Lexicon MPX1 output distortion

[morls]

Hi,
I was wondering if anyone could offer advice on diagnosing a fault with my MPX1?
The unit fires up and runs all diagnostics perfectly, and sounds great for around 5 minutes. Then the output becomes distorted. I can still hear the processing and dry sound intact underneath, but it sounds like an overall softening/distortion and loss of amplitude. Because this happens across both channels equally and only after a period of running, I was hoping someone could suggest where to start looking for the cause.
Thanks
Stephen

 

[scott2000]

Power supply is usually a good place to start checking when speaking of both channels. Could be something  simple ... .. or not....


Maybe check some voltages  to get started until others familiar with this jump in........

Service Manual

https://elektrotanya.com/lexicon_mpx1_sm.pdf/download.html#dl

 

[morls]

Thanks Scott,
I replaced the battery 3 years ago, and put in a new PS module at the same time, so hopefully these are still ok. Do you think it worth recapping after the PS output hits the main board? Attached a grab from schematic:

 

[morls]

I've been checking voltages, and have a question (I'm a newbie so please bear with me). When I hold the multimeter probe on the test point there is an initial reading which then drops as I hold the probe in place. Is this normal, and so I should take the first stable reading (after 1s or so?). Is it a sign of cheap multimeter/low multimeter battery?
Thanks again for the service manual Scott.
Stephen

 

[scott2000]

Is this normal,

Most likely normal..... Probably get readings before even touching anything if you look... just check until the readings stabilize more or less.....

Where are you measuring? At the "from power supply " points on your image?

What are they telling you?

What they say when you have your issue pop up as well may be worth looking at...

Hopefully someone familiar with these may have some direction....

Lots going on in there from glancing at the manual..........

Be steady when poking around in there.... Don't want to make matters worse if you can help it . Mistakes and slip ups can come with the territory ,especially when starting out so, be aware of this before deciding if this is something you want to learn on too far......

 

[Monte McGuire]

I may have a fuzzy recollection, but I recall a similar problem caused by the output muting circuitry. According to the schematics (which agrees with my recollection) there are a set of J108 JFETs that pull down the output lines to mute the output, and somehow, these went permanently 'on' in my unit, presumably from ESD or some overvoltage patching error. Removing the FETs solved the problem. There are a set of BAV99 dual clamp diodes that should prevent the FETs from blowing up, but maybe these can fail by themselves and cause related problems - also worth checking.

However, your device might be different - the mute signal might be improperly asserted because of some logic problem, causing the output to be 'stomped on' after warm up. A simple fix is still to remove the muting FETs but you'll lose the turn on muting feature.

 

[morls]

Voltages at "from power supply" OK. Aside from the shifting voltage I noted earlier all test points test OK.

Disconnected the small PCB holding input and output pots, just to make sure these weren't the issue. Still getting noise on output with these out of circuit.

Monte, I noticed that muting circuit too, and was wondering if this would explain the noise remaining after disconnecting LOUTPOT and ROUTPOT. Maybe these JFETs are the culprit.

I'm running all the software diagnostics from the service manual at the moment, so once these finish I might try removing those JFETs.

Thanks again,
Stephen

 

[morls]

I've removed transistors Q1-4 from the mute circuit, but the issue remains.

There's another component of the fault I didn't mention - as well as distortion there is a hum at around 275Hz.

I'll got through the Analog Audio Signal Tracing procedure from the service manual today.

 

[morls]

Making some progress...
Tracing the analog signal has identified issues at 2 ICs immediately after the input pot. The signal is great at input (pin3) on each, but there is an intermittent fault with the outputs from pins 1 and 7. This is the same for both L and R (U11 and U10 respectively).

Because this happens with both ICs I'm wondering if something further along is the fault, which is then what I'm seeing at output pins. Does this sound feasible?

The attachment MPX1-2 is the schematic of the area in question. U10 and U11 are the ICs.

When the fault is present, the AC waveform on scope is truncated above (pin 1) and below (pin 7). I'll post pics below.

Any suggestion? I hope it isn't U8, the AD/DA chip.

Thanks
Stephen

 

[morls]

sorry about the messy pics...
the left wave is pin 3, middle is pin 1 (no fault), right is pin 1 (fault). The wave is inverted at pin 7, so the fault shows as described above.

 

[Khron]

If that's the case, i'd rather point my attention at that... "soft saturation"(?) circuitry on the bottom there.

Try removing those four 10 ohm resistors and 470pF caps, and see if the fault's still there.

If it goes away, it'll be either the circuitry, or those caps (i'm assuming they're ceramics).

 

[morls]

Thanks Khron, I'll try that tomorrow (Tues evening here in Aus)
BTW, I removed the FETs in the muting curcuit Monty mentioned.

 

[Khron]

Those FETs are on the output anyway.

"Garbage in, garbage out" Ie. if the crap's being introduced at the input here, messing with the output won't to squat about it.

It wouldn't be the first time ceramic caps crapped out, or transistors failed / got noisy.

 

[morls]

It wouldn't be the first time ceramic caps crapped out, or transistors failed / got noisy.

These are SMCs, so it's a tight job. :-\

I've removed C84, C87, as well as the 10 ohm resistors, but the issue remains. Does this mean the fault is in U8? This is listed in circuit diagram as CS5360, but is actually SAA7360.

 

[Walrus]

For the sake of completeness, did you measure/scope the power supply rails actually on IC10 or 11?

 

[Khron]

I'm starting to doubt the accuracy of the service docs you've found. Unless they made (at least) two different revisions of the circuits / boards? Service manual seems to be from 2000 - any dates you can see on your board?

The pinout between the CS5360 and the SAA7360, not to mention the package, is not the same. And in the schematic snippet posted above, i see no 470k ("474") resistors connected to C84 and C87, which leads me to believe the whole circuitry is, at least to some extent, quite different.

 

[rock soderstrom]

I'm starting to doubt the accuracy of the service docs you've found. Unless they made (at least) two different revisions of the circuits / boards? Service manual seems to be from 2000 - any dates you can see on your board?

The pinout between the CS5360 and the SAA7360, not to mention the package, is not the same. And in the schematic snippet posted above, i see no 470k ("474") resistors connected to C84 and C87, which leads me to believe the whole circuitry is, at least to some extent, quite different.

AFAIK, there are two versions, the old ones with 18/20 bit ad/da converters and the new ones with all 24bit converters.

 

[rock soderstrom]

These are SMCs, so it's a tight job. :-\

This is listed in circuit diagram as CS5360, but is actually SAA7360.

Your unit is the old 18 bit ad converter version. Whats your Serial number?

 

[morls]

For the sake of completeness, did you measure/scope the power supply rails actually on IC10 or 11?

Yep, I've checked this voltage.

Your unit is the old 18 bit ad converter version. Whats your Serial number?

Serial # is L0597-5512. It's the older model, but still sounds good when working.

 

[scott2000]

What's up with D3?/// Pic making it look funky??/