Tektronix 2205 Repair. Seeking advice

[JeffWW6J]

Hi folks,

I am in the process of repairing a Tek 2205 and have found that in the high voltage section, the voltage doubler diodes and cap have failed.
To start from the beginning, I was using the scope, and all of a sudden I lost the trace. Initially, I could press the trace find and get it on the screen. After a short while this would no longer work. I saw that the power and trigger lights were blinking on and off at the some time and assumed that it was a power problem. I opened the thing up and spent several (many) hours reading and looking. I tested all the semiconductors in the devil that is the switching power supply and only after removing the protective hatch did I find that the two diodes (CR975 and CR976) were open I haven't an esr tester (yet!) but the cap in the circuit didn't show much capacity with my multimeter that does have a capacitance setting.

What I am after is: Should I look further? I have the parts coming with a friend from the states, but a .01uF 4000v was hard to come by. I wouldn't want to replace those parts only to find the some thing else took them out.

Also I found it rather strange (with my limited knowledge of switchers) that with the diodes open, the supply was cycling. I would have expected this if they were shorted but open?

Ideas, suggestions?

Thanks much

Jeff
WW6J

As an aside, I know that I have a US call, but have been living in Moscow for some years now. You think it is hard for you guys to find parts...... I have another scope, a 2245 coming from Israel now, so when it arrives (in about a month!) I can look at the ripple and waveforms on this one to be sure it is right.

[TonyPhs]

Hi Jeff,

I've got a similar issue with my Gould OS3000. No HV and no trace.

I haven't checked your schematic but you should be aware that some of these high voltage diodes are actually like a stack of diodes in series and have very high forward voltage so cannot be measured with a normal multimeter in diode test mode.

What you need to do is have a voltage source, say 30v, and have a resistor of say 10k in series with the anode of the diode and measure the voltage drop across the diode.

To check reverse leakage you can reverse the diode and measure the current through the circuit. You shouldn't be able to detected any current flow.

In my example, BY187s, I had a forward voltage of around 17v and zero reverse current.

Of course the only way to be 100% sure is to test at high voltage but I didn't fancy that so much.

I also got another scope to diagnose but that broke down as well, fortunately for me that's an easier fix.

Tony

Edit: Yes as I guessed...

VG5X-1 Diode.

Diode. fast recovery 5000V 25mA, 250NS. 14V Forward drop. Max one half cycle current 3Amp. Package: axial leads, 0.16" D x 0.60"long. Silver-plated (tarnishing, cleanable) wires. Note: 5KV. High Voltage.

So if you get around 14v with the method outlined above the diodes are probably fine.

[Radio Wrangler]

The diodes in these parts are designed to break down gracefully at high reverse voltage, so the stress is shared across all the diodes. Otherwise the least leaky diode gets all the voltage and fails, then the next least leaky dies and then... and then... like a zipper!

David

[JeffWW6J]

Thank you gentlemen. I will have to find a 30V source now, but will endeavor to do so. The lights behave normally now with those parts removed, so I am still feeling good about my diag, but now need to test. Do you guys think that I need to search further or maybe just replace the parts and see what happens?

Thanks

Jeff

[TonyPhs]

From what I gather HV multipliers are prone to failure due to the stresses involved.

For a higher voltage source you could use a voltage rail in the scope itself if there's one suitable, or wire a few 9v batteries in series. The test will only tell you if the diodes are totally failed.

I don't know of a good way to determine if HV caps are leaky of have high ESR I'm afraid.

[JeffWW6J]

I learned that as well Tony, so I am hoping with fingers crossed that I found the problem. Already send the wife to the store for the 9v batteries! I'll let you know what I find out. Now scouring the 'net for an ESR meter that I can find here and afford.

[TonyPhs]

As far as I know ESR meters, certainly the ultra cheap chinese ones, are not good for less than 1uF caps and even then do not run at the right speed to properly test. From what i can gather ESR isn't normally a problem for ceramics. They leak though, that can be a problem.

Was it just the doubler you disconnected?

When I tested the caps in my tripler I found the capacitance pretty close to the nominal value. Is your meter good at picking up the capacitance of similar capacitance non HV caps? I found my HV caps to be totally open circuit. I've no idea what my problem is yet by the way.I haven't replaced the tripler section caps, although I may yet do it. I've been assured that the diodes are likely fine, given the test I carried out.

