Tek 2246MODA some buttons don't work

[tec5c]

So 4 = 0.54v is the AS3 line?

[RobII]

Correct, the one you initially pointed me to check.

[xwarp]

Ok, from bottom to top as you have it, and from bottom to top, the transistors are Q2506 to Q2501. Measuring the DC voltage on the square pads, which is the emitter,

Q2501 = 1.5
Q2502 = 1.28
Q2503 = 1.35
Q2504 = .788
Q2505 = .885
Q2506 = 1.34

Now, if you hold the positive lead on the emitter, (square pad), of Q2503, and then press AND HOLD the Channel 1 button, under the vertical mode, you should see the meter drop to .88 vdc.

[xwarp]

One more thing, all of the collectors should have 5 vdc on them, did you check that?

[RobII]

I'm a little confused.  Q2503 is the AS3 line and I am only reading .540v on that.  I did check the 5v, all are good.

Ok, from bottom to top as you have it, and from bottom to top, the transistors are Q2506 to Q2501. Measuring the DC voltage on the square pads, which is the emitter,

Q2501 = 1.5
Q2502 = 1.28
Q2503 = 1.35
Q2504 = .788
Q2505 = .885
Q2506 = 1.34

Now, if you hold the positive lead on the emitter, (square pad), of Q2503, and then press AND HOLD the Channel 1 button, under the vertical mode, you should see the meter drop to .88 vdc.

[xwarp]

Do me a favor, the readings you took, renumber them to the corresponding transistors that they are for.

Q2501 is closest to the chassis.

Nevermind that....lol....

Something on that line is holding that value down.

It could be that the pads on the switch panel need to be cleaned.

The emitter is pulled from 1.3 vdc down to ~0.8 vdc when Channel 1 mode button is pressed.

[RobII]

So I am not confusing everyone else with my wonderful way of labeling.  Based on what you are saying, here are my voltages by the transistor #

Q2506 = 1.37
Q2505 = .775
Q2504 = .960
Q2503 (AS3) = .540
Q2502 = 1.60
Q2501 = 1.64

[RobII]

Got it.  Check my previous post.  I should have done it that way from the start.

Do me a favor, the readings you took, renumber them to the corresponding transistors that they are for.

Q2501 is closest to the chassis.

[xwarp]

If you decide to pull that panel off and clean it, BE VERY CAREFUL with what you use to clean the carbon traces.

Just distilled water should be good, maybe some diluted windex.

I know that I had issues with a TDS 460 scope and some Fluke 8840's that use carbon for the circuit traces under the button pads.

[RobII]

Will do.  So are you thinking it's not a component (transistor) failure?

If you decide to pull that panel off and clean it, BE VERY CAREFUL with what you use to clean the carbon traces.

Just distilled water should be good, maybe some diluted windex.

I know that I had issues with a TDS 460 scope and some Fluke 8840's that use carbon for the circuit traces under the button pads.

[RobII]

LOL, I just saw your edited post.  So how do you get the switch panel off?  I removed the Pot panel but couldn't figure out how to get the switch panel off.

If you decide to pull that panel off and clean it, BE VERY CAREFUL with what you use to clean the carbon traces.

Just distilled water should be good, maybe some diluted windex.

I know that I had issues with a TDS 460 scope and some Fluke 8840's that use carbon for the circuit traces under the button pads.

[xwarp]

It could be, but based on how dirty that panel is, I'd look at pulling it and cleaning it first.

If you can be careful about it, you could pull that ribbon cable, which is J/P 2501, very carefully bend the number pin out of the way and reinsert the cable.

Then, measure the voltage on the emitter of Q2503. If it reads normal, then your problem is in the panel.

[xwarp]

The front panel looks like it is attached by those black plastic pull pins. You may have to pull the front bezel off before that though.

I've already put my 2246a back together, but can check it tomorrow.

[RobII]

I'll give it a go.  Thanks!!!  I appreciate everyone's assistance.

It could be, but based on how dirty that panel is, I'd look at pulling it and cleaning it first.

If you can be careful about it, you could pull that ribbon cable, which is J/P 2501, very carefully bend the number pin out of the way and reinsert the cable.

Then, measure the voltage on the emitter of Q2503. If it reads normal, then your problem is in the panel.

[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.

I would replace Q2503. The availability of MPS6725 is bad. I had a quick look at Digikey and a ZTX603 should work. But there are other possible replacements. Where did you buy your components?

[RobII]

Thank you for the reply.  I was curious as well about that.  The reading was so far off I was hoping that might be part of the problem.

As for where I buy components from, I looked at Mouser.com briefly last night and same as Digikey, obsolete.  I do see some MPS6725 on Ebay listed as new. 

http://www.ebay.com/itm/MPS6725-Motorola-NPN-Darlington-Transitor-50V-1A-TO92-Lot-of-10-/321640691303?hash=item4ae3477667:g:AWsAAOSwzrxUtD7H
or
http://www.ebay.com/itm/MOTOROLA-MPS6725RLRP-One-Watt-Darlington-Transistor-NPN-Silicon-NEW-9-PKG-/190797809949?hash=item2c6c6f891d:m:mfM1fpHFP_GCHSA0t5k9WkQ

Thoughts on those?

