Electronics > Repair

HP 1727A oscilloscope with multiple issues.

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cosenmarco:
Hi Alex,
I have a pretty bad scan here of the HVPS (clearly no quality control over those scans at HP... not worth the money >:( ) as you can see in the attached pic.
If you can please share a pic where things are more readable I can try to figure out what may cause the problems

The 4 seconds you mentioned seem to me compatible with the time constant of the little circuit which activates the optocoupler driven by Q4 so I suppose what you see is the soft start on the cathode bias.

Alex_Baker:
Here is a scan I took, I hope it is better.

I measured the current passing through the gate output to the HVPS and it was on the order of microamps, I think the problem is still on A13. When I probe the base of Q13 and Q14 with my scope I have to set it to AC coupling and 50mV/div to even see a small noisy signal, so I think the gate amplifier is not being driven properly still. I just don't know where to look, Let me know what you think.

I am kind of running out of steam on this repair project...  :P

cosenmarco:

--- Quote ---When I probe the base of Q13 and Q14 with my scope I have to set it to AC coupling and 50mV/div to even see a small noisy signal, so I think the gate amplifier is not being driven properly still.
--- End quote ---
I think you have to narrow things down. You have to establish if the single parts of the circuit work in isolation as expected. For that you need to power up the circuit separately, provide inputs and measure single sections of the circuit. And you have to understand what the various parts do in isolation (which is hard!!) The Service sheet gives you a lot of information about what the various parts should do. Learn it by heart. Seek for understanding how those different parts do what they are supposed to do and test them.


--- Quote ---I just don't know where to look
--- End quote ---

--- Quote ---I am kind of running out of steam on this repair project...
--- End quote ---
I totally understand. My first serious repair project was and audio amp which I had in mind to repair since 15 years. I knew roughly which components were broken (some unobtanium diodes) and I did butcher it when I was a teenager but then picking it up at 33 I was able to understand it a bit better, then I started digging into it and after breaking it a couple of times, re-fixing it and reading a book about the design of audio amplifiers I was able to understand it completely and also pick up replacement parts for the diodes. The thing works: I listen music on it every day.

Here you got yourself a very hard project to start with!! I know you want to see it working ASAP but it wouldn't be bad if you took a break from it and started getting some smaller easier projects and learn the theory behind. The scope can wait if you have some storage room.

Alex_Baker:
When I bought this scope I was looking for some simple old CRO that I could use in parallels with mu USB scope. I did not know that what I actually was getting was a rare and very powerful scope, which is not a bad thing in and of itself, but it is also complex and hard to use on some accounts, not to mention that is is broken.  |O

I have been looking for small repair projects. I have done a number of vacuum tube radio repairs/referbs recently as well as some amplifier stuff, you may have seen the topic I started about the Marantz 4070, I enjoy working on those older solid state amps.

As for the future of the 1727, I think I need to give it a break. I don't know how long it will be, it could be a couple of months or it could be years down my electrical engineering education/career path.

Sorry to disappoint everybody who was hoping for a solution to this scopes issues, I just don't have the time or expertise to tackle a project like this right now. I suppose I will just let this topic sink into the depths of the forum for the time being, but I will leave notifications on incase somebody replies.

Thank you all,

Alex

0culus:
Don't give up, take a long break and come back to it. I'm currently doing battle with a recalcitrant Tek 454 (got it for free with nasty arcing in the HV...solved that problem but it has more) that I've put down for a week or so. Symptom is low horizontal gain on the A sweep with no obvious cause (and I've been through it with a fine tooth comb).

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