Tek 2246MODA some buttons don't work

[MarkL]

Thanks for the screenshots. They help a lot. MarkL has already analyzed them.

I checked R2536 in circuit - 1000ohms, same for the others.  I don't have an capacitance meter so I set my Fluke to ohms and checked C2523 in circuit , 463ohms.  The others (C2522, C2524, C2525 and C2526) all were reading 2.9 mohms and 3 mohms.

I think C2523 is shorted/very leaky. That would explain the 0.3V from the in circuit diode test of Q2503. A leaky C2523, which behaves like a low ohm resistor, would build a voltage divider with R2536. That would pull down the base voltage. Like in the 3rd screenshot. And Q2503 isn't switch on anymore.

I think I agree at this point.  But I've been hesitating to conclude on C2523 mainly because I don't think I've ever seen a ceramic fail like that in a low voltage circuit.

The DMM resistance readings could have gotten thrown off by a high test voltage on ohms that was turning on CR2003 with a short on the other side on the front panel board.  Or perhaps there's something else happening on the front panel board that we haven't thought of yet.  Hence, the request to re-do the scope traces with pin 4 disconnected.

The emitter would be floating at that point, so it might look pretty strange, but if the base did not return to a full 0 to 5V swing, there's really nothing else it could be except C2523.  (Barring some really strange problem with the PCB and/or traces.)

By my own logic a couple posts back it has to be C2523.  But I've already made a couple of mistakes in this thread and I wanted to be sure before popping more components off the board.

If the pin 4 test is not sounding attractive, Rob, an alternative would be to lift one side of C2523 and measure ohms on it.  I guess pick whichever one you dislike the least.  Depending on how critical C2523 is to the circuit, the scope might even work at that point.

[sync]

I think I agree at this point.  But I've been hesitating to conclude on C2523 mainly because I don't think I've ever seen a ceramic fail like that in a low voltage circuit.

I also found a faulty C2523 unlikely. For the same reason. I never saw such a ceramic cap failing. Only a few higher value MLCC SMD caps. But the measurements and waveforms matches a failed C2523 so well. Of course it could be a even more obscure fault.

If the pin 4 test is not sounding attractive, Rob, an alternative would be to lift one side of C2523 and measure ohms on it.  I guess pick whichever one you dislike the least.  Depending on how critical C2523 is to the circuit, the scope might even work at that point.

The circuit will work without C2523. If it's the culprit then the scope should be working after removing it/lift one side.

[MarkL]

The circuit will work without C2523. If it's the culprit then the scope should be working after removing it/lift one side.

And I'll add, even if the scope works after disconnecting C2523, it should still be replaced.

C2523 and R2536 (and the other RC pairs) slow down the edge for the AS lines going to the front panel, probably to reduce EMI eminating from the front panel.

[RobII]

A quick update.  Before I go lifting C2523 I wanted to isolate pin 4 from the ribbon cable and the ohm reading C2523 was the same.  Checked with the scope base of Q2503 and it has the same reading as with pin 4 inline, same for pin 6 of U2523.  I will lift out a leg of C2523 and report back.  Stay tuned and thanks again!!

[RobII]

As Prof. Farnsworth from Futurama would say, GOOD NEWS EVERYONE!!  It lives!  Lifted out a leg of C2523,



Powered on and ALL the buttons that were not working, are now working!!!!

All four channels on,



All channel buttons on,



Channel 3 (previously not working)




so to recap, I am now able to Select channel 3, select AC coupling channel 1 and 2, Scope BW, down buttons for Mode, trigger mode, source and CPLG, Slope, 5v for channel 3, Clear Display and Voltmeter.

I do plan on replacing C2523 but I want to ask an opinion on a different "issue".  After the scope has been pn for about a minute, it develops a high squeal.  Not terribly loud (to me at least) but noticeable.  I have experience working with arcade CRT chassis and that squeal usually indicates a faulty flyback.  Is it worth pursuing repairing that or just live with it considering I won't be using this scope 24/7.

I can't thank you guys enough for the guidance and patience.  Now, does anyone here know anything about troublshooting an Asteroids PCB 

[RobII]

Also, will this cap work as a replacement?

http://www.digikey.com/product-detail/en/C410C472K1R5TA7200/399-4475-1-ND/818332

[MarkL]

"Eeeexcellent!", in a Mr. Burns voice.  Congrats on the repair!

Leaky ceramics are a new one for me.

On the replacement cap, the Tektronix manual lists AVX SA101C472KAA.  Mouser carries the exact replacement, but the part you've picked out from DigiKey is the equivalent from a different manufacturer and is perfectly fine.


Squealing after a few minutes is usually from power supply components heating up, and the mechanical expansion frees pieces to move with the switching frequency.  I've had it happen with transformer windings most often, but I've also heard stories of it happening with plates inside capacitors.  Not different at all from noisy flybacks.

If it bothers you, I would take a well insulated stick and put light pressure on the main switching transformer in different places, or on other areas where you think the sound might be coming from.  It's possible the sound is coming from a location deep inside a part and there's nothing you can do about it.

I can tell you from experience that over the years high-pitched sounds will reduce in intensity (hint: I'm *not* talking about the equipment).


Asteroids was one of my favorite games.  I enjoyed the 4-2 option game.  A 6502 system shouldn't be too hard to fix.  You should start a new thread for that one and I'm sure there's a bunch more people who would dive in.  With these types of microprocessor systems (6502, 6800, Z80, etc.), it's usually handy to have a logic analyzer too.

[RobII]

Order placed from Mouser!

I have tinnitus so the squealing doesn't really bother me.  It was bothering my wife and daughter as I was troubleshooting but since I won't be using it in their presence, I'll let it go.

I'll start a new thread for my Asteroids PCB.  The game plays blind so I needed the scope so I could check the vector output etc. 

It's a good day today!!

[tec5c]

Nice to see the repair completed. Good work. 

[sync]

Congratulation! I'm glad it's working again.

Squealing after a few minutes is usually from power supply components heating up, and the mechanical expansion frees pieces to move with the switching frequency.

Just an idea about the squealing. Does the fan runs? It's not quiet so you should hear it.

[RobII]

All credit goes to you guys.  I merely followed your instructions.

@sync yes the fan does work.

[RobII]

One final update to close this out.

Old cap out.



Side by side old vs new



New cap in.



I let it run for about 30 minutes and she is still chugging along just fine.

I figured out why the case was so hard to remove, it appears to either have been dropped or had something impact one side.  The metal was pushed in by the handle and on the bottom where the plastic feet are, same deal.  Got the rubber mallet out and now the case slides on and off very easily.

Once again.  Thanks to you guys for the patience, guidance and knowledge!!

  REPAIR COMPLETED!!!