Tek 2246MODA some buttons don't work

[RobII]

I think that's pretty accurate lol.  Proof of concept at the very least.  Now to figure out getting the switch panel off.  Not sure I'm looking forward to that.

[sync]

Not quite accurate:

Sorry, I don't see your point. On RobII's photo none of the trigger mode LEDs are lit up. They are all on AS3. But one of them should glow like shown in your video.

[xwarp]

o.k., so we know that the emitter of Q2503 is low. We know that when you press a button, the output is pulled low, if you hold a button down on the AS3 line, it will hold that emitter low.

Outside the trigger led's, the other led, (for those buttons) circled, do not change state, just as seen in the video I posted.

Him isolating pin 4 of that cable should eliminate whether it's a stuck button.

I don't know about your 2246a, but mine turns on in the last state that it was in when it was turned off.

We don't know what state his was in and since he can't access the menu to do a reset, he's stuck with where it was last shut down.

[xwarp]

You know, it could be one of those led's, (in the A/B trigger source line), shorted.

Maybe one of those between DS2025 though DS2032.

ETA:

When you get home, get your meter and on diode function with the positive lead on pin 4 of J2501 cable, probe with the negative leads pins 9-16.

That should at least eliminate those trigger led's.

[RobII]

Probed with DMM positive on pin 4 and negative to pins 9-16, all had a reading of 1.76. 

Pulled pin four and reseated rest of pins, powered on and checked ground point to emitter? (square pad) .31vdc.

[xwarp]

You've now eliminated the front panel. 

[RobII]

Very cool!!  So next step is to get a replacement transistor.  I will see about getting the one Sync suggested. 

You guys are the best!!!

[sync]

Do you have any transistors laying around? For testing you can build a replacement with two normal NPNs.

[RobII]

None on hand.  I am going to ask one of the repair techs where I work.  I am excited about this!!

[MarkL]

I think Q2503 is dead. Your measurement of 0.3V between base and collector is a strong indication. A silicon junction have about 0.6 - 0.7V forward voltage. Measuring a substantially lower value means a shorted device. There could be a short elsewhere. But this is unlikely.

Why would you say that?  In circuit, it's entirely possible to have less than 0.7V between the base and collector since that junction is not forward biased.  And these are darlingtons, so you're going to see a double drop measuring between base and emitter, so it's not always 0.7V.


RobII, I'm a little confused where you're probing the negative of your DMM in all these measurements, so my apologies in advance if there's a reading that disputes what I'm about to say.

Although it may be Q2503, I think it could also be the output of U2523, the latch that outputs the scan signal.  I would check one more thing before you start pulling out transistors.

With your DMM set on *ACV* and negative lead to a ground (chassis is usually good), check the output of U2523 pin 15.  That is the output that drives Q2503.  You should see some reading there, indicating there's a changing waveform.  You can compare this to the other scan outputs on U2523, pins 12, 9, 6, 16, 5.  Those should definitely have an AC voltage on them since those buttons are working.

If you have no AC volts on pin 15, or very little (like < 0.1V), switch to DCV mode.  If you get a constant voltage close to +5V or 0V, I would say that output on U2523 is dead.

If you do see a comparable AC voltage on pin 15, check to see if you see an AC voltage on the other side of R2536, which is also Q2503's base.  You should also see an AC voltage on the emitter of Q2503.

[RobII]

Thanks for the reply.  No worries.  I was instructed to have the negative DMM probe on a ground point so I chose a PCB mount screw near the ribbon cable.  I also had it on the metal frame to see if there was any difference in my readings.  There was not.

I will certainly check what you suggest tonight.  Thank you so much for the input!

RobII, I'm a little confused where you're probing the negative of your DMM in all these measurements, so my apologies in advance if there's a reading that disputes what I'm about to say.

Although it may be Q2503, I think it could also be the output of U2523, the latch that outputs the scan signal.  I would check one more thing before you start pulling out transistors.

With your DMM set on *ACV* and negative lead to a ground (chassis is usually good), check the output of U2523 pin 15.  That is the output that drives Q2503.  You should see some reading there, indicating there's a changing waveform.  You can compare this to the other scan outputs on U2523, pins 12, 9, 6, 16, 5.  Those should definitely have an AC voltage on them since those buttons are working.

If you have no AC volts on pin 15, or very little (like < 0.1V), switch to DCV mode.  If you get a constant voltage close to +5V or 0V, I would say that output on U2523 is dead.

If you do see a comparable AC voltage on pin 15, check to see if you see an AC voltage on the other side of R2536, which is also Q2503's base.  You should also see an AC voltage on the emitter of Q2503.

[MarkL]

Great - let us know what you find.


If nothing obvious is surfacing and Q2503 is still a prime suspect, you could always swap Q2503 with another in the group to see if the problem moves.  Since it's only a 3-terminal device it should be easy.

And actually, it's sufficient to just put another in Q2503's place and then check to see if the dead buttons come alive.  Some other group of buttons will be dead since there's no driver, but this way it's less soldering if it turns out the original Q2503 was bad.  (No sense putting the pads through another round of soldering stress putting in and removing a bad transistor.)

[sync]

I think Q2503 is dead. Your measurement of 0.3V between base and collector is a strong indication. A silicon junction have about 0.6 - 0.7V forward voltage. Measuring a substantially lower value means a shorted device. There could be a short elsewhere. But this is unlikely.

Why would you say that?  In circuit, it's entirely possible to have less than 0.7V between the base and collector since that junction is not forward biased.  And these are darlingtons, so you're going to see a double drop measuring between base and emitter, so it's not always 0.7V.

