there's something else in the circuit that would cause this.

Amplifier Discussion / Hohner Pianet Amplifier Low Output
« on: April 15, 2018, 11:17:56 AM »
Not a guitar amp, but I'm hoping someone here might be able to help me out!

I've had a Hohner Pianet L for about a year now.  For those of you who aren't familiar with it, it's the first solid state electric piano that Hohner produced.  This particular model has built in speakers.  More info and schematics are here:

It's come a long way from when I bought it, when it made no sound at all.  All of the caps have been replaced as well as most of the resistors and the volume pot.  Mechanically, everything is in perfect shape.  The main problem I'm still fighting is a rather low volume output.  Compared to the other Hohner electric pianos I own (Pianet N and Cembalet N), this one just does not have the same volume.  It is very quiet until the volume pot reaches about 80%, then there is a sudden increase in volume AND distortion.  This volume level is closer to where I believe it should be, but the distortion doesn't seem right.  Overall, there is a buzzing in the background which isn't much worse than say the buzz from a Strat pickup.  The level of this buzz (and any other background noise) is affected by the volume pot in the same manner as the actual piano sound.  The output sounds the same through both the internal speakers and the output jack.  Also, I've tried three different volume pots now, all with the same results.

I've come to this forum because I'm hoping that someone with experience in SS amps in general might be able to help me out.  I've brought this up on other vintage keyboard forums, but haven't really gotten anywhere.

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