AI is pretty good troubleshooting help on this ART MPAII

[Steenamaroo]

"Is that a normal way to discharge a tube circuit, to unplug wall ac, then cycle the front On / Off?"
No. Turning a unit on when unplugged is not a reliable way make it safe.

Large capacitors really need to be discharged properly which probably requires access to the bottom side of the board.
Better short term advice, not being facetious, might be just to not touch anything,
and if you might touch something, make sure your other hand is in your back pocket first.
Never lean on the chassis and poke around with your other hand - That makes your chest the path to ground.


With regard to troubleshooting, no...not really.
That's why I went straight to replace and see where you stand.
Because the circuit oscillates rapidly you won't be able to get a reading for this side or that side.

The best case would be if your meter reads average and tells you it's higher than expected because voltage isn't getting to zero but, then, we know that anyway.

 

[CoolCat]

thanks great input and saving time and preventing damaging something.

 

[CoolCat]

appreciate all the inputs... safety too, a good reminder.

fyi Tech Support at ART responded quickly but said nothing, seems when I gave them the Serial ID, its so old a unit, they pushed me to the "old not important" folder.

add-
grabbed a old VLAII for $220 and its got the exact same LED meter and there's no bleed at all on the VLA LED meter.
On a good note the MPA isn't feeding the VLA anything so the LED "bleed" on the MPA must be internal only...for now.


off topic on channel bleed, the Aphex 207 had about the same, maybe a tiny more maybe not all in the -75 range..in Reaper.
Odd was the old ISA430 single channel caused some channel bleed to the Interface CH1/2 which wasn't seen with the other preamp outputs. I notice it has more noise but might be the home ac, and its Gate is great at silence, but channel bleed seen.

subjective, as for sound...I really couldn't hear a huge difference "in clean normal gain " modes between all the preamps.
However the MPAII with GAIN and +20 can go all Distortion like BEatles Revolution Guitar or Creams Sunshine of Your Love heavy distortion...the Aphex, JoeMeek,ISA430 doesn't do that. Also really like the Variable Impedance on the MPAII....none of my others have that at the moment.

noticed the VLA VU are orange colored and MPA is more normal white colored...both have the same LED displays.

 

[Steenamaroo]

If it puts your mind at ease any I simulated it.


The additional transistors are just to simulate different levels for each channel,
and the two resistors I connect are to simulate Q101/102 leakage.
You can see the mirroring when I connect them.




If there was an actual audio bleed issue it would be way lower on the wrong channel, and the LEDs would be bright.

* This is simulated at 1hz. The real circuit operates faster, so it appears both meters are permanently illuminated.

 

[CoolCat]

Thats interesting. So Im not familiar with this,

you are putting a signal in Ch1 1khz....Nothing in Ch 2...

And when you tie the "sim-leak resistors to the 5v supply, that gives the leakage "bleed" simulation that lights up the LEDs?

 

[Steenamaroo]

Oops, I thought I described earlier.
No, not quite.

The unit only ever lights up one LED bank at a time. It just happens so rapidly that it looks like they're both on together.
My simulation slows this down, to make the workings of it easier to see.

The extra transistors are there to fake an input level of 8LEDs for ch1, and 5 LEDs for ch2,
just so we have something clear and obvious to look at.


The first part of the video demonstrates working as intended, except slowly.
When I connect the resistors, that demonstrates what I think the fault is.


So your troubleshooting question is
"why do I see voltage at Q101 collector when it shouldn't be passing"
and given the symmetry of the fault
"why do I see voltage at Q102 collector when it shouldn't be passing".


All I can think of is leakage, demonstrated above, or something conductive on the board.



"Because the circuit oscillates rapidly you won't be able to get a reading for this side or that side."
Sorry, that's what I meant by this ^^ - You'd have difficulty troubleshooting with a meter because you'd think
"if ch1 is lit up with signal, I should be able to read 5Vdc at Q101 collector", but that won't work because it's oscillating between 5v+0 rapidly.

 

[CoolCat]

Interesting.
I was watching some beginner transistor videos a bit , on and off...npn, pnp.....

so in a real world of repair, a Bench Technician would just change the transistors?
and maybe inspect for any shorts of any kind?

too bad it wasnt the LED board, its easy to unplug and only one screw holds it in.

getting to the bottom of the PCB means full disassembly...which looks like a lot of work.

thanks for explaining the video a little more. and making it....so the leakage is just enough to barely open- the other side of LED's and mimic , and both channels are doing it identically... so that seems strange too.
for now its purely cosmetic visual which I don't care really, and if I use both channels, both light up bright and seems fine. The MPA preamp actually works fine without the display plugged in at all.

