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Author Topic: Hohner Pianet Amplifier Low Output  (Read 794 times)

Jazz P Bass

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Re: Hohner Pianet Amplifier Low Output
« Reply #15 on: May 05, 2018, 10:28:12 PM »

If you have no idea what the replacement may be and the parts work, I would suggest that you leave them be.

If there is nothing wrong with the diodes, then you will gain nothing by replacing them.
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aruption

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Re: Hohner Pianet Amplifier Low Output
« Reply #16 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.
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aruption

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Re: Hohner Pianet Amplifier Low Output
« Reply #17 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...
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aruption

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Re: Hohner Pianet Amplifier Low Output
« Reply #18 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.
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phatt

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Re: Hohner Pianet Amplifier Low Output
« Reply #19 on: May 16, 2018, 10:36:20 AM »

I'd check that you have *AC voltage* at the output of the transformer,,
If no AC on the secondary the transformer might be burnt out,, or You might have blown a mains fuse on the primary side,, though the schematic does not show that.
Having never seen one of these I'm not sure about the diode but maybe a picture of the circuit showing that diode might give us a clue.
Probably a 400 Volt 1N4004  or similar would be fine but that's just a guess.
Phil.
« Last Edit: May 16, 2018, 10:39:19 AM by phatt »
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aruption

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Re: Hohner Pianet Amplifier Low Output
« Reply #20 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?
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phatt

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Re: Hohner Pianet Amplifier Low Output
« Reply #21 on: May 16, 2018, 09:43:50 PM »

Yes it's possible that winding is burnt out.
Just make sure your meter is set to AC Volts and you are measuring the right wires.

You can always power down and un-solder one end of that 12 Volt winding then measure the DC resistance on that winding, If no (or very high) reading it's likely burnt out. :(

Keep in mind that after the bridge the DCV is positive ground.
Phil.
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aruption

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Re: Hohner Pianet Amplifier Low Output
« Reply #22 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!
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phatt

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Re: Hohner Pianet Amplifier Low Output
« Reply #23 on: May 18, 2018, 01:17:36 AM »

If enough room,,you can always install another 12 volt transformer to run that part of the circuit.
Phil.
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