Topics

585A -- time base A linearity


@0culus
 

Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean


Tom Lee
 

That symptom could be caused by a leaky timing cap for that sweep setting. If it starts off at about the correct speed on the left, but slows down as it moves to the right (e.g., square waves get progressively squished together), that would be a further indictment of the cap.

Try subbing it with a replacement cap and see if the problem goes away.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean





Albert Otten
 

On Thu, Sep 17, 2020 at 09:01 AM, Tom Lee wrote:


That symptom could be caused by a leaky timing cap for that sweep
setting. If it starts off at about the correct speed on the left, but
slows down as it moves to the right (e.g., square waves get
progressively squished together), that would be a further indictment of
the cap.
Try subbing it with a replacement cap and see if the problem goes away.

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,
However, the 0.5 msec/cm
setting is the outlier here. The settings on either side of it are in spec,
but this setting has a very apparent linearity problem; the trace is getting
shorter the further right it goes.
I second that, especially since at 0.5 cm/div the high value 5 M of R160 is switched in. The cap voltage increases when the sweep goes further to the right.. Any leakage then causes nonlinearity.

Albert


Dave Wise
 

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M, while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D. See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .

What pot are you referring to? R348 SWP CAL? Note that series resistor R349 was changed at S/N 9300.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee=ee.stanford.edu@groups.io>
Sent: Thursday, September 17, 2020 12:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

That symptom could be caused by a leaky timing cap for that sweep
setting. If it starts off at about the correct speed on the left, but
slows down as it moves to the right (e.g., square waves get
progressively squished together), that would be a further indictment of
the cap.

Try subbing it with a replacement cap and see if the problem goes away.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean






Albert Otten
 

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M,
while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D.
See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert


Dave Wise
 

I agree.
And it's unlikely for a polycarbonate (C160G) or ceramic (C160M) to go leaky, but it does happen now and then. Unless R160F has become nonlinear, the caps - and the switch insulation - are pretty much the only suspects.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Albert Otten via groups.io <aodiversen=concepts.nl@groups.io>
Sent: Thursday, September 17, 2020 9:50 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M,
while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D.
See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert


Tom Lee
 

I agree with Dave. The cap, or possibly some contamination across its terminals, is the highest-probability cause of the problem.

The timing resistor has more or less a constant voltage across it, as well as a constant current through it (the circuit is the classic Miller integrator that Tek favored until the 7B92's limitations stimulated an overdue change in tradition), so it's hard to imagine what pathologies it could possess to cause the observed behavior. If it does ultimately turn out to be the resistor, I'd be very interested in some curve tracings.

Good luck with your debug!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/17/2020 10:30, Dave Wise wrote:
I agree.
And it's unlikely for a polycarbonate (C160G) or ceramic (C160M) to go leaky, but it does happen now and then. Unless R160F has become nonlinear, the caps - and the switch insulation - are pretty much the only suspects.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Albert Otten via groups.io <aodiversen=concepts.nl@groups.io>
Sent: Thursday, September 17, 2020 9:50 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M,
while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D.
See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert











Chuck Harris
 

I would clean the capacitor and the 5M resistor with IPA
before I condemned it... but you are right about the list
of usual suspects.

-Chuck Harris

Dave Wise wrote:

I agree.
And it's unlikely for a polycarbonate (C160G) or ceramic (C160M) to go leaky, but it does happen now and then. Unless R160F has become nonlinear, the caps - and the switch insulation - are pretty much the only suspects.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Albert Otten via groups.io <aodiversen=concepts.nl@groups.io>
Sent: Thursday, September 17, 2020 9:50 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M,
while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D.
See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert













@0culus
 

Sorry I'm too tired to reply to anything individually tonight...long day at work (things are busy when you aren't normally in the office due to covid, and then suddenly you are in the office and you have everything to do), but noted on the proposed directions to take troubleshooting...thanks!!!

I'm crack it open this weekend and take a closer look.

Sean


@0culus
 

Did some poking around today. Deoxit didn't change anything. I noted, however, that the linearity issue is definitely temp related. It starts out in spec, then drifts out as the instrument warms up. I reckon this probably points right at the timing capacitors (which appear to be hidden behind the adjustable trimmers). No idea how to get to them short of considerable disassembly. Any suggestions on a modern replacement cap?

Sean