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Old 29th Jan 2020, 1:16 pm   #1
mole42uk
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Default Gould 4074 display problem

I've finaly had time to put the Gould 4074 DSO on the bench, only to find that the display is a little awry. Preliminary investigion suggests that voltages are okay and there's a believable signal on the plate outputs.

Has anyone got an idea of what I check next - the deflection amps are all SMD and I don't wish to replace many more parts than I have to!
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Old 29th Jan 2020, 2:09 pm   #2
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Default Re: Gould 4074 display problem

Hi!

Do you have a 4074 Service Manual?

I can help with extracts from my copy if need be!

This fault is caused by the right–hand X2 plate not going positive enough to deflect the beam towards the right hand edge of the screen!

I'll blow the dust off my copy tonight and come back with more fault–finding advice later!

Chris Williams
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Old 29th Jan 2020, 3:18 pm   #3
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Default Re: Gould 4074 display problem

Thanks Chris, I do have the full service manual. I have already sorted out quite a few problems with this instrument, mainly the PSU which is an interesting design.
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Old 1st Feb 2020, 4:58 pm   #4
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Default Re: Gould 4074 display problem

Are you sure the signals on the horizontal plates are ok?
They have to go differentially from 0V to 2xxV against ground potential.

If you are really sure that the electrical signals are ok the crt must be damaged. I don´t think that occurs very often but it´s possible...

It´s possible that I have a spare part...
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Old 1st Feb 2020, 5:03 pm   #5
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Default Re: Gould 4074 display problem

I checked that the plates were swinging between the 260V and 0V, but it was actually the 12V supply that was at fault. A tantalum capacitor had gone s/c and shorted out the 12V regulator in the power supply with the result that the 12V was actually 17V :-(
That is now fixed and I'm getting a useable display again although the output from the A/D boards is very noisy and the scan across the screen is non-linear so that a sine wave appears wider at the RH side than the LH side. I'm pretty sure it's going to be power supply problems. I might rebuild my spare supply and see if that works any better.
I also have a spare CRT!
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Old 1st Feb 2020, 5:19 pm   #6
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Default Re: Gould 4074 display problem

17V on 12V-supply sounds bad.
Good to hear that you are making progress.
Perhaps the DAC on the display card is buggy... But that´s only a guess. You will find the solution!

Seems with your 4074 you have pulled a big project ashore.
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Old 1st Feb 2020, 9:43 pm   #7
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Default Re: Gould 4074 display problem

I really dislike this Gould/Advance switch mode power supply. They really should have built a nice, reliable, linear one. It is quite possible to get this one working but whether it’s possible to get it working reliably is another matter!
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Old 1st Feb 2020, 10:17 pm   #8
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Default Re: Gould 4074 display problem

Indeed it´s a very "interesting" design.

Very interesting to look at but I´m sure it´s a pain in the ass to repair!
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Old 2nd Feb 2020, 8:41 am   #9
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Default Re: Gould 4074 display problem

I think I’m going to disconnect the A/D card outputs and just deal with the digital until I get the linearity correct, that should be possible.
I’m also going to go on a hunt for failed tantalum capacitors, the one that failed caused the 10Ω protection resistor to burn (at the top of a plug-in board) and so was easy to spot by eye. There may be others that are not so easy…
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Old 2nd Feb 2020, 10:40 am   #10
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Default Re: Gould 4074 display problem

Sounds like a good method.

Strangely enough till now I had no problems with burned tantalum capacitors... I was lucky so far.
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Old 2nd Feb 2020, 3:30 pm   #11
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Default Re: Gould 4074 display problem

For some reason, on this power supply, Gould had decided to use miniature resistors where 1/4W are specified. One of the volktage setting resistors was fractured, so the 12V wasn't stable. Therefore the display wasn't stable either. Change the resistor(s) and the 4074 is behaving better than ever.
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Old 2nd Feb 2020, 4:37 pm   #12
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Default Re: Gould 4074 display problem

Interesting...
In my power supplys there are normal cheap 1/4W-THT-resistors.
What kind of miniature resistor did you have?
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Old 2nd Feb 2020, 6:10 pm   #13
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Default Re: Gould 4074 display problem

Yes, one of my supply boards has normal 1/4W, but the other is populated with some very small, I'd say 1/8W resistors that are fragile. If I remember, I'll photograph them...
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Old 8th Feb 2020, 1:49 pm   #14
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Default Re: Gould 4074 display problem

Here's an update and a couple of pictures:

I mentioned that this oscilloscope uses some very small resistors which seem quite fragile. The one in the power supply had simply cracked and the intermittent connection gave all sorts of noise on the +12V rail. The first picture shows those tiny resistors alongside some 'normal' 1/4W ones

I also mentioned that some tantalum bead capacitors had exploded. The second picture here of the result when that happens

The power supply is now stable and the processor control system works. I do need to persuade the four inputs to provide a trace from each!
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Old 8th Feb 2020, 2:50 pm   #15
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Default Re: Gould 4074 display problem

Thanks for the pictures!

That´s really bad…
Small cost savings big problems later.

Good luck!
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Old 15th Feb 2020, 4:14 pm   #16
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Default Re: Gould 4074 display problem

Further on, further on....

Now I have a nice stable power supply with the correct voltages measured at the correct test points. I can sometimes see a representation of the applied signal on the screen but it isn't useable.

What I don't have is the ability to control the position of the traces - they are always off the top of the screen. The digital display corresponds to the setting of the input attenuator but the trace is either off the top or the incoming sine is distorted as though the negative side of the amplifier is struggling. In other words, the incoming sine is displayed with a flattened bottom. I suspect another tantalum has joined the choir invisible and possibly taken something with it, but the troubleshooting guide in the service manual is mute on this point.

I will get this DSO working reliably.
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Old 25th Feb 2020, 3:47 pm   #17
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Default Re: Gould 4074 display problem

A problem with ALL the traces makes it unlikely that the same fault is occuring in all the input analogue amplifiers unless something common (like the PSU) is faulty.

Does this scope use a single ADC, with multiplexing for the channels ?

That would be my guess and a problem in the multiplexer or ADC.
If it has multiple ADC's the it is nore likely to have a PSU problem.

dc
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Old 29th Feb 2020, 1:44 pm   #18
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Default Re: Gould 4074 display problem

Quote:
Originally Posted by dave cox View Post
A problem with ALL the traces makes it unlikely that the same fault is occuring in all the input analogue amplifiers unless something common (like the PSU) is faulty.

Does this scope use a single ADC, with multiplexing for the channels ?

That would be my guess and a problem in the multiplexer or ADC.
If it has multiple ADC's the it is nore likely to have a PSU problem.

dc
Since this is a Gould DSO with a seriously complex switch mode power supply, I will never rule out a power supply problem!

I have now, thanks to Noopy, obtained some extender PCBs which are in the UK waiting to be delivered to me so I will at last be able to run the scope with a board outrigged so I can measure it!
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Old 8th Mar 2020, 3:35 pm   #19
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Default Re: Gould 4074 display problem

Thanks to Noopy2014 who sent me the CAD files for the PCBs, and the guys at JLCPCB who made the boards, I now have the necessary extender boards to help troubleshoot this oscilloscope!
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Old 8th Mar 2020, 3:36 pm   #20
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Default Re: Gould 4074 display problem

Of course, every success leads to another question - why doesn't the memory retain the time and date? I changed the battery, made sure that it's charged and connected....
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