[JeffWW6J]

Tony,

I did your test, using my 12 (13.8) volt supply and the diodes seem to be ok. That doesn't please me.... Now I am searching thru anything downstream of those. This was one of Teks first foray's into switchers and it is a kluge at best. I just removed those two diodes and the cap. I can send you the schematic if that helps. Wish I could offer you some assistance too, but the other scopes that I have repaired were either in the low voltage areas or one had a bad crt. Honestly, without an isolation transformer here, the switcher scares me a bit, so this is going slowly as I don't want to probe into 1.8kv!

Sorry, btw the way the cap in question if a poly metal one and I have tested it, now out of circuit and it has 19.x nano farads and should be .01 mfd at 20%, so might just be ok too.

[TonyPhs]

I found a schematic for yours without issue, that's where I got the diode part number from.

I built myself a high voltage probe so I could check my EHT levels. I went completely overboard and it's probably good for 20kV for short periods. It allows me to see what HV is being produced without risking myself and also that the HV is being drained away when I turn the scope off.

I haven't probed my HV with a scope, just the oscillator section. If I can get the oscillator running normally, and not just amplifying the ripple on the power rail then I will be getting somewhere. The EHT on mine seems to have a ground that isn't common with the rest of the scope. Is it even safe to probe this sort of thing? I don't know.

I have a Met Poly cap on my EHT section too but I don't know what it does, it's only 400V rated so can't be doing anything particularly exciting with the HV. I'm more concerned with the actual HV caps though, mainly because there's not much else I haven't checked in the EHT section.

[JeffWW6J]

Tony, seems like those caps are worth changing! I don't guess that the 400v cap is your culprit, mine is 4kv, so it is high voltage for certain.
That is quite a probe, seems safe for sure. I went and tested all the semi conductors that are in the circuit after the hv, it uses the hv as a reference for the DC restorer. Didn't find a bad one yet. Still probing and swearing. I also posted in the Tek group on Yahoo, been a member there for years and actually forgot about them. I'll let you know what I find. Still hoping someone has an idea.

[TonyPhs]

I'm guessing it must be one or more of the high voltage caps too on mine as when I disconnected the secondary and ran the scope I got a good oscillation, something like what I'd expect to see. Before it was something like 8v peak at 100Hz. Now with no secondary load it's 20v on my scope and a rough 25khz measured on my DMM.

My second scope, although only showing a trace on around a 1/3 of the screen is helping, although I won't be hooking it up to what could be -1500v.

There's nothing besides a HV cap and a HV diode on the secondary output I tried.

Strange things these HV faults. I have around 8 HV caps to go through and I think 3 or 4 different values and ratings.

[JeffWW6J]

Looks like progress! Well, I went out today and got a Peak ESR meter, you are correct, less than 1uF it can't measure (and it is not a cheap one either). So far all the caps that it can check have pretty low ESR, so have to keep looking. I soldered in the two HV diodes and the Cap and at least am back to where I have started. Same behavior, a few minutes on and then blinking power and trigger light.

[JeffWW6J]

Now I too have made some progress. I removed Jumper W976, which isolates the Grid Bias and Z Axis modulation amplifiers, but leaves the HV intact. The power stays on! Q845 is suspect as it was replaced before I bought the scope. Onward we go!

[TonyPhs]

It does sound like progress.

I tried the other HV sections, first the multiplier and then the grid. It's still dragging the HV down and preventing oscillation. I might try a resistor load next. It might be that the transformer is just fine when unloaded but has an issue when loaded.

A forum member on here offered me a EHT unit out of their os3000. Looks like I might just take it. Maybe this fault is just beyond fixing and just need replacing.

I might whip the transformer off and see if I can see anything.

[JeffWW6J]

Tony and all,

I found the problem with my 2205. There was a transistor (Q236) in the vertical output amplifier that was shorted collector to emitter and was dragging the 102v supply down.

Waiting for parts now, but 99 44/100 percent certain that the problem has been found as save for one vertical deflector plate not functioning, the scope behaves as normal now.