Electronics > Repair

Textronix 475 needs repair

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JBrooks:
I have had a Textronix 475 scope for > 20 years and it used to work fine. Approx 10 years ago I went to use it but it was dead. Since then it has been stored in my integrale garage (reasonably dry with limited extremes of temperature).
Had a read around on the internet and got the manual.

It is a 475 with a DM44 fitted S/No. B277353. There are no mod strikes indicated on the back although there are possible manufacturing mods e.g. diode and resistor joined to wire and not using the pcb.

The fault symptons are not consistent. When I first started at this today the scope would have a backlight then 2 beams and then 1-2 sec later nothing. Whilst measuring PSU voltages I noticed that the -15V supply went from neg to pos after a few seconds. It is now just dead and I hope that nothing has been nackered.

My measurements have been made with an AVO 8 as that is all the other test gear that I have (except the Taylor 45D valve tester that is not going to be much use).

I have measured the impedances of the regulated supplies to ground as follows:

-ve terminal to chasis and with the following +ve term to test points, results:
+110V 5K7R
+50V 1KR
+15V 1R
+5V 17R
-15V 80KR
-8V 24R
unreg +50 23R

+ve terminal to chasis and with the -ve term to test points, results:
+110V 10KR
+50V 1K7R
+15V 2R
+5V 26R
-15V 1KR
-8 17R
unreg +50 12KR

Supply voltage readings:
+50 V 49.5 V
+110V +110V
+15V +1.5V
+5V +4.98V
-15V -ve going positive with ripple detected on AVO.
-8V +0.7V.
 
After reading I am thinking that the smooting caps, if not all, but more than 1 have had it. I have looked on line and probably could change all smoothing caps for approx £30 (this does need more research).

Before jumping in and cutting legs to check cap failures I thought  that I would seek some advice. When I start to look into each component failure I will also look to check the bias/reverse bias of the rectifier diodes.

Help please!

Kind regards Jonathan.

PaulAm:
Common failure modes on these scopes are: filter caps fail open and bridge rectifiers fail open or short.

It's easy to check the rectifiers with a dmm with a diode ragne.  Bridge the caps with a good one and you should be able to verify the bad cap.

Another scope is helpful to look at the ripple, although you might see it if you put your meter on an AC range.  An esr meter is also a useful tool if you have one.  Nothing works unless all supplies are in tolerance both for voltage and ripple (which should be in the mv range).

Removing the caps is nontrivial, you need a good soldering iron, a desoldering tool is very handy and some desoldering braid is absolutely required.  Some of that alloy that lowers the melting point of solder can be very useful in this repair.

Note there are 4 terminals on the cans: 3 from the can and the + terminal.  If you overheat the board you will pull traces off, so be careful.  It is possible to replace the caps without taking the entire scope apart, but it is a challenge.  You must bridge the 3 can terminals together since they used the can as a jumper for the ground tracks.  When repairing these scopes I use a small adapter board that fits a snap cap and then has the same footprint as the original cap.  Makes for a neat repair.

JBrooks:
Paul, thanks for the feedback especially about the cap cans. How do you make the adapter board, is it commercially available? If I was to wire the bridges is there a preferred method e.g. daisy change, connect back to one legend?

Only had a relatively quick look but some of the bigger caps (350mF) are difficult to source. Since the tolerances are wide I was thinking of going to the next value up; could you advise on the caps that you use with the adapter board.
thanks.

PaulAm:
Here's a link to a pcb pattern.  I just make them up using toner transfer when I need them;
  http://www.auroragrp.com/agi/tek/cap-adapter-sngl.pdf

I try to use 105C rated caps if I can find them, they're not really critical, just go to the next highest V and C rating.  They'll work fine.

I'm not following what you're asking on the bridges.

JBrooks:
Hi Paul,

By bridges I was reffering to using wire to inter connect the grounds that are are normally made by the cap cans. However, I will now look into toner transfer (just had a peek), it is probably >20 years since I have etched pcbs (using a light box). Things have changed.

many thanks.

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