Problem with a Fender MA6 Mixer Amplifier.

[Kala_12]

Hi All,
I have a problem with a Fender MA6 Mixer Amplifier.
I should mention that it is not a complete unit, it came to me minus its cabinet, power amp and PSU. It has been mounted in a home made cab with a MOSFET power amp and PSU.
The problem appears to be rf oscillation. This only seems to happen on channel 3 & 4 when I turn up the main fader. It manifests itself as a high reading on the vu meter with no sound from the loudspeaker. I did destroy a piezo tweeter and since then have put a 47ohm high wattage resistor in series with the loudspeaker when tried again.
I am thinking that a small capacitor in a negative feedback loop or from the signal path to ground, but would like advice on what value and where would be likely to do the most good.
I was able to get the circuit diagrams for this unit and attach the one for the preamp.
Thanks in advance
Stuart.

[Radio Wrangler]

Aaaargh!

Opamp 41A (I think it's marked) has full negative feedback around it and a capacitor directly to ground on its output. This is a classic no-no, except for a very few specialist amplifiers designed to take it.

The problem is RF instability, and the change in the source impedance from the main fader is going to affect things.

The output impedance of the amplifier into the shunt C gives a phase lag and this subtracts from the phase margin designed in by the compensation pole internal to the amplifier.

David

[lesmw0sec]

Hmm. You mention that it only occurs on certain channels and I assume that the circuit you added shows one of several. In this case look for the differences between the oscillating and non-oscillating channels - is there a signal line mounted too close to something? In the case of the added P.A. is there a ground return close to the offending channels?
David's point above is valid, however the IC in question is connected as a unity-gain follower and I would be surprised if 120pf to deck would cause the problem, although I am a bit puzzled as to why it was ever included.

Les.

[Radio Wrangler]

Unity gain followers are exactly the worst possible case for this because there is no attenuation in the feedback path and so the loop gain is maximal.

Some opamps (usually faster ones rather than the bog standard lot) aren't stable at unity gain and can only be used in circuits where the closed loop gain is set to some specified value or larger. THere are tricks around this... reducing what's sometimes called the 'noise gain' but I don't think they're needed here.

Methinks someone had an RFI ingress problem and dumped 120pf capacitors all over the place after the thing had been designed.

David

[Kala_12]

Hi All,
Les, you are correct in that this is one of several, as you can see in the photos, there are six identical channels. I can't take bits out to measure (I have no way of measuring capacitors), but it's just come to me, I could swap the boards round to see if it's a positional thing

According to the circuit sheets I have, it was designed/drawn in 1977.

The added bits are the power transformer (top centre), the power amp (top right) and the psu main capacitors (one of which can be seen just above the power amp) in photo 3.

I have tried disconnecting the power amp from the original part and still get the RF oscillation showing on the oscilloscope at the output, ie. the preamp out socket.

Something else has just come to me. Would it need a metal base panel as an RF screen in the same way as some older PA amps had a sheet of aluminium behind the front panel, between the preamp board and the power amp?

Stuart.

[lesmw0sec]

Still a bit puzzled by this. Certainly a bit of screening would do no harm. Have you tried just moving your hand close to the electronics in question to see if the oscillation varies? If so that might indicate capacitive coupling.
I repeat my point about earthing, even though you have isolated the P.A. I would assume that the mixer is of the "virtual earth" or summing node type and these can be very sensitive to earth routing.

Have a close look at the boards to check that the components on each are identical.

Les.

[Radio Wrangler]

Nothing to be puzzled about.

There is one section in that schematic which is a circuit very well known to have stability problems. Sure it's in an amplifier by a well known manufacturer, but it's still a well-known no-no. How it got designed in is an interesting question, but it is there. I'm not surprised that some channels give problems and others don't. Some of them will be oscillating happily at ultrasonic and RF frequencies and it's possible that this migh not get noticed without the amplifier meeting an oscilloscope. Some might be not oscillating, but exhibit a peaked response at inaudibly high frequencies.

Ironically, the better the capacitor is, the greater the risk of trouble.

