Misterious Cap, noise issue on NOVANEX AUTOMATIC 6 amp

[razabri]

Thanks all, great feedback and it's good to know the exact value and how these read if I ever stumble on a similar cap. Meanwhile, I did try and placed few different ones in there (3n9 and other too), but with no luck, so it's not the cap that's an issue and I placced it back in. Most probably Rob is right - it is what it is and it could be the design fault. I just thought it could sound cleaner if I tweak it up a bit, replace some old parts as it is from the 70s.

[Rob Strand]

I had a quick look at the circuit and it doesn't make a lot of sense.
- The input signal splits off at the 1n5 cap to the 47k resistor next it.
- The 1n5 goes off far away from where I expect.
   Then it comes back (from somewhere) to the volume control.
- The 47k goes to a larger cap to ground.    The larger cap is unlikely to let signal past that point.

So is there DC at the input of the amp?  And is playing with the guitar's volume messing
with the DC there.

Maybe the input going off a long way isn't helping the noise.

I can't see any bright cap on the volume - so that theory is out.

All really odd.   Not that I trust my tracing.

[razabri]

I had a quick look at the circuit and it doesn't make a lot of sense.
- The input signal splits off at the 1n5 cap to the 47k resistor next it.
- The 1n5 goes off far away from where I expect.
   Then it comes back (from somewhere) to the volume control.
- The 47k goes to a larger cap to ground.    The larger cap is unlikely to let signal past that point.

So is there DC at the input of the amp?  And is playing with the guitar's volume messing
with the DC there.

Maybe the input going off a long way isn't helping the noise.

I can't see any bright cap on the volume - so that theory is out.

All really odd.   Not that I trust my tracing.

Hey Rob, thanks for looking into it - I just remembered, we had a discussion about a reverb circuit and you did wonders for me back then - now I'm even thinking of maybe putting that very same reverb in this amp. I seem to come around weird bits and pieces of audio devices. The Novanex is Dutch and it seems that they still make amps and take a pride in doing stuff "their own way". Maybe that's why it doesn't make a lot of sense to you and maybe that's why they split that heat sink... Anyway, I haven't advanced a lot from that reverb discussion and my tracing would have to be actualy drawn as is. I do have a multimeter now. I have 18v AC from the transformer and that's about as far as I got in there.

[Rob Strand]

Hey Rob, thanks for looking into it - I just remembered, we had a discussion about a reverb circuit and you did wonders for me back then - now I'm even thinking of maybe putting that very same reverb in this amp. I seem to come around weird bits and pieces of audio devices. The Novanex is Dutch and it seems that they still make amps and take a pride in doing stuff "their own way". Maybe that's why it doesn't make a lot of sense to you and maybe that's why they split that heat sink... Anyway, I haven't advanced a lot from that reverb discussion and my tracing would have to be actualy drawn as is. I do have a multimeter now. I have 18v AC from the transformer and that's about as far as I got in there.

Oh yes I remember, that thread was cool.

I eventually worked out Novanex was Dutch  .    The company does a whole heap of stuff.   There is quite a large catalog for music equipment.   There were a lot of hot-shot engineers from the Netherlands in the 70s.   I guess that had to do with Philips.  The guys that stated Novanex probably came out of that culture.  They even have a number of patents, so they probably have a lot of their own ideas.  I noticed one of the Novanex amps used a Philips speaker.

[razabri]

Oh yes I remember, that thread was cool.

I eventually worked out Novanex was Dutch  .    The company does a whole heap of stuff.   There is quite a large catalog for music equipment.   There were a lot of hot-shot engineers from the Netherlands in the 70s.   I guess that had to do with Philips.  The guys that stated Novanex probably came out of that culture.  They even have a number of patents, so they probably have a lot of their own ideas.  I noticed one of the Novanex amps used a Philips speaker.

Yes, it's kind of a cool company and I think I found a shot from a fair somewhere, as they are making tube and hybrid amps now and still got that quirky design that I like. The guy said they don't do copies or base the circuits on popular ones, like Fender or Marshall, but do their own circuit design.

I think this one has a Philips speaker too, wide range, so it works well with bass too and for a small amp it has real oomph.

Still I feel that it was probably clean at low volume when it was new and there is an issue of sorts, but I can't figure it out...

I also had an idea with that reverb, to try and install it in there and "steal" the power from somewhere off the board, but not sure if that'll work. Then I thought to use one jack as clean and the other as reverb somehow... It seems that the transformer is giving AC voltage and then it is rectified with diodes, if I understand it well. I can read various voltages off the board, AC and DC.

[Rob Strand]

I also had an idea with that reverb, to try and install it in there and "steal" the power from somewhere off the board, but not sure if that'll work. Then I thought to use one jack as clean and the other as reverb somehow... It seems that the transformer is giving AC voltage and then it is rectified with diodes, if I understand it well. I can read various voltages off the board, AC and DC.