[xwarp]

Thank you for the reply.  I was curious as well about that.  The reading was so far off I was hoping that might be part of the problem.

As for where I buy components from, I looked at Mouser.com briefly last night and same as Digikey, obsolete.  I do see some MPS6725 on Ebay listed as new. 

http://www.ebay.com/itm/MPS6725-Motorola-NPN-Darlington-Transitor-50V-1A-TO92-Lot-of-10-/321640691303?hash=item4ae3477667:g:AWsAAOSwzrxUtD7H
or
http://www.ebay.com/itm/MOTOROLA-MPS6725RLRP-One-Watt-Darlington-Transistor-NPN-Silicon-NEW-9-PKG-/190797809949?hash=item2c6c6f891d:m:mfM1fpHFP_GCHSA0t5k9WkQ

Thoughts on those?

As I suggested, before you run off and start easter egging shit, pull that ribbon cable and lift up pin #4 very carefully and reinsert the cable. Then check the voltage on the emitter. At least eliminate the possibility of a stuck button, because guess what!

I turned my scope on and held down one of the buttons that you had circled in your picture, and, guess what, same button issues.

[RobII]

Trust me, I am not going to do any sort of desoldering etc before I have all input and have tested what has been suggested.  Not ignoring your suggestion at all 

As I suggested, before you run off and start easter egging shit, pull that ribbon cable and lift up pin #4 very carefully and reinsert the cable. Then check the voltage on the emitter. At least eliminate the possibility of a stuck button, because guess what!

I turned my scope on and held down one of the buttons that you had circled in your picture, and, guess what, same button issues.

[xwarp]

I must going blind because I can't find "AS3" signal on the A16 board.

Long night at work, time for bed.

[sync]

Thank you for the reply.  I was curious as well about that.  The reading was so far off I was hoping that might be part of the problem.

As for where I buy components from, I looked at Mouser.com briefly last night and same as Digikey, obsolete.  I do see some MPS6725 on Ebay listed as new. 

http://www.ebay.com/itm/MPS6725-Motorola-NPN-Darlington-Transitor-50V-1A-TO92-Lot-of-10-/321640691303?hash=item4ae3477667:g:AWsAAOSwzrxUtD7H
or
http://www.ebay.com/itm/MOTOROLA-MPS6725RLRP-One-Watt-Darlington-Transistor-NPN-Silicon-NEW-9-PKG-/190797809949?hash=item2c6c6f891d:m:mfM1fpHFP_GCHSA0t5k9WkQ

Thoughts on those?

Normally I would not buy components on Ebay. If your are lucky you get NOS. But there are also fakes. It doesn't have to be a MPS6725. There are a lot of transistors which will work fine in this circuit.

[sync]

As I suggested, before you run off and start easter egging shit, pull that ribbon cable and lift up pin #4 very carefully and reinsert the cable. Then check the voltage on the emitter. At least eliminate the possibility of a stuck button, because guess what!

I turned my scope on and held down one of the buttons that you had circled in your picture, and, guess what, same button issues.

Even with a stuck button the LEDs on the AS3 line will work. None of the trigger mode LEDs is lit up on RobII's photo.

[sync]

Trust me, I am not going to do any sort of desoldering etc before I have all input and have tested what has been suggested.  Not ignoring your suggestion at all 

If you want to do some measurements then use an oscilloscope. I'm guessing your 2246 will work enough to measure itself. Remove the ground lead from the probe so it doesn't short something accidentally. You don't need the probe ground lead because you are measuring the scope itself. Set the channel to 2V/div, DC coupling and the time base to 2ms/div. Probe the emitters of Q2503 (AS3) and of the other transistors for comparison. This is how the emitter of Q2503 looks on my 2246A. I used a DSO for easier screenshots. It looked the same on my 2246A measuring itself.

[RobII]

Ah, yes.  Understood.  Thanks.

Normally I would not buy components on Ebay. If your are lucky you get NOS. But there are also fakes. It doesn't have to be a MPS6725. There are a lot of transistors which will work fine in this circuit.

[RobII]

I will certainly give it a go this evening.

Thank you for your help.

If you want to do some measurements then use an oscilloscope. I'm guessing your 2246 will work enough to measure itself. Remove the ground lead from the probe so it doesn't short something accidentally. You don't need the probe ground lead because you are measuring the scope itself. Set the channel to 2V/div, DC coupling and the time base to 2ms/div. Probe the emitters of Q2503 (AS3) and of the other transistors for comparison. This is how the emitter of Q2503 looks on my 2246A. I used a DSO for easier screenshots. It looked the same on my 2246A measuring itself.

[xwarp]

Not quite accurate:

As I suggested, before you run off and start easter egging shit, pull that ribbon cable and lift up pin #4 very carefully and reinsert the cable. Then check the voltage on the emitter. At least eliminate the possibility of a stuck button, because guess what!

I turned my scope on and held down one of the buttons that you had circled in your picture, and, guess what, same button issues.

Even with a stuck button the LEDs on the AS3 line will work. None of the trigger mode LEDs is lit up on RobII's photo.