Id always wanted to try the combo- MPA and VLA and when it arrived, the VLA shows no issues at all. Its 2016, like new.
So 2 channels Pre/ comp for $420~450 ish... Ill get using it.
I did drop cash on new tubes assuming a lot of the problems were the tubes but it wasn't at all.

So I could buy the two transistors pretty cheap, but the labor would be a few hours and more being a novice. Returning the unit and spending $50 more on another one, that has no issues is an option to ponder...

Thanks again its great knowing its a LED display only circuit....

 

[Steenamaroo]

so in a real world of repair, a Bench Technician would just change the transistors?
and maybe inspect for any shorts of any kind?

I'm just a novice hobbyist.
I think it's a high-confidence move so yeah, I would just replace the transistors.
Someone being thorough would maybe probe with a scope to confirm what they're doing.

for now its purely cosmetic visual which I don't care really, and if I use both channels, both light up bright and seems fine. The MPA preamp actually works fine without the display plugged in at all.

Yeah, I think that's my takeaway.
Assuming I haven't made a mistake it's cosmetic so it's a question of how much, if at all, you care.

and both channels are doing it identically... so that seems strange too.

If they have both failed the same way, that being strange depends on what caused it.
If it was a 5v spike, it's not that weird.

 

[CoolCat]

I got a response from Yorkville- Art Tech Support and they said "they are sure this is the issue" and its the U409 computer chip.

AI Google- I typed U409 chip-

In the ART Pro MPA II service documentation, designator U409 refers to an MC68HRC908JK1P 8-bit microcontroller in a DIP-20 package. [1]

• Component: Freescale/Motorola MC68HC08 family 8-bit microcontroller
• Designator: U409
• Package Type: DIP-20 (Dual In-line Package, 20 pins)
• Function in Unit: Manages internal digital control logic, states, or interface operations (such as front-panel switch sensing and LED/display routing) within the hybrid tube preamplifier system.

(But searching deeper in the parts- the chip is 1. obsolete and even more is that its some Flash- required so buying a Blank wont help if not make it worse.
So it has to be "flashed" by Art Research. So Ive asked a quote for that U409-chip.
A good note is this chip is in a socket!! so it can be easily replaced, no soldering or disassembly required.)

Crucial Repair Note: Firmware Warning
If you are purchasing a replacement chip because your current U409 microcontroller is fried, a blank off-the-shelf MC68HRC908JK1P chip will not fix your ART Pro MPA II.
The microcontroller requires proprietary ART firmware code flashed onto its internal flash memory to understand how to control the LED displays, read the Mid/Side switch matrix, and operate the digital control logic.

• What to do: If the chip itself is dead, your best route is to contact ART (Applied Research and Technology) customer support directly to request a pre-programmed replacement chip with the firmware already loaded, or source a dead "parts unit" Pro MPA II to harvest a functioning, pre-programmed U409 chip.

What Happens When This Chip Fails in Your MPA II?
If the microcontroller fails, your analog audio pathways may actually still pass a signal, but your interface control will break down. Symptoms of a dead or failing MC68HC908JK3CP in an ART unit include:

• Frozen Front Panel: The buttons (such as +48v, Phase, Low-Cut, or Plate Voltage) refuse to switch states.
• Unresponsive Metering: The LED Tube Warmth trees or the display backlights stay frozen, dark, or flicker erratically.
• M/S Matrix Failure: The internal mid/side decoding logic stops executing commands completely.

(so I don't know....if its a cheap part I might get one but if its more expensive than the Unit.....Ill return it because the comment in RED..."dead or failing".....if its failing, it could continue to fail and is breaking down and will require a new chip. and more......

there's no telling 100% until parts are changed out if its the fix or not.
this Used Unit was $199 , 45 day return...so while the LED doesn't bother me, the "failing" comment is a no-go.

 

[CoolCat]

heres the chip- U409 location-

 

[Steenamaroo]

I don't see how but maybe I'll get an education.

