开云体育

Hi everyone.

Hoping that someone might have encountered this before.

So after the 492 repair, I picked up a nice, clean 2430A which was dead.

Including the Rifas, the power supply board had 10 completely failed caps and a couple or three toasted resistors, which once replaced gave the scope life once more.
/g/TekScopes/photo/301027/3891759?p=Taken%2C%2C%2C50%2C1%2C0%2C0
All the LV rails are present, although uncalibrated. I've have a look over the boards for obvious damage and tested many caps and semi-conductors for shorts, as well as reseated the socketed IC's and cleaned the contacts on the hybrids. So far no difference.

The issue is, that once the scope is turned on, the scale illumination lights up and the scope clicks through the start-up,
/g/TekScopes/photo/301027/3891763?p=Taken%2C%2C%2C50%2C1%2C0%2C0
however nothing appears on the display, until there's a beep, and then the screen glows off to the right hand side.
/g/TekScopes/photo/301027/3891765?p=Taken%2C%2C%2C50%2C1%2C0%2C0
No control button or knob makes any difference whatsoever.

If I remove jumper J100,
/g/TekScopes/photo/301027/3891766?p=Taken%2C%2C%2C50%2C1%2C0%2C0
then the scope no longer beeps and it just displays a bright dot in the centre of the screen.
/g/TekScopes/photo/301027/3891764?p=Taken%2C%2C%2C50%2C1%2C0%2C0


I'm slowly working my way through the manual but if anyone can give a heads-up as to where the problem might be found, then that would be much appreciated.

The manual states that the beep occurs when there is a short in the unregulated section of the power supply, yet I have checked all the PSU test points, IC's and caps and nothing there is obviously shorted.

One other thing which might be related is that the chassis (and therefore GND points) are at 65v DC in relation to the mains Earth potential.

Here in Brazil there is not really an Earth system used, and the mains here is 127VAC.

Anyway...

James,

You should know to fan cool the CCD IC on the bottom when out of the case. These run hot. Use a good compound to help with heat transfer from the IC to the heatsink, like MX-4 compound. This statement about external cooling and compound may be redundant not knowing what you have done. Failure of the CCD IC will cause problems.

I have a 2440 and fully recapped it using high temp, low ESR and long life types. The battery was replaced with an axial type that is double the mA/hr of the original. Once working, recalibrating was easy. I wonder if you need more recapping and battery to see if it helps. The 22mfd axial types in a photo you posted, I used ULD 47mfd because I had them. The lack of a true ground may be a problem.

Mark

Hi Mark.

I've only been running the scope out of the case long enough for the display to be shown but I do have some good thermal paste so I'll take your advice on that one, although who knows what this scope has been through before?

The reason that I mentioned the +/- 65vdc on the chassis is that it occurred to me that perhaps something had shorted to ground? I reasoned that considering all the LV rails are present, then it likely wouldn't be in the PSU and turned my attention to the HV board, but no shorts were obvious there. I wary of connecting another scope in this condition so I'll probably just create an Earth path by connecting into the Neutral line.

Regarding the battery, it seems this is the model which used two Dallas NVSRAM's.


Shortly after starting the thread, I came across a section in the manual for troubleshooting the display, which I plan to check out today. Let's see?



James

Followed the SM instructions for 'No Display' but haven't made any real progress, other than getting a dot to appear on the screen (as opposed to merely glowing off-screen) by adjusting the R587 'Horizontal Offset' pot.

/g/TekScopes/photo/301027/3892267?p=Taken%2C%2C%2C50%2C1%2C0%2C0

In fact, once the unit is fully 'on', there appears a second dot, which is slightly fainter.

/g/TekScopes/photo/301027/3892268?p=Taken%2C%2C%2C50%2C1%2C0%2C0

Also verified that the HV -1.9kV is present.
All the LV rails are present, although there is a 61v rail somewhere which I haven't yet discovered where to test (nor where it comes from).

