Tektronix 2465b repair / MPU IRQ issue?

[Nikan]

Hi there,

I have just bought a Tektronix 2465b scope from ebay, which turned out to be faulty 

So I took it apart checked all the voltages and had some problems. I repaired the psu and all the voltages/ripple are within the limits now.

Then I found out that I had the common problem with the A5 board (leaking caps), which eched some traces. I repaired those traces, and replaced the caps with some ordinary radial caps for now. I also had a problem with the DAC312 and replaced that with a genuine AD DAC312.

So far so good.
I turned the scope on and it seems to do some checks maybe gets through and stops on CH4. So I followed the repair procedure and came to the point where it says to turn the grid bias after turning the bias voltage up I get a trace but this trace/text is super washed out and jerks somewhat around. I'll attach a picture to that issue.

I think the text is saying press trigger to exit with an error message and some question marks but I'm not sure cuz I can't read it!
So if that's right the dellas chip is probably dead as well.

Also I think there is an issue with the MPU MC68B02P (MC6802) correct me if I'm wrong! The IRQ pin goes low every 3.3ms as it should but the low time varies prettey badly or is this a mistake of mine (measuring)? Also If I follow A0->A15 it seems to have the same signal but the frequency goes up with each pin. I'll attach some pictures of that as well

Hope it'll work properly one day I won't give up on it.

If there is anything else you need to know don't hesitate to ask!

For now I would like to get the washed/jerking picture fixed so I can see something on the screen. BTW if I move some jumpers I can get a super clear image which shows the dellas error so I guess I was correct with that statement... 

I am thankful for any advice, I am a newbie. I still would like to fix it cuz it is indeed a really cool scope!

Thanks in advance and best regards from Germany.

[Nikan]

Washed/jerking display

[Nikan]

I was not able to upload the other Pictures due to some weird file check issues...
So here they are

https://ibb.co/3Mn0bsv
https://ibb.co/3FLqSYJ
https://ibb.co/jhTQJYT
https://ibb.co/44xm2m2
https://ibb.co/pdhTHcr

[jonpaul]

Please see the very long thread "2465B teardown" and search

SMD caps leakage

and

NVRAM

1. Damage of leaky caps can etch traces on both side of the PCB and damage many components especiall in the DAC area. Test every part in the area of the SMD caps.

2. NVRAM has possibly lost it cal parameters, and needs replacement.

3. Proceedure:

Remove damaged parts, clean all coorsive cap leaks, repair all damaged traces
Remove NVRAM, place low profle IC socket
Put new Dallas NVRAM,
Do complete CALTakes a few hrs if experienced and have good desolder SMD equipment, and TEK CAL system.

Expect a few days (or weeks) if your experience has the learning curve or you do not have access to TEK CAL equipment.

Chuck Harris is the local Maven on these, also see the groups.io Tektronix group.

Bon chance,

Jon

[Nikan]

Seems like you didn't even read my post.
I have already repaired those traces and replaced the dac, caps and I know that the NVRAM is dead.
All things I know or already repaired... 
But thanks anyways I guess.

[BravoV]

I am thankful for any advice, I am a newbie.

Seems like you didn't even read my post.

All things I know or already repaired... 

... and the question is ?

Jonpaul is just trying to be helpful since you asked for advice yourself. 

[Nikan]

I know that he wanted to hep. But he has listed all the issues I already wrote and know about and those I have repaired.
Pointing out issues which have been solved is not going to help and is also not an advice at least not for me. His reply felt like he saw A5 board Caps... And then he listed all common issues.

It would be helpful to know if the IRQ pin of the MPU looks right... it is indeed going low every 3.3mS but I don't know if it is correct that it goes low for between 1uS and 10uS. Or if I am measured that incorrectly.

[SoundTech-LG]

Good job you repaired all that in/around the DAC ckt, but guess what?
You're still not done. The sneaky corrosion ruins resistors, and other components, not visibly apparent. If you do not remove and test them, particularly in the bridge adjustable pot ckt., you still have issues to resolve. I think there is still more work to do.

[jonpaul]

Hi again, Sorry I did not read all the notes....


my main point was just to suggest a read of the 2465B teardown thread, that the PCB corrosion can be very sneaky, and to give you my experience.


Enjoy, and bon chance!

Jon

[Nikan]

Yes exactly, you are right. The pot and around the pot were defektive resistors and some smd bypass caps as well. But I have testet all and replaced them already. I think I am missing something else...

[Nikan]

Yes right, it is indeed really sneeky! I have already read that post... But I think I have another problem. I think that corrosive juice shorted some IC's. Because I have already tested and changed all defective resistors and caps which were near that corrosive liquid.