 

[CoolCat]

yeah, the info on the Mid/Side is where I "tuned out" , way too complex for my brain.
Computer chips with all kinds of stuff going on inside multi-function etc..
So the chip is the Tech Support suggestion and were confident that's the issue. I don't know. No quote yet.
Its a 20 pin computer chip that does a lot of things. I think I read the Mid/Side alone has six functions happening in active mode, and more going on with other functions.
It is interesting this chip is in the easiest to reach and remove location and is designed with a socket to replace or upgrade.

I pressed on the Chip and it wasn't seated very good, thought I might get lucky but didn't. Still the "ghost" LED.

Been reading a lot and theres a lot of repair threads on the MPA stuff. The push button switches are common issues not sure of cost, in addition, the common Tube issues (on any brand not just Art) and the VU meter led's, which are $20 to fix.

If the programmed "flash" chip is $10~20...I might be interested. These "flash" type chips are ESD sensitive too. I've replaced them several times at work or in a guitar rack unit pretty simple with the socket-on-pcb.

 

[Steenamaroo]

I would very strongly consider just ignoring AI completely.
As demonstrated, it will quite happily just say "Yeah, I'm wrong. I made that up" if pressed.
It's not 100% useless and prompting is a skill but for average user troubleshooting, it's as close to useless as you get.

The chip you're talking about is responsible for the LEDMux signal but as far as I can work out LEDMux would need to be failing to reach both extremes (5V and 0)
in order to cause two-way bleed that you're seeing.

Maybe that's possible...Maybe it's more likely than I thought?

idk. I suppose the bottom line is if you described it as dimly lit mirroring from ch1 to 2, and from ch2 to 1, and ART are pointing to that chip as the cause,
then I guess it's that chip.

 

[CoolCat]

Actually AI or Google search...has been showing some great info on the ART MPA, granted its using some super search, but the Chip, the details popped up, the Mid/Side complex circuitry, it made it known NOT to buy the chip as it needs flashed...etc......and most the info is taken from sources like Home recording, Gerslutz, DIY or many other websites. I think the info on the OpAmp was the best imo...putting a DVM and seeing the movement voltage when a signal Ch 1 to Pin 1 or Ch2 to pin 7 and looking for bleed, that was interesting and knowing the CHip being used and looked that up. Unfortunately its soldered into the pcb instead of a socket.

I still think of the Mid/Side button too, its not just a On/Off....if it doesn't turn Off fully, both channels LEDs would see some of the source signal. possibly (I like to fault Mechanical things because they get pressed and clicked 1000's of times...and these are plastic internals...cheaper...but ok...)

The simulation you showed was over my head but it did simulate the problem.

Makes a Rane MS-1 preamp seem simple and attractive in price and troubleshooting. used on Steely Dan Album tracks...
I really do like the MPA, the Impedance knob is a nice subtle tone changer, the Overdrive is nuts, good, it can do a BigMuff fuzz sound or a really clean smooth and dual channels for cheap is pretty impressive. or get a Grace Design M101 with transferrable 20 yr Warranty Parts and Labor... worry free gear.
I hope they send the quote and part, if $25 or less....Im not spending $100 on a used $199 item I have $35 days to return...lol ...and spend time soldering in transistors if the chip doesn't work...and find out by process of elimination its the Mid?side or something else.

and what if the chip doesn't work? I doubt there's refunds on ESD sensitive stuff.

 

[Steenamaroo]

Yeah, I didn't say it was useless.
The point is if you ask it a question that you don't know the answer to, you don't know if the answer it gives is correct, or even sensible.
Any publicly available LLM will 100% bullshit you and talk about completely fabricated nonsense, and gladly admit it when pressed.

The example I pasted wasn't doctored.
That wonderfully detailed impressive description it gave me about the 8 pin complex arrangement handling low voltage logic switching, the backlight, and mains isolation,
which turned out to be no more complex that a light switch?
That's common place.

The reason I'm leaning on this is because you keep coming back to the mid/side circuit
but there is nothing you can do in the analog audio domain that will result in dim LED ghosting.

One way or other, dim LEDs means voltage present at q101/102 collectors when it shouldn't be.

There's a dozen reasons that could happen but it is happening ^.

 

[CoolCat]

totally agree AI doesn't fix or tell the exact correct answer (not yet anyway)....

its more like a faster , a bit finer tuned search engine , as I see it. Saves some time if the data can be verified like the Chip info then search the OEM spec sheet.. Its based on stuff from the internet so who knows? I was impressed on several searches that were Art MPA II specific and confirmed but the AI sources its info so its not really AI came up with the idea, its just showing some data from a source/site/forum. ...but it is faster than me going through 300 sites to find something.