Due to the 'Beep' at start-up, I feel certain that something is shorted somewhere, but why the rails are not low in that case, is a mystery to me?

2430A I had last week had no display, a 2.7Uf tant cap was loading a reg circuit (top PCB rear left)

--
Jeffeelcr

But this scope has a display (now).

Just ridiculously blurry

James,

Likely the resistors to the C and B of Q145, Q152 and Q269 collector only are bad. The originals are stressed. Use 1W types. These are the same problem in the 24x5/7 series. Even if they test alright, they could be bad in operation. Get resistors physically larger to take the higher voltage drop. Ones about 10mm long work fine. Mouser has a wait of 12 weeks for 442,000 ohm resistors. A pair of 1/2W 221,000 ohm resistors in series will work.

Mark

Thank you Mark.

I'll look into that and let you know how it goes..


James

In fact Mark, I actually have here a HV board with a blown transformer so I can grab the resistors from that . If for nothing else other than to see what happens?

Thanks again

You know how sometimes the Universe just sends you signals, perhaps to remind you that you are on the right path?

Well, this morning I received a phone call from the man who often supplies test equipment. He has a friend who buys up the assets of failed companies and when test stuff appears, I generally get a call. They just want rid of it and often pretty much give it away. I've bought too much to remember, however two items which spring to mind are spectrum analyzers, a HP 8569B for (the equivalent in Brazil) less than 20 Dollars and a Tek 492 for 35 Dollars. Both with failed PSU's and both fully functional following simple repairs. Insane! Although I mustn't forget the 'dead' (yet calibrated) Fluke 289 for 40 Dollars which only had a bad battery connection and is now my most accurate DMM.

Anyways, I digress... This morning, he rings me up to offer me two scopes for 50 Dollars. Would you believe it that one was a 2430A!?!? Lol

The other was/is a TAS 465 which he said was missing a board, but looking at the manual and a teardown, it appears to be complete. The problem being that it only displays a stationary 'dot' in the middle of the display.

So, opening up the new 2430A, revealed that on the top board, two tantalums had totally burned up, ironically including the 2.7?F one mentioned earlier in this thread..

Seemed reasonable to just switch the board from my blurry 2430A and sure enough the 'new' scope came to life, displaying the diagnostic menu et al.

Unsurprisingly it has various 'FAILS' however, it now gives the opportunity to switch boards and Hybrids in order to hone in on any problems.

The TAS 465 shall have to wait.

Addendum... The TAS 465 (which had been left plugged in but off) just went pffffftah! and dumped out a whole load of smoke. Guess I now know where to start.. :)


James

Shall keep this short, however...

swapping the PSU from the 'original' (no display) scope into the working 2430A, brings back the 'BEEP' and the glow off of the right hand side of the CRT (no display).

It is surely fair to assume that the root of the problem lays on the PSU board?


James

Further testing..

Swapping the known good PSU and top boards into the original 2430A reveals that the display problem is indeed created by the original PSU.

The original scope starts up and has a perfect display, as well as various diagnostic FAIL's, but we can take it that the HV board is functioning.


Now to figure out what is going wrong in the power supply..

James

A glow off to the right might suggest that the left horizontal plate signal was disconnected ...

Hi David, and thanks for the suggestion.

That could well be the case.

It seems that I need to figure out which part of the PSU board supplies the horizontal control circuit?

Whatever is causing this, is 100% located on that board, and nowhere else.

So far, on this original PSU board, I've changed two of the four Rifas, three resistors (which were all burned out), eight of the electrolytics, five LM358's (which had heated to the point of losing their graphics), three 1N4007 rectifiers, tested all the remaining caps for high ESR and probed across almost all of the remaining components looking for a short, but to no avail.

It's almost like the component only fails when a voltage is applied or it's put under load.

Unless someone has a better idea, I'm considering powering-up the two PSU boards side-by-side and taking readings at every point from the AC in, until the various DC outs, as surely that would reveal where the 'issue' lies?



Onwards and upwards.

James