TEK 2225 scope repair

[phil from seattle]

I'm no repair-Pro so hoping some smarter folks than I can help me.  My Tektronix 2225 (50 mhz analog, 1980s) scope died on me. Won't power on. I'm taking this as a sign that I need to replace it with a DSO. But it's been a good friend to me and it's so well designed and constructed, I want to repair it.  Probably will keep it,

Any way, the only additional symptom is that, after switching it off, the Power and Sweep LEDs light up for several hundred mS. At least there's one cap somewhere that works... 

So I got the service manual and took the case off. Dave's 2225 teardown video was a nice find. This is an amazingly well constructed and documented piece of equipment. Pretty sure it is a power supply problem. First order of business was a visual inspection. No obvious signs of burnt components, bulging electrolytics and such.

I took a number of voltage readings at various test points labeled on the bottom of the main board.  They all read low - 40-60% of what they should be. So, yeah, still sounds like the power supply.  I shut the scope down and started tracing from the mains input. The bridge rectifier diodes on the "Mains Input Board" have weird resistance measurements - all 4 measure 48K ohms in both directions. Strange. All 4 blown?  Then how is it possible to have any voltage on the test points? Thoughts? I'm thinking to desolder one end of one and see how my DMM likes it as a diode.  They are 400V 1A silicon rectifiers - pretty sure a 1N4004 is a good replacement. Might just go ahead and replace all of them. Does this make sense?  [as an aside, the scope worked fine the last time I used it and it was sitting on my bench, plugged in. There were no extraordinary electrical events that I am aware of.]

[phil from seattle]

update: diode checks out OK - guess that would have been too easy...  Next I think I need to trace into the pre-regulator on the Main Board.

[Tom45]

Measuring resistance across a diode won't work if the meter doesn't use a high enough voltage in resistance mode. For silicon diodes that would be around 0.6 volts.

The various power supplies in Tek scopes tend to have one main supply from which all other supplies derive their voltages. The manual may say where to start. If not, study the circuit diagrams to see if you can figure out the controlling supply.

It is quite possible that there is a shorted capacitor on the main supply's output. That would drag down the main supply and the others would follow. I would look at that possibility first.

[phil from seattle]

Thanks! Yes, the diodes were fine. I've been looking at the service manual for about a week now. Complex beast. There is a Tektronix manual - "Troubleshooting your Oscilloscope" that recommended taking the resistance reading.

The mains input board has a 2200 uF cap across the output to the pre-regulator section. Sounds like a possible candidate. I'll look at that next. 

[sprintcarfan]

Not sure if you've seen these videos or not.... Different vintage of oscilloscope, but it had a somewhat similar sounding problem.
https://youtu.be/Cmz8bzUsj7E
https://youtu.be/7ocancoZ02Q

[phil from seattle]

I got a little farther. Looking at the preregulator circuit, I think I found a problem. It looks like the mains input board is working ok but the first stage of the prereg is Q913. E, B and C all read +67.5V. I think that means it's shorted out. It's 10V higher than the voltage at the emitter is supposed to be which may also be a symptom. In addition, resistance between E, B and C is 0.

[phil from seattle]

After looking at the schematic some more, that shorted transistor passing 67.5V where it should have been 38V probably caused damage in a number of places. This is looking kind of grim. I'll continue to track down each one but at some point it may not be worth effort to fix.

[phil from seattle]

To close the loop on this one.  I fixed the scope.  The bare minimum was 1 cap and 2 transistors (C914, Q913 and Q923).  Though, I did replace all the power supply caps.  Total parts cost was about $7 USD.

[bd139]

Nice one!

I've got a 2225 here which is ticking on power up and the trace jumping around. Very annoying. Settles down after about 30 seconds then is ok. Perhaps once every hour or so it goes mad again. If anyone has any ideas, I'd appreciate it.

[phil from seattle]

Check out the Yahoo Group TekScopes.  Lots of knowledgeable people there.

[bd139]

Thanks - have just posted on there

[Specmaster]

Thanks - have just posted on there

I was just about to tell you about that group, I'm surprised that you didn't know about that group, theres also one for HP as well.

[bd139]

I’m aware of it. Have been a member for about 14 years on and off. Just nicer to post on here if I’m honest. Plus it’s mostly the same people ironically!