Anyway, the Yorkville tech support hasn't responded with a quote yet on the Flash/Chip and seems like a couple to few days wait time for response trend.
I cant imagine this chip would be cheap beings they need to program it, but Im curious for a quote.

two transistors would be a cheap fix. a bit of work on a weekend but Im opening up to it for Fun little project.
the transistors make sense as the problem appears to be solely the display circuit and nothing else. I tried understanding the functioning of the circuit but its a bit over my head.
I need to price the transistors, curious...

budget gear, input jacks and other things are what they are....the limit is spending on it.
The main repair shop around here is closed, dude guru retired. I wonder if it will get like Disposable TV's....all the TV repair shops closed down.
Just don't see shipping a $199 unit to Yorkville to pay for repair either. DIY or use it as-is until it dies.

 

[CoolCat]

If it puts your mind at ease any I simulated it.


The additional transistors are just to simulate different levels for each channel,
and the two resistors I connect are to simulate Q101/102 leakage.
You can see the mirroring when I connect them.




If there was an actual audio bleed issue it would be way lower on the wrong channel, and the LEDs would be bright.

* This is simulated at 1hz. The real circuit operates faster, so it appears both meters are permanently illuminated.

I've watched this like 25 times....I think I just now got it. All the LEDS are on when you attach the resistors to this circuit inducing leakage. Is that correct?
It went so fast but just now I think my light bulb went off...lol the resistors connected, and ALL Leds lit up...
I've never done simulation software like that, but aware of it being used. at TI they used SPICE sim software a lot.

 

[Steenamaroo]

I've watched this like 25 times....I think I just now got it. All the LEDS are on when you attach the resistors to this circuit inducing leakage. Is that correct?
It went so fast but just now I think my light bulb went off...lol the resistors connected, and ALL Leds lit up...
I've never done simulation software like that, but aware of it being used. at TI they used SPICE sim software a lot.

Yeah, the key thing to get is that in proper working condition, banks 1+2 do not light up at the same time.
It just looks like they do, because they alternate fast.
If I sped up my simulation to a few hundred hz, it would just look like both meters were lit a the same time.


With that knowledge you can work out that Q101 is the switch that turns on and off channel 1.
and it also also turns on and off Q102.

When one is open the other is closed, and vice versa.
LEDMux is the trigger which rapidly switches to control the whole thing.


By design, when LEDMux is 0V, Q101 passes which
1: Lights up channel 1
2: Lifts Q102 base, closing it, and turning off channel 2

then when LEDMux is 5V, Q101 shuts which
1: Kills channel 1 LEDs
2: Lowers Q102 base (by cutting off 5v to it) which lights up channel 2.

Knowing that ^^ you know there should only ever be 5V available at Q101 collector (for LEDS CH1) or Q102 collector (for LEDS CH2),
but never both.

Because you can get healthy clear readings for channel 1 and for channel 2, we know the timing and opening of both transistors is working.
Because of that, and the fact that your ghosting mirrors the level of the active channel,
we know that the "closed" transistor isn't fully closed (or by some other means it's being bypassed).

It might be possible for that to happen, two way, if LEDMux fails to go fully high and low...Say 1V-4V instead of 0-5V
but, honestly, I'm not 100% certain.
I left that out of my original suggestions because I'm not confident about it.


For science, though, if you pass that information to your LLM and ask "doesn't that make it impossible for the problem to be actual audio bleed, or M/S related then?"
you should get an interesting answer.

 

[CoolCat]

I found this video, showing the same thing of LED both lit with only one channel active. Both are lit up, bleed, leakage. But other MPA Ive seen don't show as much bleed or don't leak? interesting at 7:06 at the end...



Interesting explanation of Q1 and Q2, and transistor in general.
Seems the design doesn't keep the unused side OFF, couldn't they have added some component to improve less leakage...seems a 5v collector isn't that much to come up with something to prevent leakage/bleed. Why didn't they just have two separate Ch1 and Ch2...? why tie Ch1 - Q1 to Ch2- Q2?
I don't understand why Q1 is connected to Q2 input?

 

[CoolCat]

Heres another one showing bleed/leakage....at 3:15....so maybe this is just how the Art MPA is designed and not repairable? (without a design change)