Tek 485 Vertical Compression during Position Change


 

I have a 485 that exhibits vertical compression with vertical position change. For example, if a 4 division square wave is displayed center screen, when positioned to top or bottom screen the 4 division signal becomes 3.8 div.
This occurs either Ch1 or Ch2. The compression is apparent after U620 in the vertical amp, but not before at the output of the delay line. Replacement of U620 and U650 had no effect. I don't have a U660 to substitute.
Voltage measurements at these devices and the supply are close to those indicated on the schematic.

Any thoughts or previous experience?

Thanks,


Zentronics42@...
 

This is ringing a very dim bell. I have not been in a 485 cal document in a while but I just finished up a 465b. I beleave there is an adjustment for this I would check the verticual cal procedure just a read through If I recall correct which is most likely not This is a capacitive adjustment in the vertical amp. But if it is 1 & 2 that leads me to amp board not attenuators.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Bob Koller
Sent: Saturday, July 31, 2021 5:37 PM
To: TekScopes@groups.io
Subject: [TekScopes] Tek 485 Vertical Compression during Position Change

I have a 485 that exhibits vertical compression with vertical position change. For example, if a 4 division square wave is displayed center screen, when positioned to top or bottom screen the 4 division signal becomes 3.8 div.
This occurs either Ch1 or Ch2. The compression is apparent after U620 in the vertical amp, but not before at the output of the delay line. Replacement of U620 and U650 had no effect. I don't have a U660 to substitute.
Voltage measurements at these devices and the supply are close to those indicated on the schematic.

Any thoughts or previous experience?

Thanks,


Michael W. Lynch
 

I seem to recall that my 485 had a dirty Beam finder switch. On my machine, any vertical signal trace would be reasonably correct on CRT center and compress as the trace was positioned toward the top or bottom of the CRT screen. Cleaned that beam finder switch and the issue went away.

--
Michael Lynch
Dardanelle, AR


Tom Lee
 

Even though you've checked voltages,  you may have missed one or two. I'd double-check the common-mode voltages along the signal path leading up to U620. In particular, observe those voltages as you vary position.

Cheers
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 7/31/2021 14:36, Bob Koller wrote:
I have a 485 that exhibits vertical compression with vertical position change. For example, if a 4 division square wave is displayed center screen, when positioned to top or bottom screen the 4 division signal becomes 3.8 div.
This occurs either Ch1 or Ch2. The compression is apparent after U620 in the vertical amp, but not before at the output of the delay line. Replacement of U620 and U650 had no effect. I don't have a U660 to substitute.
Voltage measurements at these devices and the supply are close to those indicated on the schematic.

Any thoughts or previous experience?

Thanks,




Ozan
 

On Sat, Jul 31, 2021 at 02:36 PM, Bob Koller wrote:


I have a 485 that exhibits vertical compression with vertical position change.
For example, if a 4 division square wave is displayed center screen, when
positioned to top or bottom screen the 4 division signal becomes 3.8 div.
This occurs either Ch1 or Ch2. The compression is apparent after U620 in the
vertical amp, but not before at the output of the delay line. Replacement of
U620 and U650 had no effect. I don't have a U660 to substitute.
Voltage measurements at these devices and the supply are close to those
indicated on the schematic.

Any thoughts or previous experience?

Thanks,
------

Michael's suggestion of dirty beam finder switch is very likely. Network around beam finder switch sets the tail current(s) of U620 input pair. Nominal is 20mA so you should see ~ 2V across R603 and R604. Voltage at the bottom of R601 should be 5V in normal operation. If somehow the switch is dirty or if there is some other issue that limits the tail current you will see compression.

Ozan


 

Thanks for all the response!
The switches were cleaned, etc before troubleshooting began.

The problem was that R658 had more than doubled in value. Replacement solved the problem.


Michael W. Lynch
 

Bob,

Glad to hear that you scope is back in working order.

--
Michael Lynch
Dardanelle, AR