As I written, there could be a short elsewhere but it is unlikely. The base is connected to a 1k resistor and a 4.7nF capacitor. A 1k resistors doesn't turn into a short without very visible damage. The capacitor could cause the short. But it is unlikely. It's not a big MLCC SMD cap. There also could be a short in the PCB. But this is very unlikely on a two layer board.

A silicon darlington has ~1.4V between base and emitter. But when it measures 0.3V then it's dead. (Ok, BIG silicon semiconductors can measure 0.3V with the little current of a DMM. But this is not the case here).

About further measurements. The best would using a scope and look at the waveforms. Then the fault could be narrowed down easily.

[sync]

None on hand.  I am going to ask one of the repair techs where I work.  I am excited about this!!

They should handle 0.5A or more collector current, eg 2N2222. I measured about 45mA thought one LED. There are 8 LEDs connected on the AS3 line. So this would be about 360mA total (plus some mA if a switch is pressed).

For the final replacement I would go for 1A type like the original.

[MarkL]

A silicon darlington has ~1.4V between base and emitter. But when it measures 0.3V then it's dead. (Ok, BIG silicon semiconductors can measure 0.3V with the little current of a DMM. But this is not the case here).

You can't conclude it's dead.  It could simply be off (Vbe < 1.4V).

It's difficult to conclude anything with average DC volt readings taken on dynamic digital signals.

About further measurements. The best would using a scope and look at the waveforms. Then the fault could be narrowed down easily.

Agree completely.  This would take about 2 seconds with a scope, but I'm assuming all we have is the DMM.


EDIT: Fixed quote levels.

[RobII]

Only scope I have is the partially working 2246 lol.  I'll keep everyone posted.

Agree completely.  This would take about 2 seconds with a scope, but I'm assuming all we have is the DMM.

[sync]

You can't conclude it's dead.  It could simply be off (Vbe < 1.4V).

It's difficult to conclude anything with average DC volt readings taken on dynamic digital signals.

I talking about testing it with the DMM diode function. Not measuring the voltage while the scope is running.

[MarkL]

You can't conclude it's dead.  It could simply be off (Vbe < 1.4V).

It's difficult to conclude anything with average DC volt readings taken on dynamic digital signals.

I talking about testing it with the DMM diode function. Not measuring the voltage while the scope is running.

I'm sorry - you are right.  I missed that you were referring to diode test mode.

[sync]

I'm sorry - you are right.  I missed that you were referring to diode test mode.

I should made it clearer from the beginning. My conclusion that Q2503 is dead is based on the 0.3V of the diode test. The other voltage measurements are difficult to interpret.

[RobII]

Quick edit.  I didn't notice there was more than one 74HCT374.  Which one is U2523?  I checked the one on the right of the pic.



Checked pin 15 (and the others you suggested) all had an AC voltage bouncing around 1.5vac.  Also checked resistor R2536 and others near it with voltage about the same.  Is it safe to assume transistor Q2503 is the fault or is there something else I should be checking first?

With your DMM set on *ACV* and negative lead to a ground (chassis is usually good), check the output of U2523 pin 15.  That is the output that drives Q2503.  You should see some reading there, indicating there's a changing waveform.  You can compare this to the other scan outputs on U2523, pins 12, 9, 6, 16, 5.  Those should definitely have an AC voltage on them since those buttons are working.

If you have no AC volts on pin 15, or very little (like < 0.1V), switch to DCV mode.  If you get a constant voltage close to +5V or 0V, I would say that output on U2523 is dead.

If you do see a comparable AC voltage on pin 15, check to see if you see an AC voltage on the other side of R2536, which is also Q2503's base.  You should also see an AC voltage on the emitter of Q2503.

[tec5c]

Do you have a soldering iron? If so, are you comfortable in using it?

As previously suggested, you could swap it with another one of the transistors and see if it fixes the AS3 line.

For such an inexpensive component, I'd just replace it and see if it changes anything. If not, back to the drawing board.

[RobII]

I do and am pretty comfortable using it.  I agree about just replacing it.  I don't like swapping things around.  It just increases the chance of damaging something else.

Do you have a soldering iron? If so, are you comfortable in using it?

As previously suggested, you could swap it with another one of the transistors and see if it fixes the AS3 line.

For such an inexpensive component, I'd just replace it and see if it changes anything. If not, back to the drawing board.

[MarkL]

Quick edit.  I didn't notice there was more than one 74HCT374.  Which one is U2523?  I checked the one on the right of the pic.

That's the correct one.  The one nearest the connector.  (There's a component placement diagram in the service manual, fig 9-9, in the A16 processor board section.)

Checked pin 15 (and the others you suggested) all had an AC voltage bouncing around 1.5vac.  Also checked resistor R2536 and others near it with voltage about the same.  Is it safe to assume transistor Q2503 is the fault or is there something else I should be checking first?

I think it was worth confirming that a signal was coming out of pin 15 and getting to Q2503.  Latches can go bad, and one or more outputs can get stuck high or low.  There are some other bizarre failure modes, but it's probably not the case here.

I would go for Q2503 at this point, as others have already diagnosed.

EDIT: And once you get Q2503 out of circuit, you can re-do the diode test on it that sync described, to make sure it is the culprit.  Base to emitter voltage for a good one should be around 1.4V, but you were getting 0.3V in-circuit.  Base to collector for a good one should be around 0.7V.

[RobII]

Got the PCB out tonight.  Will get Q2503 out tomorrow.  I went ahead and placed an order for a couple of replacements.

Thanks for the tip on the component placement.  I didn't scroll down far enough in the PDF.  .  I will post updates.

Just another quick thanks for all the assistance thus far.  You all have been very patient and that has helped so much!!

[tec5c]

I want to see this bit of gear back to its fully functional state!

Thus, if the replacement tranny doesn't fix it, let us know and I'd be happy to help troubleshoot it further.