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Solid State Amplifiers => Amplifier Discussion => Topic started by: aruption on April 15, 2018, 11:17:56 AM

Title: Hohner Pianet Amplifier Low Output
Post by: aruption 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: http://hohner-pianet.com/hohner_pianet/pianet_L_LB.html (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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: Enzo on April 15, 2018, 12:28:06 PM
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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: tonyharker on April 15, 2018, 02:40:48 PM
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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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?
Title: Re: Hohner Pianet Amplifier Low Output
Post by: tonyharker on April 24, 2018, 05:37:14 AM
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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption on April 24, 2018, 07:08:19 AM
Is the supply voltage what is coming from the emitter of #6?
Title: Re: Hohner Pianet Amplifier Low Output
Post by: 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?
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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? 

Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: g1 on April 24, 2018, 01:50:33 PM
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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: Billythekid on April 27, 2018, 10:12:22 AM
I know this old equipment is tidious and frustrating i cant fir my ss amp proplem out
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: Jazz P Bass 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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...
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: phatt 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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?
Title: Re: Hohner Pianet Amplifier Low Output
Post by: phatt 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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!
Title: Re: Hohner Pianet Amplifier Low Output
Post by: phatt 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.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption 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!
Title: Re: Hohner Pianet Amplifier Low Output
Post by: tonyharker on May 30, 2018, 01:35:32 PM
Glad to hear it. Enjoy playing.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: phatt on May 31, 2018, 08:14:31 AM
  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!

My observation regards to circuit mods;
You mentioned in the first post that internal (speaker) or external (line out) gives the same volume jump.

Having never seen these units but messed around with similar circuits years back, I'd guess the issue is caused by the way the volume pot is used.
Looks like the volume is the 1 Meg pot after the first transistor. I assume it's activated via a leg bar underside?
It's wired as a Variable resistor and as you get close to max volume it likely upsets the bias of TR2 which = distortion

My guess here;
The point at which it jumps in volume is Max volume for that circuit design. It is a very basic circuit and you won't get more volume just distortion.

Early Valve Amp designs had the same problems. 8|

One possible thing;
Maybe ask your teck guy if that Volume pot can be rewired as a Voltage divider and add another resistor after the pot so that TR2 always has some series resistance to stop it going berserk.
You will have less max volume but more control over the leg bar.
If you then want more volume,, try other speakers or take the line out to an external Amp.
Phil.
Title: Re: Hohner Pianet Amplifier Low Output
Post by: J M Fahey on June 03, 2018, 02:59:06 PM
Phatt hit the nail in the head  :tu: and I´ll continue:
1) that is a ¨**terrible** circuit, VERY early Germanium Technology.
Very advanced ... in 1960 or so, obsolete by the next year  :o

2) that amplifier design is **terrible** : worse than Class A , besides having 25% efficiency (Class A is 50%) it passes full idle current through the speakers, a HIGH idle current I might add, it shift voice coils out of the gap, go figure.
Terrible design.
You will solve nothing by "repairing" it, just *replace*  it.

What is supply voltage?
I guess some 14 to 16V raw, before the capacitance multiplier.
Let it feed the preamp only,  which is a light load, and:

a) remove the cheesy original amplifier.
Simplest is to plain remove from the board (use a good solder sucker to clean pads first) the parts enclosed in red.
b) beef up the 1000uF main filter cap to a 4700uFx25V one.
You will have some 14/16V there which you will use to feed a:

c) TDA2003 amplifier.
Buy a ready built one from EBay for less than $5, feed it available 14/16V, connect it to speakers but first wire them in parallel, for a 2.5 ohm load.
TDA2003 is designed to drive down to 1.6 ohm, so it will be happy with that.

Now you will have som 8 to 10*clean" RMS watts, enough to play at home in a living room or similar with very good sound.
To play in a Church/Club, etc. you connect line out to an external amplifier, like you already did.

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

d) you can replace the cheesy series volume pot with a 10k to 25k one, Audio/Log preferred, but even a Linear one will perform better than the current series one.
You just connect it from coupling capacitor to ground, and wiper to TDA2003 amp board, which I guess already has an input capacitor.
If in doubt, upload board schematic an pictures when you buy it.

You will be (pleasantly) surprised :) 
Title: Re: Hohner Pianet Amplifier Low Output
Post by: aruption on June 17, 2018, 01:54:16 PM
Phil:

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:

[/quote]
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:

https://www.youtube.com/watch?v=v7gUYOnMlWU (https://www.youtube.com/watch?v=v7gUYOnMlWU)

Thanks guys for your help!
Title: Re: Hohner Pianet Amplifier Low Output
Post by: J M Fahey on June 17, 2018, 05:15:51 PM
Now that I think about it, you will probably have more Preamp signal out as well.
That cheesy power amp has very low input impedance (think about 1 k or so), and loads the volume control down a lot; I think when you remove the old one and add the TDA2003 instead, which has higher input impedance ( 20k to 40 k or so) you will now have more signal available at the signal out jack.

In fact I guess you´ll need to add a second volume control, so one for the internal amp, one for line out, OR regulate just the internal amp, and send line signal "straight" .... which in any case will be regulated at the external amp.