HP 1744A Storage Oscilloscope

[sentry7]

Hey guys,

Over the years, my HP scope worked perfectly fine with the exception that on power-up, it would sometimes go into STORE mode and get hung there; when I tried to switch it back into WRITE mode, it would switch itself back into STORE mode. Usually, I could just hold down WRITE for a few seconds and get it out of STORE.

Well recently, I turned it on, it got stuck into STORE mode, but my usual fix doesn't work anymore; it seems to be stuck indefinitely.

Does any one here have any experience/wisdom regarding this particular model? Maybe some insight on what might be the problem?

[tautech]

Welcome to the forum.

It is my understanding Store is accomplished with HV from the CRT circuitry to activate the special phosphors in a storage scope.
Normally voltages to do with the the CRT are 1-3 Kv with exception of the PDA that is 4-6 times higher.
Have you gear to check the 1-3 Kv range of voltages present?

Post up a link to a Service manual and we'll see who can help.
Plenty of links for manuals in the first thread in the Repair board.
Also grab the Tek scope trouble shooting pdf, plenty of good advice in it for any CRO repairs.

PS
Put a country flag in your profile

[sentry7]

I managed to find this link to the service manual. It's pretty dense from the looks of it.
http://www.ko4bb.com/manuals/24.254.167.131/HP_1744A_Oscilloscope_Operators_and_Service_Guide.PDF

I'll get on that TEK troubleshooting PDF.

EDIT: I'm afraid the scope itself is the only test gear I own besides a cheap RadioShack multimeter. Will a heavy duty multimeter do for measuring voltages of that range?

[Alex Eisenhut]

Those switches sometimes get physically stuck, they might look like they've popped back out but are still pressed in.

Press it in with your fingernail and slide off your nail to try to get it to pop back by itself.

See if that changes anything.

[sentry7]

Those switches sometimes get physically stuck, they might look like they've popped back out but are still pressed in.

Press it in with your fingernail and slide off your nail to try to get it to pop back by itself.

See if that changes anything.

I wish it were true, but the WRITE and STORE buttons are just NO pushbuttons that don't "lock" when you push them in like the power button. My intuition tells me that the write and store functions are just controlled by a flip-flop, but I could be wrong of course.

[tautech]

I managed to find this link to the service manual. It's pretty dense from the looks of it.
Will a heavy duty multimeter do for measuring voltages of that range?

It's a great manual, just what they used to do in the day.

You've got a bit of reading to do and I'd recommend you print any pages needed re storage function or you'll go nuts skipping back and forth through the manual.

At a glance only 120 V is used for storage and erase functions.
You might not even have to check that part, there are plenty of listed LV test points in the schematic.

But as is always best with repairs check the PSU is to spec first.
Ripple for a linear PSU (50/60 Hz) can be checked with a DMM on AC mV.

Don't forget it might be something real simple.....even the push button switch.
Just work you way bit by bit, you'll get there.

Take notes and post your findings. 

[oldway]

My intuition tells me that the write and store functions are just controlled by a flip-flop, but I could be wrong of course.

Yes, that's right.
See figure 8-51, page 8-35 of the service manual. (service sheet 16)
Check all the signals at: U4A, U7D, U7B, U10C, U9C and U2B.

[Alex Eisenhut]

Those switches sometimes get physically stuck, they might look like they've popped back out but are still pressed in.

Press it in with your fingernail and slide off your nail to try to get it to pop back by itself.

See if that changes anything.

I wish it were true, but the WRITE and STORE buttons are just NO pushbuttons that don't "lock" when you push them in like the power button. My intuition tells me that the write and store functions are just controlled by a flip-flop, but I could be wrong of course.

Ah I see. I gave away my 1741 years ago but I remember some buttons needed that extra kick. HP used those switches a lot, my 5316A needs that extra kick on some switches.

In a way, you're better off because the fix is probably electrical. Trying to repair that little bit of metal that rides in a groove can't be fun.

[sentry7]

Here's an update, guys.

I poked around with the multimeter early today, and as I did, the scope went back into write mode. So while the problem isn't fixed, I can at least recreate the problem which will give me better changes of being able to exactly pinpoint the bad component(s).

I don't know if you guys prefer the 1 problem per thread format, but there was one other minor problem.

I have a 1V external test signal going in and I have the vertical gain set to 0.2V/div. On the screen, the signal expands to one half a major division resulting in a measured voltage of 0.1V. Could there be a fault in the vertical deflection system or master vertical gain?

[VK5RC]

Does the scaling problem exist at other gain settings? What are the output / input impedances,  is there probe 10x auto correction?

[sentry7]

Does the scaling problem exist at other gain settings? What are the output / input impedances,  is there probe 10x auto correction?

The problem does persist across all the settings. There is no 10x probe compensation, and the input/output impedance is 1M shunted by a 10pF capacitance.

[tautech]

I don't know if you guys prefer the 1 problem per thread format, but there was one other minor problem.

Of course not.
Your thread title is not specific.

Is the scaling out on both channels?

[sentry7]

Is the scaling out on both channels?

Yes. Both channels are off by a factor of 0.1.

[tautech]

Is the scaling out on both channels?

Yes. Both channels are off by a factor of 0.1.

Without another scope you are forced to fault find by other methods.
Luckily HP have DC readings in the schematic.
The scope must be set up in a specified manner for these to be relevant.....RTFM
~ 90% through the manual is the schematic you need: P139 SS5 Fig 8-19.
IC A341 migh be your culprit or just a series resitor open somewhere.
Don't forget to re-socket any IC's or transistors on case of a oxidised pin/contact.
Could even be a bad inter-connect link or cable.

Fingers x'ed