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Hohner Pianet Amplifier Low Output

Started by aruption, April 15, 2018, 11:17:56 AM

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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: http://hohner-pianet.com/hohner_pianet/pianet_L_LB.html

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.


So check the volume control itself.  A broken wafer would leave the part below the break at low levels, and then once your wiper gets above the break, the signal becomes strong, plus if the circuit was relying on the pot as the path to ground it would be missing.


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.


Sounds to me like a bias problem or the transistors could have failed after all this time. - they are germanium after all.
First check that there is a voltage supply out of the bottom power transistor (No 6) - this is a hum reducing capacitance multiplier stage and should output about 15v. If this is present then the other transistors need taking out and checking for gain and leakage.

The symptoms you describe appear to be one of the transistors not conducting and the increasing signal being eventually rectified by the base/emitter junction and switching it on.  This would sound loud but distorted.
After all this time it may be necessary to replace all the transistors if you can find them or equivalents, or replace the complete amplifier for a modern one.


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.


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?


You need to adjust the 1M bias preset on the AD130 #5 which drives the speakers so that the collector is at half supply voltage. This is a class A stage driving the speakers directly.


Is the supply voltage what is coming from the emitter of #6?


Yes, although putting  7v  approx across the speakers doesn't seem right.  Try biasing at a lower voltage first say 3v and see what it sounds like. Do you know what power the speakers are?


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? 


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.


Quote from: tonyharker on April 24, 2018, 10:35:34 AM
Yes, although putting  7v  approx across the speakers doesn't seem right.  Try biasing at a lower voltage first say 3v and see what it sounds like. Do you know what power the speakers are?
I think biasing for 3V at the output tx collector will result in more DC across the speaker, not less.  The other end of the speakers is supply voltage, not ground.


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.


I know this old equipment is tidious and frustrating i cant fir my ss amp proplem out


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.