Then I used a 2K resister from /CONT to gnd to dissipate the voltage and it came close to 0V. The buttons came response which is great but after powering off it doesnt remember the settings. So I got ahead of myself and ordered 2 of these nand IC's in so i have a spare in case it happens again.

That's great, Sounds like that gate is definitely the culprit and with a little luck, changing IC8 will give you a working 'scope. In my experience, equipment that uses discrete components and basic logic gates is fairly easy to get working. Its the later stuff, with programmable bits you don't have visibility of that are difficult to fix. Mind you, 95% of faults on modern day kit like phones, iPads, etc are failed multi-layer ceramic capacitors (MLCC) shorting across one or other supply rail. Very easy to find with voltage injection and a thermal camera.

 

That's great, Sounds like that gate is definitely the culprit and with a little luck, changing IC8 will give you a working 'scope. In my experience, equipment that uses discrete components and basic logic gates is fairly easy to get working. Its the later stuff, with programmable bits you don't have visibility of that are difficult to fix. Mind you, 95% of faults on modern day kit like phones, iPads, etc are failed multi-layer ceramic capacitors (MLCC) shorting across one or other supply rail. Very easy to find with voltage injection and a thermal camera.

You are the man! BZ

 

Anyway, more importantly, what is the now disconnected pin 3 of IC8 actually doing? is it actually pulling down when the input (pins 1 & 2) are high? Check it with a pull-up to VDD of around 10k. If that's not getting down to 0.5v or so, IC8 pin 3 is shot.

I just conducted this voltage divider test:
- Without the 10k resister connected from Pin3 to VDD voltage is 4.37V on pin 3 leg
- With the 10k resister connected from Pin3 to VDD voltage is 4.37V on pin 3 leg

If it the pin 3 was outputting correctly to my understanding it should be a low impedance path to ground. (as the 2 inputs are high)
However, since I measured 4.37V at pin with 10k resister connected on DMM probing Pin 3 to ground it would suggest:
a: a very high impedance path to ground at pin 3
b: a low impedance path to VDD at pin 3
(please see my LTSpice simulation)

In either case there is a fault in IC8-3 but would would i determine if the fault is a or b in the scenarios above

That's great, Sounds like that gate is definitely the culprit and with a little luck, changing IC8 will give you a working 'scope. In my experience, equipment that uses discrete components and basic logic gates is fairly easy to get working. Its the later stuff, with programmable bits you don't have visibility of that are difficult to fix. Mind you, 95% of faults on modern day kit like phones, iPads, etc are failed multi-layer ceramic capacitors (MLCC) shorting across one or other supply rail. Very easy to find with voltage injection and a thermal camera.

Fantastic job analyzing this one Irving. Well done! One of of my broken NVR's I'll try this MLCC detection with voltage injection and thermal camera. That's sounds awesome. I'll have to ad a thermal camera to my toolkit. By the way I completely understand due to the programable bit and proprietry hard to source IC type of problems with later devices. I was supprised by the project how cool the use of IC's was back then. It simplifies the transister to transister logic but still allows you to repair the logic by replacing common IC's. Rather than having IC for specific task which makes almost impossible to source, repair and diagnose

 

In either case there is a fault in IC8-3 but would would i determine if the fault is a or b in the scenarios above

Was that with power on or off? I'm guessing ON so VDD would be 1 diode drop from +5.2Va or about 4.4v (assuming the 4.95v from previous measurements). On that basis it looks like an internal failure of IC8a, so that the output state is always '1'. Whether thats the output stage, or something else, is impossible to say but makes no difference, it's faulty & has to go. Sometimes it's important to analyse failures to avoid the problem reoccurring but given that chip is 44 years old & it's had a good life...

 

Was that with power on or off? I'm guessing ON so VDD would be 1 diode drop from +5.2Va or about 4.4v (assuming the 4.95v from previous measurements). On that basis it looks like an internal failure of IC8a, so that the output state is always '1'. Whether thats the output stage, or something else, is impossible to say but makes no difference, it's faulty & has to go. Sometimes it's important to analyse failures to avoid the problem reoccurring but given that chip is 44 years old & it's had a good life...

