Tek 2235A Horizontal Compression when warmed up

[Microgiant]

I bought a 2235A that had a PSU that was ticking. It turned out to be the 100uf caps had all gone bad. Now as it heats up the the trace compresses horizontally until you can see both beginning and end on screen.

I started repairing scopes a couple years ago and it has been a great learning experience but the more I learn the more I realize how much I dont know.

This was my first 2235. I am looking at the schematic for the horizontal amplifier. I have not tested anything yet but I am suspecting a resistor(s) is acting more like a thermistor? BTW there is no fan in this unit.

How do I troubleshoot this or is this a known issue? What is the fix?

Timothy

[nctnico]

Get a can of freeze spray and cool some of the components. I doubt is is a resistor; capacitors (even the foil ones) are more likely suspects.

[David Hess]

Before doing detailed tests, check to make sure the power supply voltages are correct when the problem is observed.

[Old Printer]

I have a 2225, but no 2235A. The first thing i did when I decided to buy this scope was to get the manuals. Tek made great service manuals and to me that alone, and the fact that most of them are easily available for the vintage scopes, makes the extra you pay for a Tek scope worth it. This is a link to Artek and the service manual for your scope. You can probably find it free elsewhere, but these manuals are searchable PDF's of the highest quality, for $12.

http://artekmanuals.com/manuals/tektronix-manuals/

[David Hess]

The 2235 and 2235A are essentially identical so either manual can be used.  The 2215A service manual could also be used.  The 2236 or 2236A service manual could be used if you ignore the timer/counter stuff.

[Microgiant]

Im using the fuzzy manual I downloaded. Also I checked the PSU for voltage and ripple. I did not replace all the caps wholesale just the ones that stopped the PSU from starting and one on the 30v rail due to ripple. Im going to see what caps are in the horizontal circuit and start with them. Thanks for the tip on the freeze spray. Im going to look for some locally and if I cant find any order some online.

Edit I also replaced a .068 RIFA just because and 2 .0022 film.

[Microgiant]

I couldnt wait and bought compressed air and just inverted the can for a freeze spray. I was able to narrow down the component to VR764 Zener 13V 2% 500mw 152-0702-00. Could this be the problem? I cant find a data sheet for this part but Zeners are temperature sensitive normally, correct?

I tried to swap it with a standard 13v 500mw zener which resulted in a nearly 50% compressed trace as soon as I turned the unit on. It flowed slightly more than 13v across it. The 152-0702-00 13V zener part flows more like 12.75v and rises ~ 5mv-15mv as temperature increases I get a ~15% compression with the original part. With the case on it gets hotter and compresses even more.

I am trying to find original manuals. I have 465, 465B, 475 and many others already. When I find them reasonable I snap them up. Would love any original 2XXX series manuals if anyone has them for sale.

[Old Printer]

I couldnt wait and bought compressed air and just inverted the can for a freeze spray. I was able to narrow down the component to VR764 Zener 13V 2% 500mw 152-0702-00. Could this be the problem? I cant find a data sheet for this part but Zeners are temperature sensitive normally, correct?

I tried to swap it with a standard 13v 500mw zener which resulted in a nearly 50% compressed trace as soon as I turned the unit on. It flowed slightly more than 13v across it. The 152-0702-00 13V zener part flows more like 12.75v and rises ~ 5mv-15mv as temperature increases I get a ~15% compression with the original part. With the case on it gets hotter and compresses even more.

I am trying to find original manuals. I have 465, 465B, 475 and many others already. When I find them reasonable I snap them up. Would love any original 2XXX series manuals if anyone has them for sale.

I love books and if I ever came across an original manual at a good price i would grab it as well, but I wanted to make sure I could work on the scope(s) if /when necessary and the Artek digital copies are extremely good quality. I have seen enough free home scans where the numbers on the schematics were unreadable.

[David Hess]

Im going to see what caps are in the horizontal circuit and start with them.

Do not touch the integration capacitors in the sweep generator.  They are a matched set which cannot be replaced.

I couldnt wait and bought compressed air and just inverted the can for a freeze spray. I was able to narrow down the component to VR764 Zener 13V 2% 500mw 152-0702-00. Could this be the problem? I cant find a data sheet for this part but Zeners are temperature sensitive normally, correct?

I am not going to go searching through the schematics for VR764 without some idea from you about where it is located on what schematic.

[Microgiant]

I touched them     

Im getting the hang of fixing tek scopes and the more I learn the more I realize just how hard it is to long distance repair from someones descriptions of what they think is important. And I dont want to hoist all my work on you for something simple. Tek scopes are fun because they are chock full of tunnel diodes, custom ICs, obsolete parts and other unobtanium.

The VR764 13V zener is bad, its out of spec and overly sensitive to temperature changes.  What is a suitable replacement?
SEMICOND DVC,DI:ZEN,SI,13V,2%,500MW,D0-7  152-0702-00

BTW a Chinese 13v 500mw zener did not work. The working part has ~12.22 volts across it when in operation. Maybe a misprint and it should be 12v?

[David Hess]

The VR764 13V zener is bad, its out of spec and overly sensitive to temperature changes.  What is a suitable replacement?
SEMICOND DVC,DI:ZEN,SI,13V,2%,500MW,D0-7  152-0702-00

BTW a Chinese 13v 500mw zener did not work. The working part has ~12.22 volts across it when in operation. Maybe a misprint and it should be 12v?

Zener diodes sometimes develop excessive leakage or drift.

12.22 volts across a 13 volt 2% zener diode may still be reasonable if it is operating below its design current.  It is difficult to say without inspecting the schematic.