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Restoring HP 1740A scope, strange waveforms seen, assistance please?

[email protected]
 

Hi all, I've been a member for a while but lurking in the background while working on the scope and looking out for any discussions about a 1740A taking place.


I now have the scope in a running condition after replacing 2 faulty transistors and now while testing the scope with the 1V test connection I discover that channel B works as expected but channel A works OK up to 0.2V on the volts/div switch, moving to 0.5V and upwards the trace goes odd.?


I was wondering if anyone could point me to the area that I need to be looking at on channel A to cure the problem. I have uploaded photos of? both channels with the settings on 0.5V and 1V so you can see the problem?


[url=https://flic.kr/p/ZpqZUY][img]https://farm5.staticflickr.com/4503/37677492692_c2c839b7dd_b.jpg[/img][/url][url=https://flic.kr/p/ZpqZUY]B channel 1V dc[/url] by [url=https://www.flickr.com/photos/158191416@N07/]Graham Butcher[/url], on Flickr


[url=https://flic.kr/p/Yr33kT][img]https://farm5.staticflickr.com/4471/37039289513_ec6ceee9c7_b.jpg[/img][/url][url=https://flic.kr/p/Yr33kT]B channel 0.5V DC[/url] by [url=https://www.flickr.com/photos/158191416@N07/]Graham Butcher[/url], on Flickr


[url=https://flic.kr/p/Yr33nM][img]https://farm5.staticflickr.com/4480/37039289623_04aa1256a5_b.jpg[/img][/url][url=https://flic.kr/p/Yr33nM]A channel 1V DC[/url] by [url=https://www.flickr.com/photos/158191416@N07/]Graham Butcher[/url], on Flickr


[url=https://flic.kr/p/Yr33qc][img]https://farm5.staticflickr.com/4446/37039289763_5022ed8666_b.jpg[/img][/url][url=https://flic.kr/p/Yr33qc]A channel 0.5V DC[/url] by [url=https://www.flickr.com/photos/158191416@N07/]Graham Butcher[/url], on Flickr


Thanks in advance for any help you can offer me.


Graham
 

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The signal is being differentiated, as if it is being coupled through a small value capacitor.? I would first look at the attenuator switch contacts on those particular v/div settings for an open or high resistance.? In other words dirty switch contacts.

Dave

On 10/15/2017 7:51 AM, graham.l.n.butcher@... [hp_agilent_equipment] wrote:

Hi all, I've been a member for a while but lurking in the background while working on the scope and looking out for any discussions about a 1740A taking place.


I now have the scope in a running condition after replacing 2 faulty transistors and now while testing the scope with the 1V test connection I discover that channel B works as expected but channel A works OK up to 0.2V on the volts/div switch, moving to 0.5V and upwards the trace goes odd.?


I was wondering if anyone could point me to the area that I need to be looking at on channel A to cure the problem. I have uploaded photos of? both channels with the settings on 0.5V and 1V so you can see the problem?


[url=][img][/img][/url][url=]B channel 1V dc[/url] by [url=]Graham Butcher[/url], on Flickr


[url=][img][/img][/url][url=]B channel 0.5V DC[/url] by [url=]Graham Butcher[/url], on Flickr


[url=][img][/img][/url][url=]A channel 1V DC[/url] by [url=]Graham Butcher[/url], on Flickr


[url=][img][/img][/url][url=]A channel 0.5V DC[/url] by [url=]Graham Butcher[/url], on Flickr


Thanks in advance for any help you can offer me.


Graham
Bob Albert
 

I am not familiar with that model (I have a 1727A) but if it is anything close to the Tektronix 465, there are attenuator switches operated from the panel control that may be making poor contact.

I must get into my 465 to make such a repair.? I have a spare 465B and hope that one has the parts I need.

Bob


On Sunday, October 15, 2017 6:05 AM, "dadhills@... [hp_agilent_equipment]" <hp_agilent_equipment@...> wrote:




The signal is being differentiated, as if it is being coupled through a small value capacitor.? I would first look at the attenuator switch contacts on those particular v/div settings for an open or high resistance.? In other words dirty switch contacts.

Dave

On 10/15/2017 7:51 AM, graham.l.n.butcher@... [hp_agilent_equipment] wrote:

Hi all, I've been a member for a while but lurking in the background while working on the scope and looking out for any discussions about a 1740A taking place.

I now have the scope in a running condition after replacing 2 faulty transistors and now while testing the scope with the 1V test connection I discover that channel B works as expected but channel A works OK up to 0.2V on the volts/div switch, moving to 0.5V and upwards the trace goes odd.?

I was wondering if anyone could point me to the area that I need to be looking at on channel A to cure the problem. I have uploaded photos of? both channels with the settings on 0.5V and 1V so you can see the problem?

[url=][img][/img][/url][url=]B channel 1V dc[/url] by [url=]Graham Butcher[/url], on Flickr

[url=][img][/img][/url][url=]B channel 0.5V DC[/url] by [url=]Graham Butcher[/url], on Flickr

[url=][img][/img][/url][url=]A channel 1V DC[/url] by [url=]Graham Butcher[/url], on Flickr

[url=][img][/img][/url][url=]A channel 0.5V DC[/url] by [url=]Graham Butcher[/url], on Flickr

Thanks in advance for any help you can offer me.

Graham
[email protected]
 

Hi everyone, many thanks for the tips and pointers, especially the one from Dave. I checked the switch contact, 1D is the only extra switch being used on all ranges from 0.5V upto and including 20V per div and this checked out just fine.?

Further investigation on the input for Channel A showed that R3 should be 990k but was reading 11M so there was the culprit. Problem being this resistor sits in what appears to be a metal shield which soldered to the ground plane and is also shown on the component map as MP1? This resistor sits across a isolation slot so I'm guessing that is meant to be providing extra protection against stuffing over voltage into the channel via the probe and the shield is contain any flash, is that right?

Anyway, purely as a test I have soldered a 1M resistor across it and tested it and it works fine so I'll need to replace it with the correct rating and stuff it back into the shield?

The other issue I have is that the delay timebase knob has broken up and left the brass boss on the shaft which I cannot get off as the grub screw just chews up the allen key head all the time. So does anyone know where I can get new knobs for the time base (both knobs) and both of the shafts as I see thats the best solution to the boss problem and there are missing grub screws so a set of knobs with them fitted seems the best option if possible.

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