It is certainly possible.  However, you can get problems with ground loops and hum depending on how and where you pickup the power.  If you were designing a reverb into an amp you would do that and the layout of the ground traces would then become important.   You might find the "best" solution is to separate power for the drive part and recovery part of the reverb circuit you then might need to use different ground points for each part.

I don't think it's possible to do anything to that amp without a schematic.  No easy way out.

Here's the Novanex catalog.  Heaps of products.

https://www.novanex.nl/image/data/Manuals/NovanexBrochure1979.pdf

This might give some hints towards a schematic,
https://www.freestompboxe%73.org/viewtopic.php?t=25188

One of the Novanex patents showed a similar circuit to the first two transistors (the ones with bass and treble).
https://patentimages.storage.googleapis.com/pdfs/US3961278.pdf

[razabri]

Great find, Rob! Wow, the catalogue looks nice and that patent doc is very nice, with all the explanations and all. Cheers for that. You made me look deeper myself and I found this article from '75 issue of International Musician, nice stuff!

http://www.muzines.co.uk/articles/novanex-auto-6/6646

Well, I'm not so sure about poking around it anymore to install that reverb, I'll probably just pack it as a separate effect, but I would like to mend it if there's anything wrong with it, considering that slight noise I'm getting with low volume and high input.

Forgot to mention - volume pot is a bit scratchy in certain positions and I tried to fresh it up with few shots of pot spray, but with no luck (this might be so because I used cheap stuff, oil based one, not the dry one), so this might have something to do with it, but I don't really see how.

I can try and draw up the circuit the way that I'm able to, perhaps you can notice something like so.

Thanks a lot, once again, great finds!

[PRR]

> volume pot is a bit scratchy ... pot spray .... oil based ..., so this might have something to do with it, but I don't really see how.

Oil is an insulator. Is this spray actually made for pots? WD-40 (descented kerosene) does terrible things to pots.

[razabri]

Oil is an insulator. Is this spray actually made for pots? WD-40 (descented kerosene) does terrible things to pots.

Actually yes, the small can of spray is made for pots (it says for pots recovery on it), and it has isopropyl and other stuff + oil, apparently "for proper turning". 

[razabri]

I don't think it's possible to do anything to that amp without a schematic.  No easy way out.

Alright, not exactly a schematic, but it's all I'm able to do - to draw a diagram as is. Note that some of the values may not be correct, as those resistors are old school and I can't quite make out a difference between colors. Also, tropical caps are left with colors... Hope this gives some insight into circuit design and what's going on in there. That small noise issue is something that happens only when the note is played, tiny bit of dirt follows it at low volume of the amp and guitar on high.

[Rob Strand]

Alright, not exactly a schematic, but it's all I'm able to do - to draw a diagram as is. Note that some of the values may not be correct, as those resistors are old school and I can't quite make out a difference between colors. Also, tropical caps are left with colors... Hope this gives some insight into circuit design and what's going on in there. That small noise issue is something that happens only when the note is played, tiny bit of dirt follows it at low volume of the amp and guitar on high.

Excellent!  I'll have a look at  it tomorrow.

It looks very straight forward.  It's not like the RG50.    When I traced it before from the PCB pics some of the traces were unclear and now I see a slightly different circuit.

[razabri]

Great! Didn't realize you already had a Novanex one to trace, that one's I guess some serious stuff, with reverb and tremolo... I have a small update, as I checked few resistors I wasn't sure of and I have new values in blue color. Mind that I don't know how to translate reading of say 140 at 2000k setting of multimeter, sorry about that, I'm still an amateur in all this.

[Rob Strand]

Great! Didn't realize you already had a Novanex one to trace, that one's I guess some serious stuff, with reverb and tremolo... I have a small update, as I checked few resistors I wasn't sure of and I have new values in blue color. Mind that I don't know how to translate reading of say 140 at 2000k setting of multimeter, sorry about that, I'm still an amateur in all this.

OK, thanks.  Some values won't measure correctly in-circuit so it's best to work off the resistor colors.   If the measurement and the colors match up then that's always a good cross-check as some colors aren't so clear on JPEG images.

I didn't trace the RG50, someone else traced that.   When said "I traced it before from the PCB pics"  I meant when I traced the Automatic 6 from your pics.   Your graphic is much easier to follow.

I also found a B100 bass amp schematic.   Both the RG50 and B100 use a circuit like the Novanex patent.  I'll see how the Automatic 6 circuit turns out.

[razabri]

That's wonderful, Rob! Thanks!

[Rob Strand]

I managed to get the preamp part done.   There could be a few parts with the wrong values.

[schematic obsolete]


I also did quick check for clipping.  The amp does look like it is going to clip.

This schematic shows two minimal mods to help reduce the distortion.
(Don't worry about the 22k it a very crude model for the guitar.)



The added resistor tweaks the gain.

It might be worth checking the voltages before the mods,

I got 19.3V on the preamp supply, 11.0V on the collector, 7.53V on the base, 6.92V on the emitter.
If these don't match-up with your measurements then maybe some of the resistors are incorrect.
In that case don't mod the amp (although you could add the 100R or 150R as that will help anyway).



