HP 1727A oscilloscope with multiple issues.

[Alex_Baker]

OK, so it seems that the gate output is intermittent, most of the time when I was testing it today it was just sitting at 4V on the scope. The two input signals comp gate and delayed gate are fine, so its being lost somewhere on A13. I am not very good at visualizing circuits so I don't know how the signal path works, and it is very hard to get a scope probe onto most of A13 to trace things.

[cosenmarco]

I don't think there is a problem is the LVPS.

I don't think either. Perhaps I wasn't clear enough: I think the LVPS is going into protection mode after the "snap" event but the cause is somewhere. Perhaps the LVPS going in protection mode causes the "snap" you hear. Sometime capacitors especially tantalums fail intermittently and unpredictably and becoming basically a quite low resistor, like 50Ohm, which trips overcurrent protection in the power supply making it hard to detect because they don't continue cooking off and releasing smoke.

One of the output transistors gets pretty hot while the other does not.

there you go... the one which is not getting hot is the suspicious one, but check all four Q13-16, the resistors and caps as well once the transistors are out of circuit.

I am not very good at visualizing circuits so I don't know how the signal path works

reverse engineering is a crucial skill but before you get efficient at it you must learn forward engineering and the basic building blocks. It'll come with time and it's a muscle you have to excercise. I suggest you watch as many "back to the basics" videos (also the other are worth) as possible from w2aew and Mateo Aboy if you want to learn the basic building blocks of circuits and how they couple together. Taking a  proper course would also help 

In the meantime, just ask... we can go through a section you don't understand. I can't promise accurate analysis: I'm just a hobbist but I have a degree in engineering and I learned some theory some years ago.

it is very hard to get a scope probe onto most of A13 to trace things.

That can be hard. Take your time to set up probing eg. solder a lead wire to make a signal accessible. Probing and measuring is the only way to understand what's going on so don't dismiss it as a quick and easy thing to do. Also record the results, maybe take screenshots of the measurements, and take notes / record yourself (may seem silly but I do it all the time).

[Alex_Baker]

All the transistors I checked on A13 have been fine.  I checked Q13-16 and several others but found no bad ones. I also did a quick test with my multimeter on all the other transistors but none had less than several Kohms between their pins, so I don't think there are any more bad ones.

I would like to do some more testing today but I am quite busy, its my birthday, 

[Alex_Baker]

I am now lucky enough to own a Flir thermal camera, I have wanted one for a while as a troubleshooting tool  . Needless to say I pointed it at my scope and especially A13, but it didn't really show me much that I didn't already know. Q15 gets hot, as well as a couple of resistors next to it, but that is about it.

Something on A13 is unhappy, I just need to figure out what!

[Alex_Baker]

I have new information! I got back to probing after a short break from this scope. I found that the gate signal on TP1 is fine (although half the amplitude it says in the manual) as long as the HV oscillator transistor is NOT plugged in.  When I do plug it in the gate signal stays there for about 4 seconds, and then fades down to constant low level in about 2 seconds. 

So does this mean that the problem could be on the HVPS? of course I wont discount other issues on A13, like the output amplitude.

I might see if there is a way I can measure the gate output current.

[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... 

[cosenmarco]

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 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.

I just don't know where to look

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

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. 

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).

[Alex_Baker]

That's how I feel with my scope, everything I have checked or tested is fine, and if its not I cannot find the cause.

I will keep the scope in storage indefinitely, given its rarity and the history mine has, but until then I will work on different projects. I just finished building a cloud chamber (look it up), it's pretty cool, the hard part is finding something good to put inside it!