Aria Sinsonido preamp repair

[James Perrett]

One thought - does this bass have a stereo output (like Rickenbacker's used to)? I'm trying to work out why they would go to the expense of dual ganged pots. Could it be that one half of the circuit is connected to one pickup while the other half is connected to the other pickup? If so, does the tone control not work on both pickups?

[Hugh Robjohns]

Yes, I wondered that too. It does appear to have a three-core output lead from the circuit board to the output socket.

[Dr R]

It is described as having stereo microphones on the web site. The output voltage at the socket is very high compared to my active bass, as well, so it may possibly drive headphones.
I'll ask, and do some more experiments.

Rich

[ef37a]

Ah! "Headphones" that makes sense of the use of the LM386s.

And stereo, could be some weird phase anomaly going on?

Dave.

[Hugh Robjohns]

Reading around, it does appear that it is intended to drive stereo headphones directly, but can also be used like a conventional electric bass to feed an amp.

H

[Dr R]

Hi All,

Hugh is correct, the bass does indeed drive headphones directly - and really very loudly as well!
The two pickups are wired in stereo, left and right. There is no discernible stereo effect on headphones, it's not as if one pickup is closer to the E and one to the G and there is a spread across the image. However shorting one pickup clearly removes the signal from one channel. (I'll post a photograph of the very Heath-Robinson "test environment" later on )
Testing the pots more comprehensively:

A - Volume

• R15= 1.004K
• R13= 1.004k/5.9ohm
• R35= 5.9ohm/1.004K
• R26=0.950K
• R24=0.950K/5.5ohm
• R46=5.5ohm/0.950K

all makes sense, operating as pair of potentiometers with 3, 4 as the wipers.

B- Tone

• R15=5.6ohm/1.056K
• R13=3.3ohm
• R35=5.5ohm/1.053K
• R26=5.6/1.056K
• R24=3.3ohm
• R46=5.5/1.052K

makes sense with 1+3 / 2+4 connected together, and then operating just as a variable resistor to change the tone.

So perhaps it's not the pot that is knackered after all? What's next to look at?

As ever - huge thanks to everyone.

Rich

[Dr R]

Photograph of "Test rig" to confirm stereo wiring from pickups to output:


Maintaining a steady two-finger plucking technique while shorting one pickup with the needle-nosed pliers was hard

The pickups in the Aria are, I think, in the tube that forms the bridge.

As you can see, it's a lot lighter than a typical bass.

[Folderol]

Ah! "Headphones" that makes sense of the use of the LM386s.

And stereo, could be some weird phase anomaly going on?

Dave.

Ding!

[Dr R]

Thanks Folderol - I take it that's a lightbulb moment?

I'm don't understand how a phase anomaly would prevent the tone control from affecting the sound, though I can see how it might remove some of the bass frequency, which is also part of the problem.

More confused than ever

Rich

[ef37a]

Thanks Folderol - I take it that's a lightbulb moment?

I'm don't understand how a phase anomaly would prevent the tone control from affecting the sound, though I can see how it might remove some of the bass frequency, which is also part of the problem.

More confused than ever

Rich

It is often the case Rich that even quite simple electronic systems display faults that seem to make no sense and a purely logical examination of the circuit (which we have not got!) throws very little light.

Once the fault has been found however it becomes blindingly obvious that "THAT" was why it went T's U! (I call it the "Rumsfeld effect")

Dave.

[Dr R]

Thanks Dave,
Most software faults follow the same arc. The difference is I sort of know what I am doing there
So what should I look for next?

[Folderol]

Thanks Folderol - I take it that's a lightbulb moment?

I'm don't understand how a phase anomaly would prevent the tone control from affecting the sound, though I can see how it might remove some of the bass frequency, which is also part of the problem.

More confused than ever

Rich

It completely explains the (otherwise incomprehensible) chip choice, and I'd be very interested to know what exactly they are doing to get a supposed 'stereo' effect. Oh, and poor LF response could be simply down to caps drying out.

I'm a bit suspicious of the tone control doing nothing at all. A simple fault would suggest only one side being affected. Also, no stereo effect at all sounds a bit strange. I wouldn't be surprised if it turned out to be something daft like a hairline fracture in a ground track.

[ef37a]

Thanks Dave,
Most software faults follow the same arc. The difference is I sort of know what I am doing there
So what should I look for next?

Folderol's postal address and bubble wrap?

Dave.

[Martin Walker]

So what should I look for next?

Folderol's postal address and bubble wrap?

[Dr R]

So what should I look for next?

Folderol's postal address and bubble wrap?

Well that's no pressure on the good gentleman then!

[Folderol]

So what should I look for next?

Folderol's postal address and bubble wrap?

Well that's no pressure on the good gentleman then!

It's OK, these are my friends... I think
If you really are stuck, send me a priv message - I'm a sucker for an unusual challenge

[ef37a]

Did I bolt you up Will? Sorry if so, I thought you had made an offer?

Dave.

[Folderol]

Now that's an expression I've not heard before - must be one of those Northern things

Anyway, I'm quite happy with the idea.

[ef37a]

Now that's an expression I've not heard before - must be one of those Northern things

Anyway, I'm quite happy with the idea.

Oh! Yes, getting "bolted up" is being volunteered/pressed into a task that you did not want and is not actually in your job descrip'!

Not to be confused with "fitted up" which is something our police never do or ever did.

Dave.

[Dr R]

If you really are stuck, send me a priv message - I'm a sucker for an unusual challenge

In that case I most definitely will, thank you.

Rich

[Folderol]

This is proving to be a very strange design indeed! I've worked out a drawing, and it's so strange I had to keep re-checking it
See here.

(Updated diagram shown above)
Cold checks with a meter suggest there are no faults, so the tone control should work, if there are no dried out caps, although with such low impedances I'm not quite sure how! The unmodified gain of the amplifers is 20, so those 1u feedback caps will behave as if they were about 20u.

Notice that DC supply to the microphones. This makes me think they are electrets

As soon as i can get into the office, I'll snaffle their all-singing cap tester and find out what the actual values are.

The 150n 'dry' caps are a guess based on the amp spec sheet.

The 0u22 ones are actually mis-printed so any dot would be off the edge of the cap. Size-wise they couldn't realistically be 22u, and it wouldn't make sense, bearing in mind the feed caps are only 33u.

All very strange

[zenguitar]

Bravo Folderol.



Andy

[Martin Walker]

And I agree with you that it's a very strange circuit!

Are you sure those 1u caps are not connected between pins 1 and 8 of the LM386?



Martin

[Folderol]

Absolutely. Pins 1, 7, 8 are as naked as a newborn baby.

Also today I did a few active tests in my {cough}kitchen{cough} workshop, using my signal generator (of DIY amp fame) and a modern true RMS multimeter. At these low frequencies they're pretty good.

I worked with an output level of 1V, which is quite reasonable as a headphone level, and as I'm not sure of the correct source impedance, I used 600ohms, 2.2k and 47k. The result was entirely consistent across all of these, which is something I didn't expect actually.

I can confirm the tone control has no measurable effect at all over the range 40Hz to 4kHz

Also, the -3dB point relative to 400Hz is 70Hz, which is way too high for a bass.

I might try tapping caps across those present (particularly those 33u coupling ones) and see what transpires.

I'm not at work tomorrow, but if I'm down that way I might drop in and see if I can borrow the cap tester - I should probably get one myself