Yep, those measurements were taken when powered on. Yes the chip is no good. I've ordered 2 waiting for them to arrive. I'll update you when I have chip installed. I hope it works

 

Hi guys. I'm to confirm the new nand cmos ic, ic8 on the horisontal control mode unit, has fixed the non-responsive buttons and the buttons selections are now being remember after powering off!
Upon putting in the socket for the 14 pin IC I did have alot of trouble getting the solder out of the holes so I could mount the socket and another problem I had was the top side of the board had pads that I couldnt solder onto as the socket plastic was in the way so I ended up having to router new traces on the back of the board. But it works so I am happy.

I big thank you to everyone involved! The 1st place prize medal goes to Irving for identifying /CONT was the likely culprit early on. Well done!

 

Irving is an expert. Very well done. Enjoy your scope. It should be good for long time but will likely have you wanting a modern DSO after you've used it for a while. Start saving.

 

Irving is an expert. Very well done. Enjoy your scope. It should be good for long time but will likely have you wanting a modern DSO after you've used it for a while. Start saving.

Thanks mate. I appreciate the time you spent helping me with the diagnostics on this.

 

I big thank you to everyone involved! The 1st place prize medal goes to Irving for identifying /CONT was the likely culprit early on. Well done!

Thanks! Fixing old kit that still has a use is fun and I enjoy the intellectual challenge. At least we had a service manual with complete schematics etc to work with.. So often with newer kit there's nothing available.

Enjoy using it! If its not a rude question, how much did you pay for it?

I bought an HP54601A 4 channel digital 'scope of a similar vintage on eBay back in 2019 for 137GBP. It worked perfectly for about 6 months and then one day it wouldn't turn on. Fortunately the service manual for it was available and it only took me a day to figure out a simple power supply fault, which cost virtually nothing to fix! I soon came to realize the limitations of a DSO with only a tiny 16kBytes memory so eventually splashed out of a 4 channel Siglent sds1104X-E with 24Mbytes of memory. But the HP is still in use though - sometimes you just need 2 'scopes!

 

Thanks! Fixing old kit that still has a use is fun and I enjoy the intellectual challenge. At least we had a service manual with complete schematics etc to work with.. So often with newer kit there's nothing available.

Enjoy using it! If its not a rude question, how much did you pay for it?

I bought an HP54601A 4 channel digital 'scope of a similar vintage on eBay back in 2019 for 137GBP. It worked perfectly for about 6 months and then one day it wouldn't turn on. Fortunately the service manual for it was available and it only took me a day to figure out a simple power supply fault, which cost virtually nothing to fix! I soon came to realize the limitations of a DSO with only a tiny 16kBytes memory so eventually splashed out of a 4 channel Siglent sds1104X-E with 24Mbytes of memory. But the HP is still in use though - sometimes you just need 2 'scopes!

Hi mate. No it's no problem. That particular scope was $30 because thats all the seller wanted for it. It had that issue we diagnosed and he didnt provide probes or anything and its not in the best nick but it didnt bother me. I am so amazed by the process you were able to shed light on and really enjoyed the experience. Even know it was quiet alot of labor. Thanks for that!

That HP54601A you have looks awesome. I would love one of those one day. By the way I convinced my dad to buy a Siglent SDS1104X-U (the same as yours but does not have signal analyzer input) about 2-3 years ago for diagnosing a problem at work. In hindsight I wish I had bout the -E variant as it is more useful for analyzing digital circuits.

By the way, I enjoyed this experience so much that I went out and bought another scope for AUD$100. It is a very similar scope but is 3 years older, 1987. The scope has a problem not so much with the digital circuits, but something to do with perhaps a transistor short circuiting. I would love to know what you think if you have the time or feel up for a challenge. I'll post up a few pictures with description of problem anyway as I love learning about the repair process and how crt oscilloscopes work. They really fascinate me

 

By the way, I enjoyed this experience so much that I went out and bought another scope for AUD$100. It is a very similar scope but is 3 years older, 1987. The scope has a problem not so much with the digital circuits, but something to do with perhaps a transistor short circuiting. I would love to know what you think if you have the time or feel up for a challenge. I'll post up a few pictures with description of problem anyway as I love learning about the repair process and how crt oscilloscopes work. They really fascinate me

Glutton for punishment! But that $30 'scope now working must be worth $60 now

Go for it... have you found the service manual yet?

 

Go for it... have you found the service manual yet?

Yep, luckily it has a service manual as I've bought expensive stuff before that has no manual and it makes the repair all the more harder. I just created the post here, I would love to hear what ideas you have and what other members thingk could be the issue with it

 

Hello,

I often use jabse to find the manuals:
https://www.jabse.com/

Bertus