Don't take my word, google around 'opamp' and 'capacitive load' It was certainly well known in 1977.

David

[lesmw0sec]

Well in that case, all the O.P. has to do is to disconnect that cap. and see what difference it makes. I will be interested to see the result.

[Radio Wrangler]

Indeed, quite an easy check.

It looks a bit like someone after the original design was done was given the job of stopping RF ingress... the proverbial taxi radio getting into the PA problem, and sprinkled them all over the place. Most places aren't a problem, but the output of a feedback amplifier is easily upset.

Look at transistor power amps and see the resistor/inductor pair in series with the output, and this is to protect the stage from being made unstable due to capacitive load from fancy speaker cable and crossover networks.

Some amps missed this out and were known to be damaged by weird boutique cables.

Cheers
David

[Kala_12]

Hi,
After reading the comment from Les and googling around 'opamp' and 'capacitive load', would it be sensible to try moving C3 from it's current position shown in blue to after R20 shown by the green?
Stuart.

[lesmw0sec]

Just disconnect it and see what happens to the oscillation. I can not believe that it is accomplishing very much - certainly won't affect the audio frequencies. As other posters have said - not a good place to stick a cap. in any case.

Les.

[Kala_12]

Hi,
Thanks. I'll try that, probably tomorrow and let you know how it goes.
Stuart.

[Kala_12]

Good afternoon folks,
Sorry it's taken so long, but I've finally got back to this little problem.
I'll start by answering the questions/comments.
#5 I tried swapping some of the boards round - the problem seems to move with the board. Also tried a sheet of aluminium to screen all the pre-amps to no avail.
#6 Not noticed anything when a hand is put near the boards. All 6 boards are identical.
#8 Disconnected said cap (C13), no change.
#10 Now I've got C13 disconnected, I might just try moving it to R20/C15.

I have now completely removed the PSU and power amp, and now power the preamps/mixer(master module)/equalizer with its own dedicated PSU.

OK. So here's what happens,
Screenshot #1, Hum (when oscilloscope probe touching a piece of paper on the bench).
Oscilloscope probe on pin 1 of IC. All with Gain = max and Main fader = max :-
Screenshot #2, Treble = min, Bass = min, - slight trace of hum, can't see any high frequency/noise
Screenshot #3, Treble = min, Bass = max, - +ve peak of hum seems inverted(?)
Screenshot #4, Treble = max, Bass = min, - quite a bit of high frequency/noise - don't know what sort of frequency we're talking about.
Screenshot #5, Treble = max, Bass = max, - the same high frequency/noise on the (distorted) hum of Screenshot #3.

A little more to come....

Stuart.

[Kala_12]

Continued....
Screenshot #6 Screenshot #4 but with timebase speeded up.
I hope the shots of the scope will be of use, I have VERY little experience with oscilloscopes.
The preamp circuit is in post #1. I've included the circuit of the master module and the equalizer boards just for completeness.

Stuart.

[Kala_12]

Hi all,
I have not got anywhere with this over the past few weeks.
I am thinking of building a little 15-0-15 PSU and powering one preamp on its own on the bench. I could still do with an idea or two to stop the oscillation. I have heard of grid stoppers and wondered if something similar could be use. If so, exactly where and what sort of value. Any ideas please?
Stuart.

[Diabolical Artificer]

Grid stoppers are for valves, this hasn't got any valves has it? Ferrite beads are used to stop oscillation, being fitted to the legs of transistors.

If I've read correctly one channel is oscillating. It might help to find the problem if you power the effected channel and one that is working from a bench PSU or similar, ground the inputs or inject a signal and go though each and compare. Take DC readings first then scope each section/stage of the channels.

Those schematics you've attached are just about unreadable, you need to attach better scan's. If you no good with computors like myself, taking a screenshot is an easy way to save schematics.

Andy.

[peter_sol]

The input transistor has no base bias On it unless a resistor has been missed out on the diagram (from the base of T1 to the emitter of T2 for instance)

[Kala_12]

Thank you Peter. I'll look into that. Not sure when as I don't get a lot of time (as you may have noticed).
Stuart.

