Tektronix 2465A repair -- almost (update FIXED)

[calin]

Last week I  scored a Tek 2465A 350Mhz scope on Craigslist for 100$ -- true it was having the TEST 05 FAIL 44 message. Since that was something I have seen and fixed before I could not resists and I bought the thing even that I had and Tek 2465 300Mhz sitting on the bench. At that price ... even if I had it for spare parts.


Long story short, after replacing only 3 electrolytic caps on the LV power supply the 2465A is baaaaack to life. It was the usual suspect, the -8V supply.  BTW the 2465 had the exact same issue when I got it . EVen more after the fix the 2465A is bang on on both vertical and horizontal calibration on all 4 channels. I cannot see any issues. SO .. verdict ...  "that's a winner" !!


But, and here is the annoyance - there is always a BUT , the 50 ohm coupling is dead, flat as GND on both CH1 and CH2.  On both channels if I use the 1M coupling on DC or AC signal is spot on, everything works a treat ... if I switch to 50 ohm is like I connect the input to GND .. goes 100% flat.  So, started going over the attenuators and channel preamps to figure out how the thing works. I am trying to confirm my theory before I go any further:


1) Both CH1 and CH2 have identical attenuator circuits. Nothing more than a bunch of relay, resistors and caps plus the actual 10x and 100x attenuators. Since the attenuator chips themselves are common on both 1M and 50 ohm I doubt there is any issue with these. The only thing left that can obviously short to ground is the 50 ohm resistors and the clamping diodes. Even the diodes I doubt because these are again common onthe signal path .. so the only culprit I can think of is the 50 ohm resistors to be fried.
2) The signal path after the attenuators is common for both 1M and 50 ohm coupling. Since the 1M path works a treat it means there is nothing wrong with the preamps on both channels.


Given 1 and 2 .. all I can say is the 50 ohm resistors. But on both channels ?!!!! WTF?! That puts my thinking in doubt. Can have been there such a moron that has managed to fry both 50 ohm inputs 


Wonder is anyone has seen this before; I never touched such voodoo electronics things like input attenuators in oscilloscopes. I guess what I said above is right but better double triple check before I cut the wire. I can live easily without the 50 ohm coupling but if it can be fixed I wold like it fixed.

[tautech]

Good fix, well done. 
It is common for the inexperienced to apply too much voltage and fry 50 Ohm inputs.
Many scopes have _{in small print} the max V input when in 50 Ohm mode and is often missed by the inexperienced.
If in doubt.... RTFM is best advice.

There was a thread recently where a scope monitored the input in 50 Ohm mode and warned the user if the resistive divider got warm. Can't remember the brand sorry.

Sounds like a straight forward fix to me.

[calin]

Actually the scope that uses that trick can be a Tek 2465 or 24xx series. I am saying that because the 2465 scopes use a 50 ohm resistor and "paired" thermistor to detect over voltage on 50 ohm - in this case heat - and trip the input overload indicator plus automatically switch to 1M coupling. I tested the thing unwillingly on my other 2465 when I was turning the  voltage up on my function generator.


BTW, I measured the resistance between center and GND at the input BNC connector... they measure about 897K in 1M coupled mode and around 50 ohm when I use 50 ohm coupling. Measured the 2465 and the 2465A .. and same result on both. So I am a little  more in doubt that my theory is right. Gotta  RTFM more ...

[tautech]

Get this pdf by Tek:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf
It has some flowcharts for troubleshooting.

Gotta  RTFM more ...

 

[calin]

Aaaannd .. drumroll .. IT IS FIXED !!! 

My theory was right .. something was shorted to ground on 50 ohm ... but not the 50ohm resistor .. It was a freaking relay latched in close ... mechanically ... both channels. I guess the 50ohm combination and the latched relay got to GND coupling somehow. There are 8 relays in the attenuator which do all kind of combination to get the correct coupling and attenuation ratios and 50ohm plus that relay closed looks like isl equal to GND coupling.

I found the thing while I was checking / tracing all paths to GND looking for shrted circuits. That relay was showing closed with no power turned ON .. gentle tap and bingo !! it came back to life. What I really suspect it happen was a shock that has closed the thing in a very violent manner. Still both channels same simptom is weird.

However now I have a fully functioning and bang on 2546A on the bench. All with the cost of 100$ !! This year Santa is old .. based on the chips inside from 87-88 ... but it came early

[tautech]

Good fix, story and result.
Thanks for sharing. 

[Galen]

Dear Calin,

I'm writing to seek your advice on the -8V problem for a Tek 2465A.

I got a Tek 2465A, no trace. Sometimes, trace appear, but always 1 line, even I input signal.
As first attempt, I measured J119 voltage, found the +10V is right to the point, and most others are also within limit, except the -8V and -5V, I got -4.7V and -2.9V respectively. As per the SM, should fix the -8V problem first. I checked the -13.1V (before regulated to -8V), it's -8.7V only.

Any advice on this based on your experience in this post?

Thanks a lot,  Best Regards
Galen

[tggzzz]

Dear Calin,

I'm writing to seek your advice on the -8V problem for a Tek 2465A.

I got a Tek 2465A, no trace. Sometimes, trace appear, but always 1 line, even I input signal.
As first attempt, I measured J119 voltage, found the +10V is right to the point, and most others are also within limit, except the -8V and -5V, I got -4.7V and -2.9V respectively. As per the SM, should fix the -8V problem first. I checked the -13.1V (before regulated to -8V), it's -8.7V only.

Any advice on this based on your experience in this post?

Thanks a lot,  Best Regards
Galen

You'll find the answer here: https://www.eevblog.com/forum/testgear/tek-2465a-8v-5v-too-low-216364/msg2761430/#msg2761430

Or use the search box at the top right.

[WastelandTek]

It is nice when Google leads you right back to your home forum.

Yep, cleaning up those attenuators worked wonders on my 2445.  Ch2 was dead in 50 Ohm mode, and once I cleaned its relays I realized that CH1 was missing a couple orders of magnitude...

A little isopropyl alcohol and plain white paper as an abrasive medium and now all is well.

I love how these things are put together, the attention to serviceability is really impressive.