Tektronix 465 Repair - no -8 v, hot Sweep IC

[elverado]

Hello all, this is my first post after registering - I can tell you have a great board here. After some research here I dove in and bought an 'as-is' Tek 465 off Craigslist for $50 USD and planning to repair it. Current condition is that it has a single dot in the center and doesn't sweep. Switches are in good shape, lights light, fan runs, and overall seems well kept just old and tarnished. I've downloaded the service manual and love the serviceability, the mostly discrete components, and wonderful detail of documentation.

I've just cracked into testing it and am following the power supply test flow chart to start. So far I've confirmed +55v and +15v are rock solid at the test points. The +5v was a bit low at 3.8ish. The -8v isn't showing any voltage at the test point but I could be testing wrong. But focusing on the 5v first per the flowchart, I used the troubleshooting steps in the manual to isolate the horizontal amp board by pulling the connector that went from the interface board to it. That stabilized the 5v rail to 5v! So I was loving the instructions ... until I recognized a smell that I believe started at that point; the dreaded 'toasted silicon' smell. So I started feeling around for hot transistors and then realized that the sweep IC on the trigger board was getting really hot For a 99% discrete piece of electronics this made me really sad - one of 3 ICs is roasting. While I'm not sure it's dead (far too hot to touch but not melted or anything) I'm a bit reluctant to keep it powered on and keep probing around when it's toasting away. It may have been doing this before I isolated the board and I may have just gotten more sensitive; but seemed to correlate.

I went into this assuming I'd be swapping caps and transistors and more to get it to work, but now am looking at buying a tough to source (but luckily available) custom Tek IC  I'm a bit reluctant to just order it, plug it in and hope for the best. I'll be tracing voltages up to  and around it, testing caps and transistors in its path, etc. but if I toast a second one upon insertion I will not be happy.

Basically I'm looking to hear from the voices of experience here - is this a part known to go belly up? Any other testing you would point me towards? Thanks!

[tggzzz]

I would start by examining every part in the PSU. Slowly, to ensure you don't miss anything Then move on to the other boards and components. You are looking for any sign of capacitor failure or component overheating. Aluminium capacitors dry out, may "degass", and high ESR->ripple. Tant beads either crack open or just go short circuit.

Carefully check and jiggle all the connectors.

When checking power supplies, don't forget that the ripple is important.

If an IC directly attached to the CRT is hot, then consider disconnecting that part of the scope - there may be an internal short.

And finally, join the Yahoo! TekScopes group, search their archives, then ask questions. They are extremely knowledgable and a friendly bunch, particularly if you ask sensible and interesting questions

[elverado]

Thanks for your tips! I will check those things ... only challenge is that to test ripple I need a second, working scope right? I'll have to plan for buying one or to see if I can borrow or use one somewhere.

I joined the TekGroup on yahoo and introduced myself there - thanks for that advice.

[tggzzz]

Your USB scope should be fine for testing the ripple, but be careful with some of the voltages, of course. If using a *10 probe on high voltages, check its voltage-vs-frequency rating (higher frequency = lower rated voltage).

I used my Analog Discovery to capture the startup behaviour of my 485's inverter. That was particularly pernicious since the inverter started working after 60s, and then wouldn't fail until the scope had been off for 6 hours => 2 tests per day It was a subtly faulty aluminium electrolytic, of course,

[tombi]

Not sure if this is the same but I had a 475 where the -8V rail failed and it turned out to be the bridge rectifier. Apparently this is a common fault as the bridges were under-speced.

I replaced it and soldered a new one in (onto the top-side to avoid having to pull the board out) and this got 8V back.

Unfortunately this or something else took out a very significant number of other components in the scope. Took me weeks to track it all down but was worth it in terms of the learning.

Tom

[PaulAm]

A very common problem with these scopes is failed filter caps.   They fail open and you get a lot of ripple in the supplies which makes the measured voltage wonky.  When you see something like 3.8 on the 5v line that's a major clue.  If you look for ripple on that, it will likely be huge.  The bridge rectifiers are another major failure point.  With these scopes it doesn't make any sense to look at anything else until all of the LV power supplies are in spec.  Ripple should be in the couple mv range on everything except +110 which I think is in the tens of mv range.

MOST of the time when you get a power supply fault, there is little to no collateral damage.

Keep the intensity turned down when you have it on so you don't burn the screen.