[Radio Wrangler]

To understand your scope traces, it's necessary to know what the volts/division and time/division scales are. More subtly it's necessary to know what it's triggering on.

You're using an oscilloscope probe. Now is it a times-10 probe or a times-1 probe? Also when there's hum on the go or RF signals it matters a lot where you stick the earth clip at the end of the probe.

There's a bit of a learning curve with scopes and some people are put off, but once you get going with one and master a few foibles and pitfalls, then they start showing you things which make understanding what's going on a lot easier.

Some of the things you do with a scope can seem very counter-intuitive, but actually make sense.

Your first picture, showing hum from just probing a piece of paper on the bench... assuming the paper is dry and an insulator, this is pick up by the exposed tip of the probe of the electric field floating around your bench area from all the various mains wiring and cables in the walls.

It's good practice to keep mains cables out of the area you're working in as much as possible just to reduce pick-up. Knowing the volts/division on the scope knob and the probe ratio will give an idea of how much hum is in the air so to speak, and how sensitive you've been setting the scope. I guess the hum is 50Hz and we've got 1 cycle over about 10 divisions so it must be set to about 2 ms per division on the timebase knob.

Both the Y amplifier and the timebase have scale knobs that change the picture in a 1-2-5-10- sequence, but they also have fine adjuster pots. Normal use of a scope is to put these fine pots into their calibrated positions (usually fully clockwise) Good scopes make the knobs click into this position. Really posh scopes put a warning light on when you take them out of this position. And some posh ones would write the settings on the screen so you'd see '2ms' written in a corner. Take the fine control out of cal and it's change to '>2ms' to tell you the rough scale and that you couldn't trust it.

With a basic (but still perfectly good for the job) scope you need to remember to keep the fine knobs in the cal positions, and when you take screen pictures to write on the settings.

I'll take some photo of the scope on my bench to show you what I mean

David

[Radio Wrangler]

OK, some photos

Number 1. Nothing is being probed, but I've moved the channel 2 and the timebase fine controls out of their 'cal' positions. Notice the red LED near the channel 2 volts/div knob near bottom left corner of the CRT. The timebase warning light is just to the left of the big timebase knob. Not all scopes have these warnings, but they are red for warning you that the scale of the picture might not be what you think it is.

Number 2 The fine knobs are back in their cal positions and the scope has no angry red lights. The green one at the right is 'power on' and there is a green light behind the volts/div knob skirt on the channel I have turned on (channel 2). Standard practice is to only take these knobs out of their cal positions on special occasions.

I've got a times 10 probe plugged into channel 2 but the scope defaults to thinking it's a times 1 probe unless you buy matching probes from tektronix (then it detects which ratio you have and moves the light behind the volts/div skirt to help you) So my knob says 10mV/div and my probe is a x10 voltage reducer, so 100mV at the probe tip will shift the trace 1 division. You need to figure out what ratio probe you're using and whether the scope can tell. If not you have to remember to multiply the volts/div on the knob to know what the scale on the screen is. This isn't a fancy scope by today's standards, it's a mid 1980s job and cost me £80 a few years ago. It is, however, still a very capable instrument.

Photo Number 3, now for some hum. This is just my finger touching the probe tip. I've had to change the scale to 500mV/div to fit the picture on the screen. I've got several volts peak-to-peak of hum on me! I set the timebase to 2ms/div to fit in one cycle. The scoe probe ground clip is dangling, I'm not touching anything else. Notic the mains waveform is 'orrible and not sinusoidal at all. There are little jumps in the waveform from various SMPS, low energy lights etc. Hum isn't as good as it used to be! Oh, and I fiddled about with the trigger controls to get a stable picture.

Number 4 Now I'm touching the probe tip and the probe's earth croc clip, all three together. Look! no hum. I'm now earthed via the probe's croc clip and the scope. Sorry for the camera shake while juggling.

Number 5 Now I'm probing an audio sig gen on the shack shelf. It's not plugged in. The probe tip is on the inner of it's output BNC connector, the ground clip is dangling.