Be extremely careful working on these amps there lots of exposed main wiring.  (in fact most amps from that era)

When/if you mod it pull the plug from the wall.

If you are confident about taking measurements take note of the mains wiring.

[Rob Strand]

Something which seemed a bit odd is the amplifier is 6W but the power supply is perhaps +/-20V full power.
The filter caps are small at 470uF so the voltage could be lower than that.

Are you sure the transformer is 18V AC or did you measure 18V AC with no load (which could mean it's a 15V AC rating transformer)?

Also from the power rating I was wondering if the speaker is 15 ohm or 8 ohm, I'm thinking 15 ohm.    If it's a Philips speaker the label will be AD7xxx/M8 for 8 ohm  AD7xxx/M15 for 15 ohm.   If you can't see the speaker you could measure it with the multimeter.   Turn the power off, wait a few mins and measure the ohms across the speaker output at the amplfier.

It would be good to measure the DC voltages on the large capacitors since it does impact on the best resistors to use in the preamp.  I'm assuming auite a high voltage but if the voltage drops under load then perhaps we need to re-tune the biasing resistor (the 470k/330k).

[Rob Strand]

OK, so with the hanging issue of the speaker transformer and DC rails, this is most of the schematic done.

[schematic obsolete]


Probably won't get anything more done tonight.  Although I'm not sure I can add anything anyway.

There should be a 10k resistor between the volume pot wiper and the power amp.
Technically it belongs to the power amp.

One of the parts bugging me is R10.  Not sure if it's 470k, 150k, other.

[razabri]

What a wonderful job you did, Rob! Even the simulation for the clipping, wow. I haven't been able to add anything today, but I hope that by tomorrow I'll have the correct measurements.

For now I can add that the speaker is indeed Philips one, and it has a writing that resembles what you already suspected - AD7080/X8, so I guess it's actually 8 Ohm one. As for the transformer, I'll measure it's output too - 18v is something that's printed on the coil cover, 0.7A too. I have measured all of the resistors and only ones that don't add up are ones in blue given in last image post.

I'm adding also some more images of the board, maybe it helps too.

Also, the guy from that 70s article about it said that it's quite usable amp with "usual mods" - wish he has been more specific about that, but your guesses about clipping sound promising. He also stated that it has it's own compression that prevents the speaker to distort, so it's never going to have it's "natural" distortion, but stay clean with volume all the way up.

Anyway, this is just wonderful what you did, as someone may actually need a schematic in the future, to repair or mode it, and it's nowhere to be found on the web, so cheers for that.

[razabri]

Oh, and all of the pots have same writing on them - RUW100, I'll measure those too.

[Rob Strand]

What a wonderful job you did, Rob! Even the simulation for the clipping, wow. I haven't been able to add anything today, but I hope that by tomorrow I'll have the correct measurements.

The idea was if it looks like it might clip in the simulation then it's probably a design issue.   If the sim looks OK then it could be a fault.   Apart from electrolytic caps drying up there's only two parts that unit.

For now I can add that the speaker is indeed Philips one, and it has a writing that resembles what you already suspected - AD7080/X8, so I guess it's actually 8 Ohm one. As for the transformer, I'll measure it's output too - 18v is something that's printed on the coil cover, 0.7A too. I have measured all of the resistors and only ones that don't add up are ones in blue given in last image post.

Awesome!

I haven't done a lot more.  I wrote down a list of unknowns and things I wasn't sure about.   Found a few minor bugs.

What I did do is simulate the power supply with different transformers and speakers - fairly labourious process trying to back-engineer a transformer spec and the speaker impedance.   So I'm glad to hear you have 18V 0.7A written on transformer because a 15V transformer didn't look like it was going to work.   The small 470uF filter caps make the voltage drop too much.  I assumed a 1.2A transformer (20VA) but only for regulation, not for power rating, so 0.7A (12.6VA) is good also.

I could not decide if the speaker should be 8 or 15 ohms.  The 8 ohms causes the power supply to drop more than the 15 ohm but the 8 ohm needs lower voltage so it all balanced out.  So 8 ohms it is.

I'm adding also some more images of the board, maybe it helps too.

Wow, way more than I expected.   Thanks!    I'll go over every image and check as much as I can.

Also, the guy from that 70s article about it said that it's quite usable amp with "usual mods" - wish he has been more specific about that, but your guesses about clipping sound promising. He also stated that it has it's own compression that prevents the speaker to distort, so it's never going to have it's "natural" distortion, but stay clean with volume all the way up.

I don't know what "usual mods" means either 

Anyway, this is just wonderful what you did, as someone may actually need a schematic in the future, to repair or mode it, and it's nowhere to be found on the web, so cheers for that.

No problem at all.  Yes, it is very handy having these schematics around for the not so popular amps.  Unless you have fixed a lot of amps it's tough trying fix them without a schematic.