Tektronix 465M HV problem

by you applying Biasing on the regulator you may have done nothing more than shut down the Oscillator.. try to vary that voltage and see if its it gradually coming on and your getting excessive current build up as it attempts to drive the output module? if this is the case then you most likely have a shorted high voltage Transformer or High Voltage tripler shorted or in many cases the high voltage unit which has both the coil and tripler build in..

Steve Turner wrote:

Thanks for your reply.

I did as you suggested, and found that lower bias voltage on Q548 results in a larger current through the fuse. Current is around 50 mA at a 3.5v bias level. At this point I probed around a bit with the scope and ... lo and behold ... the HV oscillator was oscillating after all -!! I don't know why the difference from my original observation. The waveforms look a bit funny though. I'm out of time tonight but will play with this some more tomorrow.

Thanks again for your feedback.

Steve Turner

Steve Turner wrote in news: snipped-for-privacy@4ax.com:

Did you read the circuit description in the service manual?

Yes, several times, until I sort of understood it in a general way. After Jamie's feedback I played with the "artificial" bias on the HV regulator transistor and found that this level 1) dramatically affects the current through the HV oscillator, and 2) prohibits oscillation entirely if above a certain level. Maybe this is obvious to you but 'taint to me. At least I can get some scope waveforms now, although they are not good waveforms and bear little resemblance to the ones in the service manual.

Even with the HV oscillator oscillating, the other end of the transformer doesn't show +95v like it's supposed to. Maybe the transformer is "bad"?

As much as anything, I was wondering if anyone else had had a similar experience with this model scope. If so, I wasn't able to find it with Google.

I appreciate all comments. Although I'm familiar with most electronic fundamentals in a textbook sense (having waded through Horowitz & Hill on my own), I have zero practical experience.

Steve Turner

Forgot to mention one thing ... I'd be happy to transcribe the circuit description here if anyone wants. (Maybe not as quickly as I'd like though, since I have in-laws arriving for the weekend...)

Steve Turner

Steve,

Hi!

BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to get it working 2 hours ago thanks to the schematic you've posted earlier :).

I have similar problem like the one your describing but without the blown fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did check the +95V testpoint and got ~ +13V. I also checked all the HV related components in-circuit and the +95V line for any shorts, all appeared to be OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C forward voltage was a bit off the 0.5-0.6V range. Bought the replacement part (2SC2527) this afternoon, soldered the transistor after dinner and viola! it's working.

Yours might have a different component/s thats faulty. Try to check the components off circuit.

Cheers!

Jo

Note: my email is valid once the extra 't' is removed.

Either transformer bad or too much load. Latter might be in the +95V path Or too muc hload due to partly damaged HV multiplier.

yepp, I had once a bad multiplier, causing same sympthoms.

hth, Andreas

I mean

Note: my email is valid > Steve,

Excellent!

That is very interesting information. Your symptoms sound similar except for the +32v fuse blowing. I may try this solution, if I can figure out how to get underneath the mainboard without breaking anything. Doesn't look like a trivial process.

I guess the problem is how to do this when the mainboard is still screwed down in the chassis. I have isolated a couple of electrolytics (w/axial leads) by snipping one of the leads on the top surface of the board. This can later be soldered back into place. I'm not sure I like this approach. It's easier than dismounting the component from the board, but I dislike connections held together by nothing but solder...

Thanks much for the feedback.

Steve Turner

That's what I'm afraid of. However, although the HV oscillator is oscillating in some fashion, the waveforms are quite a bit different than the ones shown in the service manual. So I'm not sure if the low voltage means that the transformer or HV multiplier is faulty, or if the transformer simply isn't being driven properly. I need to do some more looking at this, but have house guests at the moment...

Do you recall if you were able to find a replacement?

Thanks much for the comments.

Steve Turner

A shorted Q552 obviously will cause a blown up +32V unreg fuse. Besides a transistor is more likely to fail than a transformer. Accessing the main board is just the matter of locating the hidden screws. Two on the rear below the CRT neck and two on the middle near the four screws that holds the CRT itself. The rest of the five or so are visible and of the same typeas the other four.

Good luck!

Jo

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neck

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It looked to me at the outset that Q552 was the most likely culprit for the symptoms I was seeing, so it was one of the first parts that I "tested" (to my limited ability). It does not appear to be shorted outright. However, I'm willing to believe that my testing is not revealing the fault, so I think I'll go ahead and replace it with the

Thanks also for the tips on mainboard removal.

Steve Turner