Cannot delay the B sweep on a 465B


Torquil Macdonald Sørensen
 

Hi!

I have a problem with my 465B where the delay time position 10-turn pot for
the B sweep has no effect. Everything else seems to work. Please see this
youtube video for a demonstration:

https://youtu.be/17ZGEw_jOmo

The intensified portion of the A sweep trace always starts on the leftmost
part of the display no matter how much I turn the delay time position knob.
However, the holdoff knob does make the B sweep duration/length vary, in a
periodic manner as I turn the holdoff knob slowly clockwise.

Any pointers for things to check in the circuit ? The voltage rails are
fine. Anyone here seen this problem before?

I have just fixed a problem with a reduced trace length on this scope, by
"cleaning" the X1 Gain pot with IPA. Maybe something similar could cause my
B sweep delay problem?

Best regards,
Torquil Sørensen, Norway


Ozan
 

Hi Torquil,


I have a problem with my 465B where the delay time position 10-turn pot for
the B sweep has no effect. Everything else seems to work. Please see this
youtube video for a demonstration:

https://youtu.be/17ZGEw_jOmo

The intensified portion of the A sweep trace always starts on the leftmost
part of the display no matter how much I turn the delay time position knob.
However, the holdoff knob does make the B sweep duration/length vary, in a
periodic manner as I turn the holdoff knob slowly clockwise.
------
I don't have a 465B but B sweep delay circuit works similarly on several scopes. On sheet <7> Q7067 compares delay potentiometer voltage to "A ramp" voltage. Depending on where ramp crosses the delay pot voltage, B sweep starts with that delay. "A ramp" waveform (node 60) is not shown in the PDF I found but it is the same signal as node 82 which is in the manual. First check is whether the pot changes comparator threshold. Voltage on C7052 should change from 2.8V to 12.9V as you turn the delay pot.

If the delay pot voltage is OK next check is voltage at R7157 & R7152 junction. Looking at the schematic I expect it to be ~ -0.5V when "A ramp" is more that the delay pot voltage and should jump to ~ +1V when it goes below.

If you report back the observations there may be other suggestions.

Ozan


Torquil Macdonald Sørensen
 

On 26/04/2021 07:17, Ozan wrote:
Hi Torquil,

I have a problem with my 465B where the delay time position 10-turn pot for
the B sweep has no effect. Everything else seems to work. Please see this
youtube video for a demonstration:

https://youtu.be/17ZGEw_jOmo

The intensified portion of the A sweep trace always starts on the leftmost
part of the display no matter how much I turn the delay time position knob.
However, the holdoff knob does make the B sweep duration/length vary, in a
periodic manner as I turn the holdoff knob slowly clockwise.
------
I don't have a 465B but B sweep delay circuit works similarly on several scopes. On sheet <7> Q7067 compares delay potentiometer voltage to "A ramp" voltage. Depending on where ramp crosses the delay pot voltage, B sweep starts with that delay. "A ramp" waveform (node 60) is not shown in the PDF I found but it is the same signal as node 82 which is in the manual. First check is whether the pot changes comparator threshold. Voltage on C7052 should change from 2.8V to 12.9V as you turn the delay pot.

If the delay pot voltage is OK next check is voltage at R7157 & R7152 junction. Looking at the schematic I expect it to be ~ -0.5V when "A ramp" is more that the delay pot voltage and should jump to ~ +1V when it goes below.

If you report back the observations there may be other suggestions.

Ozan
Hi! Thanks! In the end I found the problem: R7069 which lies between a
55VDC supply
and Q7074/Q7075 was cracked and open circuit....It's supposed to be 6.2
kOhm. After replacing it, my 465B works perfectly!

The fault with R7069 turned out to be intermittent. Since it was open
circuit on my first measurement, I just added on 6.2 kOhm in parallel.
However, measuring after that I found 3.1 kOhm across the parallel
combination... So I though perhaps it was a cold solder joint that I had
fixed when tacking on the second 6.2 kOhm. But on closer inspection, I
found the crack, and R7069 was back to being open circuit. So I clipped
it out and replaced it with a correct resistance.

Best regards,
Torquil Sørensen


Ozan
 

Hi! Thanks! In the end I found the problem: R7069 which lies between a
55VDC supply
and Q7074/Q7075 was cracked and open circuit....It's supposed to be 6.2
kOhm. After replacing it, my 465B works perfectly!
Great news, congratulations